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Internalization‐dependent recognition of Mycobacterium avium ssp. paratuberculosis by intestinal epithelial cells

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of Johne's disease, a highly prevalent chronic intestinal infection in domestic and wildlife ruminants. The microbial pathogenesis of MAP infection has attracted additional attention due to an association with the human enteric inflammatory Crohn's disease. MAP is acquired by the faecal–oral route prompting us to study the interaction with differentiated intestinal epithelial cells. MAP was rapidly internalized and accumulated in a late endosomal compartment. In contrast to other opportunistic mycobacteria or M. bovis, MAP induced significant epithelial activation as indicated by a NF‐κB‐independent but Erk‐dependent chemokine secretion. Surprisingly, MAP‐induced chemokine production was completely internalization‐dependent as inhibition of Rac‐dependent bacterial uptake abolished epithelial activation. In accordance, innate immune recognition of MAP by differentiated intestinal epithelial cells occurred through the intracellularly localized pattern recognition receptors toll‐like receptor 9 and NOD1 with signal transduction via the adaptor molecules MyD88 and RIP2. The internalization‐dependent innate immune activation of intestinal epithelial cells is in contrast to the stimulation of professional phagocytes by extracellular bacterial constituents and might significantly contribute to the histopathological changes observed during enteric MAP infection.