The Role Of Regulatory B Cells In Control Of Immunopathogenesis Of Murine Malaria

Malaria caused by infection with a protozoan parasite, Plasmodium spp., is one of the major infectious diseases and threatens the global human health. Epidemiological studies show that malaria remains the major reason of mortality and morbidity in many parts of the world, especially in sub-Saharan Africa. Children under the age of five are the most vulnerable population affected by malaria and malaria-infected children often exhibit severe clinical symptoms such as anemia and cerebral malaria that are the amjor causes of mortality. However, the pathogenesis and immunoregulation of cerebral malaria of are still not fully understood. In malaria-endemic areas, malaria-infected people develop only partial immunity which cannot be maintained for long period to protect the hosts from re-infection. Although adults establish appropriate protective immunity after repeated infection with malaria, the immunity may decline quickly after leaving the malaria-endemic areas. Control of malaria infection currently relies on use of anti-malarial drugs. However, the rapid emergence of drug resistance in malaria parasites significantly reduces the efficacy of the drugs. Anti-malarial vaccines are considered the most efficient means to combat the infection. However, no effective malaria vaccine is available yet and development of the vaccines remains the great challenge, largely due to our incomplete understanding of the complex immunological relation between host and the malaria pathogens. In the research work reported in this thesis, we used murine malaria models, Plasmodium berghei to investigate (1) immune mechanisms involved in development and regulation of malaria-associated neurological pathology-cerebral malaria and (2) the vaccination strategies that would induce enhanced immune response and protection against malaria.1. Regulatory B cells and cerebral malariaPlasmodium infection-induced dysfunction of the nervous system, cerebral malaria, is one of the main pathological changes of malaria and cause of death. About1%of P. falciparum-infected patients develop cerebral malaria that results in10-20%mortality. The pathology of cerebral malaria is characterized by cytoadherence of the malaria parasite-infected red blood cells in the brain capillary wall, causing local capillary congestion and inflammatory cell recruitment. The cytotoxic immune cells including NK cells and cytotoxic CD8+T cells may cause damage to the capillary walls and the blood-brain barrier leading to brain hemorrhage and nervous system dysfunction.Immune regulatory cytokines-IL-10plays a key role in maintenance of immunohomeostasis and prevention from immuopathology during the course of infection such as cerebral malaria. In recent years, a subpopulation of B cells that produce IL-10is identified and research evidences show that these B cells play an important role in maintenance of immune tolerance and suppression of immune pathology. Our study tested the hypothesis that this immunoregulatory B cell population may be involved in control of development of cerebral malaria.The results show that infection with Plasmodium berghei in C57BL/6mice induced IL-10production. Increased number of IL-10-producing regulatory B cells was found in spleen cells of malaria infected mice. Transfer of IL-10-producing regulatory B cells to malaria infected mice can inhibit the pathological changes of brain blood vessels and significantly reduced the mortality of mice infected with malaria. It was also observed that transfer of regulatory B cells inhibit lymphocyte migration to the brain, but had no significant effect on the levels of parasitemia. The protective effect of regulatory B cells was abrogated when IL-10signal was blocked by anti-IL-10receptor antibody, indicating that the inhibition of cerebral malaria by regulatory B cells is mediated by IL-10. Interestingly, transfer the same number of CD4+CD25+regulatory T cells did not inhibit the cerebral malaria. This result shows that the IL-10-producing regulatory B cells, but not regulatory T cells, are critical in prevention of immunopathology of brain tissue during malaria infection. The results of this study reveal a novel mechanism of immunoregulation of cerebral malaria, and provide important information for development of prevention and intervention strategies to control cerebral malaria.2. Efficacy of live parasite vaccination strategy against malariaVaccine has been considered the most efficient means to control malaria. Great efforts have been made in last half century to develop malaria vaccine and various strategies have been proposed and tested. Unfortunately, no successful anti-malarial vaccine is available form clinical use. Subunit vaccines have been expected to be an efficient vaccine, but many trials with this type of vaccine in last few decades failed, probably due to the lack of "danger signal" which is needed for full activation of immune system and induction of strong protection. Irradiated or genetically attenuated sporozoites have been shown to be able to induce complete protection against malaria, but difficulty in mass production of attenuated parasites prevents its use as vaccine. Here, I analyzed the characteristics of immune response and the levels of protection induced by two vaccination strategy:short-term live parasite infection with drug termination and whole malaria parasite antigen in adjuvant. The aim of this study is to characterize the property of immune response induced by two means of vaccination and identify the elements of immune response that are associated with optimum immune protection. The experimental results show that the short-term live worm infections plus drug termination in BALB/C mice induced high levels of immune protection against malaria challenge, while vaccination with parasite whole antigen in adjuvant induced partial protection. Further analysis of immune responses in the two groups of vaccinated animals revealed that, in comparison with the animals vaccinated with parasite antigen plus adjuvant, short-term live parasite infection induced significantly higher levels of malaria specific antibody, higher levels of activation of dendritic cells and stronger CD4+T cell response. The Thl, Th2and regulatory cytokines responses were significantly higher in mice vaccinated by short-term live parasite infection than the mice vaccinated with parasite antigen plus adjuvant. Further study will be carried out to determine the functional association between the immune response components and the immune protection. The information obtained in this study will be helpful for rationally designing malaria vaccine and vaccination strategy.