Discussion
Intestinal homeostasis depends on the intricate signalling network
between gut microbiota, the host’s immune system, and the intestinal
epithelium integrity. It is clear that even with the knowledge there is
about IL-22 today, it’s role remains yet contradictory in the mammalian
intestine. Some studies report highly beneficial effects of IL-22 in the
intestine, while others show the opposite. Moreover, IL-22 appears to
have an opposing role also in other tissues, such as the skin, lungs,
and kidneys (reviewed in Dudakov et al., 2015). The fact that IL-22 is
produced by plethora of immune cells and it has a systemic effect in the
mammalian organism indicates that this cytokine and its production must
be strictly regulated.
One of the beneficial effects designated to IL-22 in the intestine is
the promotion of AMP secretion by intestinal epithelial cells. Several
studies have shown that IL-22 induces epithelial cells to increase the
production of AMPs, such as β-defensin 3, lipocalin-2, and a number of
Reg-family peptides (Gronke et al., 2019; Gunasekera et al., 2020; He et
al., 2022; Lindemans et al., 2015; Patnaude et al., 2021). AMPs, as the
name indicates, target microbes and are thus an integral part of the
protection at the mucosal surfaces against intestinal inhabitants. AMPs
are produced mainly by Paneth cells that reside in the bottom of small
intestinal crypts (reviewed in Goyal et al., 2021). Importantly, IL-22
has been revealed to drive the differentiation of human Paneth cells (He
et al., 2022). The location of Paneth cells creates a concentration
gradient of AMPs in the crypts, with the highest concentration of AMPs
in the bottom of the crypt, protecting the most important cell types
within the intestine – stem cells. IL-22, therefore, enhances the
protection of the source of all cell types in the intestine and thus
contributes to the maintenance of intestinal homeostasis. However,
several of the Reg-family AMPs are found to be overexpressed during UC
and CD in humans (Tsuchida et al., 2017). AMP overexpression may lead to
a significant decrease, or even elimination, of some bacterial species
resulting in an ecological disbalance of intestinal microbiota, allowing
possibly harmful bacterial species to thrive and cause damage to the
host. Additionally, elevated IL-22 in the intestine has been linked to
reduced microbial diversity because of an IL-22-induced increase in AMP
production (Gunasekera et al., 2020). Taken together, it appears that
when appropriately regulated, IL-22 induces epithelial cells to secrete
just the right amount of AMPs to ward off intestinal microbes at the
mucosal surfaces without causing dysbiosis. However, excessive amounts
of IL-22 may lead to overexpression of AMPs in the intestine which, as a
consequence, trigger microbial dysbiosis possibly leading to intestinal
disorders, such as UC and CD.
IL-22 has been reported to increase the proliferative capacity of the
intestinal epithelial cells in a number of studies
(Lindemans et al., 2015; Patnaude
et al., 2021; Zha et al., 2019). This feature of IL-22 certainly makes
it beneficial in the mammalian intestine since the rapid renewal of the
epithelium is the foundation of the symbiotic relationship between gut
microbiota and the host. Without the fast regeneration of the
epithelium, the intestinal microbes would likely overwhelm the host and
its immune system. Because IL-22 appears to induce epithelial
regeneration in the intestine, it has been suggested to use this
cytokine as a therapeutic agent in dysbiotic and intestinal inflammatory
disorders, such as IBD or NEC. However, the effect IL-22 has on ISC
proliferation remains controversial. There are studies reporting an
increase in ISC numbers upon IL-22 treatment (Lindemans et al., 2015;
Zhang et al., 2020), while one reports the opposite (Zha et al., 2019).
Intriguingly, the positive and negative effects of IL-22 on the
intestinal stem cell population was observed in vitro and in vivo
(Lindemans et al., 2015; Zha et al., 2019; Zhang et al., 2020). The
discrepancy in these results could be due to differences in tissues used
to derive organoids, the concentration of IL-22 used in the experiments,
the expression of IL-22 receptor, or the time period the of IL-22
treatment. It is noteworthy that while Zha et al. (2019) report a
reduction in Lgr5+ active stem cell population upon IL-22 treatment, TA
cell proliferation in the intestine was promoted by IL-22 in mice.
Furthermore, it has been demonstrated that IL-22 specifically induces
Paneth cell differentiation and proliferation in human, but not murine,
small intestine derived organoids (He et al., 2022). This is a major
difference in the effects IL-22 has in human and murine intestinal
models. It is, thus, possible that the differences reported in effects
of IL-22 in the intestine could be due to differences in the
experimental setups. Differences such as, the species of origin (human
vs mouse) of tissues and cell lines used for in vitro studies,
the specific intestinal sections examined, or the timing and the
duration of IL-22 treatment. It is also important to recognize that
while intestinal organoids are an excellent in vitro model to
study molecular interactions under various physiological conditions,
they have limitations. For instance, IL-22 may potentially be
differently regulated in different parts of the intestine (small vs
large intestine), or in structures not found in the organoids.
Furthermore, the presence of IL-22 regulators, such as IL-22BP varies
between in vivo and in vitro experiments. In vitromodels usually do not include such compounds, whereas they are likely
present in in vivo systems. This can consequentially lead to
different outcomes of the experiments. Therefore, a whole organism still
remains to be the best model to study complex immunological
interactions.
Another aspect that might lead to difference in IL-22 signalling outcome
is a difference in the cell types that produce IL-22. This is well
illustrated by the T cell transfer colitis model in mice. It has been
shown that IL-22 originating from memory/effector T cells induces
pathogenicity (Kamanaka et al., 2011), whereas IL-22 produced by naïve T
cells had a protective role in the T cell transfer colitis model
(Zenewicz et al., 2008). Although, it is also possible that the effects
on the intestinal tissue result from the activity of naïve T cells
themselves instead of IL-22. Next to this, different immune cell types
arrive at the site of the infection at different times. Neutrophils are
among the first immune cells to arrive at the site of infection and are
known to produce IL-22 (reviewed in Dudakov et al., 2015). However,
adaptive immune cells, such as T cells, arrive at the site of infection
much later. It allows for a sustained production of IL-22 during
different stages of intestinal infection or damage, but it may also
define the outcome of IL-22 signalling in the intestine. This might also
hold true for normal early life development. Mao and colleagues showed
that during early life, the sequential development of innate and
adaptive immunity influences IL22 signalling outcome. Innate cells (such
as ILC3s) might be activated by the microbiota before an effective
adaptive immune response develops. In their research, the authors show
differences in IL-22-dependent IEC signalling between wild-type and
Rag1−/− adult mice. They showed that the developing adaptive immune
cells (CD4+ T cells), silenced the ILC3-induced pSTAT3 signalling (as a
result of IL22R binding) in the epithelial cells of wildtype mice. They
also showed that in Rag-/- mice, in the absence of a dominant adaptive
immune response, the persistent activation of ILC3s resulted in impaired
lipid metabolism. Together these findings indicate that a possible delay
or alteration in adaptive immunity might influence IL22 effector
functions in terms of lipid metabolism as well as host microbe
interactions. A host-microbiome feedback loop also seems to exist in
mice and humans in which the microbiome can influence IL-22 production
via synthesis (of precursors) of AhR ligands and IL-22 in turn
influences the microbiota composition through its actions on AMP and
mucus (Mar et al, 2023).
Moreover, the presence of other cytokines can affect how adaptive immune
cells are activated. For instance, IL-22 together with IL-10 may have a
different effect on T cell activation compared to IL-22 together with
IL-17. These are important details to keep in mind when finding answers
to questions related to IL-22 signalling. Next to temporal differences
in IL-22 production in the intestine, there is also spatial variability.
For example, ILC3s – potent IL-22 producers – are more abundant in the
small intestine than in the large intestine (reviewed in Kim et al.,
2016). The location of IL-22-producing and IL-22 receptor-expressing
cells may therefore affect the outcome of IL-22 signalling. Moreover,
the presence of IL-22BP in the intestine is highly important in defining
the outcome of IL-22 signalling. Additionally, the presence and the
expression of IL-22 regulators differs between intestinal sections; for
example butyrate is solely produced in the colon upon microbial
fermentation. Furthermore, IL-22BP is highly expressed in the colon
compared to small intestine in healthy mice (Huber et al., 2012). This
can significantly change the results as well as interpretations ofin vivo experiments. Another important point to consider is how
far-reaching the IL-22 signalling is. When IL-22 production is elicited
at the location of damage in one specific segment of the intestine and
the cytokine remains at that site, it is likely beneficial as it induces
cell proliferation and healing where needed. However, if IL-22 moves to
another site in the intestine, or other tissues, where there is no
damage, it may induce needless epithelial cell proliferation, leading to
possibly pathological conditions.
IL-22 has been linked to increased paracellular permeability and
decreased epithelial integrity by upregulating claudin-2 protein (Wang
et al., 2017; Zha et al., 2019). This may lead to the flux of water and
solutes into the gut lumen, and this is an efficient way to remove
parasites and other pathogens from the intestine. However, a persistent
and uncontrolled flux of solutes and water into the gut may cause
diarrhoea and dehydration. Furthermore, increased permeability allows
the microbes to leave the intestinal environment and enter the tissue
surrounding the intestine, potentially leading to (acute) inflammation.
On top of that, IL-22-induced paracellular permeability, and not
proliferation of cells, has been suggested to be the reason why
organoids increase in size during in vitro experiments because of
increased claudin-2 expression (He et al., 2022; Zha et al., 2019).
Thus, the technique used to evaluate organoid growth – surface area or
proliferative capacity of the cells – could lead to different
interpretations of results.
An additional important factor in the intestine affecting IL-22 is
butyrate, a SCFA produced by some types of intestinal bacteria, and a
main energy source for intestinal epithelial cells (reviewed in
Martin-Gallausiaux et al., 2021). Interestingly, butyrate has been shown
to increase MUC13 expression in human colon organoids synergistically
with IL-22 (Patnaude et al., 2021), suggesting that it may enhance the
beneficial effects of IL-22 in mammalian intestine. Furthermore,
butyrate was shown to over-ride the disruptive effects to the epithelial
barrier elicited by IL-22 (Patnaude et al., 2021). The effects butyrate
has on IL-22 may be seen as a therapeutic opportunity. Treating patients
with intestinal pathologies with probiotics and/or prebiotics, which
stimulate butyrate-producing bacteria, together with IL-22 may increase
beneficial effects of IL-22 in the intestine. But also, by supplementing
only pre- and/or probiotics it may be possible to diminish the
pathological effects that IL-22 elicits in the intestine. The effects of
butyrate on IL-22 illustrate the diversity of signals and interactions
involved in regulating IL-22 and maintaining intestinal homeostasis.