Investigating the Early Immune Response to Mycobacterial Infection in Mice

Abstract

Mycobacterium tuberculosis is an air-borne pathogen that causes more death each year than any other bacterial infection. The current vaccine, Bacillus Calmette-Guerin (BCG), is considered one of the most widely distributed in the world, yet the incidence of tuberculosis (TB) remains a huge problem. In order to improve TB vaccines, a greater understanding of the immune response to mycobacteria is required. Often, the innate immune response is able to reduce the mycobacterial burden in the lungs but not completely clear the invading organism. Dendritic cell (DC) subsets in the murine lung have been investigated during late mycobacterial infection; however, the early immune response to mycobacteria is an under-investigated field. This thesis describes a flow cytometry gating strategy to identify the main DC subsets in the lungs of mice. These subsets were examined for signs of activation following intra-nasal infection of mice with BCG. It was found that only CD11b+ myeloid DC exhibited upregulation of any co-stimulatory molecules. A fluorescent strain of BCG was used to identify the primary cell targets of mycobacteria during the first week of infection. These targets were found to be lung macrophages and CD11b+ myeloid DC. A sustantial decrease in bacterial load in the lungs was also observed over this time period which could be explained by early innate killing of mycobacteria. Finally, preliminary tests with fluorescent mycobacteria suggests that a positive signal may be detected through a thin layer of murine tissue in vivo. Overall, this thesis highlights the need for further study into the early immune response to mycobacteria and provides preliminary examination of the dynamics of lung DC subsets in this context. These findings contribute to a greater understanding of the immune system and its initial response to mycobacteria.