Accurate diagnosis of tuberculosis in cattle may be compromised in areas where there are high rates of exposure to environmental/non-tuberculous mycobacteria (NTM). This cross reaction of immune responses to Mycobacterium bovis antigens shared with NTMs can result in reduced specificity of commonly used diagnostic tests including tuberculin skin tests and the interferon gamma assay (IFN-ɣ). In this study we assessed the cross-reactive immune responses of M. bovis (infected) and NTM exposed animals to M. bovis and M. avium tuberculin, the ESAT6/CFP10 cocktail antigen, tuberculin derived from cultures of selected NTMs, and a panel of recombinant mycobacterium tuberculosis complex (MTBC) antigens sharing homology with orthologues in NTM. Gamma interferon (IFN-ɣ) responses were measured in whole blood cultures using the IFN-ɣ assay and the IFN-ɣ elispot assay on purified peripheral blood mononuclear cells (PBMC).
We observed the expected strong IFN-ɣ response to PPD-B in the M. bovis infected animals that distinguished this group from non-infected NTM exposed cattle. The IFN-ɣ responses to PPD-N (M. nonchromogenicum), were relatively high in both infected and non-infected NTM exposed cattle, but were not significantly different to classify the true infection status of each group. The results indicated that the cross-reactive responses to PPD-B and/or PPD-A with PPD-N, likely arose from prior exposure to environmental non-tuberculous mycobacteria. The IFN-ɣ immune responses to the 10 R-Mag measured by the IFN-ɣ elispot assay revealed that three of the selected antigens, Rv3615 (ESpC), Rv0287 (esxG) and the ESAT6/CFP10, were immunogenic in the infected cattle, and distinguished the infected cattle from the non-infected NTM exposed animals. The combined data of PPDs and R-Mags derived from NTM mycobacteria may prove useful in future development of novel bTB diagnostic tests.