Molecular and Immunologic Aspects of the Nonclassical HLA Class I Antigen HLA‐G: Evidence for an Important Role in the Maternal Tolerance of the Fetal Allograft

PROBLEM: Human leukocyte antigen (HLA)-G is a major histocompatibility complex class I antigen, which is referred to as nonclassical because it displays a tissue-restricted distribution in the placenta, a reduced cytoplasmic domain, a limited polymorphism, and several isoforms. The HLA-G antigen is thought to play an essential role during pregnancy by protecting the semi-allogeneic fetus from recognition and destruction by maternal immune cells. METHOD OF STUDY: Alternative splicing of HLA-G mRNA was analyzed by Southern blot of reverse transcriptase-polymerase chain reaction products from trophoblasts of the first trimester of gestation and term placenta. The regulation of HLA-G gene expression was investigated by electrophoretic mobility shift assays using nuclear extracts from cells expressing different levels of HLA-G gene activity. Using polymerase chain reaction-single strand conformational polymorphism and sequencing, we studied HLA-G gene polymorphism in families from the Centre d'Etude du Polymorphisme Humain in Paris. To understand the function of the HLA-G molecule, cytotoxicity assays were carried out with peripheral blood mononuclear cells or polyclonal natural killer effectors cells from 30 different donors against HLA-G1 and HLA-G2 transfectants. RESULTS: Four main aspects have been elucidated: 1) The primary transcript of the HLA-G gene is alternatively spliced into five main mRNA forms: HLA-G1 (full length), HLA-G2 (minus exon 3), which encodes a membrane-bound isoform associated with beta-2 microglobulin, HLA-G3 (minus exons 3 and 4), HLA-G4 (minus exon 4), and HLA-G5 (plus intron 4), which encodes a soluble form of the HLA-G antigen; 2) specific nuclear factors bind to an important regulatory element located more than 1.2 kb from the HLA-G gene. Three specific complexes are observed in cells that show HLA-G transcriptional activity and an additional factor that could correlate with the repression of HLA-G gene expression that is detected in natural killer cells; 3) we observed an important genomic polymorphism in exon 3 but a very low polymorphism at the protein level; 4) HLA-G1 and HLA-G2 transfectants clearly demonstrated that both HLA-G isoforms are capable of inhibiting natural killer lytic activity. CONCLUSION: These results suggest that HLA-G acts as the public ligand for natural killer inhibitory receptors, thus protecting the fetus against maternal rejection.