The vagus nerve as modulator of intestinal immune homeostasis.
The gastrointestinal (GI) tract constitutes the largest mucosal surface that is continually exposed to a wide range of antigens and potential immune stimuli including various dietary antigens and commensal bacteria. For this reason, the intestinal mucosal tissue contains the largest number of immune cells participating to an elaborate network of cells and effector molecules to defend the host from the external milieu. In order to maintain homeostasis, the immune system has developed diverse regulatory strategies including additional non-immunological actors able to control the immune response. Defects in these immunological responses can lead to intestinal disorders such as inflammatory bowel disease (IBD), encompassing Crohn’s disease and ulcerative colitis.
Of note, organisms have evolved endogenous strategies in order to control immune response. The concept of an inflammatory reflex, in which the central nervous system responds to inflammatory stimuli and acts to limit inflammation, was clearly formulated by Tracey (174). This novel concept concerning the cholinergic anti-inflammatory pathway (CAIP) proposes that the vagus nerve plays a crucial role in the regulation of the immune response. This implies that a specific organ or region of the organism has its dedicated vagal innervation detecting activation of the innate immune system and providing integrated efferent vagal input to dampening the inflammatory response (174). Truthfully, vagotomy (VXG) significantly aggravated the development of septic shock and enhanced inflammation whereas electrical stimulation of the efferent vagus nerve prevented systemic inflammation (175). Moreover, our research group has provided evidence that the CAIP also modulates the intestinal immune system in a model of post-operative ileus (POI) in which the inflammation is located at the muscularis layer.
In this thesis, we have investigated the participation of the neuro-endocrine factors, such as the vagus nerve and the gastrointestinal hormone, ghrelin, in the regulation of the intestinal immune system. First, we aim to better define the CAIP mechanism in the intestine. We demonstrated that vagus nerve stimulation (VNS) is able to dampen intestinal muscular inflammation and prevent ileus via activation of the alpha 7 nicotinic acetylcholine receptor (α7nAChR). We provide evidence that the vagus nerve reduces local intestinal inflammation through a circuit that differs from that previously reported in models of systemic inflammation, i.e. independent of the spleen and T cells. Indeed, cholinergic nerve fibers arising from enteric neurons innervated by the vagus nerve modulate intestinal resident muscularis macrophages expressing α7nAChR. We therefore conclude that resident macrophages located within the intestinal muscularis are the ultimate target cell mediating the anti-inflammatory effect of VNS in POI (Chapter 3).
Taking into consideration that immune tolerance to food antigens and to intestinal microflora is essential to preserve intestinal homeostasis and prevent chronic inflammation, we hypothesized that the vagal innervation of the gut can modulate intestinal immune mucosal homeostasis and not only dampening on-going local inflammation. Our findings clearly showed that the vagus nerve is essential for the development of mucosal immune homeostasis and induction of appropriate immune response during colitis even if this mechanism is independent of α7nAChR. VGX impairs the ability to develop oral tolerance and results in increased severity of dextran sodium sulfate (DSS) colitis however these effects are not observed in α7nAChR-deficient mice. Additionally, no role for α7nAChR could be proven (either on transferred T cells or in the recipient mice) in T cell transfer colitis. Therefore our data indicate that α7AChR does not play a crucial role in the anti-inflammatory cholinergic effect to the lamina propria but most likely other nicotinic receptors are participating to this pathway (Chapter 4).
Together with the neuronal messengers able to affect the immune system, hormonal messengers have been recently discovered as potent immune-modulatory factors. Recent work suggests that the orexigenic hormone, ghrelin, has potent anti-inflammatory properties in a preclinical model of intestinal inflammation. Ghrelin is secreted predominantly from enteroendocrine cells of the stomach and acts as an appetite-regulating hormone increasing food intake and long-term regulation of body weight. Interestingly, the expression of this ghrelin receptor (GRLN-R) has been shown on innate and adaptive immune cells. However, the mechanism, by which ghrelin suppresses intestinal inflammation, is still unclear. Hence, we decided to investigate the effect of ghrelin on T cell function during colitis. In Chapter 5, we observed that the lack of ghrelin signalling in Th cells resulted in a significantly worsened colitis with increased colonic inflammation dependent on a pathological accumulation of CD4 effector T cells in the lamina propria. In line, ghrelin directly affected proliferation of Th cells and induced apoptosis while it did not influence Th cell polarization in vitro. Our observations strongly suggest that ghrelin may significantly ameliorate experimental chronic colitis by modulating T helper effector cell function in the gut. Indeed, clinical evidence showed that ghrelin is involved in chronic intestinal inflammatory diseases in patients. Higher serum ghrelin level is associated with disease activity in ulcerative colitis and Crohn’s disease patients. Currently, important amount of data regarding ghrelin and its contribution in modulating immune response are suggesting possible therapeutic properties of the ghrelin pathway. However more studies are required before translating this novel approach to the clinical setting in order to clarify the contribution of this hormone in regulating the colonic immune response.
In conclusion, the results described in this thesis provide further insights into implications of nervous system, via vagus nerve, and hormonal molecules such as ghrelin in the modulation of intestinal immune system. Hopefully, these findings and further investigations on these counter regulatory mechanisms will provide new insights in the cross-talk between the neuro-endocrine and immune system leading to the identification of new therapeutic targets to treat intestinal immune diseases.