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.
