Bovine tuberculosis: Effect of the tuberculin skin test on in vitro interferon gamma responses

Bovine tuberculosis (bTB) is a disease of zoonotic and economic importance. In many countries, control is based on test and slaughter policies and/or abattoir surveillance. For testing, cell mediated immune- (CMI-) based assays (i.e., tuberculin skin test (TST) supplemented by the interferon gamma (IFN-γ) assay) are the primary surveillance and disease control tests for bTB. The combined use of the in vivo and in vitro CMI assays to increase overall sensitivity has raised the question of whether the IFN-γ response is influenced by injection of purified protein derivatives (PPDs) for TST. Published data on the influence of the TST, applied as the caudal fold test (CFT) or the comparative cervical test (CCT), on the IFN-γ assay are contradictory. Reviewing published data and including additional data, the following conclusions can be drawn: (1) in naturally infected cattle, PPD administration for the single or repeated short-interval CCT neither boosts nor depresses PPD-specific IFN-γ production. Disparate results have been concluded from some studies using experimental infections, emphasizing the importance of confirming initial experimental-based findings with studies using cattle naturally infected with Mycobacterium bovis. (2) In cattle experimentally infected with M. bovis, PPD administration for CFT boosts PPD-specific IFN-γ production for up to 7 days without any effect on test interpretation. Importantly, in naturally infected cattle, CFT-related boosting selectively increases the in vitro M. bovis PPD (PPD-B) response 3 days after CFT, resulting in an increased PPD-B response relative to the response to Mycobacterium avium PPD (PPD-A). In non-infected cattle, it cannot be excluded that the CFT induces a mild boost of the PPD-specific response, particularly in animals sensitized to environmental, non-tuberculous mycobacteria, thus decreasing the specificity of the IFN-γ assay. (3) In general, there is a lack of data clearly characterizing the effect of TSTs on the IFN-γ assay. Further studies are required to clearly describe the effects of both CFT and CCT in non-infected animals and in naturally infected cattle, especially in low reacting infected cattle.