Characterization of Malaria Sexual Stage Development in the Human Host

Abstract

Due to an increase in malaria control programs in the last decade, the world has witnessed dramatic reductions in the number of infections and deaths caused by the malaria parasite. With malaria eradication on the global health agenda, a shift toward transmission-focused research has led to a renewed focus on a previously neglected stage of malaria: the sexual stage (gametocyte). Malaria's sexual stages are the only stages in the human host that are transmitted to the mosquito vector, and are therefore of critical importance for blocking transmission of this devastating disease. The process through which developing gametocytes sequester outside of the bloodstream during their 8-10 day maturation is not well understood and stands to be exploited as a potential target for therapeutic intervention. In Chapter 1, we discuss the current state of knowledge on the development of these stages in the human host. In Chapter 2, we investigate anatomical enrichment sites for developing gametocytes in the human host using autopsy tissue from cases of fatal malaria. Immunohistochemistry (IHC) and quantitative reverse transcriptase PCR-based assessments identified the bone marrow as a preferential enrichment site of developing gametocytes. Co-localization with host proteins revealed the enrichment of gametocytes inside the extravascular space of the bone marrow, often observed in contact with erythroblastic island structures. In vitro experiments with erythrocyte precursor cells, as well as in vivo co-localization studies demonstrated that gametocytes can develop within the cells of the hematopoietic system of the bone marrow. In Chapter 3, we present an assay and analysis tool for inferring the presence of young and mature asexual and sexual stages in the peripheral blood of infected patients based on gene expression data. We apply this assay to malaria patient cohorts and in vitro drug perturbation time course experiments, and demonstrate its use in identifying young and mature gametocyte carriers, as well as characterizing the effect of a given perturbation on parasite development. This body of work aims to contribute to the overall knowledge base for malaria’s elusive gametocytes as well as to establish tools for performing future assessments on these transmissible stages.