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.
OBJECTIVE
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.


METHODS
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.


CONCLUSIONS
These results suggest that HLA-G acts as the public ligand for natural killer inhibitory receptors, thus protecting the fetus against maternal rejection.
Moreau
P
P
Département de Recherche Médicale, Hôpital Saint-Louis, Paris, France.
Paul
P
P
Rouas-Freiss
N
N
Kirszenbaum
M
M
Dausset
J
J
Carosella
E D
ED
D017398:Alternative Splicing; D003602:Cytotoxicity, Immunologic; D005260:Female; D016332:Fetal Tissue Transplantation / Q000276:immunology; D006680:HLA Antigens / Q000096:biosynthesis / Q000235:genetics* / Q000276:immunology*; D059951:HLA-G Antigens; D015395:Histocompatibility Antigens Class I / Q000096:biosynthesis / Q000235:genetics* / Q000276:immunology*; D006801:Humans; D007108:Immune Tolerance / Q000276:immunology; D007694:Killer Cells, Natural / Q000276:immunology; D008431:Maternal-Fetal Exchange / Q000276:immunology; D011110:Polymorphism, Genetic; D011247:Pregnancy; D020033:Protein Isoforms / Q000096:biosynthesis / Q000235:genetics / Q000276:immunology; D012333:RNA, Messenger / Q000378:metabolism; D011971:Receptors, Immunologic / Q000276:immunology / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection; D014184:Transplantation, Homologous; D014327:Trophoblasts / Q000378:metabolism
D006680:HLA Antigens; D059951:HLA-G Antigens; D015395:Histocompatibility Antigens Class I; D020033:Protein Isoforms; D012333:RNA, Messenger; D011971:Receptors, Immunologic