Naturally acquired antibody responses to the components of the Plasmodium falciparum merozoite surface protein 1 complex

Abstract

Merozoite surface protein 6 (MSP6) and 7 (MSP7) of Plasmodium falciparum are peripheral membrane proteins whose cleaved products, MSP636, MSP722 and MSP719, are found on the merozoite surface as components of a non-covalently bound complex which also contains four polypeptides derived from merozoite surface protein 1 (MSP1). We have expressed both the precursor regions and the processed mature products of MSP6 and MSP7 in Escherichia coli and showed that these recombinant proteins react with human immune sera. In a set of sera collected from individuals living in malaria-endemic areas of Southern-central Vietnam, antibodies to the mature polypeptides of MSP636 and MSP722 were detected in 50.6 and 85.6% of the serum samples, whereas antibodies to the precursor regions of MSP6 and MSP7 were detected in only 12.1 and 42.5% of the serum samples, respectively. The predominant subclass of anti-MSP6 antibodies was IgG1, whereas the predominant subclass of anti-MSP7 antibodies was IgG3. In the same set of serum samples, the antibody responses to MSP119 are predominantly IgGI, whereas antibodies to merozoite surface protein 4 (MSP4) are mainly IgG3. This data is consistent with the proposition that, during malaria infection, variable proteins induce responses that are predominantly of the IgG3 isotype, and conserved proteins induce responses that are predominantly IgG1. The antibodies to MSP6, MSP7 and MSP119 all decreased at the time of infection, but increased during the convalescent period. No correlation was observed between the antibodies at the commencement of the study and absence of parasitaemia during surveillance in this population.

Plasmodium falciparum Merozoite Surface Protein 6 Displays Multiple Targets for Naturally Occurring Antibodies That Mediate Monocyte-Dependent Parasite Killing

“…50% identity and 85% similarity of amino acid residues) and an identity for an 11-amino-acid (aa) stretch (ILGWEFGGG[A/V]P) previously identified as a target of antibodies with strong antiparasite activity (9,13). Moreover, the C-terminal part of MSP6 is highly conserved in MSP6, as in the case of MSP3, whereas the N-terminal part displays polymorphisms, is proteolytically cleaved, and is less antigenic than the C-terminal part (10,18).…”

“…Differential patterns of antibody subclass have also been found against distinct regions within a single protein-e.g., a predominance of IgG3 against MSP1 Block 2 in contrast to IgG1 against MSP1 19 (3)-and between different proteins-e.g., predominance of IgG3 against MSP2 (11,15) as compared to IgG1 against RAP-1 (4) and AMA-1 (S. Singh et al, unpublished results). Within the family of another merozoite surface antigen, IgG3 is predominant against MSP4 as compared to the predominance of IgG1 against MSP5 (18,19). The factors responsible for a distinct human subclass response to different antigens are not fully understood: however, the nature of the antigen itself (6) and the cytokine milieu experienced by the responding B cells (5) may both influence the outcome.…”

Plasmodium falciparum Pf34, a novel GPI-anchored rhoptry protein found in detergent-resistant microdomains

“…Polyclonal antisera raised in mice (against Pf34 fragments, diluted to 1:500) and rabbits (RAMA, 1:1000; HSP70, 1:10,000 and Glucose-Regulated Protein GRP (BiP), 1:1,000) were used as primary antibodies. Human hyperimmune sera from malaria endemic areas of Papua New Guinea (PNG) (Marshall et al, 1997) and Vietnam ) and pooled sera from individuals in Melbourne, Australia, who have never been exposed to malaria were used to assess reactivity of Pf34 (Wang et al, 2003a). The primary antibodies were detected using anti-mouse, anti-rabbit or anti-human immunoglobulin conjugated to horseradish peroxidase (Silenus) and developed as previously described (Black et al, 2001).…”

Epitope-Specific Regulation of Immunoglobulin Class Switching in Mice Immunized with Malarial Merozoite Surface Proteins

“…Antibodies which bind Fc receptors and mediate antibody-dependent cytotoxicity, growth inhibition, or phagocytosis (so-called "cytophilic" antibodies) have repeatedly been shown to be highly effective at clearing blood stage malaria parasites, both in humans (14,43) and in mice (36,38,55). In humans, immunoglobulin G3 (IgG3) is especially valuable in this regard (4,19), and the presence of IgG3 to various merozoite-associated antigens has been linked to protective immunity in seroepidemiological studies (10,15,35,51,56).…”

“…A number of Plasmodium falciparum antigens (5,8,9,56) have been shown to preferentially induce IgG3 in humans; the first of these antigens to be characterized was merozoite surface protein 2 (MSP2) (42,52), but similar observations have now been made for a polymorphic N-terminal region (block 2) of MSP1 (9) and for MSP3 (9,37), MSP4 (57), and MSP7 (56). This bias towards IgG3 production to protein antigens is highly unusual (23) and suggests that something in the interaction of these proteins with the human immune system very efficiently triggers IgG3 class switching.…”