Cis- and trans-repression of class I major histocompatibility gene expression in Abelson virus-transformed murine leukemia

Numerous tumor cell lines of leukemic origin are known to modulate cell surface expression of major histocompatibility complex (MHC) class I antigens resulting in alterations in their immune detection and tumorigenicity. We have been studying the mechanisms responsible for attenuation of MHC class I gene expression in an H-2 heterozygous (H-2b x H-2d) Abelson-Murine leukemia virus (A-MuLV)-transformed leukemic cell line (designated R8). Here we report that treatment of the R8 cell line with the protein synthesis inhibitor cycloheximide (CHX) increased H-2Kb steady-state messenger RNA (mRNA) levels several fold. The induced H-2Kb mRNA transcripts were functional, as demonstrated by their ability to be translated into immunoprecipitable H-2Kb alloantigen. H-2Kb null variants derived from the R8 cell line were shown to be the product of both cis- and trans-acting mechanisms, insomuch as the treatment of R8-derived H-2Kb non-expressor lines with CHX re-established expression of H-2Kb mRNA to the same extent as transfection of the variant cell line with the wild-type H-2Kb gene. Such findings indicate that downregulation of MHC class I gene expression is constitutive for the R8 leukemic cell line, a phenomenon that may be related to the immature pre-B-cell phenotype of this A-MuLV transformant.