Potentiation of NK cytotoxicity by antibody-C3b/iC3b heteroconjugates

The interaction of two Burkitt lymphoma lines, Raji and Rael, with human C and NK cells was analyzed. Raji cells activate the alternative C pathway (ACP) and then bind C3 fragments. Consequently, the cells become more sensitive to lysis by CR3-bearing NK cells but not to C lysis. In contrast, Rael cells are poor ACP activators, do not bind C3 fragments, and are therefore resistant to C-dependent NK lysis. As suggested earlier, the difference between Raji and Rael could be attributed to the presence or absence of CR2, respectively, on their surface. To potentiate C- and NK-dependent lysis of target cells, we generated heteroconjugates composed of a murine antitransferrin receptor mAb and of human C C3b or iC3b. Antibody-C3b conjugates induced C3 deposition on Rael cells and elevated C3 deposition on Raji cells in human serum. Both Raji and Rael cells coated with antibody-C3b conjugates were efficiently lyzed by the cytolytic ACP in human serum. This conjugate had a small enhancing effect on target cell lysis by NK cells which could be markedly increased by combined treatment of the target cell with antibody-C3b conjugate and C5-depleted human serum. On the other hand, antibody-iC3b conjugates efficiently potentiated lysis of target cells by NK cells in the absence of serum. The iC3b-directed cytotoxicity was mediated by CR3-bearing NK effector cells. Anti-C3 but not anti-mouse Ig antibodies abrogated the activity of the antibody-iC3b conjugate. These results further demonstrate that NK cytotoxicity may be potentiated by opsonizing the target cells with C3 fragments and suggest that antibody-C3b/iC3b conjugates could be potent tools for targeting and potentiation of the lytic action of both C and NK cells against tumor cells.