The molecular basis of target cell killing by human lymphocytes and of killer cell self-protection

The cytolytic protein (C9RP) of human cytotoxic lymphocytes was isolated from large granular lymphocytes (LGL) and anti-CD3 activated cytotoxic T cells (CTL). It is immunochemically related to the channel-forming proteins of complement. Whereas LGL constitutively contain C9RP, peripheral resting CTL do not. C9RP synthesis is induced, however, in CD8+ cells upon stimulation of the T cell antigen receptor-CD3 structure. Comparison of cellular cytotoxicity and C9RP content at various times during anti-CD3 activation of CTL yielded a coefficient of correlation, r = 0.92. Isolated C9RP (Mr approximately 70,000) readily lysed a large variety of metabolically active cells tested. Certain monoclonal antibodies to C9RP inhibited target cell killing by LGL or activated CD8+ lymphocytes. Homologous restriction factor (HRF) is a normal membrane protein of blood cells that inhibits transmembrane channel formation by the membrane attack complex of complement. It has recently been found that isolated HRF (Mr approximately 65,000), bound to sheep erythrocytes, inhibited their lysis mediated by the antibody-dependent cellular cytotoxicity reaction or by isolated C9RP. Further, stimulation of resting peripheral lymphocytes with anti-CD3 resulted in increased expression of cell surface HRF. Acquisition of HRF expression conferred upon CTL relative resistance to lysis by C9RP. A soluble form of HRF (Mr approximately 65,000) was isolated from the cytoplasmic granules of LGL, which also contain C9RP, and shown to inhibit cytotoxicity of LGL and CTL. It is conceivable that HRF is opertive in self-protection of cytotoxic lymphocytes.