Measuring naturally acquired immune responses to candidate malaria vaccine antigens in Ghanaian adults

Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Ghana.
Malaria Journal (Impact Factor: 3.11). 06/2011; 10(1):168. DOI: 10.1186/1475-2875-10-168
Source: PubMed


To prepare field sites for malaria vaccine trials, it is important to determine baseline antibody and T cell responses to candidate malaria vaccine antigens. Assessing T cell responses is especially challenging, given genetic restriction, low responses observed in endemic areas, their variability over time, potential suppression by parasitaemia and the intrinsic variability of the assays.
In Part A of this study, antibody titres were measured in adults from urban and rural communities in Ghana to recombinant Plasmodium falciparum CSP, SSP2/TRAP, LSA1, EXP1, MSP1, MSP3 and EBA175 by ELISA, and to sporozoites and infected erythrocytes by IFA. Positive ELISA responses were determined using two methods. T cell responses to defined CD8 or CD4 T cell epitopes from CSP, SSP2/TRAP, LSA1 and EXP1 were measured by ex vivo IFN-γ ELISpot assays using HLA-matched Class I- and DR-restricted synthetic peptides. In Part B, the reproducibility of the ELISpot assay to CSP and AMA1 was measured by repeating assays of individual samples using peptide pools and low, medium or high stringency criteria for defining positive responses, and by comparing samples collected two weeks apart.
In Part A, positive antibody responses varied widely from 17%-100%, according to the antigen and statistical method, with blood stage antigens showing more frequent and higher magnitude responses. ELISA titres were higher in rural subjects, while IFA titres and the frequencies and magnitudes of ex vivo ELISpot activities were similar in both communities. DR-restricted peptides showed stronger responses than Class I-restricted peptides. In Part B, the most stringent statistical criteria gave the fewest, and the least stringent the most positive responses, with reproducibility slightly higher using the least stringent method when assays were repeated. Results varied significantly between the two-week time-points for many participants.
All participants were positive for at least one malaria protein by ELISA, with results dependent on the criteria for positivity. Likewise, ELISpot responses varied among participants, but were relatively reproducible by the three methods tested, especially the least stringent, when assays were repeated. However, results often differed between samples taken two weeks apart, indicating significant biological variability over short intervals.

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    • "The current data confirm the general observation that plasma levels of antibodies against sporozoite antigens (CelTOS and CSP) are comparatively lower than levels against blood stage antigens in populations with natural exposure to malaria [27]. For any of the three antigens, comparisons of OD data by sampling time point show that antibody levels were generally higher at month 0 compared independently to months 3 and 6 in the two younger age groups (one to five and six to 15 years) whilst antibody levels did not change significantly with time in the 16–30 years old group (Figure 1). "
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    • "Nevertheless, our results indicated that transfer of antibodies from PvTRAP-immunized mice into naive recipients also contributed to protection. Reports have shown that in regions where malaria is endemic there are TRAP-specific antibodies in the blood of patients that correlate with protection (30) and that immunization of humans and mice with irradiated sporozoites can induce protective antibody responses against SSP2 (TRAP) and inhibit sporozoite invasion of human hepatoma cells (28, 31, 32). Crystal structures determined recently show PfTRAP and PvTRAP in two distinct conformational states, closed and open, respectively (26). "
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    • "[14]–[20]. However, infection has only been associated with modest responses to well known pre-erythrocytic antigens (such as Thrombospondin Related Adhesion Protein (TRAP), Circumsporozoite Surface Protein (CSP), Cell-Traversal protein for Ookinetes and Sporozoites (CelTOS), Liver Stage Antigen 1 (LSA1), Exported Protein 1 (EXP1) and Merozoite Surface Protein (MSP) [21]–[27] which have never been correlated with protection. We sought to assess T-cell responses to other novel pre-erythrocytic antigens in the hope of identifying new antigenic targets for future vaccine development. "
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