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).