*3.4. IL-22*

IL-22, a cytokine of the IL-10 superfamily, was originally identified as an IL-9-induced gene in mouse T cells, and was named as IL-10-related T cell-derived inducible factor as it shares 22% amino acid identity with IL-10 [127]. IL-22 binds to a functional receptor complex composed of two chains: IL-22 receptor 1 (IL-22R1) and IL-10R2 [128].

#### 3.4.1. The Source of IL-22

IL-22 is produced from many different cell types such as activated T, NK cells and CD11c+ cells [129–131]. As mentioned above, in the intestine ILC3s are the main source of IL-22 [75].

#### 3.4.2. The Function of IL-22

IL-22 is increased in the intestine in patients with IBD as well as murine DSS colitis [132–135]. Although IL-22 increases the gene expression of other proinflammatory cytokines, such as IL-8 and TNF-α in intestinal epithelial cells, IL-22 promotes wound healing of the intestinal epithelium in vitro through stimulation of cell migration via phosphatidylinsitol 3-kinase signaling and beta-defensin-2 expression [135]. In addition, as mentioned above, IL-22 protects intestinal stem cells in graft versus host disease via activating STAT3 signaling and protects against genotoxic stress [78–80]. IL-22 knockout mice showed delayed recovery from biopsy forceps and DSS induced mucosal injury [129,130]. Due to decreased production of antimicrobial proteins, such as RegIIIβ and RegIII<sup>γ</sup>, IL-22 knockout mice have increased susceptibility to *Citrobacter rodentium* infection [134]. A recent study showed that IL-22 induces expression of H19 long noncoding RNA in epithelial cells to promote epithelial proliferation and mucosal regeneration [136]. Exogenous IL-22 also mitigates *Citrobacter rodentium* infection mediated colitis in mice with depletion of CX3CR1+ mononuclear phagocytes [77]. Local gene delivery of IL-22 into the colon promotes recovery from acute intestinal injury via STAT3 mediated mucus production [137].

#### 3.4.3. The Regulation of IL-22

Human intestinal ILC3 production of IL-22 is regulated by microbial stimulated IL-23 and IL-1β from CX3CR1+ mononuclear phagocytes [77]. IL-22 can be neutralized by its soluble receptor IL-22 binding protein (IL-22BP; also known as IL-22RA2), which specifically binds IL-22 and prevents its binding with membrane-bound IL-22R1 [138]. IL-22 is most highly expressed at the peak of DSS and biopsy induced intestinal tissue damage, whereas IL-22BP has the lowest expression at this time [139]. AhR also increases IL-22 production to protect against trinitrobenzene sulfonic acid-induced colitis [140]. A recent report showed that the receptor-interacting protein kinase 3 promotes intestinal tissue repair after DSS colitis via induction of IL-22 expression in a IL-23 and IL-1β dependent manner [141].

## **4. Concluding Remarks and Perspectives**

In conclusion, inflammatory cells and cytokines play critical roles in intestinal tissue repair. The introduction of anti-TNF-α antibodies has already been a grea<sup>t</sup> advance for IBD targeted therapy. Thus, targeting the above cells and cytokines may represent novel therapies for IBD. A recent phase II clinical trial showed that a human blocking antibody against T cell and NK cell receptor natural killer group 2D induced significant clinical remission in active CD patients after 12 weeks [142].

This review only covered some of the most important immune cell types and cytokines; others may also play an important role in wound healing. For example, IL-36γ was induced during experimental colitis and human IBD in a microbiota-dependent manner [143]. IL-36R-deficient mice showed delayed recovery after DSS-induced intestinal injury with profound IL-22 reduction and impaired neutrophil accumulation. In addition, we did provide much detail about the interaction between different cell types; for example, inflammatory monocytes may inhibit neutrophil activation in a prostaglandin E2 dependent manner [144]. Also, the bidirectional interactions between macrophages and lymphocytes were previously reviewed [145].

As discussed above, microbiota is essential in regulating neutrophil recruitment, colonic macrophage development, Treg function, and gene expression of ILCs (Figure 4). Thus, it is also critical to investigate microbiota and other emerging factors such as nutrients for developing novel targeted therapy to promote intestine repair.

**Figure 4.** Regulation of immune cells by microbiota. A. Antibiotics can inhibit the recruitment of neutrophils by inhibition of G-CSF production. B. Colonic macrophages and their secretion of IL-10 are significantly reduced in germ-free mice. C. Tregs and their production of IL-10 are reduced in germ-free mice as well as antibiotics treatment. D. The gene expression of ILCs are modulated by commensal microbiota.

**Author Contributions:** X.X. conceived the topic, wrote the manuscript and generated the figures. D.M.F. performed some literature review and wrote a portion of the draft. All authors approved the final version of this manuscript.

**Funding:** This research was funded by the National Institutes of Health (K01DK114390), a Research Scholar Grant from the American Cancer Society (RSG-18-050-01-NEC), a Research Pilot Project Grant from University of New Mexico Environmental Health Signature Program and Superfund (P42 ES025589), a Shared Resources Pilot Project Award and a Research Program Support Pilot Project Award from UNM comprehensive cancer center (P30CA118100), and a new investigator award from the Dedicated Health Research Funds at the University of New Mexico School of Medicine. The APC was funded by the National Institutes of Health (K01DK114390). D.M.F. was supported by the Academic Science Education and Research Training program at the University of New Mexico Health Sciences Center (NIGMS Institutional Research and Academic Career Development Award; K12-GM088021).

**Conflicts of Interest:** The authors declare no conflicts of interest.
