We monitored the expression of MHC class I antigens in a clonal isolate (M1) of B16-F10 murine melanoma cells, under various condition of growth in vitro. We found that, compared to exponentially growing cultures, cells from confluent M1 cultures expressed higher levels of H-2D(b) and H-2K(b) surface antigens. Expression of MHC class I antigens could be enhanced further by incubation of M1 cells in serum-free medium for 24-48 hours. Northern blot analyses indicated that the up-regulation of MHC class I antigen expression was associated with increased levels of H-2K(b) and H-2D(b) mRNAs, without a concomitant decrease in c-myc expression. Analyses of the cell cycle distribution of M1 cells stained for MHC class I antigens failed to show any differential segregation of H-2D(b)- or H-2K(b)-positive cells in any phase of the cell cycle. Enhanced MHC class I antigen expression appeared not to be due to the release into the medium of known cytokines like IFN-alpha, -beta, -gamma, TNF-alpha, IL-6 (IFN-beta(2)), and IL-1 by the M1 cells either at confluence, or after growth in serum-free medium. Taken together, our experimental results indicate that expression of MHC class I genes by M1 cells can be greatly enhanced by manipulating culture conditions; these observations are particularly important for the re-expression of the H-2K(b) gene, which was assumed to be phenotypically silent in B16 cells. These data also suggest that re-expression of H-2K(b) antigen in M1 cells at confluence may be related to cell-cell contact, consistent with the concept that H-2K(b) molecules can play a nonimmune role in cellular communication and growth control.

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