PROBLEM: Human trophoblast expression of class I human leukocyte antigen (HLA) genes is unique in that there is no classical gene expression, but nonclassical HLA-G is expressed and only by cytotrophoblast cells. This differential expression of classical versus nonclassical class I genes suggests tissue specific regulation. Recently, a negative regulatory element (NRE), 180 bp 5′ to transcription initiation was identified in a murine embryonal carcinoma cell line that markedly inhibited class I gene expression (Flanagan et al., Proc Natl Acad Sci USA 1991; 83:3145–3149).
METHOD: Here we analyzed the human HLA-A2 gene for a putative NRE sequence and determined whether such a sequence is capable of binding to factors present in a variety of class I-null cell lines.
RESULTS: Sequence analysis revealed that the NRE for human HLA-A2 is identical to that for mouse H-2Ld. Using gel shift assays with nuclear extracts (NE) from a variety of cell types, we demonstrated specific binding to the HLA-A2 NRE sequence. The choriocarcinoma cell lines JEG and BeWo and the F9 cells (all negative for classical gene expression) contained this DNA binding factor(s). This binding factor was not present in NE from lymphocytes or a variety of other cell lines that were positive for classical gene expression.
CONCLUSION: Human trophoblasts appear to have a tissue specific nuclear binding factor that may down regulate classical class I expression upon binding to the NRE sequence. The HLA-G gene does not have this NRE region thus enabling its expression by these cells.