CIITA activates the expression of MHC class II genes in mouse T cells. 
It has long been a puzzle that MHC class II molecules are expressed in human T cells after activation but not in mouse T cells; this expression is believed to play a role in the cell mediated immune response. Recently the MHC class II transactivator (CIITA) has been reported to be a major regulatory factor for both the constitutive and IFN inducible expression of MHC class II genes. Here we show that human T cells expressing MHC class II have CIITA transcripts while MHC class II-negative human T cells and mouse T cells do not. The expression of MHC class II genes in mouse T cells can be reconstituted upon transfection with the human CIITA cDNA. These data indicate that the expression of CIITA explains the expression or lack of expression of MHC class II in human and mouse T cells respectively.