The MHC class II transactivator (CIITA) acts in the cell nucleus as the master regulator of MHC class II (MHC II) gene expression. It is important to study CIITA regulation in multiple myeloma since MHC expression is central to ability of myeloma cells to present antigen and to the ability of the immune system to recognize and destroy this malignancy. Regulation of CIITA by IFN-gamma in B lymphocytes occurs through the CIITA type IV promoter (pIV), one of the four potential promoters (pI-pIV) of this gene. To investigate regulation of CIITA by IFN-gamma in multiple myeloma cells, first the ability of these cells to respond to IFN-gamma was examined. RT-PCR analyses show that IFN-gammaR1, the IFN-gamma-binding chain of the IFN-gamma receptor, is expressed in myeloma cells and IRF-1 expression increases in response to IFN-gamma treatment. Western blotting demonstrates that STAT1 is activated by phosphorylation in response to IFN-gamma. RT-PCR and functional promoter analyses show that IFN-gamma upregulates the activity of CIITA pIV, as does ectopic expression of IRF-1 or IRF-2. In vivo protein/DNA binding studies demonstrate protein binding at the GAS, E box and IRF-E sites. In vitro studies confirm the binding of IRF-1 and IRF-2 to CIITA pIV. Although multiple myeloma cells express PRDI-BF1/Blimp-1, a factor that represses both the CIITA type III and IV promoters, they retain the capability to upregulate CIITA pIV and MHC II expression in response to IFN-gamma treatment. These findings are the first to demonstrate that although PRDI-BF1/Blimp-1 diminishes the constitutive ability of these cells to present antigen by limiting CIITA and MHC II expression, it is possible to enhance this expression through the use of cytokines, like IFN-gamma.