Johne's disease, or paratuberculosis, is a chronic wasting disease that is caused by infection with Mycobacterium avium subspecies paratuberculosis (MAP). Johne's disease affects wild and domestic ruminants including livestock species such as cows. Due to factors including losses in milk production and early culling of infected animals, Johne's disease represents a major financial burden, estimated at between $250 million and $1.5 billion annually for the U.S. dairy industry alone. MAP is an obligate intracellular pathogen that is transmitted through the fecal-oral route. MAP infects the host by invading the ileum of the small intestine through M cells lining the lumen, following which it colonizes tissue-resident macrophages thereby establishing persistent infection. The immune response to MAP infection is characterized by a shift from a productive Th1 immune response to an unproductive Th2 response. As a chronic condition, MAP infection may result in the development of a regulatory T cell (Treg) population within infected animals, and these Tregs may play a role in the Th1-to-Th2 immune shift observed in animals progressing from subclinical to clinical disease. Alternatively, Tregs may be critical for controlling chronic inflammation, and their loss may result in severe widespread inflammation that results in damage to host tissues and progression into clinical disease. In this project, we sought to investigate the relationship between MAP infection and Treg activity. First, we utilized a monocyte-derived macrophage (MDM) model of MAP infection in an effort to induce the development of a Treg phenotype in naïve T cells from cows with Johne's disease. Second, we developed a method to expand the relative abundance of Tregs present in peripheral blood mononuclear cell (PBMC) populations from MAP-infected cows. Following this, expanded Tregs were used in functional assays to determine if they dampened Th1 immune responses to stimulation of PBMCs with live MAP. Finally, we developed a scheme to classify lesions from cows with Johne's disease according to histopathology and abundance of MAP within ileal and mesenteric lymph tissues. Based on this system, we graded lesions from cows with Johne's disease and measured several variables, including relative Treg abundance, within these lesions as compared to healthy control tissues. Although a Treg phenotype did not develop in naïve T cells cultured with MAP-infected MDMs, we observed a state of T cell unresponsiveness in naïve T cells from cows with clinical disease, and a state of reduced responsiveness in naïve T cells from cows with subclinical disease, as compared to responses from control cows. In our second set of experiments, we found that we were able to successfully expand bovine Tregs, and although these expanded Tregs are not MAP-specific, they are functional and do suppress Th1 immune activity generally. Within infected tissues we found that cows with clinical disease can likely be categorized further into early and late clinical disease, based on overall animal health and lesion severity. Further, we observed that Treg abundance decreases with increasing lesion severity in both the ileum and mesenteric lymph nodes. Additionally, within the ileum it was observed that the expression of many immune genes was elevated in mild lesions but was unchanged or reduced in severe lesions, whereas within the mesenteric lymph nodes the expression of many immune genes increased with increasing lesion severity. Altogether, our results suggest that T cell (and thus, Treg) unresponsiveness may be responsible for driving the immune shift seen in cows progressing from subclinical to clinical Johne's disease. The ability to expand non-specific Tregs from bovine peripheral blood is a novel development in bovine immunology and will be useful for future researchers. Finally, our data suggests that a robust CD4+ T cell and inflammatory response to MAP occurs in the ileum of infected cows, followed by the development of T cell unresponsiveness that likely contributes to a loss in control of MAP infection. This in turn would allow progression of Johne's disease from early to late clinical disease and ultimately the demise of the infected host.
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