Soluble MHC II-peptide complexes induce antigen-specific apoptosis in T cells

Soluble major histocompatibility (MHC) class II molecules in association with antigenic peptide recognize T cell receptors (TCRs) on CD4+ T cells. Such recognition of MHC II-peptide complexes by T cells in the absence of costimulatory signals is known to induce T cell nonresponsiveness. The present study describes that recognition of TCRs by MHC class II-peptide complexes induces antigen-specific apoptosis in a T cell clone independently of nonresponsiveness. Apoptosis was demonstrated in a murine T cell clone (4R3.9) restricted for IAk in association with a peptide analog of myelin basic protein [MBP(1-14)A4]. A dose- and time-dependent T cell death was observed upon incubation of 4R3.9 T cells with purified IAk-MBP(1-14)A4 complexes. The specificity of T cell apoptosis was shown by incubating 4R3.9 T cells with irrelevant IAs-MBP(90-101) complexes. The DNA fragmentation as a result of apoptosis was demonstrated by agarose gel electrophoresis and by pulsing T cells with BrdU followed by the detection of BrdU-labeled DNA fragments using an antibody enzyme-linked immunosorbent assay. The expression level of two regulatory intracellular proteins, bcl-2 and bax, involved in apoptosis showed a decrease in bcl-2 and an increase in bax with time. Finally, the nuclear shrinkage and chromatin condensation, typical hallmark of apoptosis, have been demonstrated by transmission electron microscopy of complex-treated T cells. Since the T cell clone (4R3.9) used in this study failed to show nonresponsiveness by IAk-MBP(1-14)A4 complexes, our results suggest that apoptosis induced by purified MHC class II-peptide complexes may involve distinct pathways rather than T cell nonresponsiveness. Such antigen-specific apoptosis may have significant clinical relevance in deleting autoreactive T cells in various autoimmune diseases.