By: Jyothi Rengarajan, PhD
In 1882, Robert Koch identified the “tubercle bacillus” M. tuberculosis as the causative agent of TB; a discovery that revolutionized medical science and paved the way for improved sanitation and public health measures that were critical in reducing the burden of TB and other infectious diseases in Europe and North America. Interestingly, rest, sunlight and good nutrition in sanitoria helped many patients recover from TB, even before the advent of antibiotics. To this day, we still do not understand why some people progress to full-blown, uncontrolled active TB disease after infection, while others contain and limit disease pathology, while still others control infection in an asymptomatic state that we call latent TB infection, or LTBI. These are questions that animate my own research interests. Understanding the molecular mechanisms that define how M. tuberculosis evades the immune system to cause disease as well as understanding how humans control TB are active areas of research in my laboratory at the Emory Vaccine Center. We hope that our studies will provide fundamental insights into how this pathogen interacts with immune cells and lead to identifying new targets that could be developed into better vaccines and therapies for TB.
Overall, TB remains a disease that is intertwined with poverty and thrives in the overcrowded settings that characterize increasing urbanization around the globe. While the majority of TB cases still occur in resource-limited countries, TB disproportionately affects vulnerable and marginalized populations, including in the U.S., where the Centers for Disease Control (CDC), reported 9,582 new TB cases in 2013. In the metro-Atlanta area, a TB outbreak during the past year has affected several homeless shelters and the state of Georgia reported x TB cases in 2013.
During outbreaks such as this one, as well as for routine diagnosis in parts of the world with high burdens of TB, identifying cases of active TB and initiating treatment is the cornerstone of TB control and public health efforts. Currently, diagnosis of pulmonary TB relies on extensive evaluations of clinical symptoms, X-ray assessments and direct detection of mycobacteria in a patient’s sputum, which is essentially mucus that is coughed up from the lower airways. The most widely used sputum-based test involves direct microscopic detection of bacteria in sputum smears, but the test is poorly sensitive and a high proportion of TB cases are smear-negative. Nucleic acid amplification-based tests are more sensitive for diagnosing TB disease but do not differentiate between live and dead bacteria and therefore are not useful for monitoring clearance of bacteria during and after treatment. Culturing M. tuberculosis from sputum is currently the gold standard for diagnosing TB and for monitoring treatment response, but takes three to six weeks for results due to the slow growth of M. tuberculosis. Sputum samples are also difficult to obtain, particularly in children, elderly and weakened/bed-ridden patients and therefore, blood-based tests are attractive alternatives to sputum-based tests. However, no blood-based tests for diagnosing active TB are currently available.
We enrolled individuals in the metro Atlanta area who had asymptomatic LTBI and treatment-naive patients who were diagnosed with active TB. We then followed these TB patients during and after their 6-month anti-TB treatment regimens. We were excited to find that the frequencies of M. tuberculosis-specific T cells that expressed immune markers CD38, HLA-DR and Ki-67 accurately identified active TB patients with 100% specificity and greater than 96% sensitivity. Importantly, we were able to validate the ability of these biomarkers to accurately classify active TB and LTBI in an independent cohort from South Africa, in collaboration with our colleague Dr. Cheryl Day. These markers also distinguished individuals with untreated TB from those who had successfully completed anti-TB treatment and correlated with decreasing bacterial loads during treatment. In fact the decrease in expression of our biomarkers during treatment mirrored sputum conversion in all patients. These findings show that blood-based biomarkers have the potential to accurately diagnose TB and discriminate between active and latent TB. Blood-based biomarkers will be particularly useful in situations where sputum-based diagnosis of TB is more difficult. Because these biomarkers provide a gauge of M. tuberculosis load within individuals, they could also have utility as surrogate markers of treatment response and as predictors of treatment efficacy, cure and relapse in patients undergoing treatment for drug-susceptible as well as drug-resistant TB. We are now interested in evaluating these biomarkers in larger studies in TB-endemic areas and across a broader spectrum of M. tuberculosis infection, including extra-pulmonary TB and in HIV-infected populations.