Major histocompatibility complex (MHC) class I–deficient cell lines were used to demonstrate that the MHC class II transactivator (CIITA) can induce surface expression of MHC class I molecules. CIITA induces the promoter of MHC class I heavy chain genes. The site α DNA element is the target for CIITA-induced transactivation of class I. In addition, interferon-γ (IFNγ)–induced MHC class I expression also requires an intact site α. The G3A cell line, which is defective in CIITA induction, does not induce MHC class I antigen and promoter in response to IFNγ. Trans-dominant–negative forms of CIITA reduce class I MHC promoter function and surface antigen expression. Collectively, these data argue that CIITA has a role in class I MHC gene induction.
