Mosquito Bite-Induced Controlled Human Malaria Infection with Plasmodium vivax or P. falciparum Generates Immune Responses to Homologous and Heterologous Preerythrocytic and Erythrocytic Antigens

Abstract

Seroepidemiological studies on the prevalence of antibodies to malaria antigens are primarily conducted on individuals from regions of endemicity. It is therefore difficult to accurately correlate the antibody responses to the timing and number of prior malaria infections. This study was undertaken to assess the evolution of antibodies to the dominant surface antigens of Plasmodium vivax and P. falciparum following controlled human malaria infection (CHMI) in malaria-naive individuals. Serum samples from malaria-naive adults, collected before and after CHMI with either P. vivax (n = 18) or P. falciparum (n = 18), were tested for the presence of antibodies to the circumsporozoite protein (CSP) and the 42-kDa fragment of merozoite surface protein 1 (MSP-1₄₂) of P. vivax and P. falciparum using an enzyme-linked immunosorbent assay (ELISA). Approximately 1 month following CHMI with either P. vivax or P. falciparum, >60% of subjects seroconverted to homologous CSP and MSP-1. More than 50% of the subjects demonstrated reactivity to heterologous CSP and MSP-1₄₂, and a similar proportion of subjects remained seropositive to homologous MSP-1₄₂ >5 months after CHMI. Computational analysis provides insight into the presence of cross-reactive responses. The presence of long-lived and heterologous reactivity and its functional significance, if any, need to be taken into account while evaluating malaria exposure in field settings.

Keywords: CHMI; CSP; MSP-1; Plasmodium falciparum; Plasmodium vivax; antibodies; heterologous; homologous; malaria.

FIG 1

Malaria-naive subjects generate immune responses to homologous antigens (Ag) 1 month after controlled human malaria infection. (A) Following CHMI via the bite of five mosquitoes infected with either P. vivax (α-Pv sera) or P. falciparum (α-Pf sera), more than two-thirds of exposed subjects seroconverted to homologous CSP and MSP. (B and C) IgM and IgG antibodies to both CSP (B) and MSP (C) are detectable.

FIG 2

At least half of the subjects in each group recognize heterologous antigens. (A) One month after exposure, ≥50% of the subjects showed reactivity to heterologous CSP and MSP. (B) Almost all the heterologous anti-CSP responses after PvCHMI and PfCHMI were IgM. (C) After PfCHMI, similar numbers of subjects had IgM and IgG antibodies directed against PvMSP, while after PvCHMI, higher anti-PvMSP IgM responses were observed.

FIG 3

Homologous erythrocytic responses persist over time, while heterologous responses wane. (A) Homologous preerythrocytic (CSP) responses decrease, but similar numbers of subjects demonstrate positive antibody responses to homologous MSP on day 28 versus day 159 after homologous challenge. (B) Heterologous preerythrocytic and erythrocytic responses decrease at day 159 postchallenge. Each bar shows IgM and IgG responses; numerical values denote the percentages positive for each isotype.

FIG 4

Magnitude of homologous and heterologous responses 1 month after CHMI at a single serum dilution of 1:500 to PvCSP, PvMSP, PfCSP, and PfMSP. (A) P. vivax challenge induces both IgM and IgG responses to both homologous preerythrocytic and erythrocytic antigens. The majority of heterologous responses were IgM. The horizontal line represents the median value. (B) After P. falciparum CHMI, a large percentage of individuals show homologous and heterologous reactivity, but no IgG responses to either homo- or heterologous CSP were seen. The horizontal line represents the median value. There was a positive Spearman correlation between homologous IgM and IgG for PvCSP (r = 0.38) (A) as well as PfMSP (r = 0.52; P = 0.07) (B). The only heterologous correlation was seen between anti-CSP and MSP IgMs: PvCHMI sera showed a weak positive correlation between PfCSP and PfMSP (A), while the PfCHMI sera demonstrated a high correlation between PvCSP and PfMSP IgMs (r = 0.9; P = 0.04) (B). Ab, antibody.

FIG 5

Discontinuous epitopes are predicted in similar regions on the C-terminal domains of P. vivax and P. falciparum CSPs using two separate prediction tools. (A and B) Cartoon diagrams showing the locations of conformational epitopes on the 3D structures of the C-terminal domains of P. falciparum CSP (A) and P. vivax CSP (B) as predicted by ElliPro (31). (A) ElliPro-predicted scores, in descending order, for the five PfCSP epitopes are 0.717 (magenta), 0.699 (red), 0.668 (yellow), 0.668 (orange), and 0.547 (blue). (B) The scores for the five epitopes, ranked in order, for PvCSP are 0.725 (red), 0.721 (magenta), 0.647 (slate), 0.556 (yellow), and 0.503 (orange). Individual residues are marked and described in the text. (C and D) Cartoon diagrams of conformational epitopes predicted by DiscoTope (30) for the C-terminal domains of P. falciparum and P. vivax CSPs, respectively. (C) The two predicted epitopes for PfCSP are highlighted in green for a 19-residue epitope and in yellow for a 24-residue epitope. (D) The single 13-residue predicted epitope for PvCSP is highlighted in yellow.

FIG 6

Cartoon drawings showing potential antigenic epitopes on the C-terminal domains of PfMSP-1 (96 residues) and PvMSP-1 (90 residues) solution structures. (A and B) Cartoon diagrams showing the locations of conformational epitopes on the 3D structures of the C-terminal domains of P. falciparum MSP-1 (A) and P. vivax MSP-1 (B), as predicted by ElliPro (31). (A) ElliPro-predicted scores, in descending order, for the four PfMSP-1 epitopes are 0.82 (magenta), 0.602 (blue), 0.588 (orange), and 0.533 (yellow). (B) Scores for the five epitopes, ranked in order, for PvCSP are 0.716 (yellow), 0.703 (blue), 0.689 (magenta), 0.65 (light blue), and 0.547 (light orange). Individual residues are marked. The second-highest-scoring epitope for PfMSP-1 (shown in blue in panel A) consists of a total of 27 residues that are distributed in different regions in the amino acid sequence but are spatially close on the 3D structure. The lowest-scoring epitope (yellow in panel A) includes 20 residues that are from different areas of PfMSP-1 sequences but are located in proximity on the 3D structure. (C and D) Cartoon diagrams of conformational epitopes predicted by DiscoTope (30) for the C-terminal domains of P. falciparum and P. vivax MSP-1s, respectively. The predicted epitopes for PfMSP-1 (C) and PvMSP-1 (D) are shown in different colors.