Differential expression of MHC class II isotype chains

Coordinate expression of the HLA-D genes is not a general rule. Herein we r eview the data suggesting that solely professional antigen-presenting cells (APCs) display cell surface expression of the HLA-DQ isotype. We then focu s on residual transcription of certain HLA-D genes in BLS cell lines, and p ropose that this phenomenon could be linked to the residual capacity of the RFX complex to interact with the X2BP and NFY transcription factors, rathe r than to a residual binding activity of the altered RFX complex to the X1 box. Finally, we discuss different compensation mechanisms that lead to the transcription of certain HLA-D genes, through tissue-specific factors, onc ogenes, or tumor suppressor genes.