Date of Degree
PhD (Doctor of Philosophy)
John T. Harty
Infection with Plasmodium species leads to nearly 400,000 deaths a year despite widespread use of mosquito bed nets, insecticides, and anti-malarial drugs. To date, there is not a licensed vaccine capable of providing complete protection from Plasmodium infection to vaccinees. Whole parasite vaccination of humans and rodents can achieve complete protection in vaccines, but the dose of sporozoites, number of administrations, and production concerns in generating these types of vaccines will likely prevent these approaches from achieving worldwide use. However, the protective immunological responses against Plasmodium parasites engendered by these vaccination approaches can be studied and aid in the development of advanced subunit vaccines against Plasmodium. Using rodent models of malaria to elucidate the features of protective immunity engendered by whole parasite vaccination, it has been repeatedly shown that CD8 T cell responses directed against liver-stage parasite antigens can provide complete protection with some contribution by CD4 T cells and antibody responses depending on the model system studied. However, the quantatitive and qualitative requirements for CD8 T cell immunity against Plasmodium remains largely undefined. To enhance our understanding of how to generate protective immunity against Plasmodium, I have utilized rodent models of malaria to study the superior protection afforded from single-dose vaccination with virulent sporozoites administered under prophylatic chloroquine-cover, referred to as chemoprophylaxis sporozoites (CPS) vaccination, compared to the well-studied approach of administering radiation-attenuated Plasmodium sporozoites (RAS). RAS vaccination has long been considered the “gold standard” in vaccination due the ability of RAS vaccination to engender complete protection following sporozoite challenge of vaccinated humans and rodents. However, CPS vaccination is arguably a superior vaccination approach since it can achieve protection through less vaccine administrations relative to RAS vaccination, but the immunological basis of this enhanced CPS vaccine-induced immune response was unclear. In my study, I utilized a stringent host/parasite model to find that C57Bl/6 mice administered CPS vaccination with P. yoelii sporozoites elicit substantially higher parasite-specific CD8 T cell responses than RAS vaccination, but CPS-induced CD8 T cells were not necessary for protection following liver-stage sporozoite or blood-stage parasite challenge. CPS vaccination resulted in a low grade, transient parasitemia shortly following cessation of chloroquine treatment, which lead to the generation of potent antibody responses to blood-stage parasites; this blood-stage parasite-specific antibody response correlated with sterilizing protection in sporozoite challenged CPS-vaccinated mice. Therefore, my data provide a mechanistic basis for enhanced protective immunity elicited by single-dose CPS vaccination in a rodent model that is independent of CD8 T cells. The other portion of my work examines how CD8 T cell specificity impacts protective capacity against Plasmodium. I show that robust CD8 T cell responses of similar phenotype are mounted following prime-boost immunization against three novel Plasmodium berghei protein-derived epitopes in addition to a previously described protective, immunodominant epitope. I show that only CD8 T cells specific to sporozoite surface-expressed protein-derived epitopes, but not the intracellular protein-derived epitopes, are efficiently recognized by sporozoite-infected hepatocytes in vitro. These results suggest that antigenic targets must be efficiently presented by infected hepatocytes for CD8 T cells to eliminate liver-stage Plasmodium infection and proteins expressed on the surface of sporozoites may be good target antigens for protective CD8 T cells. Collectively, my work highlights the ability to generate protective CD8 T cell independent and dependent immunity against Plasmodium infections, whether achieved through potent blood-stage-specific antibody responses, or via numerically large monospecific CD8 T cell responses that target parasite antigens that are efficiently presented during liver-stage infection. These studies are relevant in understanding how to efficiency engender protective immunity against Plasmodium, and could aid in the advancement of subunit vaccination approaches that generate immunity through the priming of responses from multiple arms of the immune response, targeting both the liver- and blood-stages of Plasmodium.
Plasmodium species are the causative agent of malaria, a disease that leads to approximately 400,000 deaths a year. Infection prevention methods have drastically reduced the mortality rate. However, these prevention methods are not enough to lead to complete control and thus vaccination remains the best strategy to eliminate malarial disease, and lead to worldwide eradication of the Plasmodium parasite. Currently, no licensed vaccine is available that provides complete protection in vaccinated individuals. In order to develop an effective vaccine, a better understanding of what constitutes a protective anti-Plasmodium immune response is essential. To this end, rodent models of malaria have been utilized to mechanistically define the requirements of protective immunity. Previous work has shown that whole parasite vaccination protects individuals due to a cell population that specializes in killing parasite-infected cells, called the CD8 T cell. However, using a stringent rodent model of whole parasite vaccination, I found that CD8 T cells have no measurable role in protection, and in fact protection was likely due to antibody responses against the parasite. Another part of my study sought to better define an important feature of CD8 T cells, termed their specificity. I found in my examination of four specificities that only CD8 T cells that target surface-expresed parasite antigens were protective. Collectively, my studies have helped further our understanding of how to generate protective immune responses against the Plasmodium parasite and may aid in the development of an advanced vaccine that can protect humans against Plasmodium infections, and thus malarial disease.
publicabstract, CD8 T cell, immunity, malaria, Plasmodium, vaccination
Copyright 2016 Katherine L. Doll Kanne