Development of a bovine ileal cannulation model to study the immune response and mechanisms of pathogenesis of paratuberculosis.

An ileal cannulation model was developed in conjunction with a flow cytometric assay to gain a better understanding of the mechanisms of immunopathogenesis of Johne's disease caused by Mycobacterium avium subsp. paratuberculosis. Initial studies with calves showed that M. avium subsp. paratuberculosis DNA is detectable by PCR in ileal biopsies during the first months following experimental infection. Inflammatory lesions were not detected on endoscopic evaluation up to 8 months postexperimental infection. M. avium subsp. paratuberculosis DNA was detected in multiple tissues at necropsy 8 months postinfection. Examination of the activation status of epithelial lymphocytes from the jejunum and ileum from infected and control animals at necropsy revealed that none of the major subsets of lymphocytes (NK, CD2(+), and CD2(-) gammadelta T lymphocytes, or CD4 and CD8 alphabeta T lymphocytes) expressed activation molecules CD25, CD26, CD71, ACT1, or ACT16. Subsets of CD4 and CD8 T lymphocytes from control and infected animals expressed CD26. The majority of CD4 and CD8 T lymphocytes expressed CD45R0, the memory T-lymphocyte marker. An immune response to M. avium subsp. paratuberculosis was detected by 3 months postinfection, dominated by a strong proliferative response of CD4 memory T lymphocytes. The findings indicate an immune response develops following initial exposure to M. avium subsp. paratuberculosis that controls but does not eliminate the pathogen. This persistence of M. avium subsp. paratuberculosis possibly leads to erosion and dysregulation of protective immunity at later time points postinfection. Continuous access to the ileum offers an opportunity to elucidate the cellular and molecular events leading to immune dysregulation and development of chronic inflammatory ileitis.