Appendicitis protects against colitis: an exploration of underlying immune mechanisms

Abstract

Inflammatory bowel disease-(IBD) is a chronic relapsing and remitting disorder of the gastrointestinal tract characterised by inflammation. The underlying immunopathogenesis is unknown but excessive effector and defective regulatory immune responses play a significant role. The aetiology of IBD is not yet fully resolved, but interactions between genetic and environmental factors, including the gut flora, contribute significantly to the development of IBD. Appendicectomy for intra-abdominal inflammatory conditions before the age of 20 protects against colitis. The mechanism underlying this protective effect is unclear. This thesis examined the hypothesis that appendiceal regulatory T lymphocytes (Treg) mediate the protection. Despite human and murine data showing that the appendix is involved in the prevention of colitis, few studies of human or murine appendices have been reported. A novel murine model of appendicitis was created using an operative technique of band-ligation of an induced-tubular-appendix. Histological assessment showed that this model recapitulated all of the histological features of human acute appendicitis, namely mucosal ulceration, transmural neutrophilic and lymphocytic infiltration and serositis. This local pathology was associated with a systemic host response, evidenced by raised serum Creactive- protein. The impact of inflammation on the appendiceal lymphocyte constituents was assessed by flow cytometry. The inflammation caused a shift from B-lymphocyte to T-lymphocyte predominance. In particular there was a 75% increment in Treg numbers, which was restricted to juvenile mice only (< 10 weeks old). Furthermore, appendiceal Treg expressed the gut-homing chemokine-receptor, CCR9, and the intestine-specific integrin, α4β7. These cells were shown to preferentially migrate to the colonic lamina propria. Lastly, appendicitis and appendicectomy protected against TNBS-colitis which was also restricted to juvenile mice. Evidence for antigen dependence was suggested by the effect being heightened when mice were pre-sensitised against TNBS. Exploration of the colonic lamina propria (cLP) lymphocyte population showed an increase in Treg, especially CD8+Foxp3+- Treg, in “protected” mice only, which may have originated in the appendix. Intracytoplasmic cytokine detection showed that these cLP-Treg were potent producers of the regulatory cytokine, IL-10. These findings strongly suggest that appendicitis triggers an expansion of Treg which then emigrate to the colon and mediate long-lasting protection against colitis.