Induction of cell-mediated cytotoxicity by shark 19s IgM

Plasma from unimmunized nurse sharks can mediate a reaction similar to antibody-dependent cell-mediated cytotoxicity (ADCC). Normal shark plasma contains numerous natural antibodies reactive with a variety of antigens, including the target employed. Adsorption of plasma with target cells removed a significant amount of activity, suggesting involvement of antibody. Purified 19s IgM was shown to be a component of shark plasma capable of inducing cytotoxicity. These cytotoxic reactions differ from observations in homeothermic vertebrates in that shark immunoglobulin appears to bind more avidly to the effector cells than to the targets. The effector leukocytes are glass adherent, but not susceptible to carbonyl iron treatment, which clearly separates them from the phagocytic effectors of spontaneous cytotoxicity. Thus, the shark possesses leukocytes with the capability of mediating an ADCC-like reaction. These leukocytes, in concert with those mediating spontaneous cytotoxicity, could provide the shark with an effective immunosurveillance system. These data also indicate that ADCC mechanisms, with IgM as the primary effector molecule, appeared early in evolution.