Increased sensitivity of early apoptotic cells to complement-mediated lysis.

Opsonization of apoptotic cells with complement proteins contributes to their clearance by phagocytes. Little is known about the lytic effects of complement on apoptotic cells. Sensitivity of cells treated with anti-Fas antibody (Jurkat cells), staurosporine or etoposide (Raji cells) to lysis by complement was examined. As shown here, early apoptotic cells are more sensitive to lysis by antibody and complement than control cells. More complement C3 and C9 bound to apoptotic than to control cells, even though antibody binding was similar. Enhanced killing and C3/C9 deposition were blocked by benzyloxy-Val-Ala-Asp-fluoromethylketone, a pan-caspase inhibitor. Complement-mediated lysis of early apoptotic cells was also prevented by inhibitors of caspases 6, 8, 9 or 10. In contrast, caspase inhibitors had no effect on the lysis of non-apoptotic Jurkat and Raji cells. Early apoptotic Jurkat cells were also more sensitive to lysis by the pore formers streptolysin O and melittin. Sensitivity of Jurkat Bcl-2 transfectants to lysis by complement was analyzed. Enhanced Bcl-2 expression was associated with reduced C3 deposition and lower sensitivity to complement-mediated lysis. These results demonstrate that at an early stage in apoptosis, following caspase activation, cells become sensitive to necrotic-type death by complement and other pore formers. Furthermore, they suggest that Bcl-2 is actively protecting Jurkat cells from complement-mediated lysis.