Investigation of the mechanisms of anti-complement activity in Ixodes ricinus ticks

The feeding success of a tick upon a host depends on its ability to suppress host anti-tick responses which include activation of the complement system. We investigated the mechanism of inhibition of the alternative pathway of complement by salivary gland extract (SGE) of the ixodid tick species, Ixodes ricinus. SGE treatment strongly inhibited C3a generation and factor B cleavage in serum when rabbit erythrocytes were used as complement activator, but not when cobra venom factor (CVF) was used as an activator. SGE treatment strongly inhibited C3b deposition on rabbit erythrocytes, and the turnover of C3 (to C3b/iC3b) in serum. However, there was no significant effect upon the formation, stability or activity of C3 convertase (C3bBb) when formed from purified C3b, factor B and factor D. SGE treatment of isolated C3 resulted in a shift in mobility of the α-chain (by about 5 kDa). N-terminal sequencing of this species suggests that cleavage occurs at the C-terminus of the α-chain of C3. Consistent with this hypothesis, the modified α-chain was still a substrate for pre-formed convertase. The activity was specific for the α-chain of C3 but not of C3(H 2O) nor the α′-chain of C3b. It is proposed that SGE-modified C3 does not participate in convertase formation, probably having a reduced affinity for factor B.