Evidence for the expression of non-HLA-A,-B,-C class I genes in the human fetal liver.

HLA class I Ag expression was investigated in the human fetal liver. By immunostaining, mAb to monomorphic class I Ag showed widespread reactivity with both epithelial and hemopoietic cells. By contrast, mAb to polymorphic determinants showed more restricted reactivity that was confined to a proportion of hemopoietic cells: the hepatic epithelium was essentially unreactive. This suggested that the developing liver might express nonclassical HLA class I. Class I Ag were examined in membrane and cytosol fractions of mid-trimester fetal liver. Because of its broad reactivity with HLA-A,-B, mAb Q1/28 was selected to identify classical class I Ag in these studies. Immunoprecipitations were carried out against radiolabeled glycoprotein extracts of fetal liver membranes. W6/32 detected a 40-kDa product characteristic of nonclassical class I proteins, as well as a 43-kDa product, in lysates immunodepleted with Q1/28. By immunoblotting, an anti-H chain antiserum (HC) identified a Q1/28- 40-kDa component and a 43-kDa Q1/28+ component in fetal liver membrane glycoproteins. The fetal liver cytosol fraction was found to contain a 42- to 43-kDa product by W6/32-chromatography. This component partitioned to the aqueous phase upon condensation in TritonX-114 detergent and by immunoblotting was reactive with monomorphic mAb HC10 but not with Q1/28. Total RNA and polymerase chain reaction amplified class I transcripts of fetal liver were probed using oligonucleotides specific for HLA-E, -F, and -G. HLA-F, was readily detected in total RNAs by Northern analysis. HLA-E, HLA-F, and HLA-G were all detected in fetal liver by polymerase chain reaction. Differential expression of these genes may occur between the first and second trimester of liver development. Overall therefore, the human fetal liver expresses multiple class I protein products and contains transcripts for non-classical class I genes; particularly HLA-F.