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 monomorphic class I Ag reacted extensively with amnion cells, whereas polymorphic mAb reactivity was more limited and variable. Further studies were conducted on amnion cell preparations containing negligible contaminants. Northern analysis with use of locus-specific probes demonstrated that amnion expresses two class Ib genes, HLA-E and HLA-G. Radio-immunoprecipitation with use of monomorphic mAb identified two fully glycosylated cell surface class I H chains of 44 and 41 kDa; polymorphic mAbs failed to immunoprecipitate the 41-kDa product, although 44-kDa products, typical of class Ia Ag, were identified in some preparations. Class I H chains were isolated from amnion by affinity chromatography. Microsequencing revealed that the first nine residues of the N-terminus of the 41-kDa product aligned perfectly only with HLA-E. Overall, amnion at term appears to express class Ib Ag with limited class Ia Ag. HLA-G is therefore expressed in two extrafetal epithelia: amnion and trophoblast. Identification of the class Ib protein HLA-E in amnion epithelium may have implications for preterm labor that can be associated with infection of the placental membranes.