RB and a novel E2F-1 binding protein in MHC class II deficient B-cell lines and normal IFN-gamma induction of the class IL transactivator CIITA in class II non-inducible 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-transformed B-cells, and are inducible by IFN-gamma on a wide variety of cell types. Retinoblastoma protein (RB) is a tumor suppressor and functions as a transcriptional repressor by binding and inactivating the transactivator 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 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-I binding protein, most likely DP-I, was also apparently normal in both class II-positive and -negative B-cell lines. We also examined the IFN-gamma 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-gamma. CIITA mRNA is normally inducible by IFN-gamma 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.

Retinoblastoma protein regulation of surface CD74 (invariant chain) expression in breast carcinoma cells.

The HLA class II genes encode heterodimeric cell surface proteins which bind peptide antigen recognized by T-cell receptors on CD4+ T-cells. The class II proteins are inducible by IFN-gamma, and this induction requires, or is strongly enhanced, by retinoblastoma protein (RB) in a series of breast carcinoma cell lines. Loading of peptide onto the class II protein appears to be regulated by CD74, which associates with class II during their transition to the endosomal compartment, where class II binds peptide. Class II proteins and CD74 are largely regulated in concert, provoking the question, is CD74 induction by IFN-gamma affected by RB? Results described here indicate that IFN-gamma induction of CD74 surface expression in a series of breast carcinoma lines is enhanced by RB, while RB has no effect on CD74 mRNA induction. Also, neither the class II nor the CD74 promoter regions are activated by RB in cotransfection experiments where RB activates the SV40 promoter.