A new role for C-1-inhibitor in homeostasis: control of activation of the first component of human complement
Ziccardi, R.J.
Journal of Immunology 128(6): 2505-2508
1982
ISSN/ISBN: 0022-1767 PMID: 7077078 Document Number: 196905
The 1st component of human complement (C1) was reconstituted from physiologic concentrations of purified C1q, 125IC1r and 131IC1s. Activation of C1 in the presence and absence of the control protein C.hivin.1-inhibitor (C.hivin.1-In) was monitored by SDS-PAGE [sodium dodecyl sulfate-polyacrylamide gel electrophoresis] analysis by simultaneously quantifying the characteristic proteolysis of the C1r and C1s subunits. The influence of C.hivin.1-In on both the spontaneous autoactivation of C1 and activator-mediated C1 activation was studied. Spontaneous autoactivation of C1 at 37.degree. C was inhibited by purified C.hivin.1-In. Pretreatment of C.hivin.1-In at 56.degree. C for 30 min destroyed its inhibitory activity. C.hivin.1-In concentrations ranging from 0.35-2.0 times the physiologic level all inhibited spontaneous activation of physiologic concentrations of C1. C.hivin.1-In concentrations of .ltoreq. 0.25 times physiologic were ineffective in controlling spontaneous C1 activation. At 37.degree. C, C.hivin.1-In had no effect on C1 activation induced by immune complexes [erythrocyte-antibody] (EA or tetanus-anti-tetanus), but strongly inhibited that induced by nonimmune activators (DNA or heparin). At 20.degree. C, even immune complex-induced C1 activation was completely blocked by C.hivin.1-In. Therefore, the selectivity of C.hivin.1-In at 37.degree. C is based on the faster rate of C1 activation induced by immune activators compared with nonimmune activators. Thus, C.hivin.1-In may play a physiologic role in regulating nonspecific C1 activation by nonimmune activators. Specific binding of 125I C.hivin.1-In to 131I-C1 on EA at 20.degree. C was observed. SDS-PAGE analyses confirmed that C.hivin.1-In was bound not to activated C.hivin.1, but to unactivated C1 on EA. No interaction between purified C.hivin.1-In and native C1 in the fluid phase was detectable by sedimentation velocity studies in the analytical ultracentrifuge. Thus, C.hivin.1-In must interact with EA-bound C1 that is in a state conformationally distinct from native C1 but not yet proteolyzed to C.hivin.1. In addition to controlling C1 already in its activated form, C.hivin.1-In also regulates the activation of C1, thus functioning earlier in the C sequence than previously thought.