The TCR-binding region of the HLA class I alpha2 domain signals rapid Fas-independent cell death: a direct pathway for T cell-mediated killing of target cells?

TCR binding to an MHC class I/peptide complex is a central event in CTL-mediated elimination of target cells. In this study, we demonstrate that specific activation of the TCR-binding region of the HLA-A2 class I alpha2 domain induces apoptotic cell death. mAbs to this region rapidly induced apoptosis of HLA-A2-expressing Jurkat E11 cells, as determined by morphologic changes, phosphatidylserine exposure on the cell surface, and propidium iodide uptake. In contrast, apoptosis was not induced following culture with mAbs directed to other regions of the class I molecule. Death signaling by class I molecules is apparently dependent on coreceptor activation, as apoptosis is also signaled by HLA-A2 molecules, where the intracytoplasmic residues were deleted. HLA class I alpha2-mediated cell death appeared to proceed independent of the Fas pathway. Compared with apoptotic signaling by Fas ligation, HLA class I alpha2-mediated responses displayed a faster time course and could be observed within 30 min. Furthermore, class I alpha2-induced cell death did not involve observable DNA fragmentation. The apoptotic response was not affected significantly by peptide inhibitors of IL-1beta converting enzyme (ICE)-like proteases and CPP32. Taken together, activation of the TCR-binding domain of the class I alpha2 helix may result in apoptotic signaling apparently dependent on a novel death pathway. Thus, target HLA class I molecules may directly signal apoptotic cell death following proper ligation by the TCR.