SEROPREVALENCE OF IMMUNOGLOBULIN IgG ANTIBODY RESPONSE TO PLASMODIUM FALCIPARUM MEROZOITE ANTIGENS AMONG CHILDREN IN MINNA, NORTH CENTRAL, NIGERIA

Objective: A cross-sectional study was carried out in a representative cohort of children in Minna aged 6 months–17 years to determine the correlation between immunoglobulin G (IgG) antibody responses to Plasmodium falciparum merozoite antigens. Methods: Plasma samples from 93 children were exposed to Enzyme-Linked Immunosorbent Assay for the measurement of IgG antibody production against P. falciparum. Results: There was a high seroprevalence of IgG antibody against P. falciparum antigens tested with 74.20%. The seroprevalence for the male category was quite higher as compared with that of the female category, though, analysis using Mann–Whitney U-test revealed IgG antibody response to P. falciparum infection in the male was significantly different as compared to the female category (p<0.05). Furthermore, the prevalence of IgG antibody against P. falciparum antigen increased with age, with the lowest observed in 6 months–5 years 66.66%. Kruskal–Wallis H test showed a non-significant difference in the production of IgG antibody against P. falciparum antigen between different cohorts, and no correlation exists between them (p>0.05). An evidence of more than 50% was found for the production of IgG antibody by sub-microscopic parasite. On the other hand, microscopically positive P. falciparum samples recorded more seroprevalence of 68.81% as against negative samples, though significant difference between the negative and positive P. falciparum infected samples and the production of IgG antibody was not observed (p>0.05). Conclusion: This study has demonstrated a boosting immune responses by sub-microscopic parasite and also suggests a strong relationship between production of IgG antibody and malaria transmission, rather than protective immunity.

Does Antibody Avidity to Plasmodium falciparum Merozoite Antigens increase with Age in Individuals living in Malaria-Endemic Areas?

Introduction: High avidity antibodies (Abs) are acquired after a few Plasmodium falciparum infections in low transmission areas, but it remains unclear if Ab avidity to different merozoite antigens increases with age in individuals with persistent antigenemia and if so, when a fully mature Ab response occurs. Methods: The study used plasma samples collected between 1996 and 1998 from 566 individuals aged 4-84 years in Simbok, Cameroon where residents received an estimated 1.6 infectious mosquito bites/person/night. Plasma samples were examined for Ab levels (median fluorescence intensity, MFI) and Ab avidity index (AI = [MFI after treatment with 2M NH4SCN/MFI without salt] x 100) using a bead-based multiplex immunoassay for recombinant AMA1, EBA-175, MSP1-42 (3D7, FVO), MSP2 (3D7, Fc27), and MSP3. Results: Blood-smear positivity for P. falciparum declined with age from 54.3% at 4-5 years to 18% at 16-40 years and <11% at >40 years of age, although most individuals had submicroscopic parasitemia. Ab affinity maturation, based on age-related patterns of median AI, percent of individuals with AI ≥50 and strength of association between MFI and AI, occurred at different rates among the antigens: developing rapidly before age 4 years for AMA1, increasing gradually with age for EBA-175 and MSP1 until ∼16-25 years, but occurring negligibly for MSP2 and MSP3. Conclusion: In a hyperendemic area with perennial transmission, affinity maturation resulting in an increase in the proportion of high avidity Abs occurred for some merozoite antigens, in parallel with a decline in malaria slide passivity, but not for others.

Whole Cell Phenotypic Screening Of MMV Pathogen Box identifies Specific Inhibitors of Plasmodium falciparum merozoite maturation and egress

We report a systematic, cellular phenotype-based antimalarial screening of the MMV Pathogen Box collection, which facilitated the identification of specific blockers of late stage intraerythrocytic Plasmodium falciparum maturation. First, from standard growth inhibition asays, we discovered 62 additional antimalarials (EC50 ≤ 10μM) over previously known antimalarial candidates from Pathogen Box. A total of 90 potent molecules (EC50 ≤ 1μM) were selected for evaluating their stage-specific effects during the intra-erythrocytic development of P. falciparum. None of these molecules had significant effect on ring-trophozoite transition, 10 molecules inhibited trophozoite-schizont transition, and 21 molecules inhibited schizont-ring transition at 1μM. These compounds were further validated in secondary assays by flow cytometry and microscopic imaging of treated cells to prioritize 12 molecules as potent and selective blockers of schizont-ring transition. Seven of these were found to strongly inhibit calcium ionophore induced egress of Toxoplasma gondii, a related apicomplexan parasite, suggesting that the inhibitors may be acting via similar mechanism in the two parasites, which can be further exploited for target identification studies. Two of these molecules, with previously unknown mechanism of action, MMV020670 and MMV026356, were found to induce fragmentation of DNA in developing merozoites. Further mechanistic studies would facilitate therapeutic exploitation of these molecules as broadly active inhibitors targeting development and egress of apicomplexan parasites relevant to human health.