The effect of Fab fragments of rabbit anti-human immunoglobulin on the lysis of allogeneic cultured bladder tumor cells by lymphocytes from patients with urinary bladder carcinoma or by lymphocytes from healthy donors was assayed by 51Cr-release. Addition of Fab fragments of rabbit anti-human immunoglobulin strongly inhibited this cytotoxicity in most cases. Under the conditions used, 60 to 100% inhibition was frequently obtained with 10 to 100 µg/ml of immunoadsorbent purified Fab fragments specific either for the F(ab′)2-part or for the Fc part of human IgG. In those cases in which inhibition was incomplete, increasing the concentration of inhibitor had little or no effect. Fab fragments prepared from immunoadsorbent purified antibodies to an unrelated antigen (ovalbumin) did not inhibit cytotoxicity. Fab fragments that inhibited the spontaneous cytotoxicity against the tumor target cells did not inhibit specific cell-mediated lympholysis induced by mixed lymphocyte culture activation, a cytotoxic reaction known to be mediated by antibody-independent cytotoxic T cells. Experiments with Fab fragments specific for the four IgG subclasses suggested that antibodies of all four IgG subclasses may become involved in lymphocyte-mediated cytotoxicity against the tumor cells.
Previous studies showed that killing of tumor cells by lymphocytes from patients with bladder carcinoma or from healthy donors is most frequently mediated by effector cells with high avidity Fc receptors for IgG but without surface-bound immunoglobulin (SIg) or high avidity receptors for sheep erythrocytes. These previous results and our findings now suggest that a large part (but not all) of the spontaneous cytotoxicity against allogeneic tumor cells is in fact induced by antibodies and thus mediated by K cells. The inducing antibodies are probably released from SIg- cells during lymphocyte-target cell incubation.