Immunogenicity of antigens from the TbD1 region present in M. africanum and missing from "modern" M. tuberculosis: a cross- sectional study

We assessed the immunogenicity of the TbD1 region, present in M. africanum and absent from "modern" M. tuberculosis, in an ELISPOT assay using cells from confirmed M. africanum or M. tuberculosis infected TB patients without HIV infection in the Gambia.Antigens from the TbD1 region induced IFNγ responses in only 35% patients and did not discriminate between patients infected with M. africanum vs. M. tuberculosis, while PPD induced universally high responses.Further studies will need to assess other antigens unique to M. africanum that may induce discriminatory immune responses.An estimated third of the world's population is latently (asymptomatically) infected with M. tuberculosis, of whom 5-10% will progress to TB disease in their lifetime, with higher rates of progression in immune compromised people [1]. Latent infection can be diagnosed with the tuberculin skin test, or more recently with interferon gamma release assays using M. tuberculosis specific antigens ESAT-6 and CFP-10 [2]. These antigens are present in members of the M. tuberculosis complex, except for the vaccine strain M. bovis BCG [3], M. microti and the Dassie bacillus.Recent data suggest that M. africanum has a relatively large reservoir of latent infection that supports the 38% prevalence of active disease despite a lower rate of progression.Better characterization of the immune responses leading either to containment or progression of latent TB infection to disease by M. africanum or M. tuberculosis requires diagnosis of these organisms at the latent stage of infection. During latent TB infection, the bacterial load is estimated to be low, and the location of the persistent bacteria is unknown. Molecular genotyping methods rely on the isolation of bacterial DNA, which is currently not possible from patients with latent TB infection. Nevertheless, the persistent bacteria induce an immune response that can be assessed in the periphery [1].The TbD1 region is present in M. africanum but absent in "mo