# Deposition of complement regulators on the surface of Plasmodium falciparum merozoites depends on the immune status of the host

## TL;DR

The study shows that antibodies from children with malaria can better activate the immune system's complement system and block parasite evasion compared to antibodies from adults.

## Contribution

The study introduces a new malaria vaccine strategy by targeting antigens that bind to complement regulators, enhancing antibody efficacy.

## Key findings

- Antibodies from children show higher complement activation and inhibit parasite evasion more effectively than adult antibodies.
- A monoclonal antibody targeting PfMSP3 can block C1-INH binding to the parasite surface, unlike naturally acquired antibodies.
- Structural data of the monoclonal antibody bound to PfMSP3 is reported for the first time.

## Abstract

Plasmodium falciparum is responsible for the majority of malaria cases and deaths worldwide. In malaria endemic areas, natural immunity to blood stage infection is acquired over several exposures to the parasite and is thought to rely on antibodies. Antibodies can protect from severe disease through different effector functions, with complement activation lately emerging as an important feature of protective humoral responses to malaria. Plasmodium parasites have however evolved several mechanisms to evade complement attack, including the recruitment of complement down-regulatory proteins like Factor H (FH) and C1 esterase inhibitor (C1-INH). In this study, we report that merozoite-specific antibodies acquired naturally after infection activate the complement cascade in an exposure-dependent manner. Using plasma samples from convalescent children and exposed adults collected respectively in Hohoe and Accra (Ghana), we show that the ability to fix C1q and activate the classical pathway is similar for antibodies deriving from the two donors groups. However, downstream complement activation shown as deposition of the membrane attack complex (MAC) is strikingly higher with antibodies from children compared to antibodies from adults. Moreover, we demonstrate that antibodies from naturally exposed children can interfere with the merozoite recruitment of FH, but not of C1-INH. With the aim of neutralizing parasite evasion of the complement classical pathway, we develop a murine monoclonal antibody targeting PfMSP3, the binding partner of C1-INH on the merozoite surface. We demonstrate that this antibody can effectively block the binding of C1-INH to the parasite surface, unlike the naturally acquired ones. Using cryogenic electron microscopy, we obtain a low-resolution structure of the monoclonal antibody in complex with PfMSP3, which is the first reported structural data for this antigen. We propose targeting parasite antigens binding to complement down-regulators, together with leading vaccine candidate antigens, as a novel strategy to enhance the efficacy of future malaria vaccines.

The immune mechanisms mediating clinical immunity to malaria are still not fully understood, but antibodies are considered the main defense towards the symptomatic blood stage of the disease. Recent studies have highlighted the importance of complement activation in malaria immunity, however malaria parasites are capable of evading complement recognition. In this study we provide novel insights on how the complement system is activated or evaded in the presence of malaria-specific antibodies developed after natural infection. We show that antibodies from convalescent children can inhibit parasite growth and neutralize complement evasion more effectively than antibodies from immune adults. Moreover, we demonstrate that vaccination with a malaria antigen involved in complement down regulation can induce antibodies that can restore complement activation state better than naturally acquired antibodies. We suggest including such antigens in future malaria vaccines to obtain superior antibody efficacy through the activation of the complement system. These findings can contribute to advance malaria vaccine development.