Differentiation of hemolytically active fluid-phase and cell-bound human C1q by an ant venom-derived polysaccharide

A purified polysaccharide (PS) from the venom of the ant Pseudomyrmex sp. causes the activation of human C1 in serum. We have investigated 1) the site where the PS acts to cause C1 activation, and 2) whether the recognition unit of human C1 (C1q) has a configuration that differs in the fluid phase compared to when it is bound to cells. The PS caused the activation of C1 in human serum as measured by C4 and C2 consumption, but it did not cause C4 and C2 consumption in guinea pig serum. However, both guinea pig and human C1 from diluted serum, and partially purified C1 from both sources, were blocked by the PS during the transfer from EA to EAC4 at μ=0.15 and 30°C. The blocking is interpreted as a fluid-phase binding reaction because the PS had no effect on human cell-bound C1, and ³H-PS was not taken up by cell-bound C1. In addition, the PS did not interfere with the disassembly of C1r and C1s of cell-bound C1 by the C1 inhibitor. The PS had no effect on highly purified human C1q in the fluid phase and no effect during the transfer of C1q at μ=0.15 and 30°C from EA to EAC4. However, the same C1q when cell bound was functionally inactivated by the PS, and ³H-PS was taken up by the cell-bound C1q. We ascribe the activities of the PS on macromolecular C1 (activation, blocking of transfer) to its action on C1q, and propose that the PS causes C1 activation because it binds to an exposed site(s) on C1q. This site(s) is not available to the PS when macromolecular C1 is cell-bound, probably because of an obstruction by C1r and/or C1s. The site(s) also is not available to the PS on purified fluid-phase C1q, only when purified C1q is attached to cells, or when it is attached to C1r and C1s. Thus, the site(s) for the PS on C1q is different from the six binding subunits that serve as the recognition unit for macromolecular C1 when it first is bound to the Fc portion of immunoglobulin before activation.