Activation of class II MHC genes requires both the X box region and the class II transactivator (CIITA).

CIITA, a gene that can complement a transcriptional mutation of the major histocompatibility complex (MHC) class II genes, was tested for its ability to function as a coactivator, CIITA cDNA clones isolated showed alternative RNA splicing, but only one splice site combination was able to restore class II MHC gene expression. DNA-mediated transfection experiments showed that CIITA directs its activity through the X box element; the presence of CIITA leads to the formation of a higher order complex at the X box region; and CIITA contains a potent activation domain. These findings support the hypothesis that CIITA directly interacts with the MHC class II-specific transcription factors and is required for expression.