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

Abstract

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.

Complement factor H in host defense and immune evasion

“…For example, some microbes express proteolytic enzymes that inactivate C3b on their own surface or contain glycophosphatidylinositol-anchored complement inhibitors [127]. An additional way for pathogens to escape complement attack is by recruiting (i.e., hijacking ) complement inhibitors from the host, such as FH (and C4BP) [128–130]. Indeed, some pathogens mimic a host surface by capturing these soluble complement regulators or, alternatively, target and inactivate complement inhibitors with secreted proteins/proteases [48, 131]; once a given pathogen has recruited complement inhibitors on to its own surface, it is likely protected from the complement attack as if it were self .…”

“…In this regard, human FH from a blood meal has been found to bind the luminal surface of the mosquito’s midgut and, via its cofactor activity, prevent complement activation on these surfaces, allowing the mosquito to tolerate the ingestion of large quantities of human blood. Interestingly, the merozoites from Plasmodium falciparum (i.e., the invasive form of the malarial parasite released on erythrocyte rupture) have been shown to evade the complement system by capturing FH and FHL-1 from serum (via the Pf92 protein) and down-regulating the AP [130]. …”

Evaluating Complement-Mediated Humoral Immunity to P. falciparum Blood Stages

“…It is possible that the impact of complement on invasion is different in standard growth assays as used by Biryukov et al compared to Boyle et al (2010), or depending on parasite strains used. However, we and others have previously included normal sera in growth assays using a number of different parasite lines; reduced or comparable growth has been consistently reported, but not enhanced invasion compared to heat inactivated sera ((Campbell et al, 1979, Chulay et al, 1981, Kennedy et al, 2015) and unpublished data Boyle, Beeson et al). It seems more likely that the major difference between the findings is the type of antibodies used; naturally acquired and vaccine induce human and rabbit polyclonal antibodies against different merozoite antigens by Boyle et al compared to murine mAb or vaccine induced human antibodies to the C terminal of MSP1 in the case of Biryukov et al…”

The Use of Filter-Purified Merozoites to Assess Anti-Merozoite Immunity

“…Boyle and Beeson state that other studies have failed to see an increase in invasion when using serum and cite three reports (Campbell et al, 1979, Chulay et al, 1981, Kennedy et al, 2015). The studies by Campbell et al and Chulay et al were carried out with P. falciparum grown in human RBCs exposed to serum from immune and non-immune Aotus monkeys.…”

“…Recent work has shown that merozoites can recruit the soluble complement regulator factor H (Kennedy et al, 2015, Rosa et al, 2015). We are concerned that filtration could also strip factor H or other complement regulators off merozoites making them more susceptible to complement.…”