Beyond Binding: The Outcomes of Antibody-Dependent Complement Activation in Human Malaria

Abstract

Antibody immunity against malaria is effective but non-sterile. In addition to antibody-mediated inhibition, neutralisation or opsonisation of malaria parasites, antibody-mediated complement activation is also important in defense against infection. Antibodies form immune complexes with parasite-derived antigens that can activate the classical complement pathway. The complement system provides efficient surveillance for infection, and its activation leads to parasite lysis or parasite opsonisation for phagocytosis. The induction of complement-fixing antibodies contributes significantly to the development of protective immunity against clinical malaria. These complement-fixing antibodies can form immune complexes that are recognised by complement receptors on innate cells of the immune system. The efficient clearance of immune complexes is accompanied by complement receptor internalisation, abrogating the detrimental consequences of excess complement activation. Here, we review the mechanisms of activation of complement by alternative, classical, and lectin pathways in human malaria at different stages of the Plasmodium life cycle with special emphasis on how complement-fixing antibodies contribute to protective immunity. We briefly touch upon the action of anaphylatoxins, the assembly of membrane attack complex, and the possible reasons underlying the resistance of infected erythrocytes towards antibody-mediated complement lysis, relevant to their prolonged survival in the blood of the human host. We make suggestions for further research on effector functions of antibody-mediated complement activation that would guide future researchers in deploying complement-fixing antibodies in preventive or therapeutic strategies against malaria.

Keywords: Plasmodium falciparum erythrocyte membrane protein 1; classical complement pathway; complement regulatory proteins; immune complexes; infected erythrocytes; malaria.

Figure 1

The life cycle of Plasmodium falciparum. P. falciparum requires two hosts to complete its life cycle, the mosquito, and the human. Gametocytes are ingested by a female Anopheles mosquito during a blood meal. The gametocytes transform into gametes that fertilise to form zygotes and migrate through the mosquito gut wall to form oocysts. The oocysts rupture and release sporozoites that reach the salivary gland of the mosquito ready to be transmitted into another human host. The injected sporozoites reach the liver, and within hepatocytes, the parasites undergo division (liver stage) before rupture to release merozoites into the bloodstream. The merozoites infect new erythrocytes to form ring-stage parasites that mature into trophozoites and schizonts within the infected erythrocytes (IEs). The rupture of schizonts releases a new generation of merozoites to infect erythrocytes (blood stage). A small proportion of these parasites develop into gametocytes within the IEs, which are ingested by a female Anopheles mosquito for the continuation of the life cycle [reviewed in (3)].

Figure 2

Possible mechanisms of complement activation in malaria. The binding of C1q to antigen-antibody immune complexes (ICs) activates the classical pathway. The lectin pathway can be activated through the binding of mannose with the parasite proteins expressed on schizont surface. The serine proteases MASP1 and MASP2 (Mannan-binding lectin-associated serine proteases 1 and 2) bind to MBL in lectin pathway, and C1r and C1s bind to C1q to cleave two inactive proenzymes C4 and C2 in serum to produce C3 convertase (C4b2a). The alternative pathway is activated either by circulating plasmodial antigens, or products of schizont rupture like hematin. The spontaneous hydrolysis of C3 leads to the cleavage of factor B in serum by an active serum protease called factor D to form a distinct C3 convertase of the alternative pathway (C3bBb). The C3 convertases cleave C3 into C3a and C3b. The products of C3 and C5 cleavage, C3a and C5a, act as anaphylatoxins and interact with immune cell receptors (C3aR and C5aR) to drive inflammation. The assembly of C3b with the C3 convertases produces a C5 convertase (C4b2a3b/C3bBb3b) that can cleave C5 into C5a and C5b. The C5b recruits the factors C6, C7, C8, and C9 for the assembly of the membrane attack complex (MAC) (C5b-C9 or terminal complement complex) for cytolysis (14). [Figure adapted from (15)].

Figure 3

The immune complexes (ICs) are formed when antigens and antibodies unite. They can be formed when the circulating plasmodial antigens cross-link with antibodies in the plasma (A) or antibodies can bind with the antigens expressed on the surface of sporozoites, merozoites, or IEs as shown in (B). The circulating ICs (as in A) can also get deposited on the surface of the parasite or on other cells like uninfected erythrocytes, promoting complement deposition on host cell surfaces. These ICs recruit C1q when the globular head domains of C1q bind with the Fc constant region (B) of ICs to activate a cascade of downstream events of the classical complement pathway.

Figure 4

Possible mechanisms of resistance of IEs to classical complement attack. As shown in (A), if the number of PfEMP1 target sites on IEs are below a certain threshold level, complement is not activated even in the presence of an ample supply of antibody and complement. In (B), the presence of complement regulatory proteins such as the membrane bound CD59, inhibits the MAC assembly, to prevent lysis of IEs. The soluble complement regulatory factors [factor I (FI) and factor H (FH)] are also recruited onto the schizont surface (1) and activate FH-like protein 1 (FHL-1) (2) to prevent C3b deposition (3) on the schizont surface, thus preventing complement activation. The knob-restricted expression of PfEMP1 on IEs in (C) provides an evolutionary advantage to the parasite to evade classical complement attack. In (D), both PfEMP1 and C1q compete for the same binding site on IgM. Initial binding of IgM with PfEMP1 causes PfEMP1 clustering and prevents IgM-C1q interaction.