Astrocytes can function as antigen-presenting cells (APC) upon expression of class II major histocompatibility complex (MHC) antigens, which are induced by interferon-γ (IFN-γ). Previous data from this laboratory had shown that the cytokine tumor necrosis factor-α (TNF-α) enhances IFN-γ-mediated class II antigen expression on astrocytes. We have now investigated the effect of IFN-γ and TNF-α on class II MHC mRNA expression in astrocytes using Northern blot analysis. Astrocytes do not constitutively express mRNA for class II MHC. Kinetic analysis of class II MHC mRNA expression in IFN-γ-treated cells demonstrated an 8 h time lag, which was followed by an increase over the next 16 h. Optimal expression of class II mRNA was detected after a 24 h incubation with IFN-γ. This level of expression was further enhanced by the simultaneous addition of IFN-γ and TNF-α to the astrocytes, while TNF-α alone had no effect on class II mRNA expression. TNF-α does not act by increasing the stability of IFN-γ-induced class II mRNA, indicating its action is not at that specific level of post-transcriptional control. Furthermore, astrocyte class II mRNA expression was inhibited when cycloheximide (CHX) was added together with IFN-γ or IFN-γ/TNF-α, and when CHX was added up to 4 h after treatment with IFN-γ or IFN-γ/TNF-α. These results indicate that astrocyte class II mRNA expression is mediated by newly synthesized proteins induced by IFN-γ and/or IFN-γ/TNF-α. The expression of class II antigens on astrocytes, and cytokine modulation of their expression, may be important in the initiation and perpetuation of intracerebral immune responses. © 1990.