Regulation of HLA class I transcription in T cells.

Cortical thymocytes express a very low level of HLA class I Ag on their cell surface. As these cells mature to medullary thymocytes and peripheral T cells, HLA class I expression increases to a high level. Leukemic T cell lines arrested at various stages of development were used to examine the regulation of HLA class I expression. Nuclear runoff analysis demonstrated that expression differences in these cell lines were the result of a difference in transcription rates. Electrophoretic mobility shift analysis with the upstream regulatory region of HLA-A2 as a probe for HLA class I-specific DNA binding proteins indicated that DNA binding activity in the cell extracts paralleled the transcriptional activity of the cells. The predominant DNA binding activity in these T cell lines was mapped, by DNase I protection analysis, to a conserved inverted repeat of the MHC class I enhancer. Thus HLA class I expression in these stage-specific T cell lines correlates with the amount of this enhancer binding activity in these cells suggesting that this commonly found DNA binding factor may, in this system, act to regulate HLA class I expression in a very specific manner.