Induction of human complement activation without cytolysis by mouse monoclonal antibodies to human leukocyte antigens

Ten monoclonal antibodies to human leukocyte subsets that had previously been shown to lyse their respective target cells in the presence of rabbit serum as complement source were evaluated for their cytolytic capacity with human complement. Four of the ten were lytic with human complement. All were of IgM type. Antibodies were also evaluated for their capacity to induce C3 binding to target cells. With this method we could demonstrate that, indeed, 3 of the 6 noncytolytic antibodies had the capacity to initiate the human complement activation process and to induce C3 binding. Two of these 3 antibodies were of IgM class (VIT3 and VIM13), one of IgG3 (562). From the practical point of view the most interesting of these 3 antibodies is the nonmitogenic anti-CD3 pan-T cell antibody VIT3. Therefore, this antibody was analyzed in more detail. VIT3 antibody concentrations needed to induce detectable C3 binding to human T cells are very low (down to 1 ng VIT3/ml). Human serum as complement source can also be considerably (100X) diluted before C3 binding becomes undetectable. Activation of C3 is a prerequesite for VIT3-induced C3 binding, and bound C3 seems to lack the C3a fragment. Bound C3, in contrast to the quickly occuring antigenic modulation of the CD3 complex and the simultaneous disappearance of the antibody coat, remains expressed also after prolonged incubation at 37 degrees C. C3 fragments bound to T cells after activation with VIT3 are also recognized by cells bearing C3 receptors of types CR1 and CR2.