Immunopathological aspects of equine inflammatory bowel disease

Intestinal inflammation is a major welfare issue for horses causing colic, diarrhoea and an inability to thrive. One of the diagnoses with poor prognosis is inflammatory bowel disease (eqIBD). Equine IBD comprises different entities of chronic idiopathic enteropathies, namely eosinophilic gastroenteritis, granulomatous enteritis and lymphoplasmacytic enterocolitis. An exaggerated immune reactivity towards still unidentified antigens is suggested in the disease. The aim of this thesis was therefore to elaborate methods to characterize cellular infiltrations and to determine cytokine profiles associated with various forms of eqIBD. Jejunal tissues were used to describe the immune cell populations in situ in healthy horses and in well characterised IBD-afflicted horses. T cells, regulatory T cells, B cells, IgM- IgG- and IgA-secreting plasma cells and MHC II-expressing cells were labelled by immunohistochemistry and quantified by image analysis. The inflammation in IBD horses was dominated by T cells and MHC II-expressing cells, whereas B cells and plasma cells were decreased compared with healthy horses. These features were most prominent in granulomatous IBD. The regulatory T cells followed the infiltration pattern of T cells, but were not significantly increased in diseased horses. Thus the histopathology in the studied forms of eqIBD shows similarities with a delayed hypersensitivity reaction. Seven reference genes were evaluated and their optimal combination determined for seven equine intestinal segments (the duodenum, mid-jejunum, ileum, caecum, right ventral and dorsal colon and rectum). Segments from healthy horses expressed IL-12, IL-17A, IL-23, TLR4, but not IFN-α. The relative expression of the three interleukins and TLR4 was analysed using quantitative PCR in rectal biopsies showing chronic or chronic active simple proctitis from horses with clinical signs of eqIBD. Horses with chronic active proctitis had increased expression of IL-17A and TLR4 compared with healthy horses, whereas horses with chronic proctitis had decreased expression of IL-12, indicating a TH17 involvement in chronic active disease. Taken together, techniques were established to discern patterns of immune reactions and could demonstrate differences between entities when applied to eqIBD material