Alteration of the structural integrity of TLR signalling components is often associated with profound clinical outcome and susceptibility to various infections or autoimmune disorders. During conditions of floral translocation, peripheral TLR-9 signalling is a crucial mediator of polymicrobial sepsis. Moreover, in other conditions in which bacterial translocation occurs [for example, during irradiation and human immunodefiency virus (HIV) infection] peripheral PD98059 chemical structure TLR-4 signals enhance the activation status of both CD4+ and CD8+ T cells [10]. However, under most circumstances

the tissues of the GI tract are exposed constantly to TLR ligands harboured by the commensal gut flora. Mice deficient phosphatase inhibitor library in TLR-9 display increased frequencies of Tregs within intestinal effector sites and reduced levels of constitutive interleukin (IL)-17- and interferon (IFN)-γ-producing effector T cells [9]. Complementing this, lamina propria dendritic cells (DCs) lacking exposure to gut flora DNA, induce Treg conversion in vitro. Furthermore, Tregversus effector T cell disequilibrium in TLR-9−/− mice restricts immune responses to oral infection with the pathogen Encephalitozoon cuniculi.

Impaired intestinal immune responses were recapitulated in mice treated with antibiotics and were reversible after reconstitution with gut flora DNA [9]. Thus, signals derived from the gut flora act as adjuvants of immune responses for priming intestinal responses against

oral pathogens via modulation of the equilibrium between Treg and effector T cells. Intestinal epithelial cell (IEC) expression of TLRs has also PTK6 proved to be important in maintaining the homeostatic host–microbiome relationship, and to involve unexpected subtleties. For example, TLR-9 is expressed on both the apical (luminal-facing) and basolateral surfaces of the epithelial cell layer, but only basolateral ligation triggers an inflammatory signal, while apical binding is inhibitory [11]. The capacity of IECs to control immune responsiveness extends to the production of thymic stromal lymphopoietin (TSLP) and IL-25, influencing the Th phenotype balance in a manner which can make or break effective immunity [12]. The structure and composition of the gut flora reflect natural selection at both the microbial and host levels, and show perturbations in GI dysfunction. For example, modified gut floral composition is found in inflammatory bowel disease (IBD) patients [13]. Furthermore, the presence of certain bacteria can aggravate small intestinal immunopathology following oral infection.