CIITA-DEPENDENT AND CIITA-INDEPENDENT CLASS-II MHC EXPRESSION REVEALED BY A DOMINANT-NEGATIVE MUTANT

The MHC class II transactivator gene (CIITA) coordinately controls the expression of the three major human class II genes, HLA-DR, HLA-DQ, a nd HLA-DP. Indeed, patients with one form of MHC class II immunodefici ency disease, due to defective CIITA genes, lack expression of all thr ee isotypes. Nevertheless, there is substantial evidence that human cl ass II genes are not always coordinately regulated, raising the possib ility that CIITA-independent, isotype-specific class II regulatory pat hways exist. To address this issue, we have generated a dominant negat ive mutant of CIITA that lacks the acidic transcription-activating N t erminus, but retains the proline/serine/threonine-rich domain. Three n ewly produced anti-CIITA mAbs revealed that this mutant protein lacked N-terminal epitopes. In this study, we report that this CIITA dominan t negative mutant repressed the constitutive expression of all three c lass II isotypes in human EBV-B cell lines, as well as IFN-gamma-induc ed class II transcription in HeLa cells. However, in a CIITA-deficient , EBV-transformed B cell line, clone 13, the dominant negative mutant did not alter the endogenous expression of the HLA-DQ gene. Taken toge ther, these data demonstrate the existence of both CIITA-dependent and -independent class II regulatory pathways. Furthermore, our data prov ide evidence that the Tatter pathways can be isotype specific.