Soluble gC1qR/p33 accelerates the rate of complement activation in C1 inhibitor (C1‐INH) deficient serum

Abstract

C1‐INH is a serine protease inhibitor (serpin) whose deficiency results in a life threatening disease─angioedema. Its primary function is to regulate key enzymes of both the complement and kinin generating systems; but it also plays a key role in controlling the integrity of C1. These studies were undertaken to test the hypothesis that gC1qR, which activates both the complement and kinin systems, can accelerate complement activation in C1‐INH deficient serum. To achieve this, we first prepared C1‐INH depleted serum (NHS−C1inh) by affinity depletion on protein A‐immobilized anti‐C1‐INH, followed by reconstitution with 20 μg/ml C1q and 20 mM CaCl2 and MgCl2. Depletion was verified by ELISA, and its activity by a standard hemolytic assay. Incubation (60 min, 37º C) of NHS alone, followed by measurement of residual hemolytic activity showed no loss in function when compared to untreated NHS. However, NHS−C1inh lost 30–50% of its function due to spontaneous autoactivation. Importantly, whereas incubation of NHS with 5μg/ml gC1qR or aggregated IgG resulted in complete activation after 60 min, it took only 15 min for the same level of activation in NHS−C1inh. This was reversed when the NHS−C1inh was reconstituted with C1‐INH. The data suggest that gC1qR, can trigger and/or exacerbate the inflammatory process associated with angioedema by enhancing activation of complement. [Supported by NIH‐NIAID Grant R01 AI‐060866]