Characterization of human lymphoid cell-mediated antibody-dependent cytotoxicity (LDAC).

Cytotoxicity was studied in a model system using chicken erythrocytes (Ch RBC) labelled with Cr as target cells and human peripheral blood lymphoid cells as effector cells. human lymphoid cells are highly efficient in destroying target cells coated with anti-target cell antibody, the mean net percentage cytotoxicity of lymphoid cells from fifty-eight control subjects being 64·27±2·06 (SEM). In the absence of antibody the mean net percentage cytotoxicity was 6·90±0·46. As little as 10 ng rabbit anti-Ch RBC IgG was required to cause significant target cell lysis. Studies on the nature of the lymphoid cell-dependent cytotoxic antibody showed that it is localized in the 7S IgG region of whole serum and that an intact Fc region is required; the F(ab') fragment obtained by pepsin digestion of IgG was inactive although able to inhibit the cytotoxic activity of the whole undigested IgG. Investigation of the kinetics of LDAC showed that when antibody was added to the final culture medium target cell lysis progressed rapidly (detectable within 2 hr) and linearly with time up to 8 hr. Thereafter the rate of lysis decreased reaching a maximum after 12 hr culture. With cultures containing target cells which have been pre-incubated with antibody, lysis occurred even more rapidly, detectable within 30 min and reaching a maximum after only 3–4 hr culture. The maximum cytotoxicity in this system was, however, lower than that obtained when antibody was added directly to the culture medium. Cytotoxicity could be inhibited by the addition of aggregated human IgG, as little as 5 μg causing 100% inhibition of target cell lysis. Study of the nature of the effector lymphoid cell showed, first, that viable cells were required, twice frozen/thawed lymphoid cell suspensions being inactive; secondly, that active protein synthesis by the effector cell was not an essential prerequisite, pretreatment of lymphoid cells with mitomycin C having no significan effect on their ability to lyse antibody-coated target cells but significantly reducing their ability to transform in response to the mitogen PHA; thirdly, that the effector cell is non-phagocytic, non-plastic or glass-adherent and does not bear surface immunoglobulin; and fourthly that significant cytotoxicity is detectable even with a total lymphoid cell to target cell ratio of 1:2.