Project

Regulatory T cells; Antigen specificity; Mucosal immunology; Commensal flora; germ-free; Toll-like receptor; gnotobiotic; T helper type 17

Lead
Lay summary
Our intestines are home to an immense load of non-pathogenic commensal bacteria that outnumber our own body's eukaryotic cells by ten fold. Despite the presence of these 'foreign' organisms the intestinal immune system does not mount an inflammatory response against the commensal flora. Instead, a state of armed truce between the microflora and the intestinal immune system is established. We have co-evolved to live in healthy mutualism with our intestinal flora which means that both, we as hosts as well as the bacteria that live in our intestines profit from this situation. For example, while we provide the bacteria with a nutrient rich environment, the bacteria in turn help us to digest compounds that would otherwise be unaccessible to us. While this homeostasis is stably maintained in healthy individuals, a breakdown can trigger inflammatory bowel disease (IBD), which is a chronic condition that is thought to be due to an inappropriate and exaggerated mucosal immune response to normal constituents of the commensal microflora.T helper type 1 (Th1) responses are mounted in response to viral or bacterial infection while Th2 responses are involved in allergic diseases and in responses to parasites such as helminths. The more recently described Th17 cells are involved in autoimmune responses but may play an important role in keeping the intestinal flora in check. Another important T helper cell lineage are regulatory T cells (Treg) that control or down-modulate immune responses. This research project will address how intestinal Treg induce and maintain intestinal immune homeostasis by controlling inappropriate Th1, Th2, and Th17 against intestinal bacteria to avoid detrimental inflammatory immune responses in the absence of pathogens. In particular, this research proposal will investigate how intestinal Treg sense the presence of harmless intestinal commensal bacteria.Understanding the fundamental mechanisms that lead to induction and maintenance of intestinal immune homeostasis in response to intestinal colonization with commensal bacteria is crucial for understanding the defects in homeostasis that must occur in the initiation of IBD. Elucidating the mechanism by which Treg control and regulate intestinal immune responses in the absence of pathogens might lead to the development of new therapeutic approaches to treat or even reverse chronic intestinal inflammation.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

BackgroundOur intestines are home to an immense load of non-pathogenic commensal bacteria that outnumber our own body’s eukaryotic cells by ten fold. Despite the presence of these ‘foreign’ organisms that, when encountered by the systemic immune system, induce an immediate and potent inflammatory response, the mucosal intestinal immune system does not mount an inflammatory response against the commensal flora. Instead, a state of armed truce between the microflora and the intestinal immune system is established. While this immune homeostasis is stably maintained in healthy individuals, a breakdown of this homeostasis can trigger chronic inflammatory bowel disease (IBD) characterized by an inappropriate immune response against the commensal flora.Many studies using a variety of different mouse colitis models have provided a wealth of knowledge about the pathological immune mechanisms involved in spontaneous and induced inflammatory intestinal disease in experimental mice. In particular, Fiona Powrie’s T cell transfer model has revealed the importance of intestinal regulatory T cells (TReg) in controlling intestinal inflammation and homeostasis. Other studies have revealed the critical role of T helper 1 (TH1) cells, and more recently TH17 cells, in mediating intestinal inflammation.A central role for the intestinal microflora in inducing a balanced mucosal immune response has been suggested. Indeed, the composition of the complex microflora is known to have an effect on intestinal T cell proportions, and the presence of segmented filamentous bacteria (SFB) has been shown to specifically induce TH17 cells in wild-type mice. In addition, alterations in the composition of the intestinal microflora have been observed in individuals with IBD, which has led to the hypothesis that intestinal dysbiosis may be a driving factor in mediating IBD. However, to date no intestinal pathogen or specific microflora composition has been directly shown to induce IBD. My research has revealed that colonization of germ-free mice with a limited and precisely defined commensal microflora leads to a robust FoxP3+ TReg response specifically in the intestinal lamina propria. I have also found that in the presence of a defective regulatory T cell response (in SMARTA mice), colonization with this same limited commensal microflora leads to a TH1 and TH17 effector response in the intestinal lamina propria even in the absence of SFB. My research suggests that control of these TH17 and TH1 responses is intrinsically mediated by intestinal TReg that are activated and expand upon colonization. This induction and expansion seems to be mediated by mucosal dendritic cells in a Toll-like receptor-dependent manner.HypothesisBased on my findings, I hypothesize that intestinal colonization leads to Toll-like receptor-dependent de novo generation of inducible TReg (iTReg) that are required to establish and maintain intestinal immune homeostasis in an antigen-specific manner.Aims of the project and experimental plansThis research proposal will investigate the antigen-specificity requirements for FoxP3+ regulatory T cells that are activated and expand upon colonization of germ-free mice. In addition, the role of de novo generated inducible iTReg and the Toll-like receptor signaling requirements of iTReg in the induction of immune homeostasis will be investigated. I will also address to what extent FoxP3- IL-10-producing regulatory type 1 (Tr1) cells are induced upon colonization as well as their antigen-specificity requirements. This proposal is divided into three independent specific aims:Specific Aim 1:To investigate the requirement for antigen-specificity of FoxP3+ TReg in establishing intestinal immune homeostasis upon colonization. Specific Aim 2:To determine the Toll-like receptor signaling requirements for establishment of homeostasis by de novo in vivo generation of iTReg upon colonization of germ-free mice or after low dose DSS treatment of colonized mice. Specific Aim 3:To assess the induction of FoxP3- IL-10-producing regulatory type 1 (Tr1) cells upon colonization and their requirement for antigen-specificity.Significance of the workInflammatory bowel disease (IBD) is a chronic condition that is thought to be due to an inappropriate and exaggerated mucosal immune response to normal constituents of the commensal microflora. Understanding the fundamental mechanisms that lead to induction and maintenance of intestinal immune homeostasis in response to intestinal colonization with commensal bacteria is crucial for understanding the defects in homeostasis that must occur in the initiation of IBD. Recent work in the field of mucosal immunology elegantly demonstrates the complex interactions of the mucosal CD4+ T cell compartment with the commensal microflora. The proposed research should reveal requirements for commensal bacteria-mediated induction of regulatory T cells in the induction and maintenance of intestinal CD4+ T cell homeostasis upon colonization and hopefully provide insight for the establishment of new treatment protocols for IBD.