The effect of tuberculin testing on the development of cell-mediated immune responses during Mycobacterium bovis infection

Protection against tuberculosis (TB) is associated with Th1-type cell-mediated immunity (CMI). Whilst the intradermal injection of partially purified derivatives of tuberculin (PPD) represents the classic test assessing the delayed type hypersensitivity (DTH) response used in both humans and cattle for diagnosing TB, it has been suggested that the test may modulate host CMI responses. To investigate the kinetics of the development of the DTH response and its subsequent effect on CMI responses, groups of 6-month old calves were inoculated intranasally with 8 × 104 cfu of Mycobacterium bovis, subjected to the comparative intradermal tuberculin test (TT) using bovine and avian PPD (PPD-B, PPD-A) at various time intervals post-infection, and immune responses compared. These included DTH, lymphocyte proliferation, IgG production, and synthesis of the cytokines: IFN?, IL-10, IL-4, IL-6, and IL-13. All animals were subjected to post-mortem examination. The kinetics of the development of the DTH response assessed in the TT was such that infected cattle could be identified as early as 3 weeks post-infection, which correlated with the detection of an antigen-specific IFN? response. Transient increases in plasma-derived IFN? as a result of TT during an established TB infection were more pronounced when blood was stimulated with PPD-A compared with PPD-B stimulation. This has the potential to mask diagnosis of infection as a result of the stronger avian-bias if the IFN? test is used the week following TT. Disease pathology was not affected by TT. A transient failure to a second TT was observed in 1 of 30 animals and the time (post-infection) at which the TT is administered may be of significance. In serum, IgG responses to PPD-B, which were undetectable prior to TT, were elevated after TT and were most pronounced in cattle that were TT at 6 weeks post-infection. Other cytokines were also affected by the TT; IL-4 mRNA levels increased and IL-6 mRNA levels decreased, whilst PPD-B specific IL-10 protein synthesis was enhanced. These observations may offer the potential for further diagnostic assays that could complement the TT and IFN? test.