We have prepared C3b covalently linked to IgG via a hydroxylamine-sensitive bond between the C3b alpha' chain and sites predominantly, but not exclusively, located in the IgG heavy chain. This C3b species displays relative resistance to inactivation by factors H and I when compared with free C3b. This resistance appears to be due entirely to reduced affinity of C3b-IgG for factor H. Resistance to inactivation is not conferred on C3b by binding to another serum glycoprotein of similar size, ceruloplasmin, and may be a special property of IgG. C3b-IgG demonstrates an enhanced capacity to consume serum C3 relative to C3b. These alterations of the behavior of C3b when bound to IgG may in part explain the augmentation of alternative pathway activity by IgG. In addition, IgG-induced protection of C3b might influence both complement-mediated killing and phagocytosis of bacteria, as well as modify the in vivo handling of IgG-containing soluble immune complexes.

During its differentiation in the insect vector to a stage infective for the mammalian host, Trypanosoma cruzi becomes resistant to lysis by the alternative pathway of complement. To elucidate the mechanism of complement evasion, we studied control of complement activation on the surface of the noninfective epimastigote and the infective culture-derived metacyclic trypomastigote stages (CMT) of T. cruzi. It was found that the predominant form of complement component C3 on epimastigotes is C3b, whereas the majority of C3 on CMT is in the form of the hemolytically inactive fragment iC3b, which cannot participate in C5 convertase formation or lead to deposition of the lytic C5b-9 complex. Our results also showed that C3 binds by a covalent ester linkage to surface molecules of different molecular weight in the epimastigote stage and CMT. Binding studies with purified complement components indicated that CMT do not support efficient formation of an alternative pathway C3 convertase. C3b on the parasite surface fails to bind the amplification component, factor B, rather than showing enhanced binding of the control component, factor H. These results identify the biochemical basis for evasion of complement-mediated killing in T. cruzi and reveal a mechanism for developmental regulation of complement activation.Protozoan parasites have developed a variety of mechanisms to evade the immunologic defense mechanisms of the mammalian host. Thus, vertebrate-stage parasites and infective vector-stage parasites typically resist direct serum killing and can also evade ingestion or intracellular killing by the phagocytic cells of the host. In contrast, most noninfective vector-stage parasites are susceptible to direct lysis by serum and to phagocytosis and destruction by monocytes or macrophages. For example, the epimastigote stage (Epi) of Trypanosoma cruzi, which multiplies in the gut ofthe vector, is efficiently lysed in human serum by means of alternative complement pathway (ACP) activation (1). In contrast, the infecting metacyclic trypomastigote stage, the amastigote stage (which multiples intracellularly in the vertebrate host), and the disseminating bloodstream trypomastigote form are not lysed when incubated in human serum (1-3). The membrane changes that control the transformation of these parasites from activators of the ACP to nonactivators are incompletely understood, as are the molecular interactions of the ACP with the parasite surface of various life-cycle stages.Activation of the ACP is currently thought to be mediated and regulated by the following mechanisms: continuous low-grade fluid-phase generation of complement component C3b or water-hydrolyzed C3 occurs normally in serum and results in the random deposition of C3b on all particles (4). The subsequent interaction of this randomly deposited C3b with proteins of the ACP is determined by the nature of the particle surface (5-7). On alternative pathway activators, C3b binds factor B in preference to factor H. C3b that bears B cannot interact with the i...

In vitro studies were performed utilizing sera from a strain of guinea pigs with a total absence of hemolytically active C4. Previous studies in these animals have demonstrated normal complement-dependent inflammatory reactions, suggesting that they are able to bypass their deficiency of C4. In vitro studies with C4-deficient serum also indicate normal activation of late-acting C components. Thus, endotoxin was capable of fixing normal amounts of the late components of complement (C3-9) in these sera, but did not fix C1 and C2. Antigen-antibody complexes fixed both early and late components of complement, although components beyond C4 were fixed less efficiently than in normal sera. Therefore, both in vivo and in vitro evidence indicates that the C4-deficient guinea pigs possess an alternate pathway for activation of late-acting complement components. Antigenic analysis of C4-deficient serum utilizing both guinea pig anti-C4 antibody and rabbit anti-C4 antibody suggests an absolute deficiency of C4-like molecules. Sera from animals with C4-deficiency were found to have one-half the normal level of C2. Sera from five of eight animals tested had 10–20% normal C1 activity. C3-9 assayed as a complex was normal.

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