Novel antigens used to detect cell-mediated immune responses over time in Mycobacterium avium subsp. paratuberculosis infected cattle

Early stage Mycobacterium avium subsp. paratuberculosis (MAP) infection of cattle can be detected by measuring specific cell mediated immune responses, using the interferon gamma (IFN-γ) test. Available IFN-γ tests are using purified protein derivatives of MAP (PPDj) which are crude products consisting of undefined antigens with possible cross reactions toward other environmental bacteria. The objective of the study was to optimize the IFN-γ test using different types of novel antigens for stimulation. Fourteen novel antigen candidates were selected for testing, including 4 peptides of the ESAT-6 family and 10 hypothetical proteins: 4 latency proteins, 3 secreted proteins, 2 proteins not present in Mycobacterium avium subsp. avium (MAA) and 1 from an immunological hot spot region. To determine variation of IFN-γ responses, three repeated tests was done with 4 and 5 week intervals on the same 30 heifers from a known MAP infected herd. Determination of cut-off for each antigen was based on samples from a non-infected herd, including 60 heifers. Based on PPDj stimulations, more than 50% of the heifers tested MAP positive at the first two samplings, whereas only 20% tested positive at third sampling. The resulted showed that PPDj detect a high percentage as MAP positive animals, as this crude antigen mixture is expected to induce non-specific IFN-γ production. However, the tested latency antigens, some secreted proteins and some peptides of the ESAT-6 family detected a comparable high percentage of animals as MAP positives. By combining novel antigens higher specificity might be obtained.