Complement in malaria immunity and vaccines.

Developing efficacious vaccines for human malaria caused by Plasmodium falciparum is a major global health priority, although this has proven to be immensely challenging over the decades. One major hindrance is the incomplete understanding of specific immune responses that confer protection against disease and/or infection. While antibodies to play a crucial role in malaria immunity, the functional mechanisms of these antibodies remain unclear as most research has primarily focused on the direct inhibitory or neutralizing activity of antibodies. Recently, there is a growing body of evidence that antibodies can also mediate effector functions through activating the complement system against multiple developmental stages of the parasite life cycle. These antibody-complement interactions can have detrimental consequences to parasite function and viability, and have been significantly associated with protection against clinical malaria in naturally acquired immunity, and emerging findings suggest these mechanisms could contribute to vaccine-induced immunity. In order to develop highly efficacious vaccines, strategies are needed that prioritize the induction of antibodies with enhanced functional activity, including the ability to activate complement. Here we review the role of complement in acquired immunity to malaria, and provide insights into how this knowledge could be used to harness complement in malaria vaccine development.

Recruitment of Human C1 Esterase Inhibitor Controls Complement Activation on Blood Stage Plasmodium falciparum Merozoites.

The complement system is a front-line defense system that opsonizes and lyses invading pathogens. To survive, microbes exposed to serum must evade the complement response. To achieve this, many pathogens recruit soluble human complement regulators to their surfaces and hijack their regulatory function for protection from complement activation. C1 esterase inhibitor (C1-INH) is a soluble regulator of complement activation that negatively regulates the classical and lectin pathways of complement to protect human tissue from aberrant activation. In this article, we show that Plasmodium falciparum merozoites, the invasive form of blood stage malaria parasites, actively recruit C1-INH to their surfaces when exposed to human serum. We identified PfMSP3.1, a member of the merozoite surface protein 3 family of merozoite surface proteins, as the direct interaction partner. When bound to the merozoite surface, C1-INH retains its ability to complex with and inhibit C1s, MASP1, and MASP2, the activating proteases of the complement cascade. P. falciparum merozoites that lack PfMSP3.1 showed a marked reduction in C1-INH recruitment and increased C3b deposition on their surfaces. However, these ΔPfMSP3.1 merozoites exhibit enhanced invasion of RBCs in the presence of active complement. This study characterizes an immune-evasion strategy used by malaria parasites and highlights the complex relationship between merozoites and the complement system.

Recruitment of Factor H as a Novel Complement Evasion Strategy for Blood-Stage Plasmodium falciparum Infection.

The human complement system is the frontline defense mechanism against invading pathogens. The coexistence of humans and microbes throughout evolution has produced ingenious molecular mechanisms by which microorganisms escape complement attack. A common evasion strategy used by diverse pathogens is the hijacking of soluble human complement regulators to their surfaces to afford protection from complement activation. One such host regulator is factor H (FH), which acts as a negative regulator of complement to protect host tissues from aberrant complement activation. In this report, we show that Plasmodium falciparum merozoites, the invasive form of the malaria parasites, actively recruit FH and its alternative spliced form FH-like protein 1 when exposed to human serum. We have mapped the binding site in FH that recognizes merozoites and identified Pf92, a member of the six-cysteine family of Plasmodium surface proteins, as its direct interaction partner. When bound to merozoites, FH retains cofactor activity, a key function that allows it to downregulate the alternative pathway of complement. In P. falciparum parasites that lack Pf92, we observed changes in the pattern of C3b cleavage that are consistent with decreased regulation of complement activation. These results also show that recruitment of FH affords P. falciparum merozoites protection from complement-mediated lysis. Our study provides new insights on mechanisms of immune evasion of malaria parasites and highlights the important function of surface coat proteins in the interplay between complement regulation and successful infection of the host.

Characterization of Inhibitors and Monoclonal Antibodies That Modulate the Interaction between Plasmodium falciparum Adhesin PfRh4 with Its Erythrocyte Receptor Complement Receptor 1.

Plasmodium falciparum parasites must invade red blood cells to survive within humans. Entry into red blood cells is governed by interactions between parasite adhesins and red blood cell receptors. Previously we identified that P. falciparum reticulocyte binding protein-like homologue 4 (PfRh4) binds to complement receptor 1 (CR1) to mediate entry of malaria parasites into human red blood cells. In this report we characterize a collection of anti-PfRh4 monoclonal antibodies and CR1 protein fragments that modulate the interaction between PfRh4 and CR1. We identify an anti-PfRh4 monoclonal that blocks PfRh4-CR1 interaction in vitro, inhibits PfRh4 binding to red blood cells, and as a result abolishes the PfRh4-CR1 invasion pathway in P. falciparum. Epitope mapping of anti-PfRh4 monoclonal antibodies identified distinct functional regions within PfRh4 involved in modulating its interaction with CR1. Furthermore, we designed a set of protein fragments based on extensive mutagenesis analyses of the PfRh4 binding site on CR1 and determined their interaction affinities using surface plasmon resonance. These CR1 protein fragments bind tightly to PfRh4 and also function as soluble inhibitors to block PfRh4 binding to red blood cells and to inhibit the PfRh4-CR1 invasion pathway. Our findings can aid future efforts in designing specific single epitope antibodies to block P. falciparum invasion via complement receptor 1.

More than just immune evasion: Hijacking complement by Plasmodium falciparum.

Malaria remains one of the world's deadliest diseases. Plasmodium falciparum is responsible for the most severe and lethal form of human malaria. P. falciparum's life cycle involves two obligate hosts: human and mosquito. From initial entry into these hosts, malaria parasites face the onslaught of the first line of host defence, the complement system. In this review, we discuss the complex interaction between complement and malaria infection in terms of hosts immune responses, parasite survival and pathogenesis of severe forms of malaria. We will focus on the role of complement receptor 1 and its associated polymorphisms in malaria immune complex clearance, as a mediator of parasite rosetting and as an entry receptor for P. falciparum invasion. Complement evasion strategies of P. falciparum parasites will also be highlighted. The sexual forms of the malaria parasites recruit the soluble human complement regulator Factor H to evade complement-mediated killing within the mosquito host. A novel evasion strategy is the deployment of parasite organelles to divert complement attack from infective blood stage parasites. Finally we outline the future challenge to understand the implications of these exploitation mechanisms in the interplay between successful infection of the host and pathogenesis observed in severe malaria.

The use and abuse of heme in apicomplexan parasites.

SIGNIFICANCE: Heme is an essential prosthetic group for most life on Earth. It functions in numerous cellular redox reactions, including in antioxidant defenses and at several stages of the electron transport chain in prokaryotes and eukaryotic mitochondria. Heme also functions as a sensor and transport molecule for gases such as oxygen. Heme is a complex organic molecule and can only be synthesized through a multienzyme pathway from simpler precursors. Most free-living organisms synthesize their own heme by a broadly conserved metabolic pathway. Parasites are adept at scavenging molecules from their hosts, and heme is no exception. RECENT ADVANCES: In this review we examine recent advances in understanding heme usage and acquisition in Apicomplexa, a group of parasites that include the causative agents of malaria, toxoplasmosis, and several major parasites of livestock. CRITICAL ISSUES: Heme is critical to the survival of Apicomplexa, although the functions of heme in these organisms remain poorly understood. Some Apicomplexa likely scavenge heme from their host organisms, while others retain the ability to synthesize heme. Surprisingly, some Apicomplexa may be able to both synthesize and scavenge heme. Several Apicomplexa live in intracellular environments that contain high levels of heme. Since heme is toxic at high concentrations, parasites must carefully regulate intracellular heme levels and develop mechanisms to detoxify excess heme. Indeed, drugs interfering with heme detoxification serve as major antimalarials. FUTURE DIRECTIONS: Understanding heme requirements and regulation in apicomplexan parasites promises to reveal multiple targets for much-needed therapeutic intervention against these parasites.