A new role for C-1-inhibitor in homeostasis: control of activation of the first component of human complement.

Abstract

Abstract The first component of human complement (C1) was reconstituted from physiologic concentrations of purified C1q, 125I C1r, and 131l C1s. Activation of C1 in the presence and absence of the control protein C1-inhibitor (C1-In) was monitored by SDS-PAGE analysis by simultaneously quantifying the characteristic proteolysis of the C1r and C1s subunits. The influence of C1 -In on both the spontaneous autoactivation of C1 and activator-mediated C1 activation was studied. Spontaneous autoactivation of C1 at 37°C was inhibited by purified C1-In. Pretreatment of C1-In at 56°C for 30 min destroyed its inhibitory activity. C1-ln concentrations ranging from 0.35 to 2.0 times the physiologic level all inhibited spontaneous activation of physiologic concentrations of C1. However, C1 -In concentrations of 0.25 times physiologic and less were ineffective in controlling spontaneous C1 activation. At 37°C, C1 -In had no effect on C1 activation induced by immune complexes (EA or tetanus-anti-tetanus), but strongly inhibited that induced by nonimmune activators (DNA or heparin). At 20°C, even immune complex-induced C1 activation was completely blocked by C1-In. Therefore, the selectivity of C1-In at 37°C is based on the faster rate of C1 activation induced by immune activators compared with nonimmune activators. Thus, C1 -In may play a physiologic role in regulating nonspecific C1 activation by nonimmune “activators.” Specific binding of 125I C1-In to 131I C1 on EA at 20°C was observed. SDS-PAGE analyses confirmed that C1-In was bound not to activated C1, but to unactivated C1 on EA. However, no interaction between purified C1 -In and native C1 in the fluid phase was detectable by sedimentation velocity studies in the analytical ultracentrifuge. Thus, C1-ln must interact with EA-bound C1 that is in a state conformationally distinct from native CI but not yet proteolyzed to C1. In conclusion, we have found that in addition to controlling C1 already in its activated form, C1 -In also regulates the activation of C1, thus functioning earlier in the C sequence than previously thought.