Characterisation of immune mechanisms underlying the natural history of tuberculosis infection. (360G-Wellcome-064353_Z_01_A)

Tuberculosis (TB) is an enormous and increasing public health burden. Basic and translational research have potential to help control this epidemic, particularly in two key areas: improved diagnosis and an improved vaccine. Recent genomic advances have identified the RD1 genomic segment which is present in M. tuberculosis but absent from BCG.RD1 gene products thus present the opportunity to develop more specific tests for diagnosing MTB infection. Using a highly sensitive, yet rapid, ex vivo interferon-? (IFN-?) enzyme-linked immunospot (ELISPOT) assay, I enumerated T cells specific for ESAT-6, the first RD1 gene product to be identified, in TB patients, contacts and controls. These proof-of-principle studies have shown that this rapid T cell-based assay detects active and latent MTB infection with high sensitivity and specificity, and has the potential to improve tuberculosis control. I now propose to rigorously validate the assay (and incorporate a second RD1 gene product, CFP10) in the relevant target populations where it could be of major benefit. I will validate its clinical utility for the rapid diagnosis of active TB in a low prevalence population and for the diagnosis of TB meningitis in a high prevalence population. I will also apply this assay to investigate the basic parameters and pathways of MTB transmission and the natural history of TB in distinct epidemiologic settings, including populations with a high prevalence of both TB and HIV. Rational development and clinical evaluation of new TB vaccines requires an understanding of correlates of protective immunity. The host response largely determines the outcome of MTB infection, with long-term immunological control of latent bacilli being the best clinical outcome. My cross-sectional studies of TB patients and contacts suggest that ESAT-6 specific T cells may correlate with clinically-defined protective immunity. I am to follow through this observation with longitudinal studies of T cells specific for ESAT-6 (and other RD1 gene products) in recently exposed healthy contacts, using multiple techniques of ex vivo analysis. I will also study the kinetics of these MTB-specific T cell populations in HIV-positive individuals with latent MTB infection during anti-retroviral therapy, to see if they correlate with immune reconstitution. Simultaneously, I will investigate the effects of these CD4 and CD8 T cells on intracellular MTB in vitro. These studies will shed light on the phenotype, function and antigen-specificity of T cells associated with immunological control of MTB in vivo, and should thus assist the development and evaluation of new TB vaccines.