Strain-specific immunity may drive adaptive polymorphism in the merozoite surface protein 1 of the rodent malaria parasite Plasmodium chabaudi.

Clinical immunity against malaria is slow to develop, poorly understood and strongly strain-specific. Understanding how strain-specific immunity develops and identifying the parasite antigens involved is crucial to developing effective vaccines against the disease. In previous experiments we have shown that strain-specific protective immunity (SSPI) exists between genetically distinct strains (cloned lines) of the rodent malaria parasite Plasmodium chabaudi chabaudi in mice [Cheesman, S., Raza, A., Carter, R., 2006. Mixed strain infections and strain-specific protective immunity in the rodent malaria parasite P. chabaudi chabaudi in mice. Infect. Immun. 74, 2996-3001]. In two subsequent studies, we identified the highly polymorphic Merozoite Surface Protein 1 (MSP-1) as being the principal candidate molecule for the control of SSPI against P. c. chabaudi malaria [Martinelli et al., 2005; Pattaradilokrat, S., Cheesman, S.J., Carter R., 2007. Linkage group selection: towards identifying genes controlling strain-specific protective immunity in malaria. PLoS ONE 2(9):e857]. In the present study, we sequenced the whole msp1 gene of several genetically distinct strains of P. chabaudi and found high levels of genetic diversity. Protein sequence alignments reveal extensive allelic polymorphism between the P. chabaudi strains, concentrated primarily within five regions of the protein. The 3'-end sequence region, encoding the C-terminal 21 kDa region (MSP-1(21)), which is analogous and homologous to MSP-1(19) of Plasmodium falciparum, appears to have been subject to balancing selection. We have found that the strains with the lowest sequence identity at MSP-1(21) (i.e. AS/CB and AJ/CB) induce robust and reciprocal SSPI in experimental mice. In contrast, two strains that do not induce reciprocal SSPI are identical at the 21 kDa region. Final identification of the region(s) controlling SSPI will provide important information to help guide decisions about MSP-1 based vaccines.