Mycobacterium tuberculosis ( Mtb ) isocitrate dehydrogenases show strong B cell response and distinguish vaccinated controls from TB patients

Abstract

Proteins released from Mycobacterium tuberculosis (Mtb) during late logarithmic growth phase are often considered candidate components of immunogenic or autolysis markers. One such protein is isocitrate dehydrogenase (ICD), a key regulatory enzyme in the citric acid cycle. We have evaluated the immunogenic properties of two isoforms of Mtb ICD and compared them with the control antigens heat-shock protein 60 and purified protein derivative (PPD). PPD lacks the sensitivity to distinguish between bacillus Calmette-Gué rin (BCG)-vaccinated and tuberculosis (TB)-infected populations, and, therefore, epidemiological relevance of PPD in BCG-vaccinated regions is debatable. We show that Mtb ICDs elicit a strong B cell response in TB-infected populations and can differentiate between healthy BCG-vaccinated populations and those with TB. The study population (n ‫؍‬ 215) was categorized into different groups, namely, patients with fresh infection (n ‫؍‬ 42), relapsed TB cases (n ‫؍‬ 32), patients with extrapulmonary TB (n ‫؍‬ 35), clinically healthy donors (n ‫؍‬ 44), nontuberculous mycobacteria patients (n ‫؍‬ 30), and non-TB patients (culture negative for acid-fast bacteria but carrying other infections, n ‫؍‬ 32). The Mtb ICDs showed statistically significant antigenic distinction between healthy BCG-vaccinated controls and TB patients (P < 0.0001) and those with other infections. Although extrapulmonary infections could not be discriminated from healthy controls by heat-shock protein 60 (P ‫؍‬ 0.2177), interestingly, the Mtb ICDs could significantly (P < 0.0001) do so. Our results highlight the immunodominant, immunosensitive, and immunospecific nature of Mtb ICDs and point to an unusual property of this tricarboxylic acid energy cycle enzyme. T uberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a major threat to the human population, being responsible for Ϸ2-3 millions deaths every year worldwide (1-3). The secret of the pathogen's success is its ability to escape the host immune system and remain undetected in lungs for decades. In only 10% of infected people, the number being higher in immunocompromised patients, does TB erupt as a full-blown disease (4). Delay in diagnosis and treatment impedes the downstream management and control of the disease. With the increasing emergence of multidrug resistant strains and coinfection with HIV, the problem is further compounded (5-7). Early diagnosis, therefore, is a matter of utmost concern not just for TB disease management but also for epidemiological investigations (8). Current diagnostic tools for TB often lack sensitivity and can be time consuming. TB diagnosis in developing countries largely banks on tuberculin skin tests and staining and culture methods. The epidemiological relevance of the tuberculin test with purified protein derivative (PPD) is questionable in areas where bacillus Calmette-Guérin vaccination is compulsory because PPD is not sensitive enough to distinguish between vaccinated and infected individuals (9). Microscopic determination ...

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