Discoordinate expression of invariant chain and MHC class II genes in class II transactivator-transfected fibroblasts defective for RFX5

MRC class II deficiency or bare lymphocyte syndrome is a severe combined im munodeficiency caused by defects in MHC-specific transcription factors In t he present study, we show that fibroblasts derived from a recently identifi ed bare lymphocyte syndrome patient, SSI, were mutated for RFX5, one of the DNA-binding components of the RFX complex, Despite the lack of functional RFX5 and resulting MHC class II-deficient phenotype, transfection of exogen ous class II transactivator (CIITA) in these fibroblasts can overcome this defect, resulting in the expression of HLA-DR, but not of DP, DQ, and invar iant chain, The lack of invariant chain expression correlated with lack of CIITA-mediated transactivation of the invariant chain promoter in transient transfection assays in SSI fibroblast cells. Consequently, these CIITA tra nsfectants lacked Ag-presenting functions.