Abstract
Plasmodium falciparum and Plasmodium vivax cause the majority of human malaria cases. Research efforts predominantly focus on P. falciparum because of the clinical severity of infection and associated mortality rates. However, P. vivax malaria affects more people in a wider global range. Furthermore, unlike P. falciparum, P. vivax can persist in the liver as dormant hypnozoites that can be activated weeks to years after primary infection, causing relapse of symptomatic blood stages. This feature makes P. vivax unique and difficult to eliminate with the standard tools of vector control and treatment of symptomatic blood stage infection with antimalarial drugs. Infection by Plasmodium is initiated by the mosquito-transmitted sporozoite stage, a highly motile invasive cell that targets hepatocytes in the liver. The most advanced malaria vaccine for P. falciparum (RTS,S, a subunit vaccine containing of a portion of the major sporozoite surface protein) conferred limited protection in Phase III trials, falling short of WHO-established vaccine efficacy goals. However, blocking the sporozoite stage of infection in P. vivax, before the establishment of the chronic liver infection, might be an effective malaria vaccine strategy to reduce the occurrence of relapsing blood stages. It is also thought that a multivalent vaccine comprising multiple sporozoite surface antigens will provide better protection, but a comprehensive analysis of proteins in P. vivax sporozoites is not available. To inform sporozoite-based vaccine development, we employed mass spectrometry-based proteomics to identify nearly 2,000 proteins present in P. vivax salivary gland sporozoites. Analysis of protein post-translational modifications revealed extensive phosphorylation of glideosome proteins as well as regulators of transcription and translation. Additionally, the sporozoite surface proteins CSP and TRAP, which were recently discovered to be glycosylated in P. falciparum salivary gland sporozoites, were also observed to be similarly modified in P. vivax sporozoites. Quantitative comparison of the P. vivax and P. falciparum salivary gland sporozoite proteomes revealed a high degree of similarity in protein expression levels, including among invasion-related proteins. Nevertheless, orthologs with significantly different expression levels between the two species could be identified, as well as highly abundant, species-specific proteins with no known orthologs. Finally, we employed chemical labeling of live sporozoites to isolate and identify 36 proteins that are putatively surface-exposed on P. vivax salivary gland sporozoites. In addition to identifying conserved sporozoite surface proteins identified by similar analyses of other Plasmodium species, our analysis identified several as-yet uncharacterized proteins, including a putative 6-Cys protein with no known ortholog in P. falciparum.
Highlights
Malaria is a major global infectious disease, responsible for nearly 429,000 deaths and 212 million new cases annually (World Malaria Report 2016, WHO)
Though all clinical symptoms are attributable to the blood stages, it is only by attacking the transmission stages before the formation of hypnozoites that an impact on the burden of vivax malaria can be achieved
A total of 1711 P. vivax proteins were identified from 3.5 × 106 sporozoites bearing the VK210 haplotype of Proteogenomic analysis of Plasmodium vivax salivary gland sporozoites circumsporozoite protein (CSP), of which 1492 (87.2%) were identified by at least two nondegenerate peptides
Summary
Malaria is a major global infectious disease, responsible for nearly 429,000 deaths and 212 million new cases annually (World Malaria Report 2016, WHO) This disease, found in much of the tropical and subtropical regions of the world, is caused by parasites of the genus Plasmodium, transmitted to humans by the bite of infected anopheline mosquitoes. With only one exposure to infectious mosquito bite, P. vivax can initiate one symptomatic infection but a series of reoccurring onsets of malaria episodes that, if not treated, can last for months. These recurring infections are due to a distinctive property of P. vivax liver infection: formation of hypnozoites, a portion of P. vivax liver-stage parasites that becomes dormant and can reactivate weeks to months or even years later [4]
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