RB and A novel E2F‐1 binding protein in MHC class II deficient B‐cell lines and normal IFN‐γ induction of the class II transactivator ciita in class II non‐inducible RB‐defective tumor lines

Abstract

The major histocompatibility (MHC) class II genes encode cell surface proteins that bind antigenic peptide for presentation to T‐cells. The class II proteins are expressed constitutively on B‐cells and EBV‐transformed B‐cells, and are inducible by IFN‐γ on a wide variety of cell types. Retinoblastoma protein (RB) is a tumor suppressor and functions as a transcriptional represser by binding and inactivating the transactivator E2F‐1. RB‐ defective tumor lines are non‐inducible for MHC class II by IFN‐γ, or very weakly inducible, but transfection of 2 different lines with RB expression vectors re‐establishes or substantially enhances class II inducibility. Therefore, we examined the RB status of a series of B‐cell mutants that are defective in class II expression, generated either in vitro or derived from Bare Lymphocyte Syndrome (BLS) patients. Nuclear matrix‐bound RB was detectable in all cases, indicating that loss of RB is not responsible for decreased class II expression in these lines. A second E2F‐1 binding protein, most likely DP‐1, was also apparantly normal in both class II‐positive and ‐negative B‐cell lines. We also examined the IFN‐γ induction of CIITA in RB‐defective lines. CIITA is a class II gene transactivator known to be defective in one form of BLS and to be required for the induction of MHC class II by IFN‐γ. CIITA mRNA is normally inducible by IFN‐γ in class II non‐inducible, RB‐defective lines, and in one line, re‐expression of RB has no effect on CIITA mRNA induction levels. Thus, the block in MHC class II inducibility in RB‐defective cells is not due to a block in CIITA inducibility.