| Abstract: | Increasing knowledge about DosR regulon-encoded proteins has led us to produce novel Mycobacterium tuberculosis antigens for immunogenicity testing in human populations in three countries in Africa to which tuberculosis (TB) is endemic. A total of 131 tuberculin skin test-positive and/or ESAT-6/CFP10-positive, human immunodeficiency virus-negative adult household contacts of active pulmonary TB cases from South Africa (n = 56), The Gambia (n = 26), and Uganda (n = 49) were tested for gamma interferon responses to 7 classical and 51 DosR regulon-encoded M. tuberculosis recombinant protein antigens. ESAT-6/CFP10 fusion protein evoked responses in >75% of study participants in all three countries. Of the DosR regulon-encoded antigens tested, Rv1733c was the most commonly recognized by participants from both South Africa and Uganda and the third most commonly recognized antigen in The Gambia. The four most frequently recognized DosR regulon-encoded antigens in Uganda (Rv1733c, Rv0081, Rv1735c, and Rv1737c) included the three most immunogenic antigens in South Africa. In contrast, Rv3131 induced the highest percentage of responders in Gambian contacts (38%), compared to only 3.4% of Ugandan contacts and no South African contacts. Appreciable percentages of TB contacts with a high likelihood of latent M. tuberculosis infection responded to several novel DosR regulon-encoded M. tuberculosis proteins. In addition to significant similarities in antigen recognition profiles between the three African population groups, there were also disparities, which may stem from genetic differences between both pathogen and host populations. Our findings have implications for the selection of potential TB vaccine candidates and for determining biosignatures of latent M. tuberculosis infection, active TB disease, and protective immunity.Tuberculosis (TB) remains an ongoing health crisis of global dimensions. The African Region has the highest incidence rate per capita (363 per 100,000 population) and includes 10 of the 22 most high-burden countries in the world (38). It has been estimated that one-third of the world''s population is latently infected with Mycobacterium tuberculosis. Human immunodeficiency virus type 1 (HIV-1)-infected individuals have a risk of about 5 to 10% per year of progression from latent infection to active TB (4), compared to 2 to 23% in a lifetime for HIV-1-seronegative individuals (24). The only currently licensed vaccine against TB is Mycobacterium bovis bacillus Calmette-Guérin (BCG), which has highly variable efficacy against adult pulmonary TB (6). The use of BCG in HIV-1-infected or -exposed infants may be contraindicated (11). The investigation of safe and effective TB vaccines is thus highly prioritized.The discovery of the precise mechanisms underlying protective anti-TB immunity calls for the identification of new biomarkers (17). A clearer understanding of which M. tuberculosis antigens evoke effective immune responses and how they are associated with protection or disease is required. Promising antigens that have been identified as immunodominant include the alpha-crystallin homologue (also known as the M. tuberculosis 16-kDa protein; Rv2031c, HspX) (9), alpha-cystallin 2 (Acr2; Rv0251) (36), Ag85A (Rv3804) (33), Hsp65 (Rv0440) (23), ESAT-6 (Rv3875) (5), and CFP10 (Rv3874) (32), some of which are currently being tested as potential TB vaccine candidates (28). The search for novel protective antigen(s) has been facilitated by expression profiling of M. tuberculosis laboratory strains cultured under conditions of hypoxia and nitric oxide stress, which are thought to resemble conditions that mycobacteria encounter in situ during latent infection (31). Voskuil et al. (35) showed that hypoxia and low concentrations of nitric oxide induced expression of a 48-gene dormancy survival regulon (DosR) believed to be associated with latency. A selection of these proteins has been tested for immunogenicity in relevant mouse models and has established the importance of the regulon in latent infection (26, 29, 34). In addition, human studies of latently M. tuberculosis-infected healthy adults living in areas where tuberculosis is not endemic have shown T-cell responses to selected DosR regulon-encoded antigens, suggesting a role in maintenance of the asymptomatic phase of latent infection (18, 30). In order to gain better insight into protection against TB and to expand our current understanding about proteins encoded by the DosR regulon that are targeted by the human immune system, we have tested 51 antigens, spanning the entire 48 genes of the DosR regulon (35), in geographically diverse human populations from three countries in Africa to which TB is endemic. Twenty-five of the antigens studied here have been tested previously (18); however, we report results with an additional 26 new antigens. The immunogenicity of the entire set of DosR regulon-encoded protein antigens in a high-TB-burden African context is described for the first time. |
