Improving diagnostic tests for tuberculosis: mycobacterial antigen-stimulated cytokine biomarkers for diagnosis and monitoring therapy

© 2017 Dr Vanessa Olivia CliffordTuberculosis (TB) is the leading cause of infectious disease mortality worldwide. Accurate and timely diagnosis of both active TB and latent TB infection (LTBI) is essential to controlling the TB pandemic. Current diagnostic tests for TB have several limitations, including poor performance in children, in the immunocompromised and in patients with extra-pulmonary TB. At present, commercial immunodiagnostic tests for TB do not distinguish between active TB and LTBI. In addition, there is no established biomarker that functions as a ‘test of cure’ for TB. Such a biomarker would allow treatment to be tailored to individual response, potentially decreasing treatment duration and reducing drug toxicity. My PhD research project focussed on evaluating candidate mycobacterial antigen-specific cytokine biomarkers to improve the performance of immunodiagnostic tests for TB in children and adults. The first section of this thesis focuses on cytokines to diagnose and distinguish between active TB and LTBI. It includes a systematic review of cytokine biomarkers that have been previously evaluated for their ability to distinguish between active TB and LTBI. Two separate research studies, one in a cohort of children, and one in adults, are then described. The first was a follow-up study in a new paediatric cohort to validate the role of several candidate biomarkers previously identified by our research group that distinguish between active TB and LTBI. Following on from this study in children, I led a larger study to evaluate the role of these cytokine biomarkers in the diagnosis of active TB and LTBI in adults, and to determine their specificity for TB infection. This study confirmed the findings in children that mycobacterial antigen-stimulated IL 1ra, IL-10 and TNF- responses are significantly higher in individuals with active TB than LTBI, although there was considerable overlap in responses between the two groups. The second section of this thesis focuses on cytokines for monitoring anti-tuberculous therapy. It includes a published systematic review of cytokines that have previously been evaluated for this purpose. This is followed by a published study in an adult patient cohort which investigated whether mycobacterial antigen-specific cytokine responses change during treatment for TB. It was found that mycobacterial antigen-specific IL-1ra responses decreased during therapy in both active TB and LTBI. An additional ex vivo research study, also published, confirmed that changes in cytokine response observed during therapy were unlikely to be due to any immunomodulatory effects of anti-tuberculous treatment. IL-1ra was therefore identified as a candidate ‘test of cure’ biomarker that should be investigated in future longitudinal studies