Transcription of HLA class II genes in the absence of B-cell-specific octamer-binding factor.

HLA-DR and other human class II histocompatibility genes are expressed by Epstein-Barr virus-transformed B-lymphocyte cell lines but not by most T-cell leukemia lines. We determined by transcriptional run-on experiments that regulation of class II expression in these cells is at the level of gene transcription; nuclei isolated from B-cell lines actively transcribe class II mRNA, whereas nuclei from non-class II-expressing T-cell lines and from the class II transactive factor-deficient B-cell mutant 6.1.6 do not. In searching for DNA-binding proteins which might regulate transcription, we found both a ubiquitous (B1) and a B-cell-specific (B2) factor which bind to the octamer sequence ATTTGCAT 52 base pairs 5' of the cap site in the DR alpha gene. We examined the relationship of these factors to DR alpha transcription. HUT-78, a T-cell line which expresses class II mRNA constitutively, contains only the ubiquitous B1 octamer-binding factor also found in non-class II-expressing T-cell leukemias. Human fibroblast, HeLa, and melanoma cell lines similarly contain only the ubiquitous factor, even when these cells are induced to express class II mRNA by treatment with gamma interferon. Both B1 and B2 binding factors are present in the B-cell mutant 6.1.6, which nevertheless fails to transcribe class II mRNA. Although we have not ruled out the requirement of B-cell-specific octamer-binding factor B2 for class II expression in B cells, it is clear that in other cells substantial DR alpha transcription occurs in the absence of this factor.