Allelic family-specific humoral responses to merozoite surface protein 2 (MSP2) in Gabonese residents with Plasmodium falciparum infections

Abstract

Merozoite surface protein 2 (MSP2) expressed by Plasmodium falciparum asexual blood stages has been identified as a promising vaccine candidate. In order to explore allelic family-specific humoral responses which may be responsible for parasite neutralization during natural infections, isolates from individuals with either asymptomatic infections or uncomplicated malaria and residing in a Central African area where Plasmodium transmission is high and perennial, were analysed using MSP2 as polymorphic marker. The family-specific antibody responses were assessed by ELISA using MSP2 synthetic peptides. We observed an age-dependence of P. falciparum infection complexity. The decrease of infection complexity around 15 years of age was observed simultaneously with an increase in the mean number of MSP2 variants recognized. No significant difference in the P. falciparum genetic diversity and infection complexity was found in isolates from asymptomatic subjects and patients with uncomplicated malaria. The longitudinal follow-up showed a rapid development of immune responses to various regions of MSP2 variants within one week. Comparing humoral responses obtained with the other major antigen on the merozoite surface, MSP1, our findings suggest that different pathways of responsiveness are involved in antibody production to merozoite surface antigens.

Fig. 1

Schematic diagram of P.falciparum merozoite surface protein 2 (MSP2) gene and biotinylated peptides. The MSP2 gene could be subdivided into five blocks: blocks 1 and 5 code for N terminal and C terminal regions, respectively, which are conserved. Blocks 2 and 4 code for semiconserved regions which are similar within each MSP2 allelic family, 3D7 and FC27 types. These regions flank the central variable region (polymorphic block 3) which consists either of GGSA-like sequences or of 32 amino acid repeats characteristic of the 3D7 and FC27 types, respectively. All peptides were biotinylated at N terminal (*) and amidated at C terminal.

Fig. 2

(a) Infection complexity estimated by merozoite surface protein 2 central block polymerase chain reaction genotyping in isolates from 0 to 4, 5–14 and 15–49-year-old-subjects. The infection complexity (IC) refers to the mean number of distinct P. falciparum genotypes per infected individual. ◊, Uncomplicated malaria; ○, asymptomatic infections. (b) Mean number of recognized peptides by plasma samples from 0 to 4, 5–14, 15–39-year-old individuals. Mean number of recognized peptides represented total positive antibody responses to MSP2 biotinylated peptides over the total number of responders in each age group. ▪, Uncomplicated malaria; □, asymptomatic infections.

Fig. 3

Profile of total IgG responses in serum samples from Gabonese patients. (a) Responses to P. falciparum crude extract. Responders refer to positive plasma against schizont crude extract. Scores 1, 2, 3 and 4 represented seropositivity to crude P. falciparum crude extract at inverse dilutions 400, 1600, 6400, 12800, as described elsewhere [11]. (b) Antibodies prevalence to MSP2 peptides in plasma samples from uncomplicated malaria group. Percentage of responders represents the proportion of individuals with total antibodies against at least one biotinylated peptide. □, Day 0; , day 7.

Fig. 3

Profile of total IgG responses in serum samples from Gabonese patients. (a) Responses to P. falciparum crude extract. Responders refer to positive plasma against schizont crude extract. Scores 1, 2, 3 and 4 represented seropositivity to crude P. falciparum crude extract at inverse dilutions 400, 1600, 6400, 12800, as described elsewhere [11]. (b) Antibodies prevalence to MSP2 peptides in plasma samples from uncomplicated malaria group. Percentage of responders represents the proportion of individuals with total antibodies against at least one biotinylated peptide. □, Day 0; , day 7.