Dendritic cells and regulatory T cells in the gastrointestinal tract of healthy and diseased dogs with inflammatory bowel disease

Alongside the gastrointestinal border, intestinal homoeostasis depends on strong regulatory capacity but further requires a distinct inflammatory activity to prevent penetration of intestinal microorganisms. Concerning this, dendritic cells (DCs) are crucial for recognition of foreign structures and therefore initiate and orchestrate specific immune responses. Moreover, they can induce immunosuppression towards food proteins and commensal bacteria to avoid excessive and prolonged immune responses. Particularly, this so-called oral tolerance mainly requires the induction of forkhead box P3 (Foxp3) positive regulatory T cells (Tregs) in the mucosal immune system. Breakdown of intestinal homoeostasis including a loss of oral tolerance constitutes a critical step in the pathogenesis of inflammatory bowel disease (IBD) – a group of chronic enteropathies of unknown aetiology occurring in both, men and dogs. As little is known about these cells in the canine gut, the aim of this study was to investigate DCs and Foxp3+ Tregs within the gastrointestinal immune system of healthy dogs. Additionally, gastrointestinal biopsies of dogs suffering from IBD were examined to test the hypothesis that DCs and Tregs are involved in its pathogenesis. Furthermore, Foxp3+ Tregs were analysed in the mucosa of dogs with spontaneous intestinal nematode infections constituting an additional inflammatory disease of the canine alimentary tract. In this study, gastrointestinal samples were initially examined by histopathology according to established guidelines. Subsequently, immunohistochemistry and multicolour immunofluorescence were used to evaluate the distribution and phenotypes of these cells within the canine gut-associated lymphoid tissue (GALT). Additionally, lamina propria (LP) DCs and Foxp3+ Tregs were quantified and related data were analysed statistically. DCs and Tregs were widely distributed in all compartments of the canine healthy GALT. Within the small intestinal LP, the majority of DCs expressed cluster of differentiation (CD) 1a8.2, CD11c and major histocompatibility complex (MHC) class II, whereas significantly lower amounts were positive for CD11b. In contrast, CD80+ and CD86+ DCs were absent from the healthy LP but occasionally located in Peyer’s patches (PPs) and mesenteric lymph nodes (MLNs). Additionally, DCs located beneath the surface epithelium displayed cytoplasmic dendrites projecting between enterocytes. Concerning different age groups, the amounts of LP Foxp3+ Tregs were increased in middle-aged dogs compared to juvenile and older individuals. In canine IBD, both DCs and Foxp3+ Tregs were significantly decreased in the gastrointestinal LP. In contrast, Foxp3+ Tregs were increased in the intestinal mucosa of dogs with nematode infections. The complex network of DCs and Tregs in the GALT suggests their crucial involvement in canine mucosal immunology. Expression of CD1a8.2, CD11c and CD11b but absence of CD80 and CD86 indicate canine LP DCs to be immature and responsible for mediation of oral tolerance in the steady state. The presence of transepithelial cytoplasmic dendrites further suggests direct sampling of luminal antigens by canine DCs. The increase of intestinal Tregs in middle-aged compared to juvenile dogs illustrates a progressive establishment of oral tolerance, which is in contrast to their decline in older individuals reflecting an increasing immunosenescence in the canine elderly. The reduction of Tregs and tolerogenic DCs in the mucosa of dogs suffering from IBD elucidates a loss of intestinal immune regulation and oral tolerance that could mediate exacerbating immune responses towards luminal antigens. The decline of LP DCs further suggests a reduced capacity in antigen recognition and presentation that may result in disturbances of intestinal microbiota in canine IBD. In contrast, enrichment of Tregs in the LP of dogs with nematode infections indicates a parasite-induced increase in immune regulation that could enable longer survival in the host and prevention of mucosal inflammation. This may suggest that oral application of parasites could represent a cutting-edge strategy for therapeutic management of IBD.