Interactions of commensal bacteria with the host immune system

The intestinal microbiota plays role in intestinal homeostasis via interactions with the epithelium and innate and adaptive immune mechanisms of the gut thereby profoundly shaping mammalian mucosal immunity and tolerance. However, in some diseases, such as inflammatory bowel disease (IBD), the microbiota may assume a pathologic character accentuating the damage to the host. Despite the recent advances in our understanding of the microbiota structure at different stages of life and the perturbations associated with disease, our understanding of its functional impact and the contribution of individual microbial components on health is still in its infancy. The aim of this thesis was to contribute to our knowledge and understanding of several aspects of the interactions between gut commensal bacteria and host, focusing on one of the most abundant colonic microbes, F. prausnitzii, for the detailed mechanistic studies. In <strong>Chapter 2</strong>, we investigated whether polarized intestinal epithelial cells regulate inflammatory responses to microbes by secreting the chemokine IL-8 in a vectorial fashion depending on the location of the Toll-like receptor (TLR) stimulus. In the Caco-2 model of polarized epithelium, apical stimulation with TLR2 and TLR5 ligands resulted in the apical secretion of IL-8. The CXCR1 receptor for IL-8 was expressed only on the apical membrane of Caco-2 cells and differentiated epithelial cells in the human small intestine and colon suggesting that IL-8 has an autocrine function. Transcriptome analyses revealed that Caco-2 cells respond to stimulation with IL-8, supporting the hypothesis of autocrine signalling via CXCR1. We speculated that this mechanism might be an evolutionary adaptation to the coexistence with microbiota whereby bacterial microbe-associated molecular patterns are utilized to trigger repair mechanisms in epithelial cells. In <strong>Chapter 3</strong>, mono-associated and conventional mice were used to gain new insights into the interaction of bacteria with the host mucosa and mucosal-associated lymphoid tissue. The vast majority of the bacteria are kept at a distance by the host physical and chemical barriers except for segmented filamentous bacteria which appear to have unique and specialized physical interactions with the epithelium. Chapter 4, 5 and 6 focused on the immunoregulatory mechanisms of the human commensalF. prausnitzii, which has been proposed to have anti-inflammatory properties and is underrepresented in IBD patients with active disease. In <strong>Chapter 4</strong>, we compared the effects of F. prausnitziiand other commensal bacterial strains on human monocyte derived dendritic cells (hDCs). F. prausnitziistrain A2-165 induced a very different cytokine profile in hDCs compared with four other F. prausnitziistrains and eight other commensal strains, this was due to the high levels of secreted IL-10 and low levels of IL-12. The effects of F. prausnitziiA2-165 and another commensal strain on the induction of mucosal T cell responses to the model antigen ovalbumin (OVA) in vitroand in vivowere studied in <strong>Chapter 5</strong>. Here we demonstrated that F. prausnitziiA2-165 had immunoregulatory properties in vitrowhich may be related to its strong capacity to induce IL-10 by both DCs and T cells. These effects translated in vivointo enhanced OVA-specific T cell proliferation in the nose-draining cervical lymph nodes (CLNs) after combined nasal application of F. prausnitziiA2-165 and OVA. Strikingly, this was associated with a reduction in the differentiation of IFN-&gamma; secreting T cells in the CLNs. In contrast, a different commensal, Clostridium hathewayi82-B, significantly decreased the percentage of dividing OVA-specific T cells in the CLNs and spleen. In <strong>Chapter 6</strong>, we investigated the immunomodulatory effects of F. prausnitziistrain A2-615, strain HTF-F which forms a biofilm in liquid culture and the extracellular polymeric matrix (EPM) purified from strain HTF-F. The protective capacity of the two strains and the EPM were investigated in the DSS-induced colitis model. Both F. prausnitziistrains attenuated the disease activity index, but the biofilm producing strain HTF-F conferred increased protection compared to strain A2-165 and this was partly due to the immunomodulatory properties of the EPM. <strong>Chapter 7</strong>summarizes and discusses the results of the thesis in the context of wider literature on host-microbe interactions and mucosal immunology. Furthermore, the implications of this work on intestinal health and the possible directions for future research are discussed