Abstract
The cross-reactivity of mycobacterial antigens in immune-based diagnostic assays has been a major concern and a criticism of the current tests that are used for the detection of paratuberculosis. In the present study, Mycobacterium avium subsp. paratuberculosis recombinant proteins were evaluated for antigenic specificity compared to a whole-cell sonicate preparation (MPS). Measures of cell-mediated immunity to M. avium subsp. paratuberculosis antigens were compared in calves inoculated with live M. avium subsp. paratuberculosis, M. avium subsp. avium (M. avium), Mycobacterium kansasii, or Mycobacterium bovis. Gamma interferon (IFN-γ) responses to MPS were observed in all calves that were exposed to mycobacteria compared to control calves at 4 months postinfection. Pooled recombinant M. avium subsp. paratuberculosis proteins also elicited nonspecific IFN-γ responses in inoculated calves, with the exception of calves infected with M. bovis. M. avium subsp. paratuberculosis proteins failed to elicit antigen-specific responses for the majority of immune measures; however, the expression of CD25 and CD26 was upregulated on CD4, CD8, gamma/delta (γδ) T, and B cells for the calves that were inoculated with either M. avium subsp. paratuberculosis or M. avium after antigen stimulation of the cells. Stimulation with MPS also resulted in the increased expression of CD26 on CD45RO(+) CD25(+) T cells from calves inoculated with M. avium subsp. paratuberculosis and M. avium. Although recombinant proteins failed to elicit specific responses for the calves inoculated with M. avium subsp. paratuberculosis, the differences in immune responses to M. avium subsp. paratuberculosis antigens were dependent upon mycobacterial exposure. The results demonstrated a close alignment in immune responses between calves inoculated with M. avium subsp. paratuberculosis and those inoculated with M. avium that were somewhat disparate from the responses in calves infected with M. bovis, suggesting that the biology of mycobacterial infection plays an important role in diagnosis.