HLA-G polymorphisms: ethnic differences and implications for potential molecule function

HLA-G gene 14-bp deletion variant protects Iranian subjects against chronic hepatitis B infection

To investigate whether 14-bp Ins/Del polymorphism in HLA-G gene is associated with the risk of chronic hepatitis B (CHB) infection. This study was performed on a total of 396 individuals including 199 CHB patients and 197 healthy subjects from a south-east Iranian population. We genotyped 14-bp Ins/Del polymorphism in the HLA-G gene using polymerase chain reaction method. The results of our study revealed that the HLA-G 14-bp deletion polymorphism was associated with a reduced risk of CHB at both allele and genotypic levels. The 14-bp Del allele and Ins/Del genotype were more frequent in control group than in CHB patients (37% vs 28% for Del allele with OR = 0.68 and p-value = .015; 73% vs 52% for Ins/Del genotype with OR = 0.43 and p-value = .001) and both were protective factors against CHB. However, no difference was found in the distribution of HLA-G 14-bp genotypes among subjects with varied levels of HBV DNA or hepatic enzymes (p > .05). Our findings, for the first time, suggest that the HLA-G 14-bp Ins/Del polymorphism may be a marker for genetic susceptibility to CHB infection.

Insights into HLA-G Genetics Provided by Worldwide Haplotype Diversity

Human leukocyte antigen G (HLA-G) belongs to the family of non-classical HLA class I genes, located within the major histocompatibility complex (MHC). HLA-G has been the target of most recent research regarding the function of class I non-classical genes. The main features that distinguish HLA-G from classical class I genes are (a) limited protein variability, (b) alternative splicing generating several membrane bound and soluble isoforms, (c) short cytoplasmic tail, (d) modulation of immune response (immune tolerance), and (e) restricted expression to certain tissues. In the present work, we describe the HLA-G gene structure and address the HLA-G variability and haplotype diversity among several populations around the world, considering each of its major segments [promoter, coding, and 3′ untranslated region (UTR)]. For this purpose, we developed a pipeline to reevaluate the 1000Genomes data and recover miscalled or missing genotypes and haplotypes. It became clear that the overall structure of the HLA-G molecule has been maintained during the evolutionary process and that most of the variation sites found in the HLA-G coding region are either coding synonymous or intronic mutations. In addition, only a few frequent and divergent extended haplotypes are found when the promoter, coding, and 3′UTRs are evaluated together. The divergence is particularly evident for the regulatory regions. The population comparisons confirmed that most of the HLA-G variability has originated before human dispersion from Africa and that the allele and haplotype frequencies have probably been shaped by strong selective pressures.

Amniotic fluid soluble human leukocyte antigen-G in term and preterm parturition, and intra-amniotic infection/inflammation

OBJECTIVE

Circulating soluble human leukocyte antigen-G (sHLA-G) has been associated with pregnancy complications, and determination of sHLA-G concentrations in amniotic fluid (AF) has been reported in normal pregnancies. Our aim was to determine if the AF concentrations of sHLA-G change with advancing gestation, spontaneous labor at term, and in patients with spontaneous preterm labor (PTL) with intact membranes, as well as in those with preterm prelabor rupture of membranes (PROM), in the presence or absence of intra-amniotic infection/inflammation (IAI).

STUDY DESIGN

This cross-sectional study included the following groups: (1) mid-trimester (n = 55); (2) normal pregnancy at term with (n = 50) and without (n = 50) labor; (3) spontaneous PTL with intact membranes divided into: (a) PTL who delivered at term (n = 153); (b) PTL who delivered preterm without IAI (n = 108); and (c) PTL with IAI (n = 84); and (4) preterm PROM with (n = 46) and without (n = 44) IAI. sHLA-G concentrations were determined by ELISA. Non-parametric statistics were used for analysis.

RESULTS

(1) Among patients with PTL, the median AF sHLA-G concentration was higher in patients with IAI than in those without IAI or women that delivered at term (p < 0.001 for both comparisons); (2) Similarly, patients with preterm PROM and IAI had higher median AF sHLA-G concentrations than those without IAI (p = 0.004); (3) Among patients with PTL and delivery, those with histologic chorioamnionitis and/or funisitis had a higher median AF sHLA-G concentration than those without histologic inflammation (p < 0.001); and (4) The median AF sHLA-G concentration did not change with advancing gestational age.

CONCLUSIONS

AF sHLA-G concentrations are elevated in preterm parturition associated to IAI as well as in histologic chorioamnionitis. We propose that sHLA-G may participate in the regulation of the host immune response against intra-amniotic infection.

HLA-A2 class I antigens in couples with recurrent spontaneous abortions

Class I human leukocyte antigen (HLA) antigens, locus A and B, were typed in fertile and infertile couples in cases where one of the spouses carried the HLA-A2 antigen. HLA-class I typing data were obtained from 282 participants, 63 fertile couples and 78 infertile couples with recurrent spontaneous abortions (RSA). Locus A antigens were grouped into eight broad specificities (A1, A2, A3, A9, A10, A11, A19, A28) and locus B antigens were grouped, according to HLA epitopes, in two classes (BW4 and BW6). Although the number of cases is small, significant differences in the distribution of locus A antigens were found between HLA-A2-positive (A2+) women from fertile and infertile couples. HLA-A3, A11 and A28 cross-reacting antigens were absent in women from fertile couples and present in women from infertile couples. HLA-A19, which is associated with amino acid triplets of low immunogenicity, was significantly more often observed in A2+ fertile than in infertile women. An excess of the BW4 epitope was found in A2(-) husbands from infertile couples compared to fertile ones. The results of this study support the idea that in the presence of the HLA-A2 molecule the distribution of HLA-A and B loci antigens may be different in fertile couples compared to couples with recurrent spontaneous abortions. It can be suggested that the HLA-A2 molecule, in context with specific genotypes, may contribute to the overall maternal immune response in normal and disturbed pregnancy.

HLA-G DNA sequence variants and risk of perinatal HIV-1 transmission

Background

HLA-G gene is a non-classical MHC class 1 molecule that is highly expressed in the trophoblast at the maternal-fetal interface. In an attempt to elucidate possible immunological mechanisms facilitating protection of infants born to human immunodeficiency virus type (HIV-1) infected mothers, we have been studying genetic variations in the coding and untranslated regions of HLA-G antigen between HIV-1-infected mothers and their infected or uninfected infants. This study investigated whether HLA-G DNA sequence variants are associated with perinatal HIV-1 transmission.

Results

Genomic DNA samples were obtained from a nested case-control study of 34 mother-child pairs co-enrolled in a cohort of the Perinatal AIDS Collaborative Transmission Study in New York. The samples were from two groups predominantly of African-American and Hispanic origin: In the first group, both mother and child were HIV-1-infected; in the second group, only the mother was infected while the child remained uninfected. Genotyping of HLA-G gene were performed on the extracted DNA from peripheral blood mononuclear cells using PCR based sequencing and restriction fragment-length polymorphism analyses.

Among the studied HLA-G exons, dissimilarities in HLA-G DNA sequence variants between the HIV-1 non-transmitting mother child pairs were mostly observed in exon 8-3'-untranslated region at nucleotide positions T3742A, C3743T, G3777C (P = 0.001). Non-transmitting HIV-1 mother child pairs exhibited dissimilarities at nucleotide position C3743T allele with decreased risk of perinatal HIV-1 transmission, compared with HIV-1 transmitting mother-child pairs carrying this allele (odds ratio 0.02 [95% confidence interval 0.00–0.15] P = 0.00001). In addition, heterozygous dissimilarities at nucleotide positions C634G and 714 insT/G in the 5'-upstream regulatory region were observed between the mother child pairs of the HIV-1-non-transmitting group while homozygous similarities of C634C, and either 714insG/G or mother-child pairs with similar 714insT/G were observed among the transmitting group in the same region.

Conclusion

This study identified new variants in the HLA-G gene and provides further evidence that dissimilarities in the HLA-G DNA sequence variants could influence the transmission of HIV-1 from infected mothers to their infants.

Secretion of sHLA-G molecules in malignancies

Our clinical studies revealed significantly increased soluble HLA-G (sHLA-G) plasma levels in patients suffering from malignant melanoma, glioma, breast and ovarian cancer. Specific ELISpot assays demonstrate that sHLA-G molecules expressing intron-4 sequences are preferentially secreted by peripheral blood monocytes. In vitro, the sHLA-G secretion of monocytes and tumor cells was strongly enhanced by TH1 cytokines like IFN-alpha, -beta, -gamma whereas TH2 cytokines (e.g. IL-4, -10) had minor effects. As sHLA-G can inhibit the functions of T and NK cells high concentration of these molecules should systemically or at the tumor side reduce the immune surveillance and thus favour the progression of cancer.

Genotyping: the HLA system and embryo development

The human major histocompatibility complex (MHC), in addition to its role in the regulation of cell-cell interactions in the immune response, also influences reproductive success. Human leukocyte antigen-G (HLA-G) is an MHC class I gene of particular interest in reproductive biology because of its specific expression on fetal cytotrophoblast cells, and its reported involvement both in protection of the developing fetus from destruction by the maternal immune response and in the prevention of maternal pre-eclampsia. HLA-G has 15 known alleles at the DNA level, and allelic frequency varies among ethnic groups. This study describes the results of an inaugural attempt to correlate an HLA-G genetic polymorphism with pregnancy outcome in a patient population undergoing IVF. The study group was composed of 102 Caucasian women. A maternal HLA-G genetic polymorphism was investigated by polymerase chain reaction (PCR) analysis of DNA collected from granulosa cells surrounding oocytes harvested for the IVF procedure. While no statistically significant correlation was identified in this initial study, larger studies examining DNA from trios of mother, father and offspring are planned.

Mother-to-child discordance in HLA-G exon 2 is associated with a reduced risk of perinatal HIV-1 transmission

Definitive genetic parameters correlating with mother-to-child transmission (MCT) of HIV have not been fully established. We screened for the potential correlation between HLA-G variants and MCT, in a cohort of mother-child pairs. Discordance in exon 2 of HLA-G was significantly more common among non-transmitting (93%) than transmitting mother-child pairs (40%). Our results suggest that mother-child pairs both carrying the identical mutation in HLA-G exon 2 may be at higher risk of MCT of HIV-1.

A study of human leukocyte antigen G expression in hydatidiform moles

OBJECTIVE

Human leukocyte antigen G (HLA-G) is a nonclassic major histocompatibility gene normally expressed only in extravillous trophoblasts throughout pregnancy. It may be responsible in part for the successful evasion of the hemiallogenic trophoblasts from maternal immune surveillance. We investigated whether HLA-G is expressed in molar pregnancies.

STUDY DESIGN

We examined 5 complete hydatidiform mole specimens and 5 partial hydatidiform mole specimens to determine whether HLA-G is expressed by immunohistochemistry and by RNA in situ hybridization analysis.

RESULTS

We found that both the protein and RNA of HLA-G is expressed in complete and partial hydatidiform moles.

CONCLUSION

HLA-G RNA and protein are expressed in molar pregnancies. HLA-G expression is independent of embryonic development and may therefore be an integral part of placental development. Furthermore, expression of HLA-G in the complete hydatidiform mole, a naturally occurring androgenote, confirms expression of the paternal allele of HLA-G. Imprinting of HLA-G is therefore unlikely to play a role in protecting fetal trophoblasts from maternal immune rejection.

HLA-G polymorphisms and molecule function--questions and more questions--a review

Human leukocyte antigen (HLA)-G is a non-classical HLA-class I antigen which is predominantly expressed on invasive trophoblastic cells and is postulated to be a mediator of maternal-fetal tolerance. HLA-G interacts with NK cells, can present nonamer peptides and binds CD8 in an analogous manner to classical HLA-I. The HLA-G protein exists in soluble and membrane-bound isoforms generated through alternative splicing. Although initially considered to be non-polymorphic, variations of the HLA-G DNA sequence have been reported which led to the definition of a limited number of HLA-G alleles including the Null-allele G*0105N. Whereas the HLA-G DNA sequence shows a high degree of conservation in positions which are essential for classical HLA-I molecule functions, polymorphic sites in HLA-G are not congruent with sites of high nucleotide variability in classical HLA. The identification of two females with recurrent spontaneous abortions who are homozygous for the G*0105N Null-allele re-opens the discussion about the role of HLA-G in pregnancy and underlines the need of a systematic analysis of the different hypotheses of HLA-G function in vivo.