The late inflammatory phase
This involves activation of mast cells with secretion of histamine and proteases which, in turn, activate resident macrophages and leukocytes and alter intestinal barrier functions. All of these mechanisms trigger inflammation accompanying postoperative ileus and constitute potential pharmacological targets.
Mast cells (MCs) are involved in immunological phenomena, especially as effectors in allergic and anaphylactic processes (Galli et al., 2008). In the digestive tract, mast cells play a regulatory role in vascular and epithelial permeability and immune defence in particular. Indeed, studies in murine peritonitis models have shown that, in animals deficient in mast cells, mortality due to bacterial sepsis is increased (Echtenacher et al., 1996). In the intestine, the greatest number of mast cells are found in the mucosal and submucosal layers, whereas they are less represented in the muscular and serosal layers. Extrinsic afferent nerve endings as well as enteric neurons are in close contact with mucosal mast cells and about 70% of mucosal mast cells interact directly with nerve fibres, with an additional 20% located within 2 mm (Buhner et al., 2017). It is a well-established fact that activation of mast cells generates epithelial and neuromuscular dysfunctions and promotes visceral hypersensitivity. Alteration in digestive motility is a direct consequence of these phenomena, potentially triggering postoperative ileus (The et al., 2008). De Jonge et al . investigated the administration of mast cell stabilisers such as Ketotifen and Doxantrazole in a murine model of ileus: pre-treatment of the mice with both molecules reduced inflammation (measured by myeloperoxidase immunodetection in the entire ileum wall and, more particularly, in the muscle layer) and reduced gastric emptying time (de Jonge et al., 2004).
The activation pathways of mast cells are multiple and a distinction can be made between immune and non-immune pathways. The classical stimulus for MCs activation is the cross-binding of cell surface-bound immunoglobulins E (IgE) to its high-affinity receptor FcεRI by an allergen in a sensitised individual (Galli et al., 2008). This leads to a cascade of phosphorylation and transcription of factors such as AP-1, MITF and STAT-5, and to degranulation and cytokine production (Rivera and Gilfillan, 2006) . MCs also express receptors for IgG (FcγRI), other Ig-associated receptors, complement fraction and toll-like surface receptors (Rivera and Gilfillan, 2006). Regarding non-immune pathways, MCs can be activated by neurotransmitters (acetylcholine, histamine, serotonin, dopamine, epinephrine) and neuropeptides such as substance P and calcitonin-related gene peptide (CGRP) (Wang et al., 2014). The latter two are involved in the inflammatory and motor response after a surgical procedure. Indeed, the use of a CGRP antagonist decreases transit recovery time in a murine model of ileus (Plourde et al., 1993; Zittel et al., 1994). These pathways are prospective pharmacological targets and will be discussed in detail in the ”potential targets” section. MCs are also activated by growth factors such as nerve growth factor (NGF) (Barreau et al., 2004) or hormones such as CRF (Vanuytsel et al., 2014). MCs also play a role in the activation of nociceptive signals via bidirectional interactions with neurons of the enteric nervous system (Cenac et al., 2010). MCs activation leads to degranulation and release of newly synthesised (cytokines and lipid mediator) and stored (histamine, heparin, proteases) active substances (Wouters et al., 2016). These substances play a major role in regulating vascular and epithelial barrier function, the latter being a potential pathway for activation of resident macrophages. Epithelial permeability is an important element because its alteration results in the passage of bacteria into the lamina propria (Santos et al., 2001). This barrier function is partially regulated by interaction with the protease activated receptor-2 (PAR-2) on the basolateral part of the enterocytes (Hyun et al., 2008; Vergnolle, 2016). PAR-2 is also activated by chymase and tryptase - mediators secreted by MCs (Jacob et al., 2005). This results in redistribution of the tight junction and an increase in paracellular permeability to macromolecules (Martínez et al., 2012).
The resident macrophages and their activation pathway are the lynch pin of the inflammatory phase of the ileus (Asano et al., 2015). These macrophages are housed in the muscularis near the mesenteric plexus (Kalff et al., 1998). They play a central, predominant role in postoperative ileus despite the fact that there are still unknown areas regarding their activation pathway and the mechanisms they generate. In physiological conditions, outside of any inflammatory stress or infectious aggression, notably bacterial, they are quiescent (Kalff et al., 1999a).
The activation of resident macrophages leads to a cascade of reactions resulting in the production of chemokines (monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1α (MIP-1α)), pro-inflammatory cytokines (Tumor necrosis Factor alpha (TNFα), interleukin: IL1β, IL6) and integrins, and results in an up-regulation of cell adhesion molecules (intercellular adhesion molecule-1 (ICAM-1), lymphocyte function-associated antigen-1(LFA-1)) in the endothelium (Türler et al., 2007). This results in the passage of pro-inflammatory cells, particularly leukocytes, into the intestinal muscularis by diapedesis (Kalff et al., 1999b). The invasion of these cells leads to up- regulation of inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX2) resulting in increased nitric oxide (NO) and prostaglandin (PGs) production (Eskandari et al., 1999; Kalff et al., 2000; Kreiss et al., 2003). These phenomena inhibit contractile smooth muscle activity, which is further potentiated by the secretion of PG stimulating the afferent spinal nerves (Grant et al., 2007; Cenac et al., 2010; Forsythe, 2019).
The mechanisms and activation of the inflammatory pathway and resident macrophages (located in the myenteric plexus between longitudinal and circular muscle layer) are multiple with varying levels of evidence. The initial neurological phase via intense activation of afferent fibres leads to the release of pro-inflammatory neuropeptides (Stakenborg et al., 2020). Numerous studies have highlighted the role of calcitonin gene-related peptide (CGRP) and substance P (SP) (Bueno et al., 1997; Rekik et al., 1997). Indeed, CGRP appears to activate resident macrophages and mast cells, although this pathway (mast cells) has been debated in the light of recent study (Glowka et al., 2015). The intestinal cholinergic pathways constitute another route of interaction between neurons and macrophages. There is proximity between enteric neurons and resident macrophages in the myenteric plexus (Stakenborg et al., 2020). The secretion of acetylcholine (Ach) by enteric neurons, stimulated by vagal efferences, leads to activation of α7-subtype of the nicotinic acetylcholine receptor (7αAChR) (de Jonge and Ulloa, 2007). Thus, activation of the 7αAChR ’s leading to inhibition of TNF (Huston et al., 2006) production and induction of regulatory T cells (Rosas-Ballina et al., 2011).
Tissue damage and dehydration of intestinal tissue caused by intestinal manipulation led to the release of factors such as ATP, HMGB1 and IL-1alpha. Activation by damage-associated molecular patterns (DAMPS) during cellular damage is another pathway (Lotze et al., 2007). Finally, when the intestinal barrier is altered, bacterial invasion or their products such as liposaccharides interact with Toll-Like Receptors (TLR). All of these phenomena lead to activation of TLR and receptors for advance glycation end product (RAGE) (Hori et al., 2001). This generates an intracellular signalling pathway involving the activation of kinases (p38, JNK/SAP) causing phosphorylation of transcription factors such as nuclear factor kB (NF-kB), signal transducers and activator of transcription (STATS), and early growth response protein 1 (EGR-1) leading to the transcription of pro-inflammatory genes (Hommes et al., 2003; de Jonge et al., 2005; Wehner et al., 2009).