AbstractIt is not known how individuals chronically exposed to malaria transmission have adapted to enable a life relatively free of malaria symptoms despite the ongoing persistence and replication of Plasmodium species in their bloodstreams for much of the time. It has been supposed that a combination of innately determined and acquired factors contribute to the maintenance of apparently good health in the face of chronic parasitisation by inhibiting the inflammatory events triggered by parasite toxins. Candidate mechanisms proposed to mediate this state of malarial “tolerance” include enhanced production of the biological messenger nitric oxide (NO; which may down-regulate events such as the release of endogenous pyrogens) and neutralising antibodies against the proposed P. falciparum toxin, glycosylphosphatidylinositol (GPI). Prior to this study, it was assumed that each mechanism was either boosted (NO) or induced (anti-GPI antibodies) by malaria infection in proportion to the level of parasitemia, which in highly endemic regions is commonly highest in early childhood and lowest in adulthood.
As much of the evidence that informed these two hypotheses was derived from in vitro and animal studies, the purpose of this study was to test each of them under field conditions in populations chronically exposed to intense malaria transmission. The first project was a small cross-sectional pilot study involving adults in Papua province (formerly Irian Jaya) and the second enrolled children and adults from Madang, Papua New Guinea (PNG) who were given treatment to eradicate parasitemia and then followed longitudinally to evaluate its effect. Total systemic NO production was estimated along with cellular production by peripheral blood mononuclear cells (PBMCs), thought to be most receptive to induction by infection and high output NO production. Anti-GPI immunoglobulin (Ig)M, IgG and IgG subclass antibodies were also determined cross-sectionally and longitudinally.
The results of the study presented in this thesis argue against both of these hypotheses. Although systemic NO production and PBMC NO synthase (NOS) activity were significantly higher at both study sites than in Darwin controls, there was no evidence that NO production was age-dependent and no absolute requirement v for malaria parasitemia to induce NO was demonstrated. The disassociation of systemic and PBMC NO production suggests that other cellular sources may be at least as contributory as PBMCs to the overall high basal levels of NO. Genetic polymorphisms thought to influence disease outcome through altered NO production in other geographical locations were either not found or found not to influence NO production/NOS activity in the PNG study subjects. This suggests that linkage disequilibrium with other critical polymorphisms may have explained the results elsewhere, that other polymorphisms are more important in differential NO production in PNG, and/or that more care needs to be taken interpreting the results of disease association studies. Anti-GPI antibody production was shown to be agedependent, with increasing prevalence, abundance and persistence with increasing age. IgG predominated over IgM and was short-lived, which was shown to be most likely due to a skewing of anti-GPI IgG subclass production to the more transient IgG3 over IgG1. As very few young children with high parasitemias produced antibodies, they are very unlikely to be the sole mediator of tolerance, if at all.
This study focuses attention on other potential mechanisms for mediating tolerance, the understanding of which could inform strategies aimed at reducing death from malaria, as well as the burden of disease. Conversely, the results of this study can potentially inform further investigation of other proposed anti-toxic roles for NO and anti-GPI antibodies in retarding disease severity.
|Date of Award
|Nicholas Anstey (Supervisor)