Regions of differences encoded antigens as targets for immunodiagnosis of tuberculosis in humans

Tuberculosis is one of the major global health problems causing nearly 2 million deaths every year. It continues to be a leading cause of morbidity and mortality in developing countries. Accurate early diagnosis and proper treatment can control the spread of tuberculosis in the community. Currently used diagnostic tests have certain limitations such as low sensitivity and suboptimal turn-around times. Hence, introduction of diagnostic methods that are comparatively more sensitive and specific can increase the efficiency of strategies to control tuberculosis. In recent years, there has been a remarkable progress in identifying new and potentially useful antigens for diagnosis of both latent and active tuberculosis. Regions of differences (RD) encoded proteins are among such promising candidate antigens (RD antigens). Some of these antigens are encoded by regions of differences located in the genome of Mycobacterium tuberculosis, M. africanum, M. bovis but are absent in all the Bacillus Calmette Guerin substrains and many of the environmental mycobacteria. Over the past few years, RD antigens, particularly RD-based diagnostic methods such as improved tuberculin skin testing, interferon-gamma release assays, and RD1-based serological assays are being tested and have shown promising results. This article provides an overview of the use of RD antigens in the immunodiagnosis of tuberculosis infection and disease.