Immunity induced by live malarial sporozoites under chloroquine treatment is strain-specific

Abstract

To-date, data is limited on Plasmodium vivax pre-erythrocytic (PE) stage specific protective immunity and its strain-specificity. We addressed the latter issue in an analogous non-human primate model, P. cynomolgi in toque monkey (Macaca sinica), using two immunologically and genetically distinct strains, Pc746 and PcCeylon. We immunized two groups of monkeys with either Pc746 (n=5) or PcCeylon (n=4) strain by giving bites of 2-4 sporozoite-infected Anopheles tessellates mosquitoes/monkey under chloroquine (starting from day 4 after infective bites) and premaquine (one month after infective bites) treatment to abrogate blood infections. Subsequently, a group of unimmunized monkeys (n=4) and the two groups of Pc746 and PcCeylon immunized monkeys were given a mixed-strain sporozoite challenge infection (2-5 infective mosquitoes from each strain/monkey) 140 and 100 days respectively after infective bites. Using PyrosequencingTM assay based on SNPs in msp1 and csp genes, we quantified proportions of parasites of the two strains in the challenge infections using parasite DNA collected for 5-8 consecutive days from the day which parasitaemia reached 0.03%. In the mixed-strain challenge infection, proportion of parasites of a particular strain was significantly lower in the monkeys immunized against the homologous strain (P<0.05- Mann-Whitney U test), compared to those in the monkeys immunized against the heterologous strain. This shows induction of strain-specific protective immunity following a single live sporozoite/choloroquine immunization which persists for 3-5 months. We also show a pan-specific effect of this immunity on blood-stage parasites as indicated by early reduction of parasitaemia (P<0.05- Mann-Whitney U test) in the immunized monkeys compared to the non-immunized ones during the challenge infections. These findings may have implications on P. vivax sporozoite vaccines.