Immunity to malaria: importance of exposure and parasite polymorphism

Abstract: Malaria continues to carry an intolerable burden of disease and mortality, predominantly on children in Sub-Saharan Africa. An efficacious vaccine would be a powerful tool in the combat against malaria. Further understanding of the mechanisms of naturally acquired immunity to the polymorphic Plasmodium falciparum parasite would aid vaccine development. The studies presented in this thesis contribute to the understanding of how immune responses are acquired in relation to exposure to the polymorphic P. falciparum parasite. More specifically, they investigated how the genetic diversity of the P. falciparum parasites encountered during childhood affects the risk of malaria (uncomplicated and severe), as well as whether maintenance of immune memory against P. falciparum antigens is maintained without repeated re-exposure. The thesis also included a description of a novel technique for dispersion of frozen blood clots for optimized DNA extraction and polymerase chain reaction (PCR). Immunity against severe malaria has been proposed to be acquired after single infections. A matched case-control study was nested within a birth cohort on the Kenyan Coast where three-monthly blood samples up to two years, with the aim to study how exposure patterns early in life affects the risk of developing severe malaria. P. falciparum infections detected at least once from birth conferred an increased risk of severe malaria and especially if infections with two or more parasite clones were ever detected. To optimize DNA extraction for the study above, a novel method was developed for dispersion of frozen residual clots saved from blood collected without anticoagulant for serum preparation. Compared to two commercial methods, high-speed shaking was most succesful in clot dispersion before DNA extraction and generated the highest sensitivity of PCR detection. Whether the genetic diversity of asymptomatic P. falciparum infections carried through the dry season affects the risk of malaria in the ensuing transmission season was investigated in a setting with intense but strictly seasonal malaria transmission in Mali. Individuals with multiclonal infections before the malaria season had a delayed time to their first febrile malaria episode and a lower incidence of malaria compared to individuals who were smear negative or only carried one parasite clone. The longevity of immune responses to malaria after a single exposure was assessed in travelers treated for malaria in Stockholm, Sweden, 1-16 years previously. P. falciparum-specific memory B-cells were maintained in a majority of study participants for up to 16 years without re-exposure, whereas circulating cognate antibodies were not detected. In conclusion, a single encounter with the parasite can induce immunological memory to P. falciparum antigens that lasts for many years. Severe malaria can occur in young children despite previous exposure, a finding that argues against the idea that severe malaria develops only after the first encounters with the parasite. Persistent asymptomatic infections, especially with genetically diverse infections, contribute to maintenance of immune responses that protect from malaria. This thesis presents different concepts and mechanisms of naturally acquired immunity to malaria that might be valuable in the development of malaria vaccines and other control strategies.