Demonstration of C3d and the terminal complement complex on thymic large granular lymphocytes and common thymocytes after incubation with naturally thymocytotoxic serum.

Guinea pig thymocytes were incubated in vitro with normal human or rabbit serum, which resulted in lysis of a major part of the cells, or with autologous serum causing lysis of a fraction (30%) of the cells. By using antibodies against human C3d and a neoepitope on the terminal complement complex (TCC), activation of both the initial and terminal part of the complement cascade was demonstrated on the surface of thymocytes incubated in the presence of serum. With human serum both types of antigen were detected. With rabbit serum only TCC was detected since immunoglobulins were bound to thymocytes and prevented specific demonstration of C3d by the antirabbit secondary antibody. With autologous serum only C3d was demonstrated, due to lack of cross-reactivity of the monoclonal anti-TCC antibody with guinea pig. Heat-inactivated sera or human serum devoid of IgM neither caused lysis nor resulted in complement activation. Addition of heat-inactivated serum restored the complement activating ability of IgM-depleted serum, indicating that heat stabile IgM is an obligate but sufficient requirement for complement activation in this system. The TCC epitope was also identified on a considerable number of granulated cells, on the basis of morphology classified as large granular lymphocytes.