The intestine is the largest surface of the body that is exposed to the external environment. Vast arrays of resident microbes colonize the human intestine to form the intestinal microbiota (or microflora). This is estimated to consist of up to 100 trillion microbial cells and constitutes one of the most densely populated bacterial habitats known in biology1-3. In the GIT bacteria colonize an intestinal niche where they survive and multiply4. In return, the microbiota endows the host with additional functional features not encoded on its genome1. For example the microbiota stimulates the proliferation of epithelial cells, produces vitamins that hosts cannot produce by themselves, regulates metabolism and protects against pathogens4-6. Given this beneficial relationship, some authors have suggested that humans are "super-organisms" or "holobionts" that are a mix of bacterial and human genes7,8. Given the beneficial impact of the microbiota on the (human) host, the intestinal immune system needs to tolerate commensal microbes to enable their existence in the lumen but also kill the pathogens that invade from luminal side9-11. The intestinal immune system has developed mechanisms to distinguish between harmless and potentially harmful luminal microbes; however these mechanisms are not yet well understood12. Maintaining intestinal integrity requires a tightly regulated immune homeostasis to keep the balance between tolerance and immunity13. An imbalance in immune homeostasis contributes to the induction of intestinal diseases such as inflammatory bowel disease (IBD)3,14.
There are two major types of IBD: Crohn's disease (CD) and ulcerative colitis (UC). Patients with these diseases usually suffer from rectal bleeding, severe diarrhea and abdominal pain15,16. The single cause of IBD is still unknown, but a combination of genetic factors, environmental influences and dysregulated immune responses might be the key event for disease development15.
Animal models for IBD have been used for over 50 years. In the last few decades new IBD model systems have been developed to test the various hypotheses concerning the pathogenesis of IBD17,18. The best-characterized model of chronic colitis is the T-cell transfer model that induces disruption of T-cell homeostasis19,20. This model involves transferring naive T cells from immunocompetent mice into hosts that lack T and B-cells (such as RAG-/- and SCID mice)16,21. The development of disease in this model is monitored for 3-10 weeks by evaluating the presence of diarrhea, reduced physical activity, and loss of body weight. This is so called the wasting syndrome16. Compared to the healthy mice the colonic tissue of transplanted hosts is thicker, shorter and heavier16. Using the T cell transfer model, it is possible to understand how different T cell populations can contribute to the pathogenesis of IBD22. The T cell transfer model does not analyze the interactions between APCs and T cells in the disease process in an antigen-specific manner. It has been shown that an interaction between myeloid cells and lymphoid cells could be responsible for the development of intestinal inflammation23. Although many aspects of IBD have been clarified, the initial events that lead to the disease development still need to be clearly understood.
It has been shown that in the absence of microbiota transfer colitis cannot be established24. Recently, several theories suggest that IBD could be a result of an immune response against commensal bacteria25. Authors have also proposed that commensal bacteria are essential to induce inflammation in the distal intestine26. In germ free (GF) animals the intestinal immune system is generally impaired27,28, but a colonization of these mice with a mixture of specific-pathogen-free bacteria results in the development of the fully-competent intestinal immune system29. Hence, the microbiota seems to be a key element in the pathogenesis of IBD, either as a mechanism that predisposes to or protects against the development of intestinal inflammation30,31. Current theories suggest that IBD is a result of microbial imbalance, called dysbiosis, in genetically predisposed patients32, but it is not clear yet if the dysbiosis is the cause or the consequence of the disease12. Considering the role of microorganisms in the development of IBD, in vitro experiments showed that CD4+ T cells can be activated by APCs pulsed with intestinal bacteria33,34.
Moreover, it has been shown that antigens from different commensal bacterial species, such as E. coli, Bacteroides, Eubacterium and Proteus, are able to activate CD4+ T cells35. This indicates that presentation of bacterial antigens to T cells is of importance for the development of IBD. To reduce the complexity of multiple antigens derived by the microflora in the disease process, an E.coli strain has been created that produces the OVA antigen. Transfer colitis was induced by injecting OVA-specific T cells into RAG-/- animals colonized with OVA-expressing E. coli.
This model is based on recent evidence suggesting that CX3CR1+ MPs, a major cell subset in the colonic lamina propria (cLP)36, are interacting with CD4+ T cells during transfer colitis37. MPs sample the intestinal lumen for particulate antigen, such as bacteria, using their dendrites36, 38,39. Previous studies demonstrated that MPs can also take up soluble antigens, such as OVA, introduced into the intestinal lumen40,41. Given the abundance of CX3CR1+ MPs in the cLP, it is possible that these cells can sample luminal bacteria and interact with CD4 T cells. Confocal imaging of mice transplanted with OVA-specific CD4+ T cells colonized with E. coli CFP-OVA, show that CX3CR1+ MPs are in contact with OT-II CD4+ T cell during the development of antigen-driven colitis. This model enables the study of the antigen presentation process between intestinal APCs and T cells specific only for particular antigen-expressing bacteria in the gut lumen.