Marie-Odile Joannes

Gylna Loko

Jacqueline Deloumeaux

Roger Chout
email: roger.chout@chu-guadeloupe.fr

Therese Marianne-Pepin

Joannes Mo

Marianne-Pepin Thérèse

A IFN gamma polymorphism with infections in sickle cell disease

Keywords: sickle cell, infection, polymorphism, genetic association, cytokine IFN gamma polymorphism and sickle cell disease sickle cell, infection, polymorphism, genetic association, cytokine Abbreviations: IFNγ interferon gamma, SCD sickle cell disease, Hb haemoglobin, SNP single nucleotide polymorphism, MBP mannose binding protein

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Sickle cell disease (SCD) is an inherited disorder caused by a point mutation in the β-globin gene resulting in the substitution of valine for glutamic acid at the position 6. This substitution leads to the presence of the sickle haemoglobin (HbS). Sickle haemoglobin is an atypical Hb which stands for normal Hb (HbA). About fifty million persons are affected by this haemoglobinopathy all over the world. In most of countries, SCD is a major public health problem. SCD is the first genetic disease in the Caribbean islands. This haemoglobinopathy is characterized by high mortality and morbidity, and various vaso occlusive symptoms very debilitating by their frequency and consequences. Besides, patients with SCD have to face very severe complications such as severe anemia, stroke, acute chest syndrome, osteonecrosis of femoral and humeral heads, infections…

In the last twenty years, before the systematic use of vaccines and prophylactic penicillin, bacterial infections were the leading cause of death in young children with SCD; nevertheless it is still a significant cause of morbidity in older patients [START_REF] Leikin | Mortality in children and adolescents with sickle cell disease[END_REF][START_REF] Gill | Clinical events in the first decade in a cohort of infants with sickle cell disease[END_REF]. Patients with SCD have an increased susceptibility to infection particularly with encapsulated organisms: Streptococcus pneumoniae, Haemophilus influenzae, Salmonella spp [START_REF] Granoff | Haemophilus influenzae infections in Fresno County, California: A prospective study of the effects of age, race, and contact with a case on incidence of disease[END_REF][START_REF] Lobel | Clinicopathologic characteristics of septicemia in sickle cell disease[END_REF][START_REF] Powars | Is there an increased risk of Haemophilus influenzae septicemia in children with sickle cell anemia?[END_REF][START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Magnus | Recurrent infections in homozygous sickle cell disease[END_REF].

Bacterial infection may also be caused by intracellular organisms such as Chlamydia pneumoniae, Mycoplasma pneumoniae, Salmonella spp, Escherichia coli [START_REF] Magnus | Recurrent infections in homozygous sickle cell disease[END_REF][START_REF] Shulman | The unusual severity of Mycoplasmal pneumonia in children with sickle-cell disease[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF] and extracellular pathogens such as Staphylococcus aureus.

Meningitis and septicaemia are the more serious infections and most of the time are caused by Streptococcus pneumoniae and Salmonella spp [START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF][START_REF] Robinson | Pneumococcal meningitis in sickle-cell anemia[END_REF][START_REF] Overturf | Bacterial meningitis and septicemia in sickle cell disease[END_REF][START_REF] Onwubalili | Sickle cell disease and infection[END_REF][START_REF] Wong | Infection caused by Streptococcus pneumoniae in children with sickle cell disease: Epidemiology, immunologic mechanisms, prophylaxis, and vaccination[END_REF].

These infections are all the more severe since they may cause other complications such as ostemomyelitis and leg ulcers.

Patients with SCD are at risk of osteomyelitis which is specific in SCD since it often occurs in ischemic bone. Moreover, this complication affects frequently femur, humerus, vertebra, ribs and sternum. Osteomyelitis occur far more frequently in patients with SCD than in general population and the offending pathogens are generally Salmonella and Streptococcus pneumoniae [START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF][START_REF] Robinson | Pneumococcal meningitis in sickle-cell anemia[END_REF][START_REF] Overturf | Bacterial meningitis and septicemia in sickle cell disease[END_REF][START_REF] Onwubalili | Sickle cell disease and infection[END_REF][START_REF] Wong | Infection caused by Streptococcus pneumoniae in children with sickle cell disease: Epidemiology, immunologic mechanisms, prophylaxis, and vaccination[END_REF].

As for leg ulceration, this is a common complication in SCD. Because of its frequency, chronicity and resistance to therapy, this complication is an important cause of morbidity.

Leg ulcers vary considerably in size and can be very painful. Bacterial infection can worsen this complication. They appear to occur either spontaneously or as a result of local trauma and often persist for long durations of time.

The incidence of leg ulcers varies greatly:

-70 percent of patients over 15 years old was reported in Jamaïca (14), -5 percent of patients over 15 years old in the United Sates [START_REF] Koshy | Leg ulcers in patients with sickle cell disease[END_REF], 10 to 15 percent of patients over 15 years old in West Africa [START_REF] Adedeji | Haematological factors associated with leg ulcer in sickle cell disease[END_REF] , -less than 1 percent of patients over 15 years old in India and Saudi Arabia [START_REF] Kar | Sickle cell disease in Orissa State, India[END_REF][START_REF] Padmos | Two different forms of homozygous sickle cell disease occur in Saudi Arabia[END_REF]. All these clinical manifestations vary within patients which remains unexplained. SCD is extremely heterogeneous phenotypically. Some patients are often sick whereas others never present complications. This matter of fact drew many scientists' attention. Reasons of this variability are not clear. Genetic and environmental causes seem to be involved but are still the subject of discussions.

Haplotype differences cannot exclusively explain this variability.

Nowadays, the exact reasons of the propensity of sickle cell patients to develop infections are still unknown. Genetic basis of this susceptibility has been considered by scientists in the last ten years. Some authors investigated the polymorphism of the gene encoding the Fc Receptor (FcγRIIA) [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF], and the polymorphism of the gene encoding the Mannose Binding protein (M.B.P) [START_REF] Neonato | Genetic polymorphism of the mannose-binding protein gene in children with sickle cell disease: Identification of three new variant alleles and relationship to infections[END_REF] as they considered these genes as potential factor risks for infections in patients with sickle cell disease. Polymorphism of HLA system [START_REF] Tamouza | Infectious complications in sickle cell disease are influenced by HLA class II alleles[END_REF]and the gene encoding the myeloperoxidase [START_REF] Costa | Association of the G-463A myeloperoxidase polymorphism with infection in sickle cell anemia[END_REF] have also been studied.

Because interferon gamma (IFNγ) plays a key role in immune response against infection, we considered this interferon as a genetic modulator of infection in sickle cell patients.

Interferon gamma is the sole type II interferon. It is a pleiotropic cytokine produced by activated T cells and natural killer cells.

Antigen-presenting cells such as mononuclear phagocytes, dendritic cells, are target of IFNγ.

In response to an infectious stimulus, this cytokine enhances the expression of Major Histocompatibility Complex (MHC) class II and class I molecules on the surface of those cells and so, increases antigen processing and presentation to T cells. IFNγ can also activate microbicidal effector functions of macrophages In humans, IFN gene is located on chromosome 12 (12q15). It consists in four exons and three introns [START_REF] Gray | Structure of the human immune interferon gene[END_REF].

In this study, we focused on the mutation located in the first intron at nucleotide + 874 mutation + 874 G/A [START_REF] Pravica | A single nucleotide polymorphism in the first intron of the human IFN-γ gene: Absolute correlation with a polymorphic CA microsatellite marker of high IFN-γ production[END_REF].

There are previous reports of an association of this single nucleotide polymorphism (SNP) and infectious diseases such as tuberculosis [START_REF] Tso | Association of interferon gamma and interleukin 10 genes with tuberculosis in Hong Kong Chinese[END_REF] and severe acute respiratory syndrome [START_REF] Chong | The interferon gamma gene polymorphism +874 A/T is associated with severe acute respiratory syndrome[END_REF].

We hypothesized that this SNP is associated with infectious complications in sickle cell patients.

MATERIAL AND METHODS

This is a case-control study on sickle cell patients attending the Centre Intégré de la Drépanocytose (Centre Hospitalier du Lamentin, Martinique). The present study had local ethics committee approval, and written informed consent was obtained from all participants.

Seventy-two patients were included. Thirty-seven of them had already suffered from infections such as osteomyelitis, septicaemia. The others (35 patients) had never experimented any infection. There were 40 women and 32 men from 20 to 66 years old.

DNA isolation

Genomic DNA was isolated from peripheral blood collected on EDTA using a salting out method [START_REF] Miller | A simple salting out procedure for extracting DNA from human nucleated cells[END_REF].

Genotyping

Polymorphism of IFNγ was determined using the CYTGEN kit (One Lambda, Inc) following manufacturer's instructions.

Statistical analysis

The association of IFNγ polymorphism and infections in sickle cell patients was evaluated by the chi-square test and p values smaller than of 0.05 were considered statistically significant.

RESULTS

In the case group, forty-three infections such as septicaemia, osteomyelitis, leg ulcerations, and infections of the urinary tract were counted.

Pathogens more frequently involved in those infections were Staphylococcus aureus (34.9%), Escherichia coli (27.9%) and Klebsiella(11.6%) (see table 1).

Table 2 presents the frequency of the different type of infections and the pathogens involved in these infections.

Results of IFNγ typing are presented on table 3.

Twenty-five patients in the control group (77.1 %) presented AA genotype whereas in the case group, there were seventeen (48.6%).

Only eight patients in the control group (22.9 %) presented TT or TA genotype whereas in the case group, there were nineteen (51.4%).

There are significantly more patients with infections carrying the T variant than patients without infections (χ² = 6.23 ; p = 0.013).

T allele seems to be a risk factor for infection in sickle cell patients. Several mechanisms are believed to contribute to the susceptibility of sickle cell patients to infections, a major factor being the early loss of splenic function [START_REF] Winkelstein | Deficiency of pneumococcal serum opsonizing activity in sickle-cell disease[END_REF][START_REF] Hand | Deficiency of serum bactericidal activity against Salmonella typhimurium in sickle cell anaemia[END_REF]. However, this justification is insufficient since functional splenectomy often occurs after the first infectious complications.

DISCUSSION

The unanimous opinion is that opsonic defect is a factor contributing to infection, but the reasons of this defect are still discussed.

Nowadays, the exact reasons of the propensity of sickle cell patients to infection are still unknown. In the last ten years, genetic basis of the variability of the clinical course of SCD has been considered. There are a lot of candidate genes from the immune system for an association with infections in sickle cell patients.

Scientists investigated polymorphism of the gene encoding Fc human immunoglobulin G

Receptor II A (FcγRIIA) [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF], and polymorphism of the gene encoding the mannose-binding protein as they considered these genes as potential risk factors for infections in patients with sickle cell disease.

Norris found that the H/H 131 FcγRIIA genotype is overrepresented in sickle cell children with a history of Haemophilus influenzae type b infection whereas other studies suggested just the opposite: underrepresentation of H/H 131 genotype in individuals with a history of encapsulated organism infection [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF].

Neonato found an association between low-producing MBP genotypes and the absence of infection in children with SCD [START_REF] Neonato | Genetic polymorphism of the mannose-binding protein gene in children with sickle cell disease: Identification of three new variant alleles and relationship to infections[END_REF].

Tamouza found an association between HLA class II alleles and infections in sickle cell patients. According to this study, HLA-DQB1*03 is associated with severe infections whereas HLA-DRB1*15 confers a protective effect against infections [START_REF] Tamouza | Infectious complications in sickle cell disease are influenced by HLA class II alleles[END_REF].

More recently, Costa and co-workers investigated the gene encoding the myeloperoxidase as a genetic factor increasing susceptibility to infection in sickle cell patients [START_REF] Costa | Association of the G-463A myeloperoxidase polymorphism with infection in sickle cell anemia[END_REF]. They studied the G to A polymorphism of the promoter of this gene (position -463) and found that the presence of A allele increased the susceptibility to infections.

Our results suggest that IFNγ gene could be involved in infectious complications in sickle cell patients. Indeed, the frequency of T allele is significantly higher in patients who had already suffered from infections than in patients who had never suffered from any infection.

Several authors found associations between some genes and the susceptibility to infections in sickle cell patients. They found that these genes might contribute to this susceptibility. So, one can consider that the propensity of sickle cell patients to develop infection could be the result of the modulation of various genes since numerous genetic associations have been found.

Our results revealed, for the first time, that the + 874 T/A IFNγ SNP may be a genetic modulator of infectious complications in sickle cell patients (χ² = 6.23; p = 0.013). We found an association between T allele and those complications.

These investigations have to be furthered in a larger sample in order to confirm this association. Such a result is of interest since it can provide a better management of sickle patients with the T variant.

TABLES

  Patients with homozygous SCD (SS patients) are at increased risk of infection with Streptococcus pneumoniae, Haemophilus influenzae type b, Salmonella spp, Escherichia coli and Klebsiella spp. In our population, the offending pathogens involved in the infections occurring belonged to this list.

Table 1 . Frequency of the different offending pathogens identified in this population

 1

		Number of
	Pathogens		%
		events
	Bacteriodes ovatus	2	4.7
	Enterobacter	3	7
	Escherichia coli	12	27.9
	Klebsiella	5	11.6
	Proteus mirabilis	1	2.3
	Pseudomonas aeruginosa	1	2.3
	Salmonella sp.	4	9.3
	Staphylococcus aureus	15	34.9

Table 2 . Frequency of the different type of infections, and pathogens incriminated

 2

	Infections
		Pathogens involved	%
	N = 43
	Septicaemia	Klebsiella	25
	N = 8 (19%)	Escherichia coli	25
		Bacteriodes ovatus	12.5
		Staphylococcus	12.5
		Enterobacter	12.5
		Salmonella sp	12.5
	Osteomyelitis	Salmonella sp	50
	N = 4 (9%)	Bacteriodes ovatus	25
		Staphylococcus aureus	25
	Leg ulceration	Staphylococcus aureus	81.25
	N = 16 (37%)	Enterobacter	6.25
		Proteus mirabilis	6.25
		Pseudomonas aeruginosa	6.25
	Infections of the	Escherichia coli	66.7
	urinary tract	Klebsiella	20
	N = 15 (35%)	Enterobacter	6.7
		Salmonella	6.7

Table 3 . Interferon gamma polymorphism at position +874 in sickle cell patients

 3

		Control	Case
	Genotype	n = 35	n = 37	χ² p value p c value
		F (%)	F (%)
	AA	27 (77.1) 18 (48.6)
	TT+TA	8 (22.9) 19 (51.4) 6.23 0.013	0.024

ACKNOWLEDGEMENTS

We would like to thank the staff of the Centre Intégré de la Drépanocytose for their help in recruiting patients. We thank the staff of the laboratory of the Centre Hospitalier du Lamentin.

We also thank the Conseil Régional de la Martinique for their financial support.


Joannes Mo

Roger Chout
email: roger.chout@chu-guadeloupe.fr

Marianne-Pepin Thérèse

Infectious complications in sickle cell disease and HLA polymorphism

Keywords: sickle cell, infection, polymorphism, genetic association, HLA class II alleles sickle cell, infection, polymorphism, genetic association, HLA system SCD sickle cell disease, Hb haemoglobin, HLA human leukocyte antigen

Infectious complications are a leading cause of morbidity and mortality in patients with sickle cell disease. The exact reasons of the propensity of sickle cell patients to infection are not clear and are matter of debate.

Besides, there is a striking variability of the clinical course of the disease between patients. This drew many scientists' attention.

Genetic factors have been investigated as potential factor risks for infection in sickle cell patients, for example: HLA system; genes encoding Fc Human Immunoglobulin G receptor IIA, mannose-binding protein, and myeloperoxidase.

Associations have been found, but all scientists agree that investigations have to be furthered.

We investigated an association between HLA DRB1 and DQB1 polymorphism and infectious complications in sickle cell patients.

Genotype frequencies were different between cases and controls for DRB1 locus. Indeed, HLA-DRB1*11 frequency was significantly higher in patients without infections than in patients with infections (χ² = 5.02; p = 0.025).

INTRODUCTION

Sickle cell disease (SCD) is a haemoglobinopathy characterized by the presence of the sickle haemoglobin (HbS), an atypical Hb which stands for normal Hb (HbA).

It is an autosomal recessive disorder. In 1956, Ingram determined that a point mutation in the gene coding the β chain of the haemoglobin molecule resulted in a single amino acid substitution (valine for glutamic acid) in the β globin chain, which, when present on both chromosomes in a patient, leads to sickle cell haemoglobin or Hb S [START_REF] Ingram | A specific chemical difference between the globins of normal human and sickle-cell anaemia haemoglobin[END_REF]. SCD is the most important haemoglobinopahy since fifty millions persons are affected all over the world. In most of countries this pathology is a major public health problem.

SCD is very frequent in Africa, North America (USA), South America (Brazil) and in the West Indies. It is also found in population of Maghreb (Algeria, Morocco, and Tunisia), Sicily, Greece, the Middle East, and Saudi Arabia. SCD spread in Western Europe: France, Britain, Portugal, Belgium, Germany… In Martinique, an island of the French West Indies, 11% of the population is thought to have the sickle cell trait. (People with sickle cell trait have inherited the HbA gene form one parent and the HbS gene from the other).

During the third month of life of a sickle cell patient, vaso-occlusive crisis can occur. People with SCD also have to face anaemia, repeated infections and various debilitating complications such as stroke, acute chest syndrome, osteonecrosis of the femoral and the humeral heads, retinopathy, cardiomegaly, leg ulceration….

Acute painful vaso-occlusive crises are the most common and earliest, clinical manifestation of SCD. It is the hallmark manifestation of this disease. They are characterized by pain and fever.

Sickle cell disease is characterized by periods of stability, punctuated by episodes of severe pain involving the back, chest, abdomen, and joints. This syndrome is referred to as the acute painful crisis or vaso-occlusive crisis [START_REF] Bailey | Is the painful crisis of sickle-cell disease due to sickling? [17[END_REF][START_REF] Serjeant | The painful crisis of homozygous sickle cell disease: Clinical features[END_REF].

Before the systematic use of vaccines and prophylactic penicillin, bacterial infections were the leading cause of death in young children with SCD. Nevertheless, despite this prophylactic program, infections are still a significant cause of morbidity in older patients. [START_REF] Leikin | Mortality in children and adolescents with sickle cell disease[END_REF][START_REF] Gill | Clinical events in the first decade in a cohort of infants with sickle cell disease[END_REF]. Patients with SCD have an increased susceptibility to infection particularly with encapsulated organisms: Streptococcus pneumoniae, Haemophilus influenzae, Salmonella spp [START_REF] Granoff | Haemophilus influenzae infections in Fresno County, California: A prospective study of the effects of age, race, and contact with a case on incidence of disease[END_REF][START_REF] Lobel | Clinicopathologic characteristics of septicemia in sickle cell disease[END_REF][START_REF] Powars | Is there an increased risk of Haemophilus influenzae septicemia in children with sickle cell anemia?[END_REF][START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Magnus | Recurrent infections in homozygous sickle cell disease[END_REF].

Bacterial infection may also be caused by intracellular organisms such as Chlamydia pneumoniae, Mycoplasma pneumoniae, Salmonella spp, Escherichia coli [START_REF] Magnus | Recurrent infections in homozygous sickle cell disease[END_REF][START_REF] Shulman | The unusual severity of Mycoplasmal pneumonia in children with sickle-cell disease[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF] and extracellular pathogens such as Staphylococcus aureus.

Meningitis and septicaemia are the more serious infections and most of the time are caused by Streptococcus pneumoniae and Salmonella spp [START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF][START_REF] Robinson | Pneumococcal meningitis in sickle-cell anemia[END_REF][START_REF] Overturf | Bacterial meningitis and septicemia in sickle cell disease[END_REF][START_REF] Onwubalili | Sickle cell disease and infection[END_REF][START_REF] Wong | Infection caused by Streptococcus pneumoniae in children with sickle cell disease: Epidemiology, immunologic mechanisms, prophylaxis, and vaccination[END_REF].

Besides, patients with SCD are at risk for osteomyelitis which is specific in SCD since it often occurs in ischemic bone. Moreover, this complication affects frequently femur, humerus, vertebra, ribs and sternum.

Osteomyelitis occur far more frequently in patients with SCD than in general population and the offending pathogens are generally Salmonella and Streptococcus pneumoniae [START_REF] Zarkowsky | Bacteremia in sickle hemoglobinopathies[END_REF][START_REF] Wright | Septicemia caused by salmonella infection: An overlooked complication of sickle cell disease[END_REF][START_REF] Robinson | Pneumococcal meningitis in sickle-cell anemia[END_REF][START_REF] Overturf | Bacterial meningitis and septicemia in sickle cell disease[END_REF][START_REF] Onwubalili | Sickle cell disease and infection[END_REF][START_REF] Wong | Infection caused by Streptococcus pneumoniae in children with sickle cell disease: Epidemiology, immunologic mechanisms, prophylaxis, and vaccination[END_REF].

Furthermore, leg ulceration is a common complication of SCD. Because of its frequency, chronicity and resistance to therapy, it is an important cause of morbidity.

Leg ulcers can be very painful and can be worsened by a bacterial infection.

They appear to occur either spontaneously or as a result of local trauma and often persist for long durations of time.

All these clinical manifestations vary within patients which remains unexplained.

Besides, the reasons of the extreme susceptibility of sickle cell patients to infection are matter of debate.

Several mechanisms are believed to contribute to this susceptibility, a major factor being the early loss of splenic function. [START_REF] Winkelstein | Deficiency of pneumococcal serum opsonizing activity in sickle-cell disease[END_REF][START_REF] Hand | Deficiency of serum bactericidal activity against Salmonella typhimurium in sickle cell anaemia[END_REF]. However, this justification is incomplete since functional splenectomy often occurs after the first infectious complications

The unanimous opinion is that opsonic defect is a factor contributing to infection, but the reasons of this defect are still discussed.

Nowadays, the exact reasons of the propensity of sickle cell patients to infection are still unknown.

Genetic bases of this susceptibility have been considered by scientists in the last twenty years. Some authors investigated the polymorphism of the gene encoding the Fc Receptor (FcγRIIA) [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF], and the polymorphism of the gene encoding the Mannose Binding protein (M.B.P) [START_REF] Neonato | Genetic polymorphism of the mannose-binding protein gene in children with sickle cell disease: Identification of three new variant alleles and relationship to infections[END_REF] as they considered these genes as potential factor risks for infections in patients with sickle cell disease. Polymorphism of HLA system [START_REF] Tamouza | Infectious complications in sickle cell disease are influenced by HLA class II alleles[END_REF] and the gene encoding the myeloperoxidase [START_REF] Costa | Association of the G-463A myeloperoxidase polymorphism with infection in sickle cell anemia[END_REF] have also been studied.

Because HLA system (Human Leukocyte Antigen) plays a key role in immune system, scientists investigated the potential association between this system and infectious complications in sickle cell disease.

HLA system is a gene complex encoding glycoproteins involved in differentiating self from non-self. Human histocompatibility antigens are known as Human Leukocyte Antigen Human histocompatibilty molecules, known as HLA molecules, are encoded by genes located on the short arm of chromosome 6.

There are three different types of HLA genes:

-class I with HLA-A, B and C genes, HLA-E, F and G genes and many other genes which functions are still unknown, -class II with HLA-DR, DP and DQ notably, -Class III. These genes encode other proteins related to the immune response. For example, genes for C4, C2, and B factor, components of the complement, and genes encoding critical cytokines such as TNF α and β (Tumour Necrosis Factor).

In this study we investigated HLA system class II genes association with infections in sickle cell patients.

MATERIAL AND METHODS

This is a case-control study on sickle cell patients attending the Centre Intégré de la Drépanocytose (Centre Hospitalier du Lamentin, Martinique). The present study had local ethics committee approval, and written informed consent was obtained from all participants.

Seventy-two patients were included. Thirty-seven of them, the cases, had already suffered from infections such as osteomyelitis, septicaemia. The others (35 patients), control patients, had never experienced any infection. There were 40 women and 32 men from 20 to 66 years old.

DNA isolation

Genomic DNA was isolated from peripheral blood collected on EDTA using a salting out method [START_REF] Miller | A simple salting out procedure for extracting DNA from human nucleated cells[END_REF].

HLA typing

HLA class II typing was performed by polymerase chain reaction sequence specific oligonucleotide (PCR-SSO).

For HLA class II genes, the RSSO2B1 and RSSO2QB1 genotyping kits (One Lambda, Inc) were used respectively for HLA-DRB1 and HLA-DQB1 typing. Those kits were used according to the specifications of the manufacturer.

Statistical analysis

The association of the polymorphism of HLA system and infections in sickle cell patients was evaluated by the chi-square test and p value less than 0.05 was considered statistically significant.

RESULTS

In the case group, forty-three infections such as septicaemia, osteomyelitis, leg ulcerations, and infections of the urinary tract were counted.

Pathogens more frequently involved in those infections were Staphylococcus aureus (34.9%), Escherichia coli (27.9%) and Klebsiella (11.6%) (see table 1).

Table 2 shows the distribution of HLA-DRB1 alleles.

In this population, DRB1*13 and DRB1*15, were the more frequent alleles respectively with 20.8 and 19.4%.

In the control group, the predominant allele was DRB1*13 (13.6%) followed by DRB1*15 (17.1%) and DRB1*11 (17.1%). Likewise, for the cases, the predominant allele was DRB1*13 (23%), then, DRB1*15 (21.6%).

We could notice that the frequency of DRB1*11 was higher in patients without infections (17.1%) patients with infections (5.4%). Chi-square test revealed that this difference was significant (χ² = 5.02, p = 0.025).

Results of frequency of HLA-DQB1 alleles are presented in table 3.

The predominant alleles were DQB1*06 (40.3%), then DQB1*05 (18.7%) and DQB1*03 (18.7%).

In the control group, the predominant allele was DQB1*06 (37.1%) followed by DQB1*05 (21.4%). For the cases, the predominant allele was DQB1*06 (43.2%) as well, but the second was DQB1*03 (18.9%).

There was no significant difference between the distribution of HLA-DQB1 alleles in patients with infections and in patients without infections.

DISCUSSION

Patients with homozygous SCD (SS patients) are at increased risk of infection with

Streptococcus pneumoniae, Haemophilus influenzae type b, Salmonella spp, Escherichia coli and Klebsiella spp. In this population, the offending pathogens were Staphylococcus aureus (34.9%), Escherichia coli (27.9%) and Klebsiella (11.6%).

There are a lot of candidate genes from the immune system for an association with infections in sickle cell patients.

Scientists investigated the polymorphism of the gene encoding Fc human immunoglobulin G

Receptor II A (FcγRIIA) [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF], and the polymorphism of the gene encoding the mannosebinding protein (MBP) [START_REF] Neonato | Genetic polymorphism of the mannose-binding protein gene in children with sickle cell disease: Identification of three new variant alleles and relationship to infections[END_REF] as they considered these genes as potential risk factors for infections in patients with sickle cell disease.

Norris found that the H/H 131 FcγRIIA genotype is overrepresented in sickle cell children with a history of Haemophilus influenzae type b infection whereas other studies suggested just the opposite: underrepresentation of H/H 131 genotype in individuals with a history of encapsulated organism infection [START_REF] Norris | Relationship between Fc receptor IIA polymorphism and infection in children with sickle cell disease[END_REF].

Neonato found an association between low-producing MBP genotypes and the absence of infection in children with SCD [START_REF] Neonato | Genetic polymorphism of the mannose-binding protein gene in children with sickle cell disease: Identification of three new variant alleles and relationship to infections[END_REF].

More recently, Costa and co-workers investigated the gene encoding the myeloperoxidase as a genetic factor increasing susceptibility to infection in sickle cell patients [START_REF] Costa | Association of the G-463A myeloperoxidase polymorphism with infection in sickle cell anemia[END_REF]. They studied the G to A polymorphism of the promoter of this gene (position -463) and found that the presence of A allele increased the susceptibility to infections.

Because HLA system plays a key role in the immune system, scientists investigated the potential association between this system and infectious complications in sickle cell disease. They found significantly more patients without infectious carrying HLA-DRB1*15 allele than did patients with infections (χ² = 10.47, p c = 0.01). They also found significantly more patients with severe infections carrying HLA-DQB1*03 allele than did the others (χ² = 9.41, p C = 0.01). These results suggest a protective effect of HLA-DRB1*15 allele against infections in patients with sickle cell disease. On the other hand, patients with SCD carrying HLA-DQB1*03 allele would be at increased risk for infectious complications [START_REF] Tamouza | Infectious complications in sickle cell disease are influenced by HLA class II alleles[END_REF].

On the contrary of Tamouza, we didn't found these associations. In our population, HLA-DRB1*15 frequency was not significantly different between cases and controls (respectively 21.6% and 17.1%). Likewise, HLA-DQB1*03 frequency was almost the same between the cases and the control group (respectively 18.9% and 18.6%). This could be explained by the fact that there is a linkage disequilibrium between DQB1*03 and alleles. Since those DRB1 alleles (DRB1*13 and DRB1*15) are well represented in this population (for the cases, respectively 23% and 21.6% and for the control group, respectively 18.6% and 17.6%), we could explain the reason why DQB1*03 frequency is almost the same between the control group and the cases. However, our results showed that HLA-DRB1*11 was significantly more frequent in patients without infections than in patients who had already suffered from infections (χ² = 5.02, p= 0.025, p C = 0.026). Our results suggested that HLA-DRB1*11 would have a protective effect against infections.

In conclusion, our results suggest that HLA class II alleles could influence the clinical course of sickle cell disease. Indeed, we found that HLA-DRB1*11 allele could have a protective effect against infection.

Since other associations have been found between genetic factors and infections, we can consider that the propensity of sickle cell patients to develop infection could be the result of the action of various genetic modulators.

As our study has been conducted on a very small sample, and our results refute Tamouza's, the tests should be performed in a larger population.

  (H.L.A). HLA molecules bind peptide fragments derived from pathogens and display them on cell surface for recognition by the appropriate T cells. Each T cell bears a cell surface receptor (Tcell receptor or TCR) that typically recognizes a peptide determinant bound to a molecule encoded by the HLA system.

  Tamouza et al. worked on a population of 80 patients with sickle cell disease. Forty-three of them have presented major infectious complications (meningitis, septicaemia or osteomyelitis), the others (37 patients) have never experimented such complications. The most frequent microrganisms involved in infections in the group of 43 patients were Streptococcus pneumoniae, Salmonella spp, Staphylococcus aureus and Haemophilus influenzae.

Table 1 . Frequency of the different offending pathogens identified in this population

 1

		Number of
	Pathogens		%
		events
	Bacteriodes ovatus	2	4.7
	Enterobacter	3	7
	Escherichia coli	12	27.9
	Klebsiella	5	11.6
	Proteus mirabilis	1	2.3
	Pseudomonas aeruginosa	1	2.3
	Salmonella sp.	4	9.3
	Staphylococcus aureus	15	34.9

Table 2 . Allele frequencies at DRB1 locus

 2

	Séries alléliques	Control patients	Cases
			n = 70	n = 74	χ²	p	p c
	DRB1*	AF	%	AF %
	01	5	7.1	3	4		NS
	03	7	10	11 14.9		NS
	04	1	1.4	1	1.3		NS
	07	6	8.6	3	4		NS
	08	5	7.1	8	10.8		NS
	09	1	1.4	2	2.7		NS
	10	1	1.4	1	1.3		NS
	11	12	17.1	4	5.4 χ²=5.02 p=0.025 0.026
	12	2	2.9	4	5.4		NS
	13	13	18.6	17 23		NS
	14	2	2.9	2	2.7		NS
	15	12	17.1	16 21.6		NS
	16	3	4.3	2	2.7		NS
	AF: allele frequency		NS : not significant

Table 3 . Allelic frequencies at DQB1 locus

 3

	Séries alléliques	Control patients	Cases
			n =70	n =74	χ² p	p c
	DQB1*	AF	%	AF %
	02	14	20	12 16.2	NS
	03	13	18.6	14 18.9	NS
	04	2	2.9	4	5.4	NS
	05	15	21.4	12 16.2	NS
	06	26	37.1	32 43.2	NS
	AF: allele frequency		NS: not significant

ACKNOWLEDGEMENTS

We would like to thank the staff of the Centre Intégré de la Drépanocytose for their help in recruiting patients. We thank the staff of the laboratory of the Centre Hospitalier du Lamentin.

We also thank the Conseil Régional de la Martinique for their financial support.


Session 1: Popular arts for the people? The shift in antagonism between high and low culture

When high meets low: the decorative means of Cubism

Claire Le Thomas, independent scholar When discussing innovation in avant-garde art movements, art historians often focus on influences from "fine art" and neglect other sources of inspiration. Until recently, the historiography of Cubism hasn't really take into consideration the contribution of popular culture or applied art (slide 2) to the formal or technical solutions elaborated by Braque and Picasso during their collaboration. This paper aims to reevaluate the role of two decorative sources:

-First, Braque's training as a housepainter or painter-decorator, a well-known but scarcely studied influence -Second, ordinary decorative practices using paper and glue to make decorative and useful objects, a recent field of historical research full of potential for art history.

Through this paper, I would thus like to question the traditional distinction between high and low art and the manner in which art historians usually analyse the interaction between them. To understand this interaction, we must adopt an anthropological approach: it is not merely a question of technical borrowings, influence from the decorative arts often has a deep-rooted source, as a way of thinking and using skills, techniques and materials.

A case study: Braque's tricks of the trade

"The tradesman's and housepainter's skills should, for the artist, be the most vigorous material expression of painting1 "(slide 3). This 1913 quotation from Guillaume Apollinaire demonstrates that from the start, George Braque's artisanal origins were a well-known source for Cubism: his grandfather was a house painter in Argenteuil; his father founded a house painting and decorating business in Le Havre and Braque was expected to take over the business2 . Hence, he was apprenticed, first with his father and then, with some of his father's former associates, given as examples: stencilled letters, painting that incorporates sand, and fake wood and fake marble -processes (slide 4) he taught Picasso and that other Cubists, like Gris and Laurens, imitated.

Rupalay and

Stencilling letters (slide 5) was a basic skill for decorative painters. Indeed, a large part of the trade consisted in painting storefront and commercial signs on which names, goods, opening hour, menus... appear. The letters could be stencilled, but were usually painted by hand. Forming letters in different styles (slide 6) was then a common requirement for house painters: they knew different alphabets (roman, italian, gothic, egyptian, antique…) and different ways of ornamenting letters (slide 7) with shadows, perspective, colours… They were also able to modify the size and shape of letters in order to make them fit the sign. This kind of technical skill is clearly noticeable in some of Braque's and Picasso's paintings: in Picasso's Souvenir du Havre (slide 8), we can recognize the roman capital letter and in Le violon (Mozart kubelick) (slide 9), Braque pointedly left the chalk line (cordeau or parallel guidelines) used for the letters KUBELICK visible.

The second technique usually associated with Braque's decorative training is paint that incorporates sand (slide 10). In his interview with Dora Vallier, Braque directly attributed his practice of adding sand to his paint to create a granular, stonelike surface to his apprenticeship with Laberthe. Indeed, learning to fabricate the paints and colors was the cornerstone of the trade. House-painters did not use ready-mixed industrial paints, such as Ripolin.

Each paint was made in a camion (slide 11) (a fireproof pail with a handle), following a precise recipe for the required tint. Thus, they mixed pre-ground pigments, sold as powder, flakes, and sticks, or in barrels, with a liquid (water, glue, turpentine, oil, or varnish) to fabricate the desired shade and texture.

Imitating wood and marble (slide 12) is the third decorative technique Braque learned during his apprenticeship. Fake wood and fake marble were decorative painters' masterpieces: they showed the painter's ability to imitate nature and to deceive the eye. Many specialised tools and brushes (slide 13 ) were developed for this work: the veinette or spalter (brushes made for graining and striations), the badger brush to soften the overall effect… Among the tools were steel or leather combs (slide 14) which the painter ran through the fresh paint to mimic the straight or undulating lines of wood grain -a technique often seen in Cubism and sometimes voluntarily misappropriated as in Picasso's Poet, where a decorator's comb is used for the hair and moustache. This is the first, and most familiar, interpretation: Cubism appropriated the house painter's tricks of the trade. Decorative painters' means were "a benefit derived from the beauties of the artisan's work4 " to quote André Salmon. Bus this analysis was not really developed: Braque's apprenticeship and technical training have never been described in detail to support this interpretation and therefor, another material connected to the house painting trade has received less consideration: wallpaper (slide 15). During the second half of the 19 th century the use of wallpaper spread in society and decorative painters were involved in the business of hanging it. -Stencilling letters (slide 22) recalls drizzling paint onto an item to get a negative silhouette.

From tricks to breakthrough: technique as thinking and visual scheme

-Adding sand to paint (slide 23) can be compared to covering objects, like frames, with sand, which might then be painted, to get a united surface and hide the recycled materials.

-Ripolin is employed to coat craft creations because it is cheap, robust and ready-made (there is no need to prepare the paint)

-And above all, they used found objects (slide 24). Indeed, domestic crafters were masters of collage or assemblage processes: they used pine cones, shells or seeds to decorate frames; they made pictures out of stamps or cut images from newspapers or engravings; they decorated lampshades by sticking on fabrics and plants. To conclude, Cubist's example of reusing house painters' skills and domestic crafting techniques demonstrates that high and low interactions are multi-layered (slide 29). They involve not only technical borrowing but also conceptual thinking, materialized thoughts one can apprehend best with an anthropological approach: first not to miss meanings that lie behind technical acts; second to take into consideration the impact of everyday life on creation, the ways common experiences find their way into art. Artists don't find inspiration exclusively in high art and its practice, they draw answers from the common creative culture they are embedded in. Just as Michael Baxandall6 or David Banash7 revealed, visual and technical abilities move from one human field of action to another.

Therefore, when studying creative processes, one must keep in mind that are no valid distinctions between commonplace craft practices and the practice of professionally trained artists. Invented categories, such as high and low art, can blind the art historian by creating an artificial division between areas of creativity.

  Rosney in Le Havre, and Laberthe in Paris. Contemporary chroniclers and art historians 3 underlined the tricks of the trade that Braque borrowed from his formative training in decorative painting. 3 techniques are usually

  Nevertheless, a second level of interpretation offers a more fertile understanding of these decorative borrowings (slide 16): as Clement Greenberg demonstrated, stencilled letters, painting with sand, fake wood and wallpaper were essential breakthroughs in the history of the Cubist movement. They enabled Braque and Picasso to move beyond the hermetic phase of Analytic Cubism in 1911-12. Emphasising the flatness of the painting substrate, they initiated a new kind of three-dimensional representation where the depicted object projects toward the viewer without any trace of volume: the successive flat planes arranged one above the other indicate each depicted object's position within the painting. I would go further than Greenberg: I would argue that this radically new method of representing space derived directly from Braque's formative training as a painter-decorator. Decorative painters hardly ever made use of Albertian perspective: they didn't treat the wall as an open window onto a space behind (slide 17). Instead of seeking to break through the flat surface, their trompe l'oeil effects created an impression of projecting relief: depicted objects seemed to stand in front of the depicted background, like a bas-relief. Filage for example consisted in making strips to mimic wood or stone boards and moulding. This was precisely the path taken by Braque around 1908 while studying Cezanne's works: objects seemed to project in front of the picture plane rather than recede behind it. Therefore, the so-called flatness of Cubism and its inverted perspective (projection toward the picture plane, rather than recession from it) transposed to easel painting ways of representing reality that were common in decorative painting. Likewise, the fusion of figure and ground achieved by Braque during the Cézannian and Analytic phases of Cubism corresponded to decorative painters' approach to representation, which involved superimposing and blending layers on the painted surface to the point where distinctions between them disappear. Using highly diluted paint, glazes, and badger brushes, they strove to erase the transitions between the successive layers. In Braque's hands, the Cubist system of faceting succeeded in unifying the different planes in the painting in a rather similar manner. Finally, Synthetic Cubism's representation of space as an overlay of flat planes matched decorative painters' ways of thinking about their work (slide 18). Imitating wood or marble meant mastering the layering of transparent glazes to re-create the complex tonal and textural effects of wood and marble. Beginning with a ground glaze in the colour of the type of wood or marble they wanted to imitate, they next painted successive glazes to create wood or marble patterns: the network of the grain and the marble's rippling veins. Each of these glazes had to be as thin as possible, and the more layers there were, the more convincing the imitation would be because the pattern of the wood or marble would seem to have greater depth. Hence, decorative painters conceived and perceived their work in terms of flat layers, just as happens in Synthetic Cubism. Arguably, the housepainter's very mode of thinking influenced the Cubists' invention and treatment of space from 1908 to 1912. Consequently, it may not be a coincidence that Cubism's prevailing colours -brown, grey, green… -are similar to those used for striping and for faking wood and faking marble. This first case study shows that, during creation, boundaries between categories blur; every creative practice can help achieve the artist's goals. Braque took advantage of his formative training to find solutions to Cubism's problems. Cubism is full of decorative tricks of the trade but this influence occurred in a deep-rooted way. Technical borrowings from the decorative painter's trade were possible because techniques are "materialized thoughts" as anthropologist Pierre Lemonier said 5 . They bring into play not only tools and gestures but also representations, unconscious schemes that print their mark on technical behaviours, and it is precisely these conceptual schemes, the subconscious structure beneath techniques, that Braque imported to Cubism. 5 LEMONNIER Pierre, « Technique », in BONTE Pierre, IZARD Michel (dirs.) : Dictionnaire de l'ethnologie et de l'anthropologie, Paris, Presses Universitaire de France, 1991, p.697-698 Everyday life experience in art, Cubism's debt to ordinary decorative practice Understanding the interaction between high and low therefore means investigating the artist's technical or visual expertise in order to apprehend fully how techniques and the way of thinking they involve may be transferred from artistic category to another. The second example (slide 19) I would like to analyse draws the same conclusion but from a more unconventional research field for art historians: that of amateur decorative practices. These leisure activities, which happened inside the home, consisted in making useful or decorative objects such as (slide 20) -A cardboard flower pot holder or lampshade ornamented with drawings or glued-on pictures -Weaving an umbrella stand -Turning a half barrel into a flower pot -Decorating jewel or candy boxes with silk, embossed or gold paper… These DIY and creative hobbies, to use present-day expressions, proliferated during the end of the 19 th century and would have been immediately familiar to Braque and Picasso. Living in Montmartre, a very specific neighbourhood of Paris, both poor and rural, they knew this alternative creative culture intimately. It was a shared culture of making (slide 21): Braque, Picasso, and their friends at that time, occasionally created everyday objects with recycled material. Picasso crafted some of his notebooks and a doll for Van Dongen's daughter; Gris made his own slippers by cutting up old shoes. Thus, the practices of self-creating decorative objects they were surrounded by found their way into Cubism, resurfacing through the three-level reading I have previously elaborated. First, ordinary decorative practices were a technical repertory for Cubism as they encompassed all Cubist processes:

  At a deeper level, ordinary crafts appear to have played a key role in Cubism's evolution. As with Braque's decorative training, they provided solutions to Cubist problems, offering, for instance, stylistic and artistic effects that contributed to the new representation of space and allowed Braque and Picasso to experiment with stylistic diversity. Indeed, some popular amateur techniques, consisted in creating images entirely with layers of paper (slide 25). The image is obtained through the superposition of papers or fabrics; the design is the result of the cutting and changes of colour and texture… creating an inverted perspective just like in Analytic Cubism. The different layers give the image volume; the depicted figure projects toward the viewer. The ribbon is behind the hand, the glued plant comes out of the frame, akin to the violin in Braque's painting. Pasted material can also have a textural or coloristic function (slide 26): the visual effect of the work is created by the association of different pieces of fabric or paper, by slight differences in surface and tint. Furthermore, ordinary crafts drew Braque's and Picasso's attention to the potential of heterogeneity. Thanks to their varied materials and techniques, these domestic craft objects have no stylistic unity (slide 27), just as happens in synthetic cubism where the same work may combine passages of cubist fragmentation (objects, the figure) with illusionistic signs (like guitar strings, rope, stencilled letters). This stylistic heterogeneity is matched by that of materials (slide 28), with the mixture of chromos, lace and ribbons or newspapers, musical scores and wallpaper… At the deepest level, by employing these techniques Braque and Picasso imported the conceptual thinking that lies behind the practices of ordinary crafters into the realm of high art. Braque and Picasso not only found in domestic craft an inspiring example of what can be done with non-artistic means but their concept of creation was affected by this material and technical culture. Indeed, they transposed the enforced freedom from rules of ordinary crafters, transforming it into an avant-garde method: the ability to make art with anything and in any way. Ordinary decorative practice thus provided moral support for the invention of collage and assemblage.

Apollinaire, Les Peintres cubistes, p. 83.

Brassaï, The Artists of My Life, p. 19.

Apollinaire, Salmon, Rubin, Gopnik and Varnedoe, Leal, Daix...

André Salmon. La jeune sculpture française, p.13-14. More recently, William Rubin wrote, "many of Braque's innovations in Cubist practice-such as using stencilled letters, charging pigment with sand, and simulating wood and marble-had genuinely artisanal origins"; William Rubin. Picasso and Braque: Pioneering Cubism, p. 19.

Michael Baxandall, L'oeil du Quattrocento, L'usage de la peinture dans l'Italie de la Renaissance, Paris, Gallimard, 1985, 254p. [1 ère ed. : Painting and Experience in Fifteen Century Italy, Oxford, Oxford university Press, 1972; trad. : DELSAUT Yvette].

David Banash, Collage culture. Readymades, Meaning, the Age of Consumption, Amsterdam/New York, Rodopi 2013


H Boulze

L Fleitout

E Klein

C Vigny

C Klein

Vigny

Keywords: Creep and deformation, Numerical modelling, Seismic cycle, South-America, Subduction zone processes, Time-series analysis

Over the last decade, three major subduction earthquakes, Maule M w 8.8 (2010), Illapel M w 8. 3 (2015) and Iquique M w 8.1 (2014), occurred in Chile and generated significant post-seismic deformations. These large scale and long lasting deformations can be quantified with modern GNSS precise positioning and highlight visco-elastic processes in the asthenosphere. Here, we calculate the ratios of cumulative post-seismic displacements after 5 years over the co-seismic offsets. We find that at any distance from the trench, ratios are similar for the three earthquakes despite their different magnitudes which imply induced stresses that are more than one order of magnitude apart. This observation suggests that the post-seismic deformation is related to the same effective viscosity for the Downloaded from https://academic.oup.

Large earthquakes induce stress changes in the mantle over large distances and depths. On a timescale of a few years, only the part of the mantle with the lowest viscosity, named asthenosphere, is able to creep significantly. Thus, post-seismic deformation is a "natural experiment" that constrains the rheology of the asthenosphere. The asthenosphere is below the thermal boundary layer which constitutes the lithosphere. The lithosphere, colder thus more viscous, is about 80 km thick for old oceans and non cratonic continents (e.g. [START_REF] Fleitout | Secondary convection and the growth of the oceanic lithosphere[END_REF][START_REF] Doin | Thermal evolution of the oceanic lithosphere: an alternative view[END_REF]. The thickness of the asthenosphere is poorly constrained, but, according to models of post-seismic deformation, it is thought to range between 100 km and 300 km [START_REF] Trubienko | Models of postseismic deformation after megaearthquakes: the role of various rheological and geometrical parameters of the subduction zone[END_REF]. One of the most important questions that remains is whether the asthenospheric viscosity is stress dependent or not. In general, the answer to this question depends on the predominant creep mechanism in the asthenosphere (e.g. [START_REF] Weertman | High temperature creep of rock and mantle viscosity[END_REF][START_REF] Karato | Deformation of earth materials[END_REF]. For example, dislocation creep is known to involve stress-dependent effective viscosity [START_REF] Weertman | The creep strength of the earth's mantle[END_REF] while diffusion creep is associated with linear rheology, i.e. a creep rate that is simply proportional to the deviatoric stress [START_REF] Gordon | Diffusion creep in the earth's mantle[END_REF]. It was also pointed out that with a power-law rheology the medium can artificially appear linearly viscous during a non steady-state process if the background stresses induced by 'steady' mantle convection are large enough [START_REF] Schmeling | On the interaction between small-and large-scale convection and postglacial rebound flow in a power-law mantle[END_REF].

Monitoring and quantification of surface deformation after a large earthquake by precise satellite

O R I G I N A L U N E D I T E D M A N U S C R I P T
Post-seismic deformations in Chile and linear viscosity in the asthenosphere 3 positioning (GNSS) should provide the means to quantify the viscous laws at play in the asthenosphere. However, while many studies have used a linear viscosity (e.g. [START_REF] Hu | Three-dimensional viscoelastic finite element model for postseismic deformation of the great 1960 chile earthquake[END_REF][START_REF] Pollitz | Transient rheology of the upper mantle beneath central alaska inferred from the crustal velocity field following the 2002 denali earthquake[END_REF][START_REF] Trubienko | Interpretation of interseismic deformations and the seismic cycle associated with large subduction earthquakes[END_REF][START_REF] Klein | Afterslip and viscoelastic relaxation model inferred from the large-scale post-seismic deformation following the 2010 Mw 8.8 Maule earthquake (Chile)[END_REF][START_REF] Hu | Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake[END_REF][START_REF] Li | Postseismic uplift of the andes following the 2010 maule earthquake: Implications for mantle rheology[END_REF], many others concluded that a power-law viscosity provides a better fit to post-seismic deformation (e.g. [START_REF] Pollitz | Mantle flow beneath a continental strike-slip fault: Postseismic deformation after the 1999 hector mine earthquake[END_REF][START_REF] Freed | Evidence of power-law flow in the mojave desert mantle[END_REF][START_REF] Freed | Stress-dependent power-law flow in the upper mantle following the 2002 denali, alaska, earthquake[END_REF][START_REF] Barbot | Asthenosphere flow modulated by megathrust earthquake cycles[END_REF][START_REF] Agata | Rapid mantle flow with power-law creep explains deformation after the 2011 tohoku mega-quake[END_REF][START_REF] Muto | Coupled afterslip Downloaded from[END_REF]Peña et al., 2019;[START_REF] Weiss | Illuminating subduction zone rheological properties in the wake of a giant earthquake[END_REF]Peña et al., 2020). A somewhat surprising observation has been made regarding far-field post-seismic deformation after the three recent megathrust earthquakes of magnitude close to 9 (Sumatra, M w 9.2, 2004; Maule, M w 8.8, 2010 and Tohoku, M w 9.1, 2011)

in [START_REF] Trubienko | Models of postseismic deformation after megaearthquakes: the role of various rheological and geometrical parameters of the subduction zone[END_REF]. The observed co-seismic and post-seismic displacements as a function of time at continuous far-field GNSS stations were found to be nearly homothetic, independently of the earthquake magnitude and geological context. This result argues for a similar effective viscosity in the asthenosphere, at least in the far-field and for earthquakes of magnitude around 9. We expand here the magnitude range of this observation using the three recent Chilean subduction earthquakes of Maule 2010 (M w 8.8), Illapel 2015 (M w 8.3) and Iquique 2014 (M w 8.1). We also investigate whether the observation is valid not only in far-field but at any distance from the trench and discuss the implications for the rheology of the asthenosphere.

GPS DATA

Data set and processing

South American networks provide a good coverage of co-seismic offsets and post-seismic displacements at the scale of the continent, at all distances from the trench. Networks are dense enough to allow us to pick stations near profiles perpendicular to the trench at the latitude of all three earthquakes. We use 52 continuous GPS stations facing the three ruptures zones, in near-field (100-300 km from the trench), mid-field (300-500 km) and far-field (500-1400 km) (Fig. 1A and Fig. S1). Station time-series [START_REF] Baez | The chilean gnss network: Current status and progress toward early warning applications[END_REF], the RAM-SAC network in Argentina (Piñón et al., 2018) and the IGM network in Bolivia. Daily coordinates are processed with the GAMIT/GLOBK suite (Herring et al., 2010a,b) and the PYACS toolbox. Twenty year time-series are expressed in ITRF2014 (Altamimi et al., 2016).

O R I G I N A L U N E D I T E D M A N U S C R I P T 4 H.

Data analysis

We correct time-series from outliers, antenna offsets and earthquakes. We estimate co-seismic offsets differently depending on the signal/noise ratio. For high ratios we simply differentiate between the day after (three days for Iquique, Section S2.2) and the day before the earthquake. For low ratios, we estimate multi-day trends to compute average positions both before and after the earthquake. We define the post-seismic deformation as the difference between after and before the earthquake (cf a description of the whole seismic cycle in [START_REF] Trubienko | Interpretation of interseismic deformations and the seismic cycle associated with large subduction earthquakes[END_REF]). Thus, we estimate pre-seismic trends and subtract them from the total displacement after the earthquake to extract the post-seismic signal (Section S2.1). We estimate the cumulative post-seismic displacement over 5 years after each earthquake. For high signal/noise ratios, we use the position one day after the earthquake at the beginning and the one week-average position at the end. For low signal/noise ratios, we use smoother parametric models. The estimation of the cumulative post-seismic displacement is sensitive to the estimation of the pre-seismic trend, especially in the far-field where numbers are small.

Analysis of the ratios of post-seismic displacements over co-seismic offsets

Co-seismic offsets decrease with distance from the trench and, as expected, the larger the earthquake the larger the offsets (Fig. 1B and Fig. S4). The cumulative post-seismic displacements also decrease with distance from the trench after an initial increase in near-field. Again, the larger the earthquake the

O R I G I N A L U N E D I T E D M A N U S C R I P T
Post-seismic deformations in Chile and linear viscosity in the asthenosphere 5 larger the displacements (Fig. 1C). On the contrary, ratios of cumulative post-seismic displacements (post) over co-seismic offsets (co), hereafter referred to as post/co ratios, increase with distance from the trench (Fig. 2A). Remarkably post/co ratios of all three earthquakes evolve in the same way and are superposed, despite different magnitudes and geometries (Fig. 2A). The post/co ratios are similar even for the Illapel earthquake where the presence of a flat slab does not seem to disrupt the horizontal deformation. This remains true over different time periods: the first two years when afterslip dominates near-field deformation as well as the last three years when visco-elastic deformation becomes dominant (see Section S4 for discussion on afterslip duration and impact).

In near and mid-field (at distances shorter than 500 km), uncertainties are small because displacements are large and post/co ratios fall well on the same linear trend. In the far-field (beyond 500 km), a decreasing trend seems to appear. However, the trend is here unclear because scattering is larger. Scattering increases with distance from the trench because displacements decrease with distance from the trench. For Illapel and Iquique, beyond 800 km both co-seismic offsets and cumulative post-seismic displacements go down to a few millimetres only, yielding less well determined ratios.

Post-seismic time series normalised by the co-seismic offsets also show the remarkable superposition independent of the earthquake size. In near-field, station MAUL (affected by Maule Eq.) and MNMI (affected by Iquique Eq.) superimpose extremely well (Fig. 2B). The same is true in far-field for station VBCA (affected by Maule Eq.) and EPSF (affected by Illapel Eq.) (Fig. 2C).

CONSEQUENCE FOR THE ASTHENOSPHERIC VISCOSITY

The post/co ratios are similar for the three recent Chilean mega-earthquakes, despite their different moments, different slip patterns and perhaps different asthenospheres. In principle, post-seismic displacements, co-seismic offsets and post/co ratios depend mainly on three parameters: The magnitude and slip distribution of the earthquakes that generated them, and the viscosity law of the asthenosphere

O R I G I N A L U N E D I T E D M A N U S C R I P T 6 H. Boulze, L.
Fleitout, E. Klein and C. Vigny that carries the deformation. Our purpose here is not to present models that would closely fit the curves of post/co ratios (Fig. 2A). We simply want to explore the impact of the asthenospheric rheology on earthquakes of different magnitudes using simple models: In particular, we compare predictions based on power-law versus linear viscosities. In the finite-element numerical models used in this section, we deliberately suppress all the various supplementary ingredients (Burgers viscosity, low viscosity channel or low-viscosity wedge, post-seismic slip on the interface) already explored in [START_REF] Klein | Afterslip and viscoelastic relaxation model inferred from the large-scale post-seismic deformation following the 2010 Mw 8.8 Maule earthquake (Chile)[END_REF] concerning the Maule earthquake. In addition, following most previous studies, we assume that the stress in the asthenosphere is only generated by the earthquake itself. At this point, the only thing that matters is to predict post/co ratios independent upon the magnitude and slip pattern of the earthquake.

Modeling details (slab geometry, boundary conditions, etc.) are given in Section S5.

We begin by recalling some general analytic scaling laws that involve time and rheology, which are exact only if one compares homothetic earthquakes, i.e. earthquakes with different magnitudes but with the same slip distribution. So, we first perform numerical tests to compare post/co ratios produced by homothetic earthquakes, using either Newtonian or power-law viscosity. Then, to approximate the real case, we compare post/co ratios produced by different co-seismic slip distributions (both along strike and along depth), combined with different magnitudes, again in the context of either Newtonian or power-law viscosity.

Asthenospheric viscosity and earthquake magnitude: analytical scaling relations from homothetic earthquakes

We consider here two earthquakes E1 and E2 with the same pattern of co-seismic slip, but different magnitudes (homothetic slips): At any point on the subduction interface, the ratio of the co-seismic slips is a constant factor λ. Because linear elasticity is involved, the ratio of the two co-seismic offsets at any point on the Earth's surface is then also λ. For a linear visco-elasticity in the asthenosphere

O R I G I N A L U N E D I T E D M A N U S C R I P T
Post-seismic deformations in Chile and linear viscosity in the asthenosphere 7 (Maxwell or Burgers), possibly variable spatially, but identical for the two earthquakes, the ratio of the post-seismic deformations is also λ everywhere and for all times. So:

co E2 = λ•co E1 and post E2 (t) = λ•post E1 (t).
The post/co ratios of the two earthquakes at a same point of the Earth surface and at a same time t are then equal at all times:

(post/co) E1 (t) = (post/co) E2 (t) (1)
If the viscosities for the E1 earthquake are everywhere k times larger than the viscosities for the E2 earthquake, then this relationship becomes:

(post/co) E1 (kt) = (post/co) E2 (t) (2)
In simple words, at any given point, the same strain will be reached after a time k-times longer if the viscosities are everywhere k-times larger.

For power-law viscosity we use the following creep law (e.g. [START_REF] Stocker | On the rheology of the upper mantle[END_REF][START_REF] Schubert | Mantle circulation with partial shallow return flow: Effects on stresses in oceanic plates and topography of the sea floor[END_REF][START_REF] Schmeling | On the interaction between small-and large-scale convection and postglacial rebound flow in a power-law mantle[END_REF]:

ε = C(J 2 ( τ )) (n-1)/2 τ (3)
where ε and τ are respectively the strain-rate and deviatoric-stress tensors and J 2 is the second invariant of the stress tensor 1/2 tr( τ 2 ). The stress exponent n varies between 2 and 6 but will be taken equal to 3 for further applications. The scalar C depends, in the mantle, upon temperature, pressure, water content, grain size, etc. It is then possibly variable spatially but assumed here to be the same for the two earthquakes which differ only by their magnitude. Then, the two post/co ratios are equal if the post-seismic motion of the smallest earthquake is computed at a time λ 2 t (Section S6.1):

(post/co) E1 (λ 2 t) = (post/co) E2 (t) (4)
In simple words, if the moment of E1 is λ-times smaller than the moment of E2, the same post/co ratios will be reached after a time λ 2 -times larger.

Implications of Eqs. 1 to 4 for post/co ratios after 5 years are investigated through a synthetic nu- are used: i) a M w 8.1 earthquake (Fig. 3A), ii) a homothetic (multiplied by 11.4) M w 8.8 earthquake (Fig. 3B).

The curves of post/co ratios in the Newtonian rheology case are identical (Newtonian-E1 vs.

Newtonian-E2) (Fig. 3D). On the contrary, the curves of post/co ratios in the non-Newtonian case differ largely (power-law-E1 vs. power-law-E2), by a factor of 10 (Fig. 3D). A direct illustration of Eq. 4 based on modelled time-series in case of power-law rheology is provided in Supporting Information (Fig. S8).

Numerical tests concur with the above analytical relationships. Both show that when the effective viscosity is the same, post/co ratios as a function of time and distance from the trench are equal, whatever the magnitude of earthquakes (assuming an identical slip pattern). The post/co ratios become strongly different when the effective viscosity differs either through power-law rheology or through linear but regionally variable viscosities in the asthenosphere (see Eq. 2, Eq. 4 and light red dot-dashed line in Fig. 3).

Asthenospheric viscosity, earthquake magnitude and slip distribution: numerical tests for non-homothetic earthquakes

We now consider the case of two earthquakes with non homothetic co-seismic slip distributions: the fictive M w 8.1 earthquake (referred to as E1, Fig. 3A) and the real Maule earthquake M w 8.8 from [START_REF] Klein | Afterslip and viscoelastic relaxation model inferred from the large-scale post-seismic deformation following the 2010 Mw 8.8 Maule earthquake (Chile)[END_REF] (referred to as E3, Fig. 3C). The Maule earthquake slip distribution presents patches extending from 32 • S to 39 • S and several deep slip patches (below 45 km depth). The fictive slip distribution of E1 corresponds to the main patch of Maule only, but with reduced N-S extent and without deep slip patches. So the magnitude is reduced but also the slip distribution is different.

The curves of the post/co ratios in the Newtonian rheology case are again almost identical (Newtonian-

O R I G I N A L U N E D I T E D M A N U S C R I P T
Post-seismic deformations in Chile and linear viscosity in the asthenosphere 9 E1 vs. Newtonian-E3, Fig. 3D). And likewise, the curves of the post/co ratios in the non-Newtonian case differ largely (non-Newtonian-E1 vs. non-Newtonian-E3, Fig. 3D), by a factor close to 10. So, the scaling laws described by Eqs. 1,2 and 4 can still be used, in first approximation, to compare the post/co ratios of earthquakes with co-seismic slip distributions that differ in both north-south lateral extent and average depth.

To better understand why the post/co ratios vary little with the co-seismic slip distribution, we plot, along the same profile the ratios of i) co-seismic offsets (co(E3)/co(E1)); ii) cumulative post-seismic displacement (post(E3)/post(E1)) and iii) von Mises stresses (( (3J 2 ) where J 2 is the second invariant of the stress tensor) at 200 km depth in the asthenosphere and at the date of the earthquake σ(E3)/σ(E1) (Fig. 3E). These ratios would all be equal to the ratio of seismic moment (11.4 in this case) if the earthquakes were homothetic. In the far-field, they are close to the seismic moment ratio of 11.4, and slightly differ only because the average depth of the co-seismic slip for the two earthquakes differs (Fig. 3E). In the near-field, all three ratios decrease as we get closer from the trench since only a little fraction of the widespread source of Maule earthquake is acting. Therefore, relations between post/co ratios, exact in the homothetic case, remain approximately valid for realistically different earthquakes.

DISCUSSION AND CONCLUSION

This study highlights the striking similarities of the ratios of post-seismic displacements over coseismic offsets after the three Chilean earthquakes. We propose a simple interpretation of these observations based on fairly basic models involving purely Newtonian or power-law rheologies. linearized regime (Fig. S9). However, the stresses in the near and mid-field just facing the rupture zone of Maule would strongly exceed the background stress during the first years after the earthquake and the strong variability of the post/co ratios that we infer for the power-law case would persist. In the unlikely presence of background stresses larger than 10 5 Pa, the post-seismic deformations induced by the three earthquakes would occur under the linearized regime.

It seems difficult to argue for a smaller intrinsic viscosity in the area affected by Illapel and Iquique earthquakes, which would cancel the potential effect of a power-law viscosity. For example, the constant C in Eq. 3 would be multiplied by 100 in the area affected by Iquique and Illapel earthquakes because of conditions hotter or wetter than in the area affected by Maule earthquake. Actually, the seismic velocities in the asthenosphere (e.g. [START_REF] Feng | Upper mantle structure of south america from joint inversion of waveforms and fundamental mode group velocities of rayleigh waves[END_REF][START_REF] Celli | The upper mantle beneath the south atlantic ocean, south america and africa from waveform tomography with massive data sets[END_REF], thought to be good indicators of temperature or water content, are slightly lower in the area affected by Maule earthquake, suggesting a somewhat lower intrinsic viscosity. Both power-law creep and the intrinsic effect would then lower the effective viscosity in the area affected by the huge Maule earthquake, making the misfit to data even worse. Such a large viscosity difference in front of the rupture zone of the three earthquakes is not supported by our observations.

Many studies favour power-law rheology to explain post-seismic deformation (e.g. [START_REF] Pollitz | Mantle flow beneath a continental strike-slip fault: Postseismic deformation after the 1999 hector mine earthquake[END_REF][START_REF] Freed | Evidence of power-law flow in the mojave desert mantle[END_REF][START_REF] Freed | Stress-dependent power-law flow in the upper mantle following the 2002 denali, alaska, earthquake[END_REF][START_REF] Barbot | Asthenosphere flow modulated by megathrust earthquake cycles[END_REF][START_REF] Agata | Rapid mantle flow with power-law creep explains deformation after the 2011 tohoku mega-quake[END_REF][START_REF] Muto | Coupled afterslip Downloaded from[END_REF]Peña et al., 2019;[START_REF] Weiss | Illuminating subduction zone rheological properties in the wake of a giant earthquake[END_REF]Peña et al., 2020). The two main reasons invoked are ei-

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Post-seismic deformations in Chile and linear viscosity in the asthenosphere 11 ther an observation (significant long-term uplift of the volcanic arc after the Aceh, Tohoku or Maule earthquakes) or a model requirement (an apparent increase in effective viscosity with time). Indeed, models with uniform linear viscosity in the asthenosphere, a flat lithosphere-asthenosphere boundary and Maxwell rheology do not predict uplift in the volcanic arc and have difficulties to fit long timeseries with a single viscosity. Power-law creep does induce short-term uplift in this region, but during a few months only. Any model involving a linear low viscosity wedge [START_REF] Trubienko | Models of postseismic deformation after megaearthquakes: the role of various rheological and geometrical parameters of the subduction zone[END_REF] or channel [START_REF] Klein | Afterslip and viscoelastic relaxation model inferred from the large-scale post-seismic deformation following the 2010 Mw 8.8 Maule earthquake (Chile)[END_REF]) also predicts long-term (> 10 years) uplift on the volcanic arc, as observed.

The visco-elasticity of the heterogeneous asthenosphere is unlikely to correspond to a Maxwell material, as demonstrated by the homogenization theory [START_REF] Ivins | Transient creep of a composite lower crust: 1. constitutive theory[END_REF]. The temporal increase in apparent viscosity can be explained by Burgers rheology. Post-seismic slip on the interface also contributes to a rapid initial deformation. Rapid deformation in the first months after the earthquake and uplift of the volcanic arc are predicted by a power-law type rheology but also by other rheological features of the asthenosphere. The comparison of deformations induced by stresses of very different magnitude (Maule vs Illapel and Iquique) provides a specific, so far unexplored argument in the debate between linear and power-law viscosity.

The surprising similarity of the post/co ratios as a function of time and distance from the trench for the three recent Chilean megathrust earthquakes suggests a rather uniform effective viscosity from North to South in the asthenosphere below the South American plate, south of 15 • S. We find that power-law creep plays no significant role in post-seismic deformation, which can be actually modelled using linear rheology. It is somewhat more difficult to know whether these observations on the post/co ratios mean that the asthenospheric rheology governing long-term mantle deformation is also linear, as the answer to this question requires an estimate of the magnitude of the background stress. However, unless we have grossly underestimated the background stress, the similarity of the deformations in S1 andS2.

  The time-dependent stresses associated with the seismic cycle are in reality superimposed on the steady (i.e. steady over a time-scale of few 100 yrs) background stresses involved in plate-tectonics and global mantle convection. In case of power-law creep, the visco-elastic response to an earthquake , L. Fleitout, E. Klein and C. Vigny becomes coupled to these background stresses. If the deviatoric stresses associated with the earthquake cycle are smaller than the background stress, then the effective viscosity governing the post-seismic deformation becomes linear although anisotropic, as discussed in detail in[START_REF] Schmeling | On the interaction between small-and large-scale convection and postglacial rebound flow in a power-law mantle[END_REF] concerning Glacial Isostatic Adjustment (GIA). Assuming a background stress of 4 × 10 4 Pa corresponding to a 5 cm/yr velocity variation across a 200 km depth asthenosphere with a viscosity of 5 × 10 18 Pa.s, the post-seismic deformation induced by Iquique and Illapel earthquakes would mostly happen under this

O

  Figure 1. A-Cumulative post-seismic displacements (cm) after 5 yrs for Maule (red), Iquique (blue) and Illapel (green) earthquakes. Epicentres (stars) and focal mechanisms from USGS. The dashed line depicts profile P. The inset depicts the studied region (N=Nazca plate, SA: South-American plate). Evolution with distance from the trench of: B-co-seismic offsets (uncertainties are exaggerated 200 times) and C-cumulative postseismic displacements over 5 yrs (uncertainties are exaggerated 50 times). B & C-Circles indicate stations where both co-seismic offsets and cumulative post-seismic displacements are measured while squares indicate stations where either co-seismic offsets or some of the post-seismic displacements are interpolated (missing data). All points have uncertainties, just hard to see if too small. Details about uncertainties are provided in Section S2.3. All values are summarised in TablesS1 and S2.

OFigure 2 .O

 2 Figure 2. A-Evolution with the distance from the trench of the ratios of post-seismic displacements over coseismic offsets for Maule (red), Illapel (green) and Iquique (blue) earthquakes. Circles and squares have the same meaning as in Fig. 1. All points have uncertainties, just hard to see if too small. East displacements scaled by co-seismic offset for two stations B-in near-field: MAUL (272 km, Maule Eq., in red) and MNMI (201 km, Iquique Eq., in blue) and C-in far-field: VBCA (1083 km, Maule Eq., in red) and EPSF (1127 km, Illapel Eq., in green). B & C-Pre-seismic trend subtracted, station distance from the trench indicated in legend.

  Boulze, L. Fleitout, E. Klein and C. Vigny are extracted from SOAM GNSS solENS, the South American daily solution produced by Klein et al. (2022) based on observations provided by the CSN network in Chile

  Downloaded from https://academic.oup.com/gji/advance-article/doi/10.1093/gji/ggac255/6628656 by guest on 04 July 2022 H. Boulze, L. Fleitout, E. Klein and C. Vigny merical example. Two fictive seismic sources with identical slip distributions but different magnitudes

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ACKNOWLEDGMENTS

We would like to thank CSN (Centro Sismologico Nacional), IPOC (Integrated Plate Boundary Observatory Chile), FENIX and CANTO (Central Andean Tectonic Observatory), RAMSAC (Red Argentina de Monitoreo Satelital Continuo), Bolivian IGM (Instituto Geográfico Militar), IGS (International GNSS service) for providing data from their respective geodetic networks. Maps are made with Generic Mapping Tools GMT (Wessel et al., 2019). The finite-element computations have been done using the Zset-Zébulon code (http://www.zset-software.com/). We thank J.D. Garaud for his help in particular for the 3D finite-element meshes. Finally, we thank our two reviewers and our editor Prof. Jörg Renner for their very constructive reviews. This work was supported by ANR grant number ANR-19-CE31-0003.

DATA AVAILABILITY

Time-series are publicly available [START_REF] Klein | A 20 year-long GNSS solution across South-America with focus in Chile[END_REF].

SUPPORTING INFORMATION

Supporting Information.pdf