We have generated a second line of mice lacking a transcription factor thou
ght to be a critical regulator of MHC class II gene expression, CIITA (for
class II transactivator), Our and the previously published lines differ in
the deletion that was engineered and by the fact that we removed the neomyc
in-resistance promoter and structural gene via the cre-IoxP recombination s
ystem. Characterization of our line led to two new findings. First, a subst
antial number of cells can express class II molecules in the absence of CII
TA, albeit at 5-fold reduced levels, most notably dendritic cells in s.c. l
ymph nodes; therefore, the CIITA gene cannot be an absolute 'master gene' c
ontrolling the expression of class II molecules, as had been thought. Secon
d, in contrast to recent results on human cell lines, CIITA is not critical
ly involved in the IFN-gamma-induced up-regulation of MHC class I genes.