Allele specificity of gamma interferon responses to the carboxyl-terminal region of Plasmodium falciparum merozoite surface protein 1 by Kenyan adults with naturally acquired immunity to malaria

Abstract

Cross-sectional seroepidemiological studies of populations naturally exposed to Plasmodium falciparum suggest an association between protection from malaria and circulating antibodies to the carboxyl terminus of merozoite surface protein 1 (MSP1). Questions remain regarding the significance of cell-mediated immunity to MSP1 in conferring protection and inducing immunologic memory. Vaccine constructs have been based on the 42-kDa recombinant MSP1 protein (MSP1(42)), which includes the 19-kDa (MSP1(19)) and 33-kDa (MSP1(33)) fragments containing the major B- and T-cell epitopes, respectively. To evaluate T-cell responses to the MSP1(33) fragment, two libraries of overlapping 18-mer peptides from the 3D7 and FVO MSP1(33) regions were used to screen a cohort of asymptomatic Kenyan adults. Gamma interferon (IFN-γ) measured by enzyme-linked immunospot assay (ELISPOT) at multiple time points assessed the magnitude and stability of these responses. The percentage of individuals with IFN-γ responses to single MSP1(33) peptides ranged from nil to 24%, were clustered among a subset of peptides, and were not consistently recalled over time. In comparison to peptide responses, IFN-γ ELISPOT responses to recombinant MSP1(42) were more prevalent, more frequently elicited by the 3D7 as opposed to the FVO allele, and more stable over time. The prevailing MSP1(33) genotype infection was 3D7, with few mixed infections and no sole FVO infections. This study demonstrates that immunity against MSP1(33) after cumulative natural infections consists of low-magnitude and difficult-to-detect IFN-γ responses. Although immunity against MSP1 alone will not confer protection against malaria, demonstrating a relative and sustained increase in T-cell immunity to MSP1 after vaccination would be a reasonable measurement of vaccine responsiveness.

FIG. 1.

Sample collection. Timeline, season, intervals, and MSP1 recombinant and peptide testing time points are depicted, as well as references to relevant tables and figures presented herein.

FIG. 2.

Frequency of IFN-γ ELISPOT responders to overlapping MSP1 33-kDa peptides from Table 1, for 3D7-specific peptides (A) and MSP1 (33-kDa) FVO-specific peptides (B) screened in August to October 2004, illustrated with bars and corresponding to the left margin legend. Magnitude of IFN-γ ELISPOT-forming units (sfu) per million PBMCs is represented by red spots and range referenced on the right margin legend.

FIG. 3.

Magnitude of IFN-γ ELISPOT responses to allelic variants of MSP1 42-kDa recombinant protein, 3D7 (A) and FVO (B), during six time periods. ELISPOT responses are reported as spot-forming units (SFU) per million PBMCs.