Protection against mycobacterial disease relies on T-cell dependent immune responses. In Johne's disease, cell-mediated immune responses predominate in subclinical disease. As disease progresses these responses diminish and can be undetectable while there is a concurrent strong humoral response, demonstrated by elevated serum anti-Mptb (Mycobacterium avium subsp. paratuberculosis) antibodies. If the infection is not eliminated by the initial T-cell response then Mptb are able to persist and proliferate resulting in lesion formation and chronic infection. Much work has been published on cellular responses throughout the course of bovine Johne's disease but knowledge of similar responses in sheep is lacking. The aim of this study was to identify changes in antigen-driven proliferation of lymphocyte subsets during the course of early ovine Johne's disease (OJD).

Merino lambs aged 4 months were randomly assigned into three groups, n = 20 per group. Two groups were orally challenged with either a clonal inoculum of Mptb (Group 1) or a gut homogenate prepared from an animal with clinical OJD (Group 2) while the third group was left unchallenged (Group 3). Blood samples were collected at 4 monthly intervals and prior to necropsy at 13 months post-challenge. Lymphocytes isolated from blood and lymph nodes (ileal, posterior jejunal and prescapular) were labelled with CFDA-SE and cultured in the presence or absence of Mptb antigen for 5 days. Cells were labelled for phenotype - CD5, CD4, CD8αβ, 86D (γδ T cells) and WC4 (B cells) - prior to data collection by flow cytometry. Faecal shedding and presence of Mptb in tissue samples were determined by culture in a radiometric system (Bactec). Diseased sheep were categorised based on histological lesion type.

Distinct differences were seen in the antigen-induced proliferative response in both blood and lymph node cells from sheep challenged with Mptb compared to the unchallenged controls. While 30% of Group 1 and 50% of Group 2 animals responded to Mptb antigen stimulation in the proliferation assay as early as 4 months post-challenge, none in Group 3 responded. The number of responders in Groups 1 and 2 continued to be higher than in Group 3 throughout the study. Differences in the phenotype of proliferating subsets in relation to infection status will be discussed.