Malaria : multiclonal infections and protective immunity

Abstract

The mortality and morbidity attributable to malaria remain high in Sub-Saharan Africa, especially among children less than five years of age. In areas of high transmission, immunity to clinical malaria is gradually acquired after repeated exposure to the polymorphic Plasmodium falciparum parasite. Increased knowledge of the interaction between the human host and the genetic diversity of P. falciparum infections is a prerequisite for understanding the mechanisms underlying acquisition of protective malaria immunity, an understanding important for the development of malaria control strategies e.g. vaccines.

This thesis has assessed how the genetic diversity of P. falciparum infections affects the risk of clinical malaria and how clearance of asymptomatic infections affects host protection. The thesis also includes establishment/development of a new technique to analyze the genetic diversity of parasite populations.

P. falciparum infections were genotyped based on sequence and size polymorphisms of the genes encoding the parasite antigens merozoite surface protein 1 and 2 (msp 1 and 2). A nested PCR method widely used to characterize parasite populations was adapted to fluorescent PCR and capillary electrophoresis in a DNA sequencer. The improved sensitivity and specificity of allelic discrimination forwards this new method as an important tool in molecular epidemiology studies and antimalarial drug trials.

Factors associated with the genetic diversity of P. falciparum infections were investigated in different transmission settings in Tanzania, Ghana and Kenya. The number of concurrent clones increased with age in all studies. Individual exposure, analyzed by antibody levels to the circumsporozoite protein, increased with age but was not associated with the number of clones in a high transmission setting in Tanzania.

The number of P. falciparum clones was correlated to the individual s subsequent risk of clinical malaria. In Tanzania, the lowest risk was found in asymptomatic children infected with 2-3 clones. In Ghana, intermittent preventive treatment administered during 6 months of the peak malaria season reduced the infection diversity. Although temporary, this reduction affected susceptibility to malaria during the following high transmission season. Infections composed of ≥2 clones again predicted a lower risk of febrile malaria, however only in children given placebo. These findings suggest that persistence of antigenically diverse P. falciparum infections is important for protective immunity and that clearance of multiclonal infections might contribute to the rebound in clinical disease observed after IPT was stopped in some studies. In an area of lower transmission in Kenya, children with ≥ 2 clones had a marked decreased risk of febrile malaria only when the parasites had been cleared with a course of an antimalarial drug. In Kenya, the number of clones was associated with level of exposure. When excluding children who remained uninfected after treatment and thus considered less exposed, the protection associated with multiclonal infections were even more evident and associated with blood stage immunity.

A reduced risk of malaria in asymptomatic individuals with persistent multiclonal P. falciparum infections suggests that controlled maintenance of diverse infections is important for clinical protection in continuously exposed individuals. The findings need to be considered in the design and evaluation of new malaria control strategies such as vaccines and interventions aiming to clear asymptomatic infections.