RB AND A NOVEL E2F-1 BINDING-PROTEIN IN MHC CLASS-II DEFICIENT B-CELLLINES AND NORMAL IFN-GAMMA INDUCTION OF THE CLASS-II TRANSACTIVATOR CIITA IN CLASS-II NONINDUCIBLE RB-DEFECTIVE TUMOR LINES

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-tran sformed B-cells, and are inducible by IFN-gamma on a wide variety of c ell types. Retinoblastoma protein (RE) is a tumor suppressor and funct ions as a transcriptional repressor by binding and inactivating the tr ansactivator E2F-I. RB-defective tumor lines are non-inducible for MHC class II by IFN-gamma, or very weakly inducible, but transfection of 2 different lines with RE expression vectors re-establishes or substan tially enhances class II inducibility. Therefore, we examined the RE s tatus of a series of B-cell mutants that are defective in class II exp ression, generated either in vitro or derived from Bare Lymphocyte Syn drome (BLS) patients. Nuclear matrix-bound RE was detectable in all ca ses, indicating that loss of RE is not responsible far decreased class II expression in these lines. A second E2F-I binding protein, most li kely DP-I, was also apparantly normal in both class II-positive and -n egative B-cell lines. We also examined the IFN-gamma induction of CIIT A in RE-defective lines. CIITA is a class II gene transactivator known to be defective in one form of BLS and to be required for the inducti on of MHC class II by IFN-gamma. CIITA mRNA is normally inducible by I FN-gamma in class II non-inducible, RE-defective lines, and in one lin e, re-expression of RE has no effect on CIITA mRNA induction levels. T hus, the block in MHC class II inducibility in RE-defective cells is n ot due to a block in CIITA inducibility. (C) 1995 Wiley-Liss, Inc.