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 speci fic tolerance, Methods. sHLA-A2 and A3 were isolated from the sera of liver transplant rec ipients by affinity chromatography, T cell bulk lines directed against HLA- A2 and HLA-A3 were generated by stimulation with HLAA2, A3+ peripheral bloo d leukocytes and B-lymphoblastoid cells. Induction of T cell apoptosis by s HLA was analyzed by adding sHLA to allospecific CTL 4 or for 24 hr before f low cytometric analysis of propidium iodide and fluorescein isothiocyanate- conjugated annexin V stained cells, T cell receptor (TCR) engagement by sHL A 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%, whe reas 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 i nduced 21% apoptosis of an antiHLA-A2 T cell line, whereas sHLA-A3 caused o nly 3% apoptosis, The antibody complexed form of sHLA was ineffective in th e induction of apoptosis. Preincubation of the T cells with anti-T cell rec eptor monoclonal antibody protected the T cells from sHLA-induced apoptosis , indicating that sHLA-TCR engagement is necessary for this process to occu r. Conclusion. TCR-mediated apoptosis of alloreactive CTL may serve as a mecha nism by which sHLA can modulate the immune response.