THE HUMAN AMNION IS A SITE OF MHC CLASS IB EXPRESSION - EVIDENCE FOR THE EXPRESSION OF HLA-E AND HLA-G

The expression of HLA class I Ag by term human amnion epithelial cells was investigated. In immunostaining and FACS analysis, mAb to monomor phic class I Ag reacted extensively with amnion cells, whereas polymor phic mAb reactivity was more limited and variable. Further studies wer e conducted on amnion cell preparations containing negligible contamin ants. Northern analysis with use of locus-specific probes demonstrated that amnion expresses two class Ib genes, HLA-E and HLA-G. Radio-immu noprecipitation with use of monomorphic mAb identified two fully glyco sylated cell surface class I H chains of 44 and 41 kDa; polymorphic mA bs failed to immunoprecipitate the 41-kDa product, although 44-kDa pro ducts, typical of class la Ag, were identified in some preparations. C lass H chains were isolated from amnion by affinity chromatography. Mi crosequencing revealed that the first nine residues of the N-terminus of the 41-kDa product aligned perfectly only with HLA-E. Overall, amni on at term appears to express class Ib Ag with limited class Ia Ag. HL A-G is therefore expressed in two extrafetal epithelia: amnion and tro phoblast. Identification of the class Ib protein HLA-E in amnion epith elium may have implications for preterm labor that can be associated w ith infection of the placental membranes.