Nucleated cell killing by complement: effects of C5b-9 channel size and extracellular Ca2+ on the lytic process

For C5b-9 channels to mediate cytolysis of a nucleated cell, a sufficient number of channels must be formed in the plasma membrane to override the compensatory mechanisms that nucleated cells might employ to survive. It is well known that nucleated cells are relatively resistant to lysis by complement in comparison to erythrocytes, and it is now evident that this resistance is due, in part, to the ability of nucleated cells to rapidly eliminate C5b-9 from the cell surface. The ability of nucleated cells to eliminate complement complexes is related to physiochemical properties of the complex, such as channel diameter, which in turn affect Ca2+ fluxes that stimulate metabolic processes involved in the elimination process. Paradoxically, these same channel properties that stimulate the defense response may also be responsible for the lethal effects of complement. To further study the role of channel size on cytolysis of nucleated cells by C5b-9, we examined the lytic efficiency of larger C5b-9 channels containing several C9 molecules in comparison with smaller C5b-9 channels containing fewer C9. We have obtained data to indicate that although the larger channels were more cytolytically potent, the channel size had little influence on the rate of cell death. In contrast, the rate of lysis of erythrocytes was substantially slower when smaller C5b-9 channels were present. In evaluating the effect of the extracellular Ca2+ concentration, [Ca2+]o, on nucleated cell lysis in the presence of a lytic number of C5b-9 complexes, it was observed that when the [Ca2+]o was increased the rate of cell death also increased. These findings suggest that lysis of nucleated cells by C5b-9, unlike erythrocytes, may not be entirely due to colloid osmotic deregulation.