Immunization strategies against pulmonary tuberculosis: considerations of T cell geography

Pulmonary tuberculosis (TB) remains a global health concern with an astounding 9 million new cases and 2 million deaths per year. This leading infectious cause of death remains highly prevalent with one third of the world's population latently infected with Mycobacterium tuberculosis (M.tb) despite routine vaccination against TB in endemic areas. The only approved TB vaccine is the Bacille Calmette-Guerin (BCG), which provides protection against childhood miliary tuberculosis and has been administered intradermally in humans for almost a century. While effective in preventing disseminated forms of TB, the BCG has variable efficacy in providing protection against pulmonary TB. Therefore, the BCG has been unable to control the instance of adult pulmonary TB which constitutes the global disease burden. Despite the fact that mechanisms underlying the lack of pulmonary protection provided by the BCG remain poorly understood, it remains the "Gold Standard" for vaccine-mediated protection against M.tb and will continue to be used for the foreseeable future. Therefore, continued effort has been placed on understanding the mechanisms behind the failure of BCG to provide sufficient protection against M.tb in the lung and to design new vaccines to be used in conjunction with the BCG as boost strategies to install protective immunity at the site of infection. Growing evidence supports that the route of immunization dictates the geographical location of TB-reactive T cells, and it is this distribution which predicts the protective outcome of such vaccine-elicited immunity. Such vaccines that are able to localize TB-reactive T cells to the lung and airway mucosa are thought to fill the "immunological gap" in the lung that is required for enhanced protection against M.tb infection. This chapter focuses on the critical importance of T cell geography when designing new immunization strategies against pulmonary TB.