Efficient repression of endogenous major histocompatibility complex class II expression through dominant negative CIITA mutants isolated by a functional selection strategy.

Major histocompatibility complex class II (MHC-II) molecules present peptide antigens to CD4-positive T cells and are of critical importance for the immune response. The MHC-II transactivator CIITA is essential for all aspects of MHC-II gene expression examined so far and thus constitutes a master regulator of MHC-II expression. In this study, we generated and analyzed mutant CIITA molecules which are able to suppress endogenous MHC-II expression in a dominant negative manner for both constitutive and inducible MHC-II expression. Dominant negative CIITA mutants were generated via specific restriction sites and by functional selection from a library of random N-terminal CIITA deletions. This functional selection strategy was very effective, leading to strong dominant negative CIITA mutants in which the N-terminal acidic and proline/serine/threonine-rich regions were completely deleted. Dominant negative activity is dependent on an intact C terminus. Efficient repression of endogenous MHC-II mRNA levels was quantified by RNase protection analysis. The quantitative effects of various dominant negative CIITA mutants on mRNA expression levels of the different MHC-II isotypes are very similar. The optimized dominant negative CIITA mutants isolated by functional selection should be useful for in vivo repression of MHC-II expression.