SCHISTOSOMA-MANSONI CIRCULATING ANODIC ANTIGEN BUT NOT CIRCULATING CATHODIC ANTIGEN INTERACTS WITH COMPLEMENT COMPONENT C1Q

Adult schistosome parasites, living in the blood vessels of their mamm alian hosts, protect themselves against immune damage in a variety of ways. In addition to the tegument, the intestinal epithelium of the bl ood-feeding worms is permanently exposed to both the innate and the ac quired immune system. In this study, we investigated whether the Schis tosoma gut-associated antigens CAA and CCA (circulating anodic antigen and circulating cathodic antigen, respectively), which are excreted i n relatively large quantities into the host's circulation, might play a role in evading complement attack. Of several complement components tested, only purified C1q showed significant binding to CAA, a negativ ely charged highly glycosylated glycoprotein. CCA, also highly glycosy lated, but neutral or slightly positively charged, did not bind to C1q . CAA bound only to the collagen-like stalks of C1q and not to the glo bular heads. No detectable interaction of CAA with precursor human C1 was found and CAA did not induce activation of C1 in whore human serum as assessed by consumption of hemolytic C4 activity. Also CAA could n ot induce activation of precursor C1 in vitro. These results suggest t hat CAA behaves like a receptor for C1q, and might be involved in prot ecting the vulnerable schistosome gut against complement-mediated atta ch.