The cytokine tumor necrosis factor α (TNF-α) alone does not induce class II major histocompatibility complex (MHC) expression in most primary cells but can regulate ongoing class II expression in either a positive or negative fashion. The mechanism(s) by which TNF-α enhances interferon γ (IFN-γ)- induced class II expression was examined in a primary cell type, the astrocyte, by transient transfection of the HLA-DRA promoter linked to a chloramphenicol acetyltransferase reporter gene (DRA-CAT). We show that TNF- α, while having no effect on its own, can synergize with IFN-γ to increase the level of promoter activity of a DRA-CAT construct. Three known sequences- W, X, and Y-are required for TNF-α enhancement of IFN-γ-induced promoter activity. The corollary effect of TNF-α on DNA-binding proteins specific for these elements was examined. A previous report described a DNA-binding protein, IFN-γ-enhanced factor X (IFNEX), which is upregulated by IFN-γ in astrocytes and is specific for the X box of the DRA promoter. In this study, we found that TNF-α alone did not induce any nuclear proteins; however, combined treatment of astrocytes with both IFN-γ and TNF-α induced a DNA- protein complex of slower electrophoretic mobility than IFNEX. The TNF-α- induced complex (TIC-X) has specificity for the X element of the DRA promoter. These results suggest a mechanism by which TNF-α enhances IFN-γ- induced class II MHC expression via the formation of TIC-X.