Downregulation of MHC class II expression by oxidant-induced apoptosis in EBV-transformed B-cells.

The expression of MHC class II molecules is actively regulated upon various cellular stimuli. Since apoptosis is an inducible cellular process, it was asked whether cells undergoing apoptosis would also modulate their expression of class II molecules. Using an EBV-transformed B-cell line, the cell surface expression of HLA-DR molecules was analyzed by fluorescence-activated flow cytometry on normal and oxidant-treated apoptotic cells. A rapid and continuous decrease in HLA-DR expression was observed in apoptotic cells. RNA analysis and semiquantitative RT-PCR of cytoplasmic beta-actin mRNA showed that apoptotic cells contain partially degraded RNA and much lower amounts of beta-actin mRNA. Nevertheless, when compared after normalization of intact mRNA amounts, the HLA-DRB mRNA signals were of similar strength in normal and apoptotic cells as determined by semiquantitative RT-PCR. Thus, the decrease in the number of class II molecules during apoptosis underlies no specific program for downregulation of HLA-DRB mRNA transcription but is due to a nonspecific degradation of RNA molecules accompanied by cell death.