The MAS-related gene (Mrg) receptor MrgE has been suggested to be expressed at all tissue levels involved in pain sensation and to influence the expression of another Mrg receptor, MrgF. Given the knowledge on the role of the enteric nervous system (ENS) in sensation, and the plasticity of enteric neurons during intestinal inflammation, it can be hypothesized that MrgE is expressed in enteric neurons, and that MrgE and MrgF change expression in intestinal inflammatory conditions. Therefore, we aimed to reveal the expression details of MrgE and MrgF in the murine ileum in normal and inflamed conditions. Using reverse transcriptase-PCR, quantitative-PCR and immunohistochemistry, we compared the ileum of non-inflamed control mice with that of two models of intestinal inflammation, i.e. intestinal schistosomiasis and chemically induced ileitis. MrgE and MrgF mRNAs were detected in control and inflamed conditions. MrgE and MrgF mRNAs showed a trend towards downregulation during intestinal schistosomiasis and a significant reduction during ileitis. MrgE and MrgF receptors were expressed in distinct enteric neuronal subpopulations, such as the sensory, secretomotor and vasodilator neurons, and in nerve fibres in the tunica muscularis and lamina propria of control and inflamed ileum. Only a minor proportion of enteric neurons co-expressed MrgE and MrgF. The number of enteric neurons expressing MrgE and MrgF receptors was significantly reduced during intestinal schistosomiasis and ileitis. This is the first report on the expression of MrgE and MrgF in the ENS in (patho)physiological conditions. The expression of MrgE and MrgF in enteric neurons was negatively affected by inflammation.

The expression of transient receptor potential vanilloid type 1 channel (TRPV1) in the enteric nervous system is still the subject of debate. Although a number of studies have reported that TRPV1 is limited to extrinsic afferent fibers, other studies argue for an intrinsic expression of TRPV1. In the present study, reverse transcriptase PCR was employed to establish the expression of TRPV1 mRNA throughout the gastrointestinal tract. Using two antibodies directed against different epitopes of TRPV1, we were able to show at the protein level that the observed distribution pattern of TRPV1 is dependent on the antibody used in the immunohistochemical staining. A first antibody indeed mainly stained neuronal fibers, whereas a second antibody exclusively stained perikarya of enteric neurons throughout the mouse gastrointestinal tract. We argue that these different distribution patterns are due to the antibodies discriminating between different modulated forms of TRPV1 that influence the recognition of the targeted immunogen and as such distinguish intracellular from plasmalemmal forms of TRPV1. Our study is the first to directly compare these two antibodies within the same species and in identical conditions. Our observations underline that detailed knowledge of the epitope that is recognized by the antibodies employed in immunohistochemical procedures is a prerequisite for correctly interpreting experimental results.

The gastrointestinal (GI) tract, just like the skin and the airways, is constantly exposed to both harmless and pathogenic organisms and hence requires a tightly regulated immune homeostasis to function properly. A central role in the regulation of this balance is played by the dendritic cells (DCs), a heterogeneous population of antigenpresenting cells that can be further divided into distinct subsets with different functions depending on the tissue they reside in. In recent years, the DC population in the lamina propria (LP) of the intestine has emerged as a key player in immune surveillance. Given the extensive innervation of the GI mucosa, these DC subsets possibly are also regulated by interactions with neuronal components. Current knowledge, be it still fragmentary, indicates that dysregulation of this neuroimmune communication leads to the onset of pathological disorders. The present review article deals with the identification and interaction of distinct subtypes of mouse intestinal LP DCs with elements of the enteric nervous system (ENS) in normal and inflammatory conditions. Furthermore, the question is addressed whether any parallels can be drawn between intestinal LP DCs and DCs residing in the skin and lung in order to gain a better insight into common or clearly distinct mechanistic pathways and the possible impact of the mucosal components in the microenvironment. The exact way in which the ENS is serving its immunomodulatory roles in the GI tract is still largely unknown, although there are significant indications for a crosstalk between LP DCs and components of the ENS. This review clearly shows that in the three different organ systems the same neurotransmitters (i.e., SP, CGRP, and VIP) reoccur, serving similar functions. Mechanistic lessons learned from other organ systems, such as the skin and lung, may be of substantial help in further exploring the nature of the neuroimmune communication between GI innervation

Being continuously exposed to a plethora of antigens ranging from food antigens to potential pathogenic organisms, the gastrointestinal (GI) tract harbors the largest collection of immune cells in the mammalian body. This immune system has to maintain a delicate balance between mounting an active immune response and maintaining tolerance. The GI tract is also home to an elaborate intrinsic nervous system, the enteric nervous system (ENS). Various in vitro studies of neuro-immune communication have suggested that vasoactive intestinal peptide (VIP), an important GI neurotransmitter, modulates mononuclear phagocytes (MNPs), i.e., dendritic cells and macrophages. Using a combined approach of reverse transcription plus the polymerase chain reaction, immunofluorescence, three-dimensional maximum intensity projections and immunoelectron microscopy, we investigate the interaction between the enteric innervation and MNPs in the ileal lamina propria (LP). We demonstrate that VIP-ergic fibers of the ENS lie adjacent to CX3CR1 MNPs and that VPAC1 is constitutively expressed on ileal CX3CR1+ cells in the LP of the mouse. We also identify, for the first time, CX3CR1 immune cells in the LP at the ultrastructural level. Our data thus reveal the in situ presence of the molecular components that are necessary for a VIP-mediated neuro-immune interaction between the ENS and CX3CR1-expressing immune cells in the LP of the ileum.

The significant upregulation of CD11c(+)  CX3CR1(+)  F4/80(-) DCs during intestinal schistosomiasis and the restriction of phagocytosis of parasitic antigens to CX3CR1-expresssing MNP indicate a crucial role for this immune cell niche in response to trematodiasis. These findings add insight into the functional specialization of LP immune cells and add to the understanding of cellular mechanisms behind helminth-based therapies.

No abstract

Some members of the Mas‐related gene receptor (Mrg) family have been suggested to play a role in mediating IgE‐independent mast cell activation, as well as in neuroimmune communication. In a previous study, we observed an increased expression of some Mrgs, such as MrgD, in the murine ileum during intestinal inflammation. To further unravel the distribution and involvement of MrgD in intestinal inflammation, we compared the ileum of non‐inflamed and Schistosoma mansoni‐infected wild‐type and MrgD−/− mice. In the wild‐type mice, in the non‐inflamed ileum, immunohistochemistry revealed no MrgD immunoreactivity (IR), whereas in the inflamed ileum, MrgD IR was observed in 5% of the myenteric neurons. Neurochemical coding revealed that the MrgD‐expressing neurons were of the intrinsic primary afferent type. Moreover, MrgD IR was detected in mucosal mast cells (MMCs) in the inflamed ileum. In the MrgD−/− mice, no MrgD IR was detected, while an increased infiltration of MMCs was observed in the inflamed ileum. The expression of MrgD in sensory neurons and MMCs in the inflamed ileum of wild‐types, and the increased MMC infiltration observed after MrgD deletion, suggest that MrgD is involved in nociceptive and mast cell responses during intestinal schistosomiasis. Future work should aim at elucidating the mechanisms underlying MrgD‐mediated effects during intestinal inflammation. Supported by FWO grant G.0179.08

Intestinal dendritic cells (DCs) are key regulators of intestinal immune homeostasis. We studied the mononuclear phagocyte (MNP) subsets of the murine ileum and mesenteric lymph nodes in a mouse model of Schistosoma mansoni‐induced inflammation. Intraperitoneally injected, fluorescently labeled S. mansoni‐related antigens, i.e. soluble worm proteins as well as soluble egg antigens, were preferentially taken up by CX3CR1+ and F4/80+ MNPs and not readily observed in other MNP populations. Using a combination of immunohistochemistry and multiparametric flow cytometry, we demonstrated a significant increase in CD11c+CX3CR1+F4/80‐ DCs in the ileal lamina propria and the mesenteric lymph nodes during schistosomiasis. Our results point to an important role of CD11c+CX3CR1+F4/80‐ DCs in the immune response to trematode infection, lending support to the presumed role of these cells as specialized sentinel cells of the intestinal lamina propria.