Expression of the three human major histocompatibility complex class II isotypes exhibits a differential dependence on the transcription factor RFXAP

Major histocompatibility complex class II (MHCII) molecules play a pivotal role in the immune system because they direct the development and activatio n of CD4(+) T cells. There are three human MHCII isotypes, HLA-DR, HLA-DQ, and HLA-DP. Key transcription factors controlling MHCII genes have been ide ntified by virtue of the fact that they are mutated in a hereditary immunod eficiency resulting from a lack of MHCII expression. RFXAP-one of the facto rs affected in this disease is a subunit of RFX, a DNA-binding complex that recognizes the X box present in all MHCII promoters. To facilitate identif ication of conserved regions in RFXAP, we isolated the mouse gene. We then delimited conserved domains required to restore endogenous MHCII expression in cell lines lacking a functional RFXAP gene. Surprisingly, we found that 80% of RFXAP is dispensable for the reactivation of DR expression. Only a short C-terminal segment of the protein is essential for this isotype. In c ontrast, optimal expression of DQ and DP requires a larger C-terminal segme nt. These results define an RFXAP domain with an MHCII isotype-specific fun ction. Expression of the three MHCII isotypes exhibits a differential requi rement for this domain. We show that this is due to a differential dependen ce on this domain for promoter occupation and recruitment of the coactivato r CIITA in vivo.