Liver transplant recipient sera derived soluble HLA mediates allele specific CTL apoptosis

Background Significant levels of donor soluble human leukocyte antigen (HLA) class I(sHLA) are present in patients after transplants, We investigated the possibility that sHLA may inhibit cytolytic T lymphocyte (CTL) activity by inducing apoptosis of the CTL, thereby serving as a mechanism for specific tolerance, Methods. sHLA-A2 and A3 were isolated from the sera of liver transplant recipients by affinity chromatography, T cell bulk lines directed against HLA-A2 and HLA-A3 were generated by stimulation with HLAA2, A3+ peripheral blood leukocytes and B-lymphoblastoid cells. Induction of T cell apoptosis by sHLA was analyzed by adding sHLA to allospecific CTL 4 or for 24 hr before flow cytometric analysis of propidium iodide and fluorescein isothiocyanate-conjugated annexin V stained cells, T cell receptor (TCR) engagement by sHLA was demonstrated using a monoclonal antibody specific for the TCR. Results. sHLA-A3 inhibited CTL activity of a HLA-A3 T cell line by 53%, whereas sHLA-A2 had no effect, sHLA-A3 also increased T cell death by 77% over the control, whereas sHLA-A2 had no significant effect. However, sHLA-A2 induced 21% apoptosis of an antiHLA-A2 T cell line, whereas sHLA-A3 caused only 3% apoptosis, The antibody complexed form of sHLA was ineffective in the induction of apoptosis. Preincubation of the T cells with anti-T cell receptor monoclonal antibody protected the T cells from sHLA-induced apoptosis, indicating that sHLA-TCR engagement is necessary for this process to occur. Conclusion. TCR-mediated apoptosis of alloreactive CTL may serve as a mechanism by which sHLA can modulate the immune response.