Genetic diversity and recombination at the C-terminal fragment of the merozoite surface protein-1 of Plasmodium vivax (PvMSP-1) in Sri Lanka

Extensive polymorphism in the genes encoding for surface antigens of Plasmodium falciparum and Plasmodium vivax has been a serious impediment for malaria vaccine development. One such antigen is the merozoite surface protein-1 (MSP-1). The MSP-1 precursor after proteolytic cleavage generates a C-terminal fragment of 42kDa (MSP-1₄₂), which subsequently produces 33kDa (MSP-1₃₃) and 19kDa (MSP-1₁₉) fragments. Since MSP-1₄₂ is currently being considered as a candidate for vaccine development against blood stage malaria it is important to catalogue the existing diversity in this antigen in natural P. vivax infections. Here we investigated the level of genetic diversity in the PvMSP-1₄₂ gene fragment in 95 single clone P. vivax infections in Sri Lanka. We observed that the PvMSP-1₁₉ fragment was highly conserved among these samples, whereas the PvMSP-1₃₃ fragment exhibited extensive diversity with 39 polymorphic amino acid positions (corresponding to 27 haplotypes, 19 of which were unique to Sri Lanka). Of these 27 PvMSP-1₄₂ haplotypes, 24 belonged to hypervariable region (HVR) T1-T7 types, while 3 haplotypes were generated by interallelic recombination between T1/T3 (HVRT8-T9) and T2/T3 (HVRT10). In addition, we analysed 107 PvMSP-1₄₂ sequences (corresponding to 62 haplotypes, H28 to H89) deposited in the NCBI GenBank database from other regions of the world. Seventy-four of these correspond to 9 of the 10 HVR types (HVR-T7 was unique to Sri Lanka). Two novel HVR types, T11 and T12, with a double recombination between HVR-T1/T3 and HVRT6/T2, were derived from South America and Thailand, respectively. T cell epitope polymorphism arising due to non-synonymous substitutions in PvMSP-1₃₃ may result in differential binding of the polymorphic peptides to class II MHC alleles, inducing different host immune responses. In conclusion, under low transmission and unstable malaria conditions prevalent in Sri Lanka, extensive allelic polymorphism was evident at PvMSP-1₃₃ due to recombination, mutation, and balancing selection. In contrast, PvMSP-1₁₉ is highly conserved_, greatly enhancing its suitability as a malaria vaccine candidate.