Human antibodies activate complement against Plasmodium falciparum sporozoites, and are associated with protection against malaria in children

Abstract

BackgroundAntibodies targeting Plasmodium falciparum sporozoites play a key role in human immunity to malaria. However, antibody mechanisms that neutralize sporozoites are poorly understood. This has been a major constraint in developing highly efficacious vaccines, as we lack strong correlates of protective immunity.MethodsWe quantified the ability of human antibodies from malaria-exposed populations to interact with human complement, examined the functional effects of complement activity against P. falciparum sporozoites in vitro, and identified targets of functional antibodies. In children and adults from malaria-endemic regions, we determined the acquisition of complement-fixing antibodies to sporozoites and their relationship with antibody isotypes and subclasses. We also investigated associations with protective immunity in a longitudinal cohort of children (n = 206) residing in a malaria-endemic region.ResultsWe found that antibodies to the major sporozoite surface antigen, circumsporozoite protein (CSP), were predominately IgG1, IgG3, and IgM, and could interact with complement through recruitment of C1q and activation of the classical pathway. The central repeat region of CSP, included in leading vaccines, was a key target of complement-fixing antibodies. We show that antibodies activate human complement on P. falciparum sporozoites, which consequently inhibited hepatocyte cell traversal that is essential for establishing liver-stage infection, and led to sporozoite death in vitro. The natural acquisition of complement-fixing antibodies in malaria-exposed populations was age-dependent, and was acquired more slowly to sporozoite antigens than to merozoite antigens. In a longitudinal cohort of children, high levels of complement-fixing antibodies were significantly associated with protection against clinical malaria.ConclusionsThese novel findings point to complement activation by antibodies as an important mechanism of anti-sporozoite human immunity, thereby enabling new strategies for developing highly efficacious malaria vaccines. We also present evidence that complement-fixing antibodies may be a valuable correlate of protective immunity in humans.Electronic supplementary materialThe online version of this article (10.1186/s12916-018-1054-2) contains supplementary material, which is available to authorized users.

Multi-functional antibodies are induced by the RTS,S malaria vaccine and associated with protection in a phase I/IIa trial

“…The following antigens used in this study were based on P. falciparum 3D7: recombinant CSP expressed in Pichia pastoris (Sanaria, Rockville, USA), synthetic (NANP) 15 peptide representing the central repeat region of CSP (NANP, Life Tein, Hillsborough, USA) [27], after 12 minutes of TMB substrate incubation. To measure the terminal phase of complement activation, we adapted the C1q-fixation assay to measure downstream complement proteins, C5b-C9, which form the membrane attack complex, as previously described [27].…”

“…Conducting large-scale functional assays with viable P. falciparum sporozoites is not feasible, as they cannot be readily cultivated in standard laboratories or obtained in large numbers. To address these technical limitations, we developed high-throughput, cell-free assays that quantify the ability of anti-CSP antibodies to fix human C1q (and other downstream complement components), or interact directly with dimeric recombinant soluble FcγR complexes as a functional surrogate [27,29,30].…”

Antibody effector functions in malaria and other parasitic diseases: a few needles and many haystacks

“…Whether antibodies toward sporozoites activate human Kupffer cells in the liver is unclear. Antibodies toward sporozoites also can bind complement and antibody‐dependent complement deposition has been shown to inhibit hepatocyte traversal and also result in sporozoite death . P. falciparum circumsporozoite protein is an abundant protein on the surface of the sporozoite; it is composed of a central repeat region of NANP and NANP‐like motifs flanked by conserved domains at the N and C termini.…”

Immune effector mechanisms in malaria: An update focusing on human immunity

“…27,38,39 More laboratory-intensive and thus less high-throughput methods have further characterized several antibody-mediated effector mechanisms. Functional antibodies against blood-stage infections include parasite growth inhibition, merozoite invasion-inhibition of RBCs, parasitized-RBC adhesion-inhibition, complement-mediated immunity, phagocytosis, antibody-mediated cellular cytotoxicity (ADCC) and neutrophil-mediated antibody-dependent respiratory burst (ADRB) [40][41][42][43][44][45][46][47][48][49][50] (Figure 1). Inhibitory antibodies that prevent sporozoites from invading hepatocytes, gametocytes from sequestering in the bone marrow, and parasitized-RBCs from adhering to host cells exist but are challenging to study in vitro and in vivo.…”