Cis-acting elements determine the locus-specific shutoff of class I major histocompatibility genes in murine S49...

Several tumors have been reported to down-regulate expression of their class I major histocompatibility molecules, potentially altering their immune recognition. To investigate this phenomenon, we are using various sublines isolated from the S49 lymphoma of the BALB/c mouse strain. These S49 tumor sublines were previously found to have shut off expression of their Kd, Dd, and/or Ld class I molecules in a locus-specific manner. Extensive Southern blot analyses indicated that there were no major chromosome aberrancies in these S49 sublines, and analyses of steady-state class I mRNA suggested that a form of transcriptional regulation was responsible for their variant class I expression. In this report, we characterize the nature of this locus-specific regulation of class I in S49 cells by producing somatic cell hybrids. Three phenotypically distinct S49 sublines were each fused to tumor cells with normal class I expression, and several of the resulting hybrids were analyzed. In every case, the class I molecules expressed by the hybrids were an exact composite of the two fusion partners. Thus, these fusions failed to rescue expression of the Kd, Dd, and/or Ld molecules shut off within each of the S49 tumor sublines. These findings indicate that this locus-specific shutoff of class I expression results from a cis-acting defect and not trans-acting factors. Because the analysis of each of three phenotypically different S49 cells implicated a form of cis-dominant regulation, we hypothesize that a common mechanism generating homologous mutations in class I genes is operative in S49 tumor cells.