Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Abstract

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Fig. 1

Parasite-induced modifications to P. falciparum-infected erythrocytes. A. During intra-erythrocytic development, P. falciparum expresses knob structures and VSAs on the surface of pigmented trophozoite IEs. PfEMP1, P. falciparum erythrocyte membrane protein 1; RIFIN, repetitive interspersed family; STEVOR, subtelomeric variable open reading frame; SURFIN, surface-associated interspersed gene family; KAHRP, knob-associated histidine-rich protein. B. Scanning (left) and transmission (right) electron microscopy (EM) shows the ultrastructural features of the IE membrane. The IE membrane is distorted by surface knob protrusions (arrows) that present the major virulence factor, PfEMP1

Fig. 2

Evaluating the antibody response to PfEMP1 using transgenic P. falciparum P. falciparum-infected erythrocytes transfected with a construct that inhibits PfEMP1 expression but does not appear to have an impact on the expression of other VSAs (referred to as ‘var promoter knockdown’). This provides a novel approach to quantify antibodies to PfEMP1 and assess its importance as an immune target. The figure shows a representative selection of serum samples that were tested for IgG binding to parental and transgenic parasites [102]. Samples were from malaria-exposed Kenyan adults (K2-K16) and non-exposed Melbourne residents (Control). IgG binding to the surface of erythrocytes infected with the transgenic parasites was markedly reduced compared to parental parasites as previously reported [102]. The horizontal dotted line represents the mean level of IgG binding to parental parasites (n = 8); bars represent mean and range of samples tested in duplicate; IgG levels are expressed as geometric mean fluorescence intensity for both graphs

Fig. 3

Approaches to overcome antigenic diversity of PfEMP1 in vaccine development. Antigenic diversity is the major challenge to developing PfEMP1 as a vaccine against malaria. The flow chart provides an overview of the two broad approaches to overcoming antigenic diversity in PfEMP1 and the steps involved in progressing vaccine candidates to the clinical trial stage. One approach is to develop a multivalent vaccine comprised of a mixture of common PfEMP1 variants that induces a broad repertoire of antibodies. A second approach is to identify conserved epitopes on PfEMP1 and develop a vaccine that targets these epitopes to induce broadly cross-reactive antibodies. As discussed in the text, there are significant challenges to overcome for each approach. It is likely that any PfEMP1 candidate vaccine antigen(s) would be included in a multi-antigen approach that includes antigens from other parasite life stages to ensure the development of a highly effective vaccine