Understanding the Determinants of Human Red Blood Cell Tropism of Understudied Plasmodium Species

Malaria control programs in the past decade have been successful in decreasing the global burden of Plasmodium falciparum, the more virulent of the Plasmodium species causing malaria in humans. However, as the public health community gears towards elimination and eradication of malaria, there is need for an attention shift towards research of neglected Plasmodium species. A central event in malaria pathogenesis is the invasion of host red blood cells (RBCs) mediated by specific interactions between parasite ligands and RBC receptors. These interactions, also called invasion pathways, can be major determinants of host tropism. Restriction in invasion due to tropism is attributed to limiting disease severity, as it limits the proliferation of the parasite in vivo. In this dissertation, we determine the host cell tropism of Plasmodium knowlesi and Plasmodium vivax, two understudied Plasmodium species. The zoonotic P. knowlesi is now the major cause of malaria in parts of South East Asia, and P. vivax is the most widespread species worldwide causing substantial morbidity. After a review of the current literature on biological and clinical features of these species and their invasion pathways in Chapter 1, we determined the tropism of P. knowlesi in human RBCs in Chapter 2. We used density-based enrichment methods to test the invasion of P. knowlesi into human RBCs of varying age. Incorporating mathematical modeling and experimental adaptation, we demonstrated that an expansion of host cell niche is required for the parasite to reach densities observed clinically. We also obtained parasite lines adapted to efficient proliferation in human RBCs. In Chapter 3, we investigated the molecular basis of increased invasion of human RBCs through whole genome sequencing analysis of newly human-adapted lines, as well as historical strains we adapted to in vitro culture. We showed that different genetic changes in P. knowlesi can lead to the upregulation of PkDBPα-DARC pathway, and have provided evidence of major divergence in invasion ligands in recent field isolates. Finally, in Chapter 4, we studied the variation in human RBC preference of patient isolates for P. knowlesi and P. vivax. We confirmed that recent human P knowlesi isolates also vary in the niche of susceptible RBCs, with a subset exhibiting a lack of restriction in invading human RBCs. We also evaluated ex vivo P. vivax patient isolates for their host RBC preference. We determined that P. vivax field isolates differ in their level of reticulocyte preference. We further found an association between increased reticulocyte preference and schizont maturation. This body of work aims to contribute to our overall understanding of human RBC tropism of Plasmodium species of public health importance and the implication this has for parasite adaptation to humans.Biological Sciences in Public Healt