A caspase-independent pathway of MHC class II antigen-mediated apoptosis of human B lymphocytes

MHC class II molecules have a crucial role in thymic selection and in gener ating Ag-specific T cell responses. There is extensive evidence for second messenger generation,ia MHC class II molecules, which can lead to apoptosis of B lymphocytes. We have examined HLA class II-mediated apoptosis in both normal and tumoral human B lymphocytes. Phosphatidylserine exposure and DN A fragmentation were observed in B cells within 24 h of stimulation via HLA class II, In marked comparison with Fas, the cell-permeable and irreversib le caspase inhibitors zVAD-fmk and DEVD-fmk failed to inhibit HLA-DR-mediat ed apoptosis, No direct activation of caspase 3 was detected, and cleavage of pro-caspase 3 was not observed. Cleavage of poly(ADP-ribose) polymerase was detected via Fas but not via HLA class II. Although phosphatidylinosito l-3-kinase has been implicated in HLA class I-mediated apoptosis, neither w ortmannin nor LY294002 affected HLA class II-mediated apoptosis, CD95-sensi tive cells were used to reveal that death occurred independently of CD95-CD 95 ligand interactions. Overall, these data reveal a pathway of HLA-DR-medi ated apoptosis that neither requires nor involves caspases, Moreover, it is phosphatidylinositol-3-kinase independent and Fas/CD95 independent. This p athway of HLA class II-mediated apoptosis could have an important role in t he regulation of APC populations or in the control of malignant B lymphocyt e proliferations.