Abstract
Infection with Mycobacterium tuberculosis (Mtb), the pathogen responsible for tuberculosis (TB), results in 1.4 million deaths annually. The escalating rate of active TB, due to co-infection with HIV and emergence of antibiotic resistant Mtb strains, makes prevention of TB through the development of an effective vaccine a global health priority. The vaccine currently in use, Bacille Calmette Guérin (BCG), fails to provide reliable protection against pulmonary TB. The development of a much needed, more effective vaccine is hindered by an incomplete understanding of the memory immune response that protects against TB.
The aim of this thesis was to characterize the CD4+ memory T cell response that mediates immunity Mtb. To identify this protective response, published adoptive transfer studies had been performed using immunodeficient recipient mice and they suggested that CD62Lhi memory CD4+ T cells from BCG-vaccinated mice were key for immunity to TB. The experiments performed in this thesis demonstrate that this model is flawed; the transfer CD62Lhi CD4+ T cells from naïve or BCG-vaccinated donors into immunodeficient mice induced severe colitis in recipients and activated cells could be detected in the lungs, even in the absence of infection. The induction of colitis induced following transfer of CD62Lhi CD4+ T cells is likely to influence protection against mycobacteria and therefore the adoptive transfer of T cells into immunodeficient mice is an inadequate model to study CD4+ memory T cell-mediated immunity to TB.
A novel adoptive transfer model was established to determine the ability of CD4+ T cell subsets to protect against TB. It was hypothesized that the transfer of CD4+ T cell subsets into CD4+ T cell receptor (TCR) transgenic mice specific for an irrelevant antigen would avoid the competition observed following transfer into immunocompetent mice, permitting adoptively transferred CD4+ T cell meditated protection against TB to be observed without the induction of colitis. Adoptive transfer of BCG-experienced T cells into CD4+ TCR transgenic mice revealed that CD62Llo, and not CD62Lhi CD4+ T cells, mediated protection against lung mycobacteria, challenging the findings obtained using immunodeficient recipients. Importantly, transferred CD4+ T cells did not alter their phenotype or induce colitis in the recipient animals in the absence of mycobacterial challenge, demonstrating the adequacy of this model to evaluate CD4+ T cell elicited protection against TB.
Since effector CD62Llo CD4+ T cells could mediate immunity to mycobacteria, whether this protective subset could be generated in the lung using traditional subcutaneous (s.c) BCG vaccination and whether the protective response could be improved by mucosal BCG administration, was investigated. Comparing the immune response induced by s.c. BCG and orally delivered lipid-formulated BCG, we determined that although both administration routes were able to induce robust CD4+ effector T cell responses shortly following vaccination, superior long-term effector responses were observed in mice that received oral BCG. These results demonstrate that oral administration of BCG improves the protective immune response in the lung compared to s.c. vaccination, which could provide enhanced immunity to pulmonary TB.
The work in this thesis significantly contributes to our understanding of the CD4+ memory T cell response that protects against TB, demonstrates a means by which immunity in the lung can be improved and establishes novel models in which the ability of CD4+ T cells to protect against TB can be assessed.