To our knowledge, this study is the first attempt at uncovering TB sub-species specific T cell responses in the natural human host. Our findings suggest that the antigens prepared from TbD1 do not induce a robust T cell response in humans. Among the responders, there was moreover no difference in TbD1 recognition between the proportion of patients infected with "modern" M. tuberculosis, which lacks the TbD1 region, and the proportion infected with M. africanum, with an intact TbD1 region. Lastly, we identified a trend towards attenuated immunogenicity of ESAT-6 and CFP-10 in M. africanum infected TB cases, on which we reported previously .
The low proportion of responders may be due to poor host recognition of these antigens, or due to the fact that the host response includes cytokines other than IFNγ or B cell rather than T cell responses. Indeed, mmpL6 is a putative transmembrane transport protein  and may induce a B cell response against its extracellular tail.
Although we did not identify instances of dual infection from the cultured sputum samples, the presence of dual infection could explain the inability of the TbD1 antigens to discriminate between those diseased with M. africanum from M. tuberculosis. Undetected dual infection in TB patients could occur when one organism was maintained in latency while the other caused TB disease, or when both organisms partook in active TB disease, but one organism outgrew the other in culture. The latter can be addressed by genotyping the isolates with species-specific primers using DNA extracted from sputum samples.
The M. africanum whole genome sequencing project is near completion, which will facilitate the search for other discriminatory antigens unique to M. africanum. The TbD1 region, shared by M. africanum and M. bovis, is the only region identified in the M. bovis genome that is absent from M. tuberculosis, so our findings may also inform veterinary research directed at distinguishing M. bovis - vs "modern" M. tuberculosis specific responses in latently infected cows.