Veronica Karina Quintuna Rodriguez
email: veronicakarina.quintunarodriguez@orange.com

Fabrice Guillemin
email: fabrice.guillemin@orange.com

Veronica Quintuna Rodriguez

VNF modeling towards the Cloud-RAN implementation

Keywords: NFV, VNF, Cloud-RAN, BBU, channel coding, multi-core, global scheduling

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I. INTRODUCTION

Network Function Virtualization (NFV) is a promising technique [START_REF] Han | Network function virtualization: Challenges and opportunities for innovations[END_REF] which provides network operators with high flexibility and agility to instantiate network functions on demand, e.g., control functions such as IP Multimedia Subsystem (IMS) or Evolved Packet Core (EPC). Instead of implementing such functions on expensive dedicated hardware, Virtualized Network Functions (VNFs) can be run on general purpose servers, ideally by using open-source software. Moreover, the possibility of instantiating on the fly control functions and configuring network elements via Software Defined Network (SDN) protocols (e.g., OpenFlow) allows network operators to create customized networks on demand, and thus, to explore new businesses offering innovative services.

Two current use cases investigated by network operators are virtual EPC (vEPC) and Radio Access Network as a Service (RANaaS). The first case is targeted at companies wishing to deploy private mobile networks by using the spectrum and the access network of a network operator. A vEPC can then be instantiated by a network operator and configured by a company according to own needs (firewall, private numbering, user's profile, etc.). RANaaS is one step further by offering to a client the possibility of deploying his own radio access network, even if the radio spectrum is still under the control of an operator. In this paper, we shall pay special attention to the RAN and explore how a RAN can be handled as a VNF by a network operator.

With regard to the implementation of NFV, one possibility is to instantiate a VNF as a software package on a Virtual Machine (VM) or a container with dedicated resources in terms of computing and storage. This is typically the case of current open-source software packages for EPC and Radio Access Network (RAN) functions, namely Open Air Interface (OAI)-cn and OAI-eNB, respectively [START_REF] Nikaein | Openairinterface 4G: an open LTE network in a PC[END_REF], [START_REF] Kaltenberger | OpenAirInterface: Open-source software radio solution for 5G[END_REF].

The basic reason for implementing a VNF on a single VM or more generally on the same data center in the case of multiple VMs is that some network functions cannot easily be dissociated (e.g., P-Gateway and MME in the case of EPC). Even when the functional decomposition of a VNF is possible, some performance constraints can limit the separation of some elements (e.g. in the case of RAN, the time budget for the TX base-band process is 1 millisecond).

On the other hand, a dedicated resource allocation to each VNF is extremely expensive since resources keep assigned even when the VNF is not active and any resource sharing model cannot be applied. In addition, beyond unused resources, the energy consumption aspects of NFV are also of prime interest. If NFV enables a split between hardware and software-based network functions, supporting VNFs on not optimized hardware may lead to additional costs in terms of energy. This is a major issue for network operators and more generally for the whole of the IT industry in the aim of reducing the energy footprint in the next future. Hence, any attempt to save energy is utmost important. In our understanding, resource pooling is one possibility to achieve energy efficiency.

In order to reach efficient resource utilization, we rely in this paper on the decomposition of a VNF into a set of sub-functions which can be executed independently of each other as much as possible. This is consistent with the notion of micro-services [START_REF] Heinonen | Mobility management enhancements for 5G low latency services[END_REF] as well as with the network slicing concept [START_REF] Zhou | Network slicing as a service: enabling enterprises' own software-defined cellular networks[END_REF]. In this perspective, a VNF is considered as a service chain that can be disseminated throughout the network. However, a major challenge is to place the various components on non co-localized data centers while meeting performance and resilience objectives [START_REF] Dietrich | Network service embedding across multiple providers with Nestor[END_REF].

The present work investigates the implementation of a VNF in the form of a chain of components to be executed on a multi-core platform. Instead of dedicating resources to each VNF, we assume that VNFs are served by one high capacity server, i.e., the global capacity is not split into virtual servers. This is motivated by the general results given by Kleinrock, chapter 5 [START_REF] Kleinrock | Queueing Systems, volume II: Computer Applications[END_REF], where it is shown that it is better to have jobs in a big queue instead of splitting jobs into sub-classes processed by dedicated servers.

In order to illustrate the implementation of VNFs, we consider the case of Cloud-RAN, which is a proposed architecture for future mobile networks [START_REF] Mobile | C-RAN: the road towards green RAN[END_REF], [START_REF] Chih-Lin | Toward green and soft: a 5G perspective[END_REF], [START_REF] Chih-Lin | Recent progress on C-RAN centralization and cloudification[END_REF]. The RAN functions are today hosted by eNodeBs (eNBs) but in the next future it is very likely that those functions can be grouped higher in the back-haul network, thus, dissociating Radio Remote Head (RRH) and Base Band Unit (BBU). In this case, centralized BBUs are executed on a large pool of computing resources that accomplish the virtualized RAN functions. Improving the response time in the execution of BBU functions is highly important because, beyond the hard real-time constraints of LTE, the runtime of some BBU functions might not have a deterministic behavior.

The goal of this work is to determine the amount of time which is possible to gain when applying parallel processing techniques in the execution of BBU functions. The resulting gain shall allow the separation of the BBU-pool and antennas (RRHs). In addition, the performance of virtual BBUs in terms of latency (i.e., runtime) determines the number of cells (MIMO/SISO) that can be hosted in the BBU-pool, as well as the required back-haul capacity.

In order to reduce the BBU runtime, we present a VNF threading model for the virtualization of BBU down-link functions. The proposed model defines threads at a finer granularity and maximizes the number of processes that can be executed in parallel. As a proof of concept, we demonstrate by simulation the gain in terms of latency which is a step towards the RAN cloudification. The simulation scenario assumes urban macrocells with heterogeneous traffic conditions for multiple UEs.

The major goal of the present study is to investigate which distance can be introduced between BBU and RRH functions and how many BBUs can be grouped together. The ultimate goal is to determine where to implement a BBU hostel and more generally a Central Office (Central Office (CO)) depending on the target radio coverage. This actually amounts to cloudifying RAN functions. For this purpose, we introduce general modeling principles for VNFs.

This paper is organized as follows: In Section II, we present the VNF model using the case of BBU functions. Implementation guidelines of a virtualized BBU are described in Section III. The scheduling strategy is detailed in Section IV. In Section V, we present the simulation results for BBU DL functions and the achieved gain in terms of latency. Concluding remarks are presented in Section VI.

II. VNF MODELING

A. Model Description

VNF modeling needs to be based on the general philosophy of high performance computing architectures which maintains thousands of threads in parallel [START_REF] Ryoo | Optimization principles and application performance evaluation of a multithreaded GPU using CUDA[END_REF]. Due to the intrinsic nature of a network function only some tasks can run independently one of each other, thus their parallel execution is not always possible.

In this context, we consider a VNF as a suite of executable processes. Each of them executes a specific sub-function in such a way that the global VNF can be realized. Some processes can be started only when the output of the previous one is available. Two such processes are necessarily executed in series. On the contrary, some tasks can run in parallel even if the subsequent task can be executed only when the output of all parallel processes is available. This leads to the representation of a VNF as depicted in Figure 1. The tasks t n-1 and t n are executed in series but t n+1 is composed of m sub-tasks t n+1,1 , . . . , t n+1,m , which can be executed in parallel. Task t n+2 can be executed only when all the tasks t n+1,j , j = 1, . . . , m are completed. With regard to the execution of VNFs on a Graphics Processing Unit (GPU)/Central Processing Unit (CPU)-based server, we can assume that the computing capacity is shared among the various active VNFs. The server can be composed of a single or several cores. In the case of a multi-core platform, we assume that cores are shared among the various VNFs. The scheduler contained in the kernel of the operating system is in charge of allocating the capacities of the cores.

Each task of a VNF can be considered as a job requiring a certain amount of service but upon service completion, a job can simply leave the system or give rise to another job with another service requirement, or to several jobs each with a certain amount of service, which can give rise to another job only when all the parallel jobs are completed.

We can hence easily see that the presence of batches can have a significant impact on those tasks which are executed in series, if the server capacity is shared among all active tasks. A VNF can momentarily receive more processing than other VNFs. Moreover, if the subsequent task of a VNF can start only when all the sub-tasks of a batch are all completed, then badly scheduling tasks can introduce significant latency in the execution of the global VNF. An abundant literature addresses the problem of batch processing and constraint based programming. In the following, we shall focus on nonpreemptive Round-Robin scheduling to test the benefit of parallel execution of sub-tasks of a VNF before considering more sophisticated scheduling algorithms.

B. The case of RAN

To illustrate the decomposition of a VNF into sub-functions or components, we consider in this section the case of RAN in the framework of LTE. Note that the current trend in 3GPP is to keep the architecture of RAN for the first phase of 5G similar to that of LTE. As a consequence, it is relevant to consider the current architecture for devising the RAN of 5G. We shall more precisely focus on distributed radio elements (RRH) and centralized BBUs with shared processing, i.e., the BBU functions are hosted in a multi-core data center.

Separating the BBU and RRH functions is motivated by many reasons, beyond the usual ones such as better computing resource utilization, energy savings, operational cost reduction, etc. Grouping several BBUs on the same data center also enables multi-point cooperation, advanced coordinated beamforming, optimal radio resource allocation, and efficient power management.

The BBU processes the data of User Equipments (UEs) as follows. Packets from the IP layer are handled by the PDCP, RLC and MAC layers before to be presented in a Transport Block (TB) format to the physical layer (PHY) as shown in Figure 2. LTE standardization defines the procedure for choosing the TBS for FDD and TDD frame configurations. In the rest of this paper, we shall consider the FDD scenario and we shall analyze only the functions involved in the transmission process (Tx) for the down-link (DL) direction. Furthermore, we shall consider only the best effort bearer, in spite of the fact that the 3GPP standards specify a multi-bearer architecture. In the horizon of 5G, it is very likely that all services will be supported by the best effort bearer. An adequate traffic management at the IP layer, possibly taking into account cross layer information (including that of the radio layer), will then be necessary to meet the QoS requirements. A quality of service architecture is however out of the scope of this paper.

The runtime of the VNF that implements the BBU functions is especially impacted by the sub-functions listed below and shown in Figure 3: The MAC layer defines the TBS and MCS which are function of the traffic load and the radio channel conditions. The radio scheduler selects the UEs to be served in each TTI, i.e., one sub-frame of 1 millisecond. The scheduler allocates resources according to some algorithm, for instance Round Robin or Proportional Fair among the various UEs which have data to receive or transmit. In general, the scheduler takes into account the radio conditions (namely Channel Quality Indicator (CQI)) to allocate radio resources while achieving some fairness criterion. In general, the CQI derives from channel models which are defined by vendors. The most Current mobile networks use high-throughput FPGA architectures to perform channel coding which can be considered as the bottleneck in the virtualization of RAN functions [START_REF] Islam Alyafawi | Critical issues of centralized and cloudified LTE-FDD radio access networks[END_REF]. As a consequence, we investigate the gain that can be achieved when virtual coding sub-functions are instantiated and executed in parallel as much as possible. We propose a multi-threading scheme which processes transport blocks by segments namely Code Blocks (CBs). LTE defines 6120 bits as the maximum Code Block Size (CBS). The CB is the data structure which triggers the encoding function. The number of parallel channel coding threads per UE depends on the TBS. When the TBS is not big enough, parallelism is not performed since only one CB is produced. Conversely, when the TBS takes the maximum standardized value, the number of parallel threads is given by TBS/CBS . See Figure 5 for an illustration.

Modulation sub-function is sequentially executed after the code block concatenation. Finally, the OFDM signal is generated after the modulation sub-function is completed.

Figure 6 shows an illustration of additional procedures required for MIMO. Note that the transport block size is also different. The aim of the threading model presented above is to investigate the performance gain in terms of execution time when running PHY sub-functions with a high level of parallelism. Concretely, we focus on the channel coding sub-function since it is the most expensive one. These claims are validated by simulation in Section V.

Looking for a high-performing virtual RAN, we also investigate the scheduling algorithm which dictates how much CPU time is allocated to each sub-function, thread and/or job. We describe general implementation guidelines in the next section.

III. IMPLEMENTATION OF A VIRTUALIZED BBU A. Implementation guidelines

The implementation of a VNF on a data center as depicted in Figure 1 calls for some implementation principles. Basically, when a software-based network function runs on a multicore platform, it generates multiple jobs which are executed according to a certain scheduling algorithm. (See Figure 7 for an illustration.)

The scheduling strategy plays a crucial role in the performance of VNFs, since it allocates the processing capacity and decides which runnable job will be executed, i.e., which processor executes a job and in what order the jobs are processed. Scheduling algorithms are implemented in the kernel of operating systems (OS), thus virtualization solutions where the resource allocation of CPU and memory is handled by an external entity (e.g., the hypervisor in the case of VM) have lower performance than those where the resource provisioning is kept by the OS (e.g.,containers).

The computing platform architecture is also important for the performance of VNFs, especially when considering the memory access time. Generally speaking, the time required to access instructions and data in memory is rarely negligible in general purpose computers.

Commodity parallel computing platforms based on GPUs can significantly increase the performance, since they give direct access to the instruction set and parallel runnable elements. The performance analysis of computer architectures and memory access mechanisms is beyond the scope of this work. It is nevertheless important to note that recent Fig. 7. BBU virtualization environment studies have compared the LTE BBU execution on GPU and CPU based architectures [START_REF] Zheng | Revisiting cloud RAN from a computer architecture point of view[END_REF]. Results show that GPU servers substantially increase the performance.

In the following, we focus our study on the fact that there is an important level of parallelism in the base-band processing of LTE. It can be exploited when applying an appropriate threading-based model that works together with the scheduling strategy of a multi-core platform.

B. Implementing BBUs in a virtualized environment

Let us consider a multi-core platform where two or more homogeneous cores are connected to a shared memory. This architecture represents a pool of resources which can host various eNBs. The main challenge is to guarantee the individual performance of each eNB while avoiding waste of resources. Thus, the computing platform processes the BBU functions of various radio network elements which are geographically distant. The front-haul capacity should also maintain the latency requirements in order to meet LTE deadlines.

We propose a container-based virtual environment in which the BBU functions run as applications. This approach improves the performance because there is a single kernel handling the BBU processes or jobs, in this way their execution uses less RAM and makes the runtime more efficient. See Figure 7 for an illustration.

We assume that all cores are controlled by a global scheduler. Partitioned scheduling is also possible in multi-core systems, however dedicating resources for a particular task or sub-function limits the performance of the VNF runtime [START_REF] Kleinrock | Queueing Systems, volume II: Computer Applications[END_REF].

The global scheduler makes its decisions based on the scheduling policy. We assume that all BBU-jobs have the same and the highest priority in the system. Thus, the scheduling policy allocates cores among the runnable BBU-threads. The scheduling strategy is detailed and illustrated in the following section.

IV. SCHEDULING STRATEGY FOR PARALLEL PROCESSING

In the framework of parallel computing on multi-core platforms, we define a global scheduler which selects the next job to be processed among those which are ready to be executed. The scheduler allocates a single processing unit (core) to each job according to the Round Robin criterion. We use nonpreemptive scheduling, in this way, a running job cannot be interrupted and it is executed until completion.

We focus on the channel coding function which is decomposed into several sub-functions that can be processed in parallel. See Figure 8 for an illustration. The execution of subfunctions occurs at the same instant on separate cores. Note that parallel computing is not possible on a single core, since only one sub-task is able to run at any instant. When sub-tasks are carried out simultaneously on one core (or even multiplecores) by time-sharing computing resources, we then refer to concurrent computing [START_REF] Pike | Concurrency is not parallelism[END_REF]. Parallel and concurrent computing can be used together, nevertheless, we focus our analysis on parallel computing.

As shown in Figure 8, we consider data decomposition in such a way that the channel coding sub-function is performed on different sub-sets of data at the same time, i.e., each parallel task works on a portion of data. Note that functional parallelism is not applicable on the PHY layer of BBUs, since modulation, channel coding and IFFT sub-functions must be executed in series.

In the framework of software-based BBUs, each LTE subframe generates a job for channel coding processing, i.e., a new job requires service every millisecond. When parallel computing is not applied, as shown in Figure 8(a), a channel coding job denoted by CC k for the k-th sub-frame is executed on a single core even when more cores are available. If the channel coding job CC k arrives when the previous one CC k-1 is not finished, the scheduler allocates to it the next free core. In the case that all cores are busy, the job is queued. This scenario is not desirable when executing base-band functions, mainly when closed-loop processes are active, e.g., the HARQ process defined in LTE.

In order to speed up the channel coding sub-function, we decompose each channel coding job CC k in a suite of subjobs. Each parallel sub-job works on a portion of BBU data. The sub-job belonging to the n-th UE allocated in the k-th sub-frame is denoted by CC k,n . Channel coding jobs arrive in batches at the computing platform every millisecond. (See Figure 8(b) for an illustration.) The batch size (i.e., the number of active jobs) is variable and corresponds to the number of UEs allocated into a sub-frame. The service time of a job in this batch varies in function of the TBS which is determined by the radio scheduler in the MAC layer. Batches are processed in a First-Come-First-Serve discipline. The global scheduler selects jobs in a circular order and assigns them one core to each.

We now consider a finer granularity in the decomposition of channel coding sub-functions. Thus, each job CC k,n of the n-th UE is split in a set of m parallel sub-jobs, where the m-th sub-job is denoted by CC k,n,m . This is illustrated in Figure 8(c). The resulting number of parallel channel coding jobs, i.e., the batch size, varies every millisecond (or more generally, according to the periodicity of radio scheduling). Thus, the batch size is the product of active UEs and the number m of CBs. Note that m can take different values for each UE. As in the preceding threading model, incoming batches join the tail of a single queue and each job gets access to a particular core under Round-Robin criterion.

We evaluate these scenarios by simulation. Performance results and simulation settings are described in the following section.

V. PERFORMANCE ANALYSIS OF A VIRTUALIZED BBU A. Simulation settings

In this section, we evaluate by simulation the execution time of virtualized BBU functions. First, we analyse the performance on a single-core platform. Then, we present the gain in terms of latency on a multi-core platform employing the threading model depicted in Figure 4. The simulation emulates a container-based virtualization environment with a global scheduling as described in Section III. We utilize the software-based RAN solution named OAI-5G, in order to determine the execution time of each runnable job.

We emulate real traffic conditions for an urban macro cell with multiple connected UEs. Concretely, we consider heterogeneous traffic where UEs with small needs in terms of radio resources coexist with UEs requiring high data rates. In addition, the simulation scenario takes place during the peakhour, i.e., we assume a greedy consumption of radio resources with the aim to evaluate the worst case. The simulation tool implements the Release 12 of the LTE specification for the PHY layer which is defined by the 3GPP TS 36.213, version 12.4.0 [START_REF]LTE, evolved universal terrestrial radio access, physical layer procedures[END_REF].

The simulation is performed for an LTE bandwidth of 20MHz, FDD transmission and SISO configuration. We focus our study on the execution of the PHY DL taskset which is composed of channel coding, modulation, and IFFT functions. We are concretely interested in the runtime performed by each TTI (1 millisecond).

The simulation selects the modulation order Q m and more concretely the MCS index I M CS based on the emulated radio conditions, i.e., the SINR. It is shown in Table I. In the same way, we define the number of Physical Resource Blocks (PRB) for an UE in function of the emulated traffic in the cell and of the data load per user. Note that for an LTE Bandwidth of 20 MHz the available number of PRB is 110 [START_REF]LTE, evolved universal terrestrial radio access, physical layer procedures[END_REF].

We can easily identify the I T BS from the I M CS looking up the table 7.1.7.1-1 presented in the LTE Physical layer specification 3GPP TS 36.213 version 12.4.0 Release 12 [START_REF]LTE, evolved universal terrestrial radio access, physical layer procedures[END_REF]. Used values are show in Table II.

B. Performance results: PHY runtime

The simulation shows the runtime of the PHY layer for a single cell and multiple connected UEs. In a first step, we do not consider the parallel execution of BBU functions. Figure 9 presents separately the runtime for the Modulation, Channel coding and IFFT sub-functions.

The execution time of the modulation sub-function depends on the modulation order and on the number of PRBs. The IFFT sub-function depends only on the number of PRBs. Finally the Channel Coding depends on the TBS, i.e., the quality of radio conditions, the amount of data to be transmitted by each UE and the traffic load in the cell. Simulation results which are based on the OAI performance show that the runtime of the whole PHY layer takes around 785 microseconds. It does not leave enough margin for processing L2 and L3 BBU functions. This fact may limit the implementation of centralized BBUs. We present below the gain that can be obtained when using implementation guidelines that were described in Section III.

C. Gain in terms of latency

In order to reduce the execution time of the BBU, we focus our analysis on the channel coding sub-function. We simulate a multi-core platform with c cores and a global scheduler managing all of them as detailed in Section IV. We evaluate the gain in terms of latency when using the threading model presented in Section II. We are able to execute one thread per UE, and even one thread by CB. It is possible to execute up to eleven threads per UE when it has the maximum number of RBs and the maximum modulation order according to the LTE-Release 12.

Figures 10(a) and 10(b) show the gain of the parallel execution of channel coding in a multi-core platform with 2 and 6 cores respectively. Results are shown for the following scenarios:

1) UEs in parallel, CBs in series: one thread per UE.

2) UEs and CBs in parallel: one thread per CB.

3) UEs and CBs in series: no parallelism.

It clearly appears that the execution of the channel coding sub-function with one thread per CB can halve the runtime even when using only twice the processing capacity. When the processing capacity is significantly increased, (e.g., for a multi-core platform with 6 cores), the runtime of the channel coding can be reduced almost five times compared with the original model which is not based on parallel threads.

VI. CONCLUSION

We have studied in this paper the execution of VNFs on multi-core platforms. Results show that NFV implementation requires an adequate computing platform, an efficient global scheduling algorithm, as well as an optimum threading model in order to minimize the runtime of VNFs.

In the case of software-based RAN functions, we defined threads at a fine granularity and as a result, the most expensive sub-function in terms of latency (namely channel coding) was strongly decreased. This fact is a step towards the Cloud-RAN implementation, since some tasks of the BBU can be moved higher in the network, this offers more possibilities in the functional split.

In a future work, we shall investigate how these subfunctions might be executed on various data-centers separated from each other. Moreover, we shall investigate how to account of the uplink and what strategy is the most suited for sharing computing resources.

Fig. 1 .

 1 Fig. 1. VNF modeling

Fig. 2 .

 2 Fig. 2. BBU elements

•

  Compression and encryption procedures of the PDCP sub-layer; • Concatenation tasks executed by the RLC sub-layer; • Radio scheduling and HARQ management handled by the MAC layer, and • Sub-functions performed by the PHY layer, which are channel coding, modulation and OFDM signal generation.

Fig. 3 .

 3 Fig. 3. VNF model of the BBU

Fig. 4 .

 4 Fig. 4. VNF threading model of the BBU

Fig. 5 .

 5 Fig. 5. PHY threading model

Fig. 6 .

 6 Fig. 6. MIMO threading model

Fig. 8 .

 8 Fig. 8. Global scheduler for parallel computing

  MCS index I M CSQm TBS index I T BS 0-9 2I T BS = I M CS 10-16 4 I T BS = I M CS -1 17-28 6 I T BS = I M CS -2Based on the TBS index, I T BS , and the number of Physical RB, N P RB , we determine the TBS from the table 7.1.7.2.1.1-1 of the LTE PHY specification 36.213 version 12.4.0 Release 12[START_REF]LTE, evolved universal terrestrial radio access, physical layer procedures[END_REF]. It is for transport blocks not mapped to two or more layer spatial multiplexing, i.e., SISO configuration. A fragment is shown in TableIII

Fig. 9 .

 9 Fig. 9. Execution time of PHY sub-functions

(a) c = 2 (b) c = 6 Fig. 10 .

 2610 Fig. 10. Parallel execution of channel coding.

(a) c = 2 (b) c = 6 Fig. 11 .

 2611 Fig. 11. Probability density of channel coding runtime.

Figures 11 (

 11 Figures 11(a) and 11(b) show the probability density distribution function for each scenario. Results show that for a multi-core platform with c = 6, the execution time of the channel coding when running one thread per CB or even one thread per UE becomes less variable (i.e., runtime values are more concentrated around the mean). This fact is relevant for the incoming cloudification of RAN functions.

TABLE II MODULATION

 II AND TBS INDEX

TABLE III TRANSPORT

 III BLOCK SIZE TABLE

				N P RB
	I T BS	4	5	6	7	8	9
	12	904 1128 1352	1608 1800	2024
	13	1000 1256 1544	1800 2024	2280
	14	1128 1416 1736	1992 2280	2600


Leif E Sander
email: erik.sander@charite.de

Johan Garaude
email: johan.garaude@inserm.fr

The mitochondrial respiratory chain: a metabolic rheostat of innate immune cell-mediated antibacterial responses

Keywords: Electron transport chain, metabolism, innate immunity, bacterial infection, pattern-recognition receptors

Upon microbial infection, cells of the innate immune system undergo profound metabolic reprogramming in order to eradicate pathogens, promote inflammation, and eventually restore tissue homeostasis. Mitochondria are at the core of these adaptations, given their dual role as metabolic hubs and innate immune signaling platforms. The mitochondrial electron transport chain (ETC) is very well characterized at the genetic, molecular, structural, and biochemical level. In contrast, the role for mitochondrial ETC and metabolites beyond fulfilling cellular ATP synthesis in innate immune cell biology was not understood until recently.

Here we discuss the latest advances in our understanding of immune functions of mitochondria and particularly the mitochondrial respiratory chain.

Introduction: of the importance of metabolism in the innate immune system.

The immune system of jawed vertebrates can be conceptually divided into two branches; the innate and the adaptive immune system [START_REF] Litman | The origins of vertebrate adaptive immunity[END_REF].

Although evolutionarily distinct, the innate and adaptive immune system are functionally closely intertwined [START_REF] Boehm | Evolution of vertebrate immunity[END_REF]. Professional phagocytes, such as macrophages or dendritic cells (DCs) continuously probe their environment for microbial invaders. They express a wide array of 'pattern recognition receptors' (PRRs), a large group of germline-encoded immune receptors that detect highly conserved molecules in microorganisms, called 'pathogen associated molecular patterns' (PAMPs) [START_REF] Janeway | Approaching the asymptote? Evolution and revolution in immunology[END_REF]. Ligation of PRRs, such as Toll-like receptors (TLRs), leads to rapid activation of intracellular signaling cascades eliciting proinflammatory and antimicrobial responses (Figure 1). Upon activation, some members of the NOD-like protein (NLR) family of PRRs can assemble into large cytosolic protein complexes, termed inflammasomes [START_REF] Martinon | The inflammasomes: guardians of the body[END_REF]. In antigen presenting cells (APCs) such as DCs, PRR activation promotes antigen processing and presentation to cells from the adaptive arm of immunity (i.e T cells and B cells), upregulation of co-stimulatory molecules and cytokine production.

Other professional phagocytes including macrophages, monocytes and neutrophils show a high capacity to engulf and destroy microbes such as bacteria.

Nevertheless, they also have the capacity to present antigen and produce various cytokines and chemokines upon PRR activation to further tailor immune responses mediated by adaptive immune cells such as T cells or B cells. Thus, PRRs represent a molecular link between innate and adaptive immunity [START_REF] Iwasaki | Regulation of adaptive immunity by the innate immune system[END_REF].

Microbes can trigger distinct and specific set of PRRs. The nature of the PRRs engaged during infection shapes the innate immune responses generated by skewing innate immune cells differentiation, which rely on complex molecular events. These events are largely viewed as a succession of phosphorylation events eventually leading to controlled gene transcription. However, early observations noted a profound switch to glucose fermentation upon activation of human and murine professional phagocytes [START_REF] Chance | Respiratory Enzymes in Oxidative Phosphorylation[END_REF][START_REF] Newsholme | Metabolism of glucose, glutamine, long-chain fatty acids and ketone bodies by murine macrophages[END_REF][START_REF] Oren | METABOLIC PATTERNS IN THREE TYPES OF PHAGOCYTIZING CELLS[END_REF]Sbarra andKarnovsky, 1960, 1959) and recent studies have further delineated the complex metabolic reprogramming that takes place upon immune cell activation (see other reviews in this issue of Mitochondrion) including macrophages and dendritic cells [START_REF] Gleeson | Metabolic reprogramming & inflammation: Fuelling the host response to pathogens[END_REF][START_REF] O'neill | Immunometabolism governs dendritic cell and macrophage function[END_REF]. As the main metabolic hubs of animal cells, mitochondria are naturally positioned at the center of these 'immunometabolic' regulations. Intriguingly, mitochondria also serve as critical signaling platforms for PRRs including RIG-I like receptors (RLR), or NLR protein 3 (NLRP3). Moreover, release of mitochondriaderived molecules during infection can activate PRRs [START_REF] Krüger | Human TLR 8 senses UR / URR motifs in bacterial and mitochondrial RNA[END_REF][START_REF] Shimada | Oxidized Mitochondrial DNA Activates the NLRP3 Inflammasome during Apoptosis[END_REF][START_REF] Zhou | A role for mitochondria in NLRP3 inflammasome activation[END_REF]. Hence, intricate molecular interactions between mitochondria and immune signaling pathways unfold during infection. In contrast, potential interactions between mitochondria or mitochondrial-derived metabolites and invading pathogens are less well studied. Innate immune cell differentiation thus results from specific combinations of signaling events and metabolic adaptations that are dictated by the biochemical composition and infectious features of the microbe encountered. Mitochondria have been found to play important function in both PRR signaling and metabolism of innate immune cells but whether these two faces of mitochondria are interconnected is only started to be understood.

A growing number of studies delineates the metabolic events that occur during innate immune cell activation placing mitochondria at the crossroads of immune signaling and metabolic adaptations. These metabolic pathways generate reductive intermediates that are eventually oxidized in the electron transport chain (ETC). While this process was generally considered as a means to supply sufficient ATP to resting professional phagocytes and APCs, it is becoming increasingly clear that the ETC's contribution in innate immunity extends beyond ATP synthesis when cells become activated. Important progress has been made in our understanding of metabolic adaptation in cells of the adaptive immune system and T cell in particular. We strongly encourage the readers willing to understand the metabolic aspects of adaptive immune cells to refer to comprehensive reviews published in this issue of Mitochondrion and elsewhere [START_REF] Buck | T cell metabolism drives immunity[END_REF][START_REF] Mills | Mitochondria are the powerhouses of immunity[END_REF]. Here we review recent works highlighting the unexpected antibacterial innate immune functions of the mitochondrial respiratory chain and mitochondrial metabolism.

Metabolic flux variations upon microbial encounter

Microbial encounters initiate complex activation and differentiation programs in phagocytes, which critically shape the nature of the immune response that will be generated. In the middle of last century, several laboratories made the observation that the phagocytes change their metabolic state upon activation raising the possibility that those metabolic adaptations would be part of this differentiation program [START_REF] Chance | Respiratory Enzymes in Oxidative Phosphorylation[END_REF][START_REF] Oren | METABOLIC PATTERNS IN THREE TYPES OF PHAGOCYTIZING CELLS[END_REF]Sbarra andKarnovsky, 1960, 1959).

Changes in metabolic pathways upon immune cell activation have been reviewed elsewhere [START_REF] Gleeson | Metabolic reprogramming & inflammation: Fuelling the host response to pathogens[END_REF][START_REF] Mills | Mitochondria are the powerhouses of immunity[END_REF][START_REF] O'neill | A guide to immunometabolism for immunologists[END_REF], and we will only provide a brief overview of the main metabolic adjustments found in macrophages upon microbial stimulation. The majority of studies investigating metabolic changes in macrophages have used selected microbial ligands, most commonly TLR4 agonist lipopolysaccharides (LPS) as a proxy for bacterial infection.

Activation of PRRs by bacterial components strongly increases phagocyte's glycolytic flux, and levels of different glycolytic intermediates are highly elevated in activated macrophages (Everts et al., 2014;[START_REF] Everts | Commitment to glycolysis sustains survival of NO-producing inflammatory dendritic cells[END_REF][START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF].

Increased in glycolytic flux and lactate production is found in macrophages upon infection with different bacterial species including Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF][START_REF] Palsson-Mcdermott | Pyruvate kinase M2 regulates Hif-1alpha activity and IL-1beta induction and is a critical determinant of the warburg effect in LPS-activated macrophages[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF], Mycobacterium tuberculosis [START_REF] Mehrotra | Pathogenicity of Mycobacterium tuberculosis is expressed by regulating metabolic thresholds of the host macrophage[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF], or Escherichia coli (E. coli) [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF]. Interestingly, this increase in glycolysis and lactate production is observed even under unlimited access to oxygen [START_REF] O'neill | Immunometabolism governs dendritic cell and macrophage function[END_REF]. For this reason, this induction of aerobic glycolysis has been repeatedly compared to the 'Warburg effect' observed in cancer cells. Surprisingly, concomitant to this increase in glycolysis, a decrease in mitochondrial oxygen consumption was observed suggesting that glucose catabolism was diverted from its use by the mitochondria [START_REF] Everts | Commitment to glycolysis sustains survival of NO-producing inflammatory dendritic cells[END_REF][START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF][START_REF] Krawczyk | Toll-like receptorinduced changes in glycolytic metabolism regulate dendritic cell activation[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF]. However, recent studies revealed that activation of TLRs others than TLR4 could increase mitochondrial oxygen consumption and engage distinct metabolic programs in human monocytes [START_REF] Lachmandas | Rewiring cellular metabolism via the AKT/mTOR pathway contributes to host defence against Mycobacterium tuberculosis in human and murine cells[END_REF] and plasmacytoid dendritic cells (D. Wu et al., 2016). Therefore, while the increased glycolytic flux is clearly a hallmark of many activated innate immune cells (macrophage, DCs, monocytes or neutrophils) other more subtle metabolic reprogramming events are likely to unfold depending on the nature of the microbial stimulus and the PRRs engaged, as well as involved cell type.

The pentose phosphate pathway (PPP), which diverts glycolytic intermediates towards nucleotide and amino acid production, is also augmented by LPS/TLR4 stimulation [START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF]. It generates reducing equivalents of NAPDH, which are required for fatty acid synthesis and contribute to cellular redox homeostasis in macrophage. A recent study by Haschemi et al. highlighted some molecular details about PPP regulation and functional consequences during murine macrophage activation. This study showed that a substantial portion of glycolysis-derived carbons is routed towards the PPP upon LPS stimulation [START_REF] Haschemi | The sedoheptulose kinase CARKL directs macrophage polarization through control of glucose metabolism[END_REF]. The authors notably identified the carbohydrate kinase-like protein (CARKL) as a sedoheptulose kinase (converting the PPP metabolite sedoheptulose to sedoheptulose-7-phosphate) that can repress LPS-induced inflammatory cytokines production by macrophage. CARKL is downregulated in LPS-treated macrophages (also called M1 macrophages or inflammatory macrophages) but upregulated in IL4-activated macrophage (or M2 macrophage), which are essential to fight parasite infection and can modulate immune responses through the production of regulatory cytokines such as IL-10.

Consistently, the authors found that loss of CARKL induced a mild activation in resting macrophage while its over-expression sensitized macrophage to M2 polarization. This suggests that CARKL may exert a pivotal function in macrophage polarization.

The role of fatty acid metabolism during professional phagocyte activation is less well understood. Fatty acid oxidation (FAO) is augmented in mouse and human monocytes primed with activators of the NLRP3 inflammasome, involving mitochondrial NADPH oxidase 4 (NOX4)-dependent ROS production [START_REF] Moon | NOX4dependent fatty acid oxidation promotes NLRP3 inflammasome activation in macrophages[END_REF]. However, IL4-activated macrophage have been shown to use FAO contrary to macrophages induced by stimulation with LPS and Interferon-γ [START_REF] Huang | Cellintrinsic lysosomal lipolysis is essential for alternative activation of macrophages[END_REF] . Pearce an colleagues used etomoxir, an inhibitor of carnitine-palmitoyl transferase 1 (CPT1), a key enzyme in the long-chain fatty acid oxidation to show that lipolysis of triacylglycerol substrates by phagocytes was important for oxidative phosphorylation elevation and expression of genes that define IL-4activated macrophages. In contrast, the genetic deletion of Cpt2, which also strongly diminished long-chain fatty acid oxidation in the mitochondria, did not alter IL-4-stimulated macrophage polarization as shown by Finkel and co-workers [START_REF] Nomura | Fatty acid oxidation in macrophage polarization[END_REF]. Fatty acid synthesis seems in contrast to be closely related to inflammatory phenotypes in macrophages. It is augmented in LPS-stimulated macrophages [START_REF] Feingold | Mechanisms of triglyceride accumulation in activated macrophages[END_REF] and fatty acid synthase is induced during sepsis through a mechanism that requires mitochondrial uncoupling protein 2 (UCP2) [START_REF] Moon | UCP2-induced fatty acid synthase promotes NLRP3 inflammasome activation during sepsis[END_REF]. More work is clearly needed to further define the exact contributions of FAO and fatty acid synthesis to the functions of macrophages and other innate immune cells during infection [START_REF] Namgaladze | Fatty acid oxidation is dispensable for human macrophage IL-4-induced polarization[END_REF].

While metabolism of various amino acids is likely to contribute to immune function [START_REF] O'neill | A guide to immunometabolism for immunologists[END_REF], glutamine has emerged as the most prominent amino acid in macrophage and DC metabolic reprogramming. Its catabolism (glutaminolysis) has been found to extensively replenish the TCA cycle and the hexosamine pathway while glycolytic product pyruvate is diverted from entry into the mitochondrion in murine macrophages [START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF].

In LPS-primed macrophages, glutamine is thought to contribute to succinate accumulation through anaplerosis of α-ketoglutarate [START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF] and is required for production of the inflammatory cytokine IL-1β [START_REF] Wallace | Glutamine and macrophage function[END_REF] and the generation of nitric oxide (NO) through arginine synthesis in murine macrophages and human monocytes (Murphy and Newsholmes, 1998), thereby contributing to antimicrobial functions of macrophages. In addition NO can inhibit mitochondrial respiration and ATP production through nitrosylation of the iron-sulfur containing ETC complex I, II and IV [START_REF] Beltran | The effect of nitric oxide on cell respiration: A key to understanding its role in cell survival or death[END_REF] thus accounting for OXPHOS shut down observed in LPS-stimulated macrophages and DCs. Consistently, glutamine deficiency reduced inflammasome activation [START_REF] He | Glutamine modulates macrophage lipotoxicity[END_REF]. Glutamine-derived succinate can also be used to generate nitric oxides [START_REF] Bellows | Glutamine is essential for nitric oxide synthesis by murine macrophages[END_REF], which in turn can inhibit the mitochondrial respiratory chain in LPS stimulated DCs [START_REF] Everts | Commitment to glycolysis sustains survival of NO-producing inflammatory dendritic cells[END_REF]. However, whether glutamine catabolism can contribute to other metabolic pathways in phagocytes still needs to be further documents. A recent study nonetheless revealed some interesting features of the use of glutamine in macrophages. Liu et al. showed that α-ketoglutarate produced by glutaminolysis accumulates in IL-4-stimulated macrophages in which it induces Jumonji domaincontaining protein 3 (Jmjd3)-dependent epigenetic reprogramming and instructs cells to augment FAO. In contrast, α-ketoglutarate represses the NF-kB pathway in LPS-primed macrophages through prolyl-hydroxylase (PHD)-dependent posttranslational modification of IKKb. The ratio between glutamine-derived αketoglutarate and succinate controls macrophage differentiation. A high αketoglutarate/succinate ratio promotes IL-4-induced macrophage differentiation, whereas a low ratio supports inflammatory macrophage generation. Hence, glutamine metabolism is a critical a key metabolic checkpoint of macrophage fate and endotoxin tolerance [START_REF] Liu | α-ketoglutarate orchestrates macrophage activation through metabolic and epigenetic reprogramming[END_REF].

Adaptations of the mitochondrial Electron Transport Chain (ETC) during microbial infection

ETC adaptations as a response to fuel source fluctuations

Most catabolic processes converge on the mitochondrial ETC by supplying electrons through the reductive equivalents of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH). The ETC comprises two electron carriers (coenzyme Q [CoQ]/ubiquinone and cytochrome c) and four respiratory complexes (complex I-IV [CI-CIV]). Respiratory complexes I, III, and IV can assemble as large molecular supercomplexes (SCs) in the mitochondrial inner membrane (for a comprehensive review see [START_REF] Enriquez | Supramolecular Organization of Respiratory Complexes[END_REF]). The transfer of electrons through the respiratory complexes generates a H + gradient that is used by the H + -ATP synthase (CV) to generate ATP. The supramolecular organization of the electron transport chain and its functional implications are a subject of passionate debate [START_REF] Cogliati | Mechanism of super-assembly of respiratory complexes III and IV[END_REF][START_REF] Lapuente-Brun | Supercomplex Assembly Determines Electron Flux in the Mitochondrial Electron Transport Chain[END_REF][START_REF] Mourier | The respiratory Chain supercomplex organization is independent of COX7A2L isoforms[END_REF][START_REF] Pérez-Pérez | COX7A2L Is a Mitochondrial Complex III Binding Protein that Stabilizes the III2+IV Supercomplex without Affecting Respirasome Formation[END_REF]. Nevertheless, recent studies have provided detailed insights into the sequential assembly of the respiratory chain [START_REF] Cogliati | Mechanism of super-assembly of respiratory complexes III and IV[END_REF][START_REF] Guerrero-Castillo | The Assembly Pathway of Mitochondrial Respiratory Chain Complex I[END_REF][START_REF] Mick | MITRAC links mitochondrial protein translocation to respiratory-chain assembly and translational regulation[END_REF][START_REF] Stroud | Accessory subunits are integral for assembly and function of human mitochondrial complex I[END_REF] and resolved the structure of the so-called respirasome (a supercomplex composed of CI, a dimer CIII and CIV) [START_REF] Gu | The architecture of the mammalian respirasome[END_REF][START_REF] Letts | The architecture of respiratory supercomplexes[END_REF]M. Wu et al., 2016) and CI [START_REF] Fiedorczuk | Atomic structure of the entire mammalian mitochondrial complex I[END_REF]. The dynamic assembly of respiratory complexes into supercomplexes may provide a structural environment favorable to assembly of CI [START_REF] Enriquez | Supramolecular Organization of Respiratory Complexes[END_REF][START_REF] Genova | Functional role of mitochondrial respiratory supercomplexes[END_REF][START_REF] Lapuente-Brun | Supercomplex Assembly Determines Electron Flux in the Mitochondrial Electron Transport Chain[END_REF], stabilize the individual respiratory complexes [START_REF] Acín-Pérez | Respiratory complex III is required to maintain complex I in mammalian mitochondria[END_REF], prevent ROS production by sequestrating reactive radicals [START_REF] Maranzana | Mitochondrial respiratory supercomplex association limits production of reactive oxygen species from complex I[END_REF], or organize electron flux to optimize the use of available substrate required for ATP synthesis and oxygen consumption [START_REF] Guarás | The CoQH2/CoQ Ratio Serves as a Sensor of Respiratory Chain Efficiency[END_REF][START_REF] Lapuente-Brun | Supercomplex Assembly Determines Electron Flux in the Mitochondrial Electron Transport Chain[END_REF]. Switch in the nature of the fuel that feed cellular metabolism alters the intramitochondrial ratios of FADH2 and NADH (herein F/N ratios) [START_REF] Speijer | Being right on Q: shaping eukaryotic evolution A kinetic theory in which FADH 2 /NADH ratios determine mitochondrial ROS formation[END_REF], which may require adjustment of respiratory chain super-assembly. The predicted F/N ratio is lowest for glucose oxidative breakdown with a ratio of 0.2 (one molecule of FADH2 formed per five molecules of NADH), but is high (0.5) for saturated long-chain fatty acids with one molecule of FADH2 per two NADH molecules [START_REF] Speijer | Oxygen radicals shaping evolution: Why fatty acid catabolism leads to peroxisomes while neurons do without it: FADH2/NADH flux ratios determining mitochondrial radical formation were crucial for the eukaryotic invention of peroxisomes and catabolic tissue[END_REF].

Interestingly, fasting modifies SCs distribution and ETC activity (Lapuente-Brun et al., n.d.), and shifting nutrient substrate from glucose to fatty acids can downregulate CI content [START_REF] Guarás | The CoQH2/CoQ Ratio Serves as a Sensor of Respiratory Chain Efficiency[END_REF]. Mechanistically, it was proposed that accumulation of the reduced form of CoQ can generate a reverse electron transfer leading to oxidation of CI subunits [START_REF] Chouchani | A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury[END_REF]. The resulting degradation of CI is likely to modify the stoichiometry of SCs to adjust the electron flow within the ETC [START_REF] Guarás | The CoQH2/CoQ Ratio Serves as a Sensor of Respiratory Chain Efficiency[END_REF]. Nutrient availability also regulates mitochondrial dynamics. However, the natural occurrence and functional roles supercomplexes still remain to be fully established.

Architectural changes of the ETC and their role in innate immune responses

Given the profound metabolic changes during activation of innate immune cells, it could be assumed that the architecture and the activity of the ETC would also vary in such conditions. A first indication came from observations in macrophages infected with Listeria monocytogenes, an intracellular Gram-positive pathogen that causes listeriosis. It was found that Listeria infection causes transient mitochondrial network fragmentation [START_REF] Stavru | Listeria monocytogenes transiently alters mitochondrial dynamics during infection[END_REF]. These alterations require pore-forming toxin listeriolysin O (LLO), but are independent of dynaminlike protein 1 (Drp1) or Opa1, two well-known proteins involved in mitochondrial fusion/fission dynamics [START_REF] Stavru | Atypical mitochondrial fission upon bacterial infection[END_REF]. In contrast, the obligate intracellular bacterium Chlamydia trachomatis maintains mitochondrial network integrity by inhibiting Drp1-induced mitochondrial fragmentation [START_REF] Chowdhury | Chlamydia preserves the mitochondrial network necessary for replication via microRNA-dependent inhibition of fission[END_REF].

Mitochondrial dynamics play a particularly important role in antiviral immunity. It was observed that RIG-I (a member of the RIG-I-like receptors or RLR), its downstream adaptors mitochondrial antiviral signaling (MAVS) and mitochondria localize around centers of viral replication upon infection by single-stranded RNA Sendai virus or 5'ppp-RNA detection [START_REF] Onoguchi | Virus-infection or 5'ppp-RNA activates antiviral signal through redistribution of IPS-1 mediated by MFN1[END_REF]. MAVS is a RLRsignaling adaptor that translocates at the mitochondria upon viral infection to form a mitochondrial antiviral signaling platform and induce type-I interferons (IFN) essential for fighting viral infection (West et al., 2011b). Consistent with the link between mitochondrial dynamics and MAVS signaling, both mitofusin proteins (MFN) 1 and 2, two well-characterized fusion regulators, can interact with MAVS [START_REF] Castanier | Mitochondrial dynamics regulate the RIG-I-like receptor antiviral pathway[END_REF][START_REF] Yasukawa | Mitofusin 2 inhibits mitochondrial antiviral signaling[END_REF]. Although the respective role of MFN1 and MFN2 in RLR signaling and the formation of MAVS platform is still not clear, other proteins involved in mitochondrial dynamics have been found to interact with proteins of the RLR/MAVS pathway during viral infection (West et al., 2011b) supporting the notion that mitochondrial dynamics is closely linked to innate immune signaling. However, whether this also occurs upon bacterial infection is still poorly appreciated. Additional research should clarify the role of mitochondrial dynamics related protein in innate immune signaling and their contribution to phagocyte-mediated antibacterial responses. Nevertheless, taken together those studies suggested that the alterations of the mitochondrial network induced by bacterial infection and inflammation are likely to affect the bioenergetics and immune signaling of innate immune cells. Accordingly, significant changes in the expression of mitochondrial carriers including the dicarboxylate carrier DIC, the citrate carriers CIC or the adenine-nucleotide carriers ACC have been associated with inflammation [START_REF] Iacobazzi | Mitochondrial carriers in inflammation induced by bacterial endotoxin and cytokines[END_REF].

Adaptations of the ETC are therefore likely to occur to meet the strong energetic requirements and metabolic adjustments of activated immune cells.

It was originally postulated that the overall mitochondrial oxygen consumption by the respiratory chain of macrophages, DCs and monocytes is reduced following TLR- [START_REF] Krawczyk | Toll-like receptorinduced changes in glycolytic metabolism regulate dendritic cell activation[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF] and Dectin-1 [START_REF] Cheng | mTOR-and HIF-1alpha-mediated aerobic glycolysis as metabolic basis for trained immunity[END_REF] activation. Thus it has been general wisdom that during infections, the ETC might serve other roles than sustaining metabolic fluxes and generating ATP, which would then mainly arise from glycolysis [START_REF] Mills | Mitochondria are the powerhouses of immunity[END_REF].

However, an increase in glucose-dependent oxygen consumption was observed within few hours after LPS stimulation of DCs (Bart Everts et al., 2014;[START_REF] Everts | Commitment to glycolysis sustains survival of NO-producing inflammatory dendritic cells[END_REF] suggesting that mitochondrial respiration is required for innate immune cell activation. In fact, phagocytosis of living Escherichia coli by macrophages can the maximal respiration capacity and induces changes in the architecture of the ETC within two hours post-infection leading to a decrease in the abundance of CI and CI-containing SCs [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF]. Consequently, CI activity is decreased upon E. coli infection. However, the overall mitochondrial respiratory capacity is maintained at least for several hours after bacterial stimulation. This is explained by an increase in mitochondrial glycerol-3-phosphate dehydrogenase activity, which allows mitochondrial recycling of cytosolic NADH and supply of electrons to the ETC [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF]. CII activity is also strongly increased and contributes to mitochondrial respiration and ATP synthesis during bacterial infection [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF]. Interestingly, the observed ETC adaptations were absent in macrophages stimulated with TLR4 agonist LPS or with heat killed bacteria. We have previously shown that murine phagocytes possess the intrinsic capacity to discriminate viable from dead bacteria through the recognition of specialized PAMPs, which we refer to as vita-PAMPs [START_REF] Sander | Sensing prokaryotic mRNA signifies microbial viability and promotes immunity[END_REF]. Bacterial RNA was identified as the first vita-PAMP, recognition of which signals microbial viability to the immune system and elicits pronounced immune responses. A signature response of macrophages and DC to live bacteria or bacterial RNA is the activation of the NLRP3 inflammasome. When we investigated the ETC adaptations following bacterial stimulation we found that bacterial RNA or live bacteria, but not their killed counterparts, initiated ETC adaptations through activation of the phagosomal NADPH oxidase and the ROS-dependent tyrosine kinase Fgr, which was shown to phosphorylate the tyrosine 604 of the CII subunit succinate dehydrogenase subunit A (SDHA) (Acin-Perez et al., 2014;[START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF].

The partner phosphatase of Fgr was recently identified [START_REF] Nath | PTPMT1 inhibition lowers glucose through succinate dehydrogenase phosphorylation[END_REF], highlighting the potential of pharmaceutical manipulation of post-translational modifications of CII activity to control inflammation and other metabolic-related diseases [START_REF] Bezawork-Geleta | Mitochondrial Complex II: At the Crossroads[END_REF]. Hence, we demonstrate that specific innate immune signals that convey an increased level of infectious threat to the immune system induce profound metabolic alterations beyond the well-described glycolytic switch [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF].

This study suggested that innate immune receptor-mediated endosomal ROS production during bacterial infection directly increases mitochondrial ETC capacity but also enhance mitochondrial CII-dependent ATP synthesis, challenging the idea that ATP demand is mostly covered by anaerobic glycolysis. Nevertheless, it remains to be fully established whether metabolic flux modulation and the subsequent changes in the proportion of reductive equivalents NADH and FADH2 contribute to those adaptations. First evidence came from a recent study by O'Neill and colleagues. They found that LPS/TLR4-activated murine macrophages accumulate succinate [START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF]. As a consequence, TLR4-induced succinate oxidation by succinate dehydrogenase (SDH -which is also CII) and elevation of the mitochondrial membrane potential drives a reverse electron transfer (RET) that promotes mitochondrial ROS production by CI. This was dampened by the over-expression of an alternative oxidase (AOX) from Ciona intestinalis, which restores the normal flow of electrons through CoQ thereby avoiding RET [START_REF] El-Khoury | Engineering the alternative oxidase gene to better understand and counteract mitochondrial defects: State of the art and perspectives[END_REF]. It is tentative to speculate here that upon LPS stimulation, the increase ATP synthesis through glycolysis allows RET to occur assuming that mitochondrial ETC is not uncoupled. This would place ATP demand as an important driver of mitochondrial changes in LPS-activated phagocytes. This study also confirmed the importance of CII for macrophage activation upon innate immune receptor stimulation, and placed CI at the core of immune signaling.

Complex I deficiency is a well-known cause of rare neurological and metabolic disorders. However, a recent study by Jin et al based on the deletion of the CI subunit Ndufs4 highlighted a key function of CI in macrophage differentiation and contribution to inflammatory responses [START_REF] Jin | Mitochondrial complex I activity suppresses inflammation and enhances bone resorption by shifting macrophage-osteoclast polarization[END_REF]. The global Ndusf4 loss results in lethal encephalomyopathy [START_REF] Kruse | Mice with Mitochondrial Complex I Deficiency Develop a Fatal Encephalomyopathy[END_REF] and decreases bone resorption and increases bone mass [START_REF] Jin | Mitochondrial complex I activity suppresses inflammation and enhances bone resorption by shifting macrophage-osteoclast polarization[END_REF], while Ndfus4-deficient macrophages exhibit a pro-inflammatory phenotype, which can be attenuated by deletion of TLR2 and TLR4 again highlighting the relationship between innate immune receptors and the ETC [START_REF] Jin | Mitochondrial complex I activity suppresses inflammation and enhances bone resorption by shifting macrophage-osteoclast polarization[END_REF]. Together with the observation that Gram-negative bacterial infections leads to a decrease in CI [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF] and that LPS stimulation induce RET and ROS production at CI [START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF], the study of Jin et al. further suggests that CI functions as a regulator of inflammatory responses by macrophages and that its downregulation or dysregulation act as an inflammatory signal.

4-The mitochondrial Electron Transport Chain (ETC): an immune rheostat during infection (~600words)

ETC byproducts as innate immune signals

Several byproducts of mitochondrial activity have been implicated in inflammatory processes (Figure 2). Shadel and co-workers recently found that perturbation of oxidative phosphorylation by moderate mitochondrial DNA (mtDNA) stress engages cytosolic signaling to initiate antiviral immune responses [START_REF] West | Mitochondrial DNA stress primes the antiviral innate immune response[END_REF]. Heterozygous 'transcription factor A, mitochondrial' (TFAM) deficiency leads to aberrant packaging of mtDNA and induces its escape to the cytosol where it is sensed by the innate immune receptor cGAS, which promotes STING-IRF3dependent induction of interferon-stimulated genes. Mitochondrial ROS (mtROS) are generated as a byproduct of ETC activity or dysfunction, and mtROS have also been identified as a critical stimulator of cytosolic immune receptors [8].

Phagocytosis of microorganisms like bacteria elicits a respiratory burst leading to the generation of high amounts of ROS, which critically contribute to bacterial clearance [START_REF] Lambeth | NOX enzymes and the biology of reactive oxygen[END_REF]. Although ROS generated by the phagosomal NADPH oxidase are considered the main component of the antimicrobial response in phagocytes, mtROS have recently emerged as important contributors to the bactericidal capacity and immune signaling in innate immune cells (West et al., 2011a). As many as 10 possible sites of ROS production have been proposed in mitochondria [START_REF] Quinlan | Sites of reactive oxygen species generation by mitochondria oxidizing different substrates[END_REF] with CI being one of the most relevant for macrophages [START_REF] Kelly | Metformin Inhibits the Production of Reactive Oxygen Species from NADH:Ubiquinone Oxidoreductase to Limit Induction of Interleukin-1beta (IL-1beta) and Boosts Interleukin-10 (IL-10) in Lipopolysaccharide (LPS)-activated Macrophages[END_REF][START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF]. In phagocytes, expression of mitochondrial uncoupling protein 2 (UCP2) dampens mtROS production [START_REF] Arsenijevic | Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production[END_REF] and TLR4 engagement decreased UCP2 expression in LPSprimed murine macrophages [START_REF] Kizaki | Uncoupling protein 2 plays an important role in nitric oxide production of lipopolysaccharide-stimulated macrophages[END_REF]. Consequently, Ucp2 -/-mice are more resistant to infection by intracellular pathogens L. monocytogenes [START_REF] Rousset | The uncoupling protein 2 modulates the cytokine balance in innate immunity[END_REF] and Toxoplasma gondii [START_REF] Arsenijevic | Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production[END_REF], and UCP2-deficient macrophages exhibit increased killing of S. Typhimurium [START_REF] Arsenijevic | Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production[END_REF].

Finally, a recent study by Kawataba and colleagues shows that treatment of mouse embryonic fibroblasts (MEFs) with chemical uncoupling molecules that decrease mitochondrial membrane potential lowers type IFN production [START_REF] Koshiba | Mitochondrial Membrane Potential Is Required for MAVS-Mediated Antiviral Signaling[END_REF]. Precisely, they observed that the increased abundance of UCP-2 leads to proton leakage from mitochondria that impairs MAVS-dependent signaling. This suggests that the regulation of the mitochondrial ETC could directly influence the outcomes of innate immune signaling. Importantly, these studies highlights the notion that induction of RET, CI-mediated ROS production and subsequent increase in antimicrobial capacity and inflammatory responses by activated macrophages all require dampening of mitochondrial uncoupling mechanisms.

In further support of the important role of mitochondrial byproducts, West et al. observed that mitochondria co-localize and potentially interact with bacteriacontaining phagosomes (West et al., 2011a). They proposed that mitochondrial ROS production is induced by cell surface TLRs (e.g. TLR2 or TLR4), in a process that requires the TLR-adaptor TRAF6 (TNF receptor associated factor 6) and the mitochondrial protein ECSIT (evolutionary conserved signaling intermediate), which was originally described as a CI assembly factor [START_REF] Vogel | Cytosolic signaling protein Ecsit also localizes to mitochondria where it interacts with chaperone NDUFAF1 and functions in complex I assembly[END_REF]. Hence, accumulating evidence suggests a direct interaction between mitochondria, or their metabolic byproducts and invading pathogens. Aside from their proposed bactericidal role, mtROS have been reported to be involved in the activation of key innate immune modules. NLRP3 is a member of the cytosolic Nod-like receptors (NLRs) family. Upon activation, NLRP3 assembles into a large cytosolic protein complex termed the inflammasome. Inflammasomes serve as a platform for autocatalytic activation of pro-inflammatory caspases, most prominently Caspase-1, which processes inflammatory cytokines IL-1β and IL-18 [START_REF] Martinon | The inflammasomes: guardians of the body[END_REF].

The NLRP3-inflammasome is activated by a variety of endogenous and exogenous stimuli during bacterial infection including bacterial pore-forming toxins or bacterial nucleic acids [START_REF] Martinon | The inflammasomes: guardians of the body[END_REF][START_REF] Sander | Sensing prokaryotic mRNA signifies microbial viability and promotes immunity[END_REF]. Similar to the observed changes in ETC SC composition, NLRP3 is also activated by bacterial RNA [START_REF] Kanneganti | Bacterial RNA and small antiviral compounds activate caspase-1 through cryopyrin/Nalp3[END_REF][START_REF] Sander | Sensing prokaryotic mRNA signifies microbial viability and promotes immunity[END_REF]. ROS production is common to many activators of NLRP3 inflammasome and mtROS seem to be critically involved in NLPR3 inflammasome activation. ETC inhibition promotes mtROS generation, which increases NLRP3 activation and IL-1β secretion [START_REF] Zhou | A role for mitochondria in NLRP3 inflammasome activation[END_REF].

Accordingly, removal of dysfunctional ROS-producing mitochondria by directed autophagy (mitophagy) inhibits NLRP3 activation and IL-1β release [START_REF] Nakahira | Autophagy proteins regulate innate immune responses by inhibiting the release of mitochondrial DNA mediated by the NALP3 inflammasome[END_REF]. However, the precise molecular mechanisms that links mtROS to NLRP3 inflammasome activation remain unresolved. Macrophages deficient for the cytosolic superoxide dismutase 1 (SOD1) produce less IL-1β due to the inhibition of caspase-1 by oxidation and glutathionylation of specific cysteines [START_REF] Meissner | Superoxide dismutase 1 regulates caspase-1 and endotoxic shock[END_REF]. In addition, mtROS generation may be dispensable for some NLRP3 activators. The oxazolidinone antibiotics linezolid, used in the treatment of methicillin-resistant S. aureus infections, can activate the NLRP3 inflammasome independently of ROS [START_REF] Ament | Linezolid: Its role in the treatment of gram-positive, drug-resistant bacterial infections[END_REF]. In this context, NLRP3 activation occurs via its binding to the mitochondrial-specific phospholipid cardiolipin [START_REF] Iyer | Mitochondrial cardiolipin is required for Nlrp3 inflammasome activation[END_REF]. In addition to their role in NLRP3 inflammasome activation, mtROS also regulate LPS-mediated production of different cytokines including proIL-1b [START_REF] Kelly | Metformin Inhibits the Production of Reactive Oxygen Species from NADH:Ubiquinone Oxidoreductase to Limit Induction of Interleukin-1beta (IL-1beta) and Boosts Interleukin-10 (IL-10) in Lipopolysaccharide (LPS)-activated Macrophages[END_REF][START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF] by stabilizing hypoxiainducible factor 1a (HIF1a) [START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF]. HIF1a has thus emerged as a key regulator of metabolic reprogramming in innate immune cells as exemplified by its important role in human monocyte functional adaptations occurring in sepsis [START_REF] Shalova | Human monocytes undergo functional re-programming during sepsis mediated by hypoxia-inducible factor-1??[END_REF].

TCA cycle-derived metabolites are essential components of immune responses

Mitochondrial-derived metabolites generated in the Krebs cycle have important inflammatory signaling functions [START_REF] Mills | Mitochondria are the powerhouses of immunity[END_REF], and many of these metabolites accumulate in macrophages upon PRR ligation [START_REF] Jha | Network Integration of Parallel Metabolic and Transcriptional Data Reveals Metabolic Modules that Regulate Macrophage Polarization[END_REF].

Accumulation of succinate is associated with an enhanced production of pro-IL-1β and diminution of IL-10 in LPS-stimulated macrophages [START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF][START_REF] Tannahill | Succinate is an inflammatory signal that induces IL-1β through HIF-1α[END_REF]. Several mechanisms have been proposed to explain how succinate regulates cytokine production. First, succinate is structurally similar to a-ketoglutarate and has been shown to inhibit dioxygenases including the HIF1a regulators prolyl 4-hydroxylasess (PHD) [START_REF] Hewitson | Structural and mechanistic studies on the inhibition of the hypoxiainducible transcription factor hydroxylases by tricarboxylic acid cycle intermediates[END_REF][START_REF] Selak | Succinate links TCA cycle dysfunction to oncogenesis by inhibiting HIF-α prolyl hydroxylase[END_REF].

Second, succinate oxidation by CII and subsequent mtROS generation from CI leads to HIF-1a activation, which in turn induces IL1B transcription. Consequently, the pharmacological inhibition of CII inhibits LPS-or bacteria-induced production of IL-1β and boosts IL-10 expression [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF][START_REF] Mills | Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages[END_REF].

Fumarate, another TCA cycle metabolite and enzymatic product of CII also accumulates in activated innate immune cells. Work from Netea and co-workers has previously demonstrated that fumarate accumulates in monocytes activated by Beta-glucans due to replenishment of the Krebs cycle through glutaminolysis [START_REF] Arts | Glutaminolysis and Fumarate Accumulation Integrate Immunometabolic and Epigenetic Programs in Trained Immunity[END_REF]. In this context, fumarate was shown to inhibit KDM5 histone demethylases leading to epigenetic imprinting and altered cytokine expression profiles, a hall-mark of the recently proposed form of innate immune memory, also referred to as trained immunity [START_REF] Arts | Glutaminolysis and Fumarate Accumulation Integrate Immunometabolic and Epigenetic Programs in Trained Immunity[END_REF][START_REF] Netea | Trained immunity: A program of innate immune memory in health and disease[END_REF]. The link between metabolic adaptations and epigenetic imprinting has long since been noted in metabolic disorders, and it will be exciting to further investigate its role in immunity. Interestingly, fumarate also exerts direct antibacterial capacities. We could show that fumarate impairs the growth of E. coli and S. Typhimurium [START_REF] Garaude | Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense[END_REF]. Dimethyl-fumarate (DMF) is an approved drug for the treatment of multiple sclerosis and psoriasis (Al-Jaderi and Maghazachi, 2016) but its potential as an antimicrobial drug has not been explored so far. Taken together, these studies place CII activation and subsequent fumarate production at the crossroads of many inflammatory signaling pathways, and moreover deliver further evidence for a direct interaction between mitochondrial molecules and invading pathogens. Another important TCA cycle metabolite driving inflammatory responses is citrate. The accumulation of citrate in TLR-activated innate immune cells is thought to support fatty acid biosynthesis, which is essential for the production of proinflammatory cytokine in DCs (B Everts et al., 2014). Citrate accumulation is likely caused by a decreased isocitrate dehydrogenase expression in LPS-activated cells. This also leads to accumulation of itaconate by decarboxylation of cis-aconitate, a citrate derivative and substrate of isocitrate dehydrogenase, by 'immune responsive gene-1' (IRG1). TLR4 activation strongly enhances IRG-1 expression in macrophages leading to increased production of itaconate [START_REF] Michelucci | Immune-responsive gene 1 protein links metabolism to immunity by catalyzing itaconic acid production[END_REF][START_REF] Naujoks | IFNs Modify the Proteome of Legionella-Containing Vacuoles and Restrict Infection Via IRG1-Derived Itaconic Acid[END_REF]. Recent studies have shown that itaconate limits the deleterious effects of excessive ROS production caused by sustained SDH (CII) activity [START_REF] Lampropoulou | Itaconate Links Inhibition of Succinate Dehydrogenase with Macrophage Metabolic Remodeling and Regulation of Inflammation[END_REF]. Pre-treatment of macrophages with itaconate prior to LPS stimulation inhibits SDH, increases succinate levels, limits HIF-1a expression, and reduces inflammatory cytokine production. Similar to fumarate, itaconate posseses direct antimicrobial effects.

IRG-1 dependent production of itaconate was shown to inhibit in vitro growth of several bacterial pathogens including Legionella pneumophila, S. enterica or M.

tuberculosis [START_REF] Michelucci | Immune-responsive gene 1 protein links metabolism to immunity by catalyzing itaconic acid production[END_REF][START_REF] Naujoks | IFNs Modify the Proteome of Legionella-Containing Vacuoles and Restrict Infection Via IRG1-Derived Itaconic Acid[END_REF]. The expression of IRG-1 is in turn regulated by interferons, which are produced upon PRR activation. The antibacterial effects of itaconate may be at least partially explained by the inhibition of the glyoxylate shunt, an essential pathway in fatty acid-and acetatedependent bacteria, which is absent in mammals. It was thus proposed that itaconate impairs microbial metabolism while sparing host cells from the potentially detrimental induction of metabolism-dependent inflammatory responses.

Concluding remarks (~150words)

Recent work revisiting old observations on metabolic reprogramming of phagocytes during infection and inflammation has placed mitochondria, and particularly the ETC, at the center of the newly discovered immuno-metabolic adaptations. The mitochondrial respiratory chain has the capacity to adjust its

Reference

Acin-Perez, R., Carrascoso, I., Baixauli, F., Roche-Molina, M., Latorre-Pellicer, A., Fernandez-Silva, P., Mittelbrunn, M., Sanchez-Madrid, F., Perez-Martos, A., release of bacterial nucleic acids leads to the activation of cytosolic and endosomal PRRs including TLR7/8, TLR9, cGAS, and RIG-I, all of which induce activate IRFs and induce the production of type-I IFNs. Bacterial nucleic acids also activate the NLRP3-and AIM2-inflammasomes. NLRP3 and other PRRS, as well as PRR adaptor proteins like MAVS localize to mitochondria.

  architecture and activity in response to innate immune signals, and thereby regulate immune responses. These findings may provide novel options for controlling immune responses. Manipulation of specific features of mitochondrial metabolism could represent a valid strategy to fine-tune immune responses. It is becoming clear that mitochondrial-derived metabolic intermediates, redox molecules, and ROS play essential roles in antimicrobial immune responses. More research is needed to deepen our understanding of the intricate cross-regulations between the mitochondrial respiratory chain and innate immune responses during infection.

Figure 2 :

 2 Figure 2: The mitochondrial electron transport chain (ETC) is at the core of innate immune responses against bacteria. In macrophage, TCA cycle produces reductive equivalents of NADH and FADH2 that provide electrons to the ETC, which in turn produces sufficient ATP to fulfill cellular energetic demands of resting cells. A significant proportion of the ETC is assembled in supercomplexes (SCs) including the respirasome (CI/CII2/CIV). Mitochondrial ROS (mtROS) production is low in resting macrophage. The engagement of innate immune receptors including Tolllike receptors (TLRs) and NOD-like receptors (NLRs) during infection by live Gram-negative bacteria is thought to divert mitochondrial metabolism from energy production towards anabolic and signaling functions. Levels of SCs are decreased, probably due to destabilization of the complex I (CI) and is associated with a decrease in CI activity. In turn, CII and glycerol-3-phosphate dehydrogenase (G3PDH, here signalized as G) activities are augmented and provide electrons to the ETC and generate reverse electron transfer, accounting for CI destabilization and mitochondrial ROS (mtROS) production. MtROS can then activate hypoxiainduced factor 1a (HIF1α) to increase expression of the pro-inflammatory cytokine pro-IL-1b, and contribute to NLRP3 activation, which assembles the inflammasome to convert pro-IL-1β into the mature biologically active cytokine IL-1β. MtROS can also promote the recruitment of mitochondrial anti-viral signaling protein (MAVS) to the mitochondrial outer membrane where it interacts with stimulator of inferno genes (STING) to stimulate the production of antimicrobial Type-I interferons. MtROS also exert direct antibacterial properties when released in the microbecontaining phagosomes. Cardiolipin translocation from the inner membrane to the outer membrane in damaged mitochondria -as it can be induced by different bacterial infection -provides a docking site for NLRP3. microbes that trigger mitochondrial damages. Reprogramming of the tricarboxylic acid (TCA) cycle lead the accumulation of citrate, which can be metabolized into itaconate thereby

Acknowledgements J.G. is an INSERM investigator and supported by the IdEx program of the university of Bordeaux (ANR-10-IDEX-03-02). L.E.S. is supported by the German research council (DFG, SA19402/1, SFB-TR84 TP C08) and the German ministry of research and education (BMBF) and EU FP-7 ERA-Net (Infect-ERA, HAPLO-Infect).

Disclosure statement

The authors declare no competing financial interests