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We examined the interaction of complement component C3 with surface molecules on Trypanosoma cruzi. Five- to six-fold more C3 was bound to epimastigotes (Epi) than to metacyclic trypomastigotes (CMT) of strain M88. Epi and CMT were surface iodinated, then incubated in C8-deficient serum, and detergent lysates were applied to anti-C3 antibody that had been coupled to Sepharose. We found that 9.20-10.24% of applied 125I-Epi protein bound to anti-C3-sepharose, compared to 2.64% binding of 125I-CMT protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that C3 was attached to 125I-Epi protein by a covalent bond. Samples eluted from anti-C3-sepharose with hydroxylamine revealed a single, major, 72 kD band, suggesting that C3b attaches almost exclusively to the 72 kD glycoprotein of Epi by a hydroxylamine-susceptible ester bond. An antiserum was prepared from lysates of serum-treated Epi that had been affinity-purified on anti-C3-sepharose. This antiserum immunoprecipitated a single 72 kD component (gp72) from surface-iodinated Epi, and specifically recognized only gp72 from Epi in immunoblots. In contrast to the results with Epi, gp72 on CMT was not found to be an efficient acceptor molecule for C3 deposition. The results are the first to evaluate the acceptor site for C3 deposition on a parasite, and they show that gp72 on Epi, but not gp72 on CMT, serves as the preferential acceptor for C3 during antibody-independent alternative complement pathway activation.

This is the first report demonstrating that C3d receptor (CR2) has functional activity in regulating complement cascade. Purified CR2 was examined for its cofactor activity in factor I-mediated cleavage of membrane-bound iC3b. CR2 plus C3b inactivator (I) released C3c from EA 125I-iC3b, and the release was inhibited when CR2 was preincubated with OKB7 monoclonal anti-CR2. Furthermore, immunoelectroblotting analysis showed crossreactivity of CR2 with 57H anti-CR1. These results indicate that CR2 has functional and antigenic similarity to CR1, thus providing a supporting evidence for placement of CR2 as a member of the recently defined gene family of C3- and C4-regulatory proteins composed of CR1, C4-binding protein, and factor H.