| Summary: | In addition to their major function in antigen presentation and natural killer cell activity regulation, HLA class I molecules may modulate T-cell activation and proliferation. Monoclonal antibodies (MoAbs) that recognize distinct epitopes of HLA class I molecules were reported to interfere with T-cell proliferation. We show here that two MoAbs (mouse MoAb90 and rat YTH862) that bind to an epitope of the alpha1 domain of HLA class I heavy chain induce apoptotic cell death of activated, but not resting, peripheral T lymphocytes. Other reference anti-HLA class I antibodies specific for distinct epitopes of the alpha1 (B9.12.1), alpha2 (W6/32), or alpha3 (TP25.99) domains of the heavy chain decreased T-cell proliferation but had little or no apoptotic effect. Apoptosis shown by DNA fragmentation, phosphatidylserine externalization, and decrease of mitochondrial transmembrane potential was observed whatever the type of T-cell activator. Apoptosis did not result from Fas/Fas-L interaction and distinct though partly overlapping populations of activated T cells were susceptible to Fas- and HLA class I-mediated apoptosis, respectively. Induction of apoptosis did not require HLA class I cross-linking inasmuch as it could be observed with monovalent Fab' fragments. The data indicate that MoAb90 and YTH862 directed against the alpha1 domain of HLA class I trigger apoptosis of activated T lymphocytes by a pathway which does not involve Fas-ligand.
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