Cytokine Gene Expression in Peripheral Blood Mononuclear Cells and Tissues of Cattle Infected with Mycobacterium avium subsp. paratuberculosis: Evidence for an Inherent Proinflammatory Gene Expression Pattern

In cattle and other ruminants, infection with the intracellular pathogen Mycobacterium avium subsp. paratuberculosis results in a granulomatous enteritis (Johne's disease) that is often fatal. The key features of host immunity to M. avium subsp. paratuberculosis infection include an appropriate early proinflammatory and cytotoxic response (Th1-like) that eventually gives way to a predominant antibody-based response (Th2-like). Clinical disease symptoms often appear subsequent to waning of the Th1-like immune response. Understanding why this shift in the immune response occurs and the underlying molecular mechanisms involved is critical to future control measures and diagnosis. Previous studies have suggested that M. avium subsp. paratuberculosis may suppress gene expression in peripheral blood mononuclear cells (PBMCs) from infected cows, despite a continued inflammatory reaction at sites of infection. In the present study, we tested the hypothesis that exposure to M. avium subsp. paratuberculosis suppresses a proinflammatory gene expression pattern in PBMCs from infected cows. To do this, we examined expression of genes encoding interleukin-1α (IL-1α), IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12p35, IL-16, and IL-18, as well as genes encoding gamma interferon (IFN-γ), transforming growth factor β (TGF-β), and tumor necrosis factor alpha (TNF-α), in PBMCs, intestinal lesions, and mesenteric lymph nodes of cattle naturally infected with M. avium subsp. paratuberculosis. Cytokine gene expression in these cells and tissues was compared to expression in similar cells and tissues from control uninfected cattle. Our comprehensive results demonstrate that for most cytokine genes, including the genes encoding IFN-γ, TGF-β, TNF-α, IL-1α, IL-4, IL-6, IL-8, and IL-12p35, differential expression in PBMCs from infected and control cattle did not require stimulation with M. avium subsp. paratuberculosis. In fact, stimulation with M. avium subsp. paratuberculosis tended to reduce the differential expression observed in infected and uninfected cows for genes encoding IFN-γ, IL-1α, and IL-6. Only IL-10 gene expression was consistently enhanced by M. avium subsp. paratuberculosis stimulation of PBMCs from subclinically infected cattle. In ileal tissues from M. avium subsp. paratuberculosis-infected cattle, expression of the genes encoding IFN-γ, TGF-β, IL-5, and IL-8 was greater than the expression in comparable tissues from control uninfected cattle, while expression of the gene encoding IL-16 was lower in tissues from infected cattle than in control tissues. Mesenteric lymph nodes draining sites of M. avium subsp. paratuberculosis infection expressed higher levels of IL-1α, IL-8, IL-2, and IL-10 mRNA than similar tissues from control uninfected cattle expressed. In contrast, the genes encoding TGF-β and IL-16 were expressed at lower levels in lymph nodes from infected cattle than in tissues from uninfected cattle. Taken together, our results suggest that cells or other mechanisms capable of limiting proinflammatory responses to M. avium subsp. paratuberculosis develop in infected cattle and that a likely place for development and expansion of these cell populations is the mesenteric lymph nodes draining sites of infection.