A membrane-associated receptor for the C1q subcomponent of complement is widely distributed among different cell types. While a number of possible physiological functions of the C1q receptor (C1qR) on different cell types have been described, the way in which C1qR regulates complement activity remains unclear. This report describes the mechanism by which C1qR regulates activation of the first component of complement, C1. Using purified components of complement, we were able to show that membrane-associated C1qR as well as detergent-solubilized C1qR, purified from polymorphonuclear leukocytes, human umbilical vein endothelial cells or an endothelial cell line, EA.hy 926, are able to inhibit complement-mediated lysis of C1q-sensitized erythrocytes. Using hemolytic assays, we were able to demonstrate that C1qR prevents the association of C1q with C1r and C1s to form macromolecular C1. In addition, incubation of C1qR with the collagen-like stalks, but not with the globular heads of C1q, inhibits the effect of C1qR. This demonstrates that C1qR exerts its complement inhibitory effect by binding to the collagen-like stalk of C1q. No complement regulatory effect of C1qR was observed on preformed macromolecular C1. These data suggest that besides such-well-known complement regulatory molecules as CD55 (DAF), CD46 (MCP), CD35 (CR1) and CD59 (HRF), C1qR too is able to regulate complement activity.

SLE is a disease characterized by the presence of multiple autoantibodies and high levels of circulating immune complexes. We studied the presence and functional relevance of autoantibodies directed against a receptor for the collagen‐like stalks of the first subcomponent of complement, also known as calreticulin (cC1qR/CaR), in patients with SLE. In a cross‐sectional study it was found that higher titres of antibodies against cC1qR/CaR are present in sera of SLE patients compared with normal donors. No association between anti‐cC1qR/CaR titres and SLE disease activity was found. Following gel filtration of SLE serum it was found that anti‐cC1qR/CaR reactivity is associated with the peak of monomeric IgG. Purified IgG from patients was able to specifically immunoprecipitate cC1qR/CaR. Since we have shown previously that cC1qR/CaR is able to inhibit the haemolytic activity of C1q, we determined a possible pathogenic role for anti‐cC1qR/CaR on complement regulation. IgG derived from SLE serum reversed the inhibitory capacity of cC1qR/CaR in a dose‐dependent fashion up to 63%, whereas IgG from normal donors had no significant effect. With respect to the capacity of anti‐cC1qR/CaR antibodies to activate neutrophils, it was found that incubation of normal neutrophils with F(ab′)2 anti‐cC1qR/CaR resulted in a very limited oxidative burst. However, cross‐linking of F(ab′)2 anti‐cC1qR/CaR on the neutrophils clearly induced neutrophil activation. Pre‐incubation of the SLE‐derived F(ab′)2 with cC1qR/CaR prevented activation of neutrophils up to 81 ± 5%. These results suggest that the presence of anti‐cC1qR/CaR antibodies in patients with SLE may modulate complement and neutrophil activation.

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Defensins are small, cationic antimicrobial peptides that are present in the azurophilic granules of neutrophils. Earlier studies have shown that defensins may influence complement activation by specific interaction with activated C1, C1q, and C1-inhibitor. In the present study, we show that the defensin human neutrophil peptide-1 (HNP-1) is able to inhibit activation of the classical complement pathway by inhibition of C1q hemolytic activity. The binding site for HNP-1 on C1q is most likely located on the collagen-like stalks, as a clear, dose-dependent binding of HNP-1 to either intact C1q or to the collagen-like stalks of C1q was demonstrated using enzyme-linked immunosorbent assay (ELISA). Besides binding of HNP-1 to C1q, also a limited binding to C1 and to a mixture of C1r and C1s was observed, whereas no binding to C1-inhibitor was found. Because binding of HNP-1 to C1-inhibitor has been suggested in earlier studies, we also assessed the binding of HNP-1 to mixtures of C1-inhibitor with either C1r/ C1s or C1. No binding was found. Using a competition ELISA, it was found that HNP-1, but not protamine, inhibited binding of biotin-labeled HNP-1 to C1q in a dose-dependent fashion. In the fluid phase, preincubation of HNP-1 with C1q resulted in complex formation of HNP-1 and C1q and generation of stable complexes. In conclusion, HNP-1 is able to bind to C1q in the fluid phase and inhibits the classical complement pathway. This mechanism may be involved in the control of an inflammatory response in vivo.

gClqBP is a 33 kDa glycoprotein that binds to the globular 'heads' of Clq. We have cloned cDNAs encoding the rat and mouse homologues of gClqBP. Comparison of the cDNAderived amino acid sequences of gClqBP reveals that either of the rodent sequences is 89.9% identical to the reported human sequence. Recombinant rat gClqBP binds avidly to human Clq. gClqBP mRNA is abundantly expressed in every rat and mouse tissue analysed. Rat mesangial cells synthesise gClqBP, but do not express gClqBP on the cell surface. In rat serum, gClqBP is present at low levels.