Class II MHC Ags are critical in the regulation of immune responses by presenting Ag to T lymphocytes, resulting in their activation and differentiation. Class II expression is rare in the normal central nervous system, but elevated expression on glial cells has been observed in several neurologic diseases. We have previously demonstrated that IFN-γ-induced class II expression in glial cells involves activation of both tyrosine kinase and protein kinase C. IFN-γ induces tyrosine phosphorylation of the tyrosine kinases Jak1 and Jak2 and of Stat1α. In addition, IFN-γ enhances expression of Stat1a mRNA and protein. We utilized antisense oligonucleotides against Stat1α to determine directly whether IFN-γ-induced activation and/or enhancement of Stat1α is involved in class II expression. Antisense oligonucleotides complementary to Stat1α mRNA were introduced in CH235-MG astroglioma cells by transient transfection; such treatment inhibited both constitutive and IFN-γ-enhanced expression of Stat1α. IFN-γ-induced class II MHC expression was also inhibited in cells exposed to Stat1α a antisense oligonucleotides. The fact that the class II promoter does not contain IFN-γ-activated sequences for binding Stat1α suggests that Stat1α must activate another protein that is directly involved in class II expression. A likely candidate is the class II MHC transactivator (CIITA). IFN-γ induction of CIITA mRNA was also inhibited in cells treated with antisense oligonucleotides against Stat1α. These findings demonstrate that Stat1α is involved in IFN-γ induction of CIITA expression, resulting in class II MHC expression.