MicrobiologyBytes: Infection & Immunity: Complement Updated: January 7, 2007 Search

THE INTERACTION OF MICROORGANISMS WITH THE COMPLEMENT SYSTEM:

PARASITES

The complex life cycle of protozoan parasites involves the developmental regulation of characteristics which allow for survival in diverse microenvironments in insect vectors and mammalian hosts. In order to establish and disseminate the infection, parasites have evolved means whereby they are resistant to the effects of intrinsic host defence mechanisms. A crucial step in the establishment of infection by blood-borne pathogens is the avoidance of direct killing by serum factors, such as the complement system. Several mechanisms of complement resistance by micro-organisms have been elucidated (230), and more recently, the mimicry of host complement components and complement regulatory proteins (CRPs) by micro-organisms has been described (231,232,233).

In the case of Trypanosoma cruzi, the causative agent of Chagas' disease, the bloodstream

trypomastigotes have evolved one or more means of avoiding complement-mediated killing (232,234,235). Within the insect vector, the conversion of the parasites from epimastigotes to infectious trypomastigotes coincides with the acquisition of resistance to lysis by the alternative complement pathway. Norris et al (232) have already described a trypomastigote specific membrane glycoprotein which inhibits the formation and stability of the alternative pathway C3 convertase, the central multi-subunit enzyme of the complement cascade, and thereby contributes to the complement resistance of the parasite. The glycoprotein was initially recovered from proteins spontaneously shed by trypomastigotes, and was characterised as having an apparent molecular mass of 160-kDa. This 160-kDa glycoprotein is biochemically and genetically related to a family of mammalian CRPs, which serve to prevent lysis of autologous cells by complement activation and amplification (232). These proteins, which include factor H, DAF, and CR1, share a binding affinity for components of the alternative and/or classical pathway C3 convertases, C3b and C4b, respectively. Although the precise mechanism of interference is not known, these CRPs bind to C3b (or C4b in the classical pathway) and competitively inhibit the uptake of the subsequent components, thereby preventing convertase formation and lysis of the cell.

A T.cruzi membrane protein has been characterised which functions to restrict complement activation on the surface of infectious forms of the parasite (232). This protein, along with most of the other surface proteins of trypomastigotes, was spontaneously released from the parasitic surface. Initial biochemical characterisations of the protein and analysis of a partial genomic clone of the gene revealed striking similarities with a family of mammalian CRPs (232). These mammalian proteins are members of the family of regulators of complement activation and restrict complement activation on autologous cells through binding interactions with C3b and/or C4b. Several of these proteins have multiple functions which affect the formation and stability of the C3 convertase as it assembles on the surface of complement activators. Among these mammalian complement regulatory elements, the T.cruzi CRP most closely resembles DAF, a membrane glycoprotein with widespread cell distribution and that is anchored in the cell membrane via a GPI (glycosyl phospatidyl inositol) linkage (236). Similar to the T.cruzi CRP, DAF bind C3b and C4b, thus regulating both the alternative and the classical complement pathways, although it does not permanently alter or destroy the complement components (237). Unlike other C3b-binding proteins, such as factor H and CR1, neither DAF nor T.cruzi CRP can serve as a cofactor for factor I-mediated proteolytic cleavage of C3b or C4b (237,238). Similar to T.cruzi CRP, human DAF is spontaneously released from the cell surface, most likely by a plasma-derived GPI-specific lipase (PIPLD) (239), although apparently not by the actions of endogenous PIPLC (240). The release may enable these proteins to enhance their complement-restricting activity by removing bound complement components from the cell surface. Whether the release of membrane proteins is augmented by ligand binding to the CRP has not been investigated, but this may provide the parasites with a rapid means of eliminating active complement components from their surface. This may not only diminish the lytic effects of complement, but also reduce the efficiency of complement-mediated opsonisation and clearance of the parasites (241).

Recently, an increase in C3 activation was associated with high levels of parasitaemia in rabbits infected with Trypanosoma evansi, another parasite from the Trypanosoma sp.. This suggests that C3 activation may play a crucial role in the control of parasitaemia in T.evansi infections possibly through complement-dependent parasite lysis, one of the principal ways by which trypanosomes are considered to be eliminated by the host (242).

Experiments were carried out to evaluate the pathways of C3 activation with sera collected from rabbits infected with T.evansi (242). The observations made demonstrated that the classical and alternative pathways of complement activation may be induced in T. evansi infections, with the activation of the classical pathway probably being dominant. Furthermore, the observed complement activation by the stable components of the parasite may be important in protecting infected animals, particularly through the activation of the alternative pathway, at periods when the antibody response of the host is compromised as a result of antigenic variation. Interactions between antibodies and macrophages are considered to be necessary if animals infected with trypanosomes are to achieve recovery by the elimination of the pathogen (243). The role of complement in the process of binding of trypanosomes to macrophages in the presence of specific antibody has been studied. In vitro studies on the mechanism of macrophage binding to trypanosomes demonstrated that an antibody must bind to the surface antigen of the trypanosome in order for the phagocyte to recognise the pathogen (244,245). The aggregation of trypanosomes observed at the optimal antigen:antibody ratio or in the presence of excess antigen, inhibited the binding. Complement caused 'clumped' trypanosomes to dissociate, and the free trypanosomes, which were presumed to be coated with antibody that had fixed complement, readily attached to surfaces of phagocytes (246).

In 1960, the results of various studies on Trypanosoma gambiense concluded that activation of complement contributed to adhesion, and that inactivation of complement decreased attachment (247). Complement was shown to contribute at the site of the antigen-antibody reaction to the creation of an environment suitable for the binding. In the presence of complement, it seems likely that the immune complex binds to complement to form the immune complex-C3 conjugate, and the conjugate is then attached to cell membranes that possess receptors for fixed C3 (248). Because the macrophage has both the Fc and C3 receptors on its cell membrane, the unaggregated trypanosomes, in the presence of excess antibody without complement, are considered to have attached to the macrophage by means of receptors for the Fc portion of the complexed antibody on the trypanosome surface (246). On the other hand, in the presence of antibody and complement, the dissociated trypanosomes are presumed to have bound to macrophages through C3 receptors, though the present results did not eliminate the possibility that attachment in the presence of antibody and complement is also through the Fc receptor. This type of binding to macrophages through C3 receptors is assumed to be important in vivo, because the fixed C3 enhances cytotoxic activity of effector cell against target cell by improving effector cell-target cell contact (249,250). The pathway by which complement is activated when it changes immune aggregates into soluble immune complexes is debatable, but it is thought that complement can fulfil the dissociating function whether it is activated by the classical pathway or by the alternative pathway, although the effect does not seem to be as rapid using the alternative pathway as at is when complement is activated by classical means.

   

© AJC 2007.