The immunologic mechanisms determining the progression of paratuberculosis from subclinical to clinical disease have not been finally determined. Induction of immunological T cell unresponsiveness has been suggested to play a role in progression of paratuberculosis. Here we focus on the T-cell immune responses in the different stages of the infection, with special focus on the possible loss of T cell activity caused by antigen-induced apoptosis.Cattle from two paratuberculosis infected herds were categorized into 3 progressive disease stages according to their cell mediated and humoral immune responses. Stages were defined as stage 0 (IFN-γ ÷, Ab ELISA ÷), stage 1 (IFN-γ +, Ab ELISA ÷) and stage 2 (IFN-γ +, Ab ELISA +). Whole blood samples were stimulated in vitro with a positive control (superantigen), M. paratuberculosis PPD (PPDj) and with PBS as a negative control. PBMC were purified and tested for induction of antigen-induced proliferation and apoptosis. By flow cytometry, a progressive antigen specific induced apoptosis (caspase-3 induction) was demonstrated in paratuberculosis infected animals in the different subclinical disease-stages but not in uninfected. Further studies showed that this antigen induced apoptosis were transferable, i.e. supernatant from apoptosis induced cultures could induce apoptosis in cultures with ParaTB naïve animals. Subsequent RT-PCR studies on an array of apoptosis genes show that Caspase 8 and FasL are upregulated in the apoptosis indicating apoptosis by Activation Induced Cell Death (AICD).

Conclusions

Altogether these results indicate that the development of clinical paratuberculosis involves progressive loss of immune responsiveness and that T-cell death by AICD may represent one of the important components leading to an ineffective immune response against M. paratuberculosis. Furthermore, it can be speculated that a yet unknown apoptosis factor in plasma may be a future diagnostic tool to predict the transition from appropriate cell mediated immune responses to inappropriate humoral responses.