Abstract
BackgroundPlasmodium falciparum apical membrane antigen-1 (AMA1) is a leading malaria vaccine candidate antigen that is expressed by sporozoite, liver and blood stage parasites. Since CD8+ T cell responses have been implicated in protection against pre-erythrocytic stage malaria, this study was designed to identify MHC class I-restricted epitopes within AMA1.MethodsA recombinant adenovirus serotype 5 vector expressing P. falciparum AMA1 was highly immunogenic when administered to healthy, malaria-naive adult volunteers as determined by IFN-γ ELISpot responses to peptide pools containing overlapping 15-mer peptides spanning full-length AMA1. Computerized algorithms (NetMHC software) were used to predict minimal MHC-restricted 8-10-mer epitope sequences within AMA1 15-mer peptides active in ELISpot. A subset of epitopes was synthesized and tested for induction of CD8+ T cell IFN-γ responses by ELISpot depletion and ICS assays. A 3-dimensional model combining Domains I + II of P. falciparum AMA1 and Domain III of P. vivax AMA1 was used to map these epitopes.ResultsFourteen 8-10-mer epitopes were predicted to bind to HLA supertypes A01 (3 epitopes), A02 (4 epitopes), B08 (2 epitopes) and B44 (5 epitopes). Nine of the 14 predicted epitopes were recognized in ELISpot or ELISpot and ICS assays by one or more volunteers. Depletion of T cell subsets confirmed that these epitopes were CD8+ T cell-dependent. A mixture of the 14 minimal epitopes was capable of recalling CD8+ T cell IFN-γ responses from PBMC of immunized volunteers. Thirteen of the 14 predicted epitopes were polymorphic and the majority localized to the more conserved front surface of the AMA1 model structure.ConclusionsThis study predicted 14 and confirmed nine MHC class I-restricted CD8+ T cell epitopes on AMA1 recognized in the context of seven HLA alleles. These HLA alleles belong to four HLA supertypes that have a phenotypic frequency between 23% - 100% in different human populations.
Highlights
Plasmodium falciparum apical membrane antigen-1 (AMA1) is a leading malaria vaccine candidate antigen that is expressed by sporozoite, liver and blood stage parasites
Combining circumsporozoite protein (CSP) with other pre-erythrocytic stage antigens and using a vaccine platform such as adenovirus vectors better able to induce class I restricted cell-mediated immunity might more effectively target the hepatic stages of infection and reproduce the immunity induced by the irradiate sporozoite vaccine
AMA1 is an integral membrane protein found in all species of Plasmodium and has traditionally been regarded as a blood stage antigen, since it is required for the invasion of red blood cells[19], monoclonal and polyclonal antibodies targeting AMA1 inhibit blood stage growth in vitro, naturally acquired anti-AMA1 antibodies correlate with protection against clinical malaria in endemic areas [20,21,22,23,24], and vaccines based on AMA1 elicit protection against blood stage infection [13,25] in animal models that appears to be antibody mediated [25,26]
Summary
Plasmodium falciparum apical membrane antigen-1 (AMA1) is a leading malaria vaccine candidate antigen that is expressed by sporozoite, liver and blood stage parasites. AMA1 is an integral membrane protein found in all species of Plasmodium and has traditionally been regarded as a blood stage antigen, since it is required for the invasion of red blood cells[19], monoclonal and polyclonal antibodies targeting AMA1 inhibit blood stage growth in vitro, naturally acquired anti-AMA1 antibodies correlate with protection against clinical malaria in endemic areas [20,21,22,23,24], and vaccines based on AMA1 elicit protection against blood stage infection [13,25] in animal models that appears to be antibody mediated [25,26]. AMA1 is expressed in sporozoites and liver stage parasites[27], and may be a suitable target for CD8+ T cell responses directed toward liver stage parasites
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