Cytokines must be carefully regulated in order to prevent excessive destruction of host tissues. One such mechanism of regulation is through host production of a cytokine-binding protein. Several cytokines have coordinating binding proteins or soluble receptors that bind to the cytokine and prevent cellular signaling by blocking the cytokine’s interaction with its cellular receptor. This property potentially allows for site-specific, microenvironmental regulation of potent cytokines. The best-described examples are IL-18 binding protein and soluble IL-6 and TNF receptors (1–3). IL-22 also has a coordinating binding protein, termed IL-22 binding protein (IL-22BP), that has high homology to one subunit of the heterodimeric IL-22 receptor. IL-22BP binds to IL-22 with greater affinity than the receptor, sequestering IL-22 so it cannot interact with cell surface bound receptor and modulate cell signaling ( Figure 1 ). IL-22 is a critical cytokine in tissue responses to inflammation, especially at sites that form a barrier between the environment and the host, such as the skin, lungs, and gastrointestinal (GI) tract (4, 5). IL-22BP has an important role in controlling the biological activity of IL-22 in healthy individuals and during infection or chronic inflammatory diseases. In this review, we will provide a foundation on IL-22 biology and then focus on IL-22BP, including its regulation, role in health and disease, and potential as a therapeutic.

IL-22 is a member of the IL-10-related cytokine family, which includes IL-19, IL-20, IL-24, and IL-26 (6). These cytokines were identified from human genome sequencing due to their shared amino acid sequence and structural similarity to IL-10. For signaling, their heterodimeric receptors share several different receptor chains that assort to generate different receptors of different cytokine specificity (6) ( Figure 2 ). All cytokine family members are involved to some degree in tissue-mediated responses during inflammation (6). IL-22 is the best-studied member and can contribute to both innate and adaptive immune responses (4, 5). The major sources of IL-22 are CD4 T cells and group 3 innate lymphocytes (ILC3s); the cytokine can also be produced by other immune cells, such as γδ T cells, natural killer (NK) cells, NK T cells, and mucosal associated invariant T (MAIT) cells (4). Like many other cytokines, IL-22 is produced upon immune cell activation, which could be an antigen-specific response to a MHCII-presented peptide for CD4 T cells or innate cytokines, such as IL-23 or IL-1β for ILC3s (5).

IL-22 signals to responsive cells through a heterodimeric receptor composed of IL-22R paired with a second chain, which is IL-10R2 in humans and IL-10Rβ in mice (7, 8). This complex is specific to IL-22 signaling, but each chain can pair with other receptors to form a receptor for related cytokines; IL-10Rα pairs with IL-10Rβ to form the receptor for IL-10, and IL-22R can pair with IL-20Rβ to form one receptor for IL-24 (9) ( Figure 2 ). Immune cells normally lack IL-22R and do not respond to IL-22 (10). The main responding cells are epithelial cells, such as keratinocytes, hepatocytes, and enterocytes, but also include other cells, such as fibroblasts (4). In these cells, IL-22 receptor binding leads to phosphorylation of Janus tyrosine kinase (JAK 1) and tyrosine kinase 2 (TYK2), promoting phosphorylation of the transcription factor STAT3. Other pathways have been shown to be activated depending on the cell type and include STAT1, AKT, and MAPKs (4). These signaling pathways lead to activation or repression of many different genes, the best studied of which are highly upregulated by IL-22. Some genes are broadly upregulated in many cell types, such as those that increase cellular proliferation or inhibit apoptosis (11). Many genes upregulated by IL-22 are tissue specific. For example, in the GI tract, IL-22 can induce mucin production in goblet cells or antimicrobial peptides in Paneth cells (12, 13). Many more genes have been shown to be positively and negatively regulated by IL-22 through microarray and RNA sequencing studies, including genes encoding nitric oxide synthase 2 (NOS2), several chemokines (CXCL12, CXCL13, CCL20) and potent systemic secreted factors such as serum amyloid A (SAA) and LPS binding protein (LBP) (4).

IL-22 can have protective or pathogenic roles in inflammation. This dual role has been best elucidated from experimental mouse models with gene-deficient mice or neutralizing antibodies (Abs). This dual-natured activity is thought to be dependent on the context of inflammation, which includes the involved tissue, levels of other cytokines, oxygen and metabolites and their derivatives, and other inflammatory mediators, like leukotrienes and prostaglandins. In general, IL-22 is pathogenic in skin inflammation (14). For example, in psoriasis, IL-22 promotes proliferation of keratinocytes, preventing proper differentiation as the cells more rapidly progress through the dermis to the epidermis (15). Anti-IL-22 Ab has shown promise in the treatment of patients with atopic dermatitis (16). In contrast, IL-22 has been shown to be both protective and pathogenic in different inflammatory models of the GI tract; it is often protective in acute models and pathogenic in chronic models of inflammation (5).

IL-22 is a potent cytokine that can regulate many critical cell pathways in tissues. As such, an individual must regulate this activity not only at the molecular and protein levels of cytokine transcription and translation, but also at a biological level to control IL-22 after it has been released by immune cells into the “environment” of the individual. This is where IL-22BP enters the cytokine story.

IL-22BP was identified as a soluble receptor homolog of IL-22R soon after the identification of IL-22R as part of the receptor for IL-22 (17–19). The gene encoding IL-22BP, IL22RA2 in humans and Il22ra2 in mice, is encoded on chromosome 6 in humans and chromosome 10 in mice (17). The gene encoding the receptor chain IL-22R, IL22RA1 in humans and Il22ra1 in mice, is encoded on chromosome 1 in humans and chromosome 4 in mice (7, 8). This suggests that the gene encoding IL-22BP may have arisen from an evolutionarily distant gene duplication event. IL-22BP shares 34% sequence homology with the extracellular domain of IL-22R (20). This shared homology extends to the secondary and tertiary structures, which allows for IL-22BP to bind IL-22 in a manner that blocks IL-22-IL-22R binding.

IL-22, encoded as a 179 amino acid protein that is an active secreted 146 amino acid protein, is a compact bundle of six anti-parallel α-helices (21). IL-22 is active as a monomer, but has been found as dimers and tetramers at high concentrations in a laboratory setting (21). Secreted IL-22BP is an approximately 24-kDa protein and forms an L-shaped structure with two fibronectin-III domains in tandem (22).

IL-22 binds to IL-22R with a high affinity and has no affinity for IL-10R2, leading to a model where IL-22 binds IL-22R, allowing binding of IL-10R2 and subsequent downstream signaling (20). IL-22 and IL-22R bind at a one-to-one ratio. IL-22BP binds to IL-22 at a site that overlaps with IL-22R, interfering with the ability of IL-22 to bind to IL-22R, and thus inhibiting cell signaling. The affinity of IL-22 and IL-22BP is K_D 1 pM compared with K_D 20 nM for IL-22 and IL-22R (20). The K_off rate for IL-22-IL-22BP is 4.7 days, whereas it is only a few minutes for IL-22-IL-22R (20, 22). The 10,000-fold higher affinity for IL-22 to IL-22BP than to IL-22R and the difference in K_off rate makes IL-22 bound to IL-22BP essentially inaccessible to IL-22R. The 2.75 Å resolution crystal structure of IL-22 bound to IL-22BP has revealed that, like IL-10 binding to IL-10R1 and IL-22 binding to IL-22R, IL-22BP contacts IL-22 using five binding loops (22).

Like IL-22, IL-22BP is primarily produced by immune cells. Unlike many immune effector molecules that are more highly expressed by activated immune cells than by their resting counterparts, IL-22BP is expressed constitutively. In healthy rodents, Il22ra2 levels are highest in the spleen and mesenteric lymph nodes and substantial in the small intestine and colon compared with other tissues, such as the thymus, heart, bladder, and liver (23). Dendritic cell (DC) production of IL-22BP has been best characterized (23–26). A recent study using single cell RNA sequencing expanded upon past studies, showing that IL-22BP is produced by distinct subset of DCs in the GI tract (26). CD11c+ DCs within the cryptopatches and isolated lymphoid follicles respond to lymphotoxin-β with production of IL-22BP. Other identified immune cell sources include eosinophils and CD4 T cells (27, 28). IL-22BP has been shown to be produced by keratinocytes (29), which are also a source of other IL-10-related cytokines, IL-19, IL-20 and IL-24, but not IL-22 (30). Additional insight into sources of IL-22BP in humans could be gleaned from databases such as the Human Cell Atlas and Immunological Genome Project (ImmGen).

Although much of IL-22BP biology is similar between mice and humans, there is some discordance between mouse and human mRNA that encodes IL-22BP. There are three different IL22RA2 isoforms in humans, but just one Il22ra2 mRNA in mice, which corresponds to human isoform 2 (31, 32). Of the human isoforms, some inhibit IL-22 and some do not. IL-22BPi1 is not secreted likely due to inclusion of exon 3 and therefore fails to bind to IL-22 (31). IL-22BPi2 binds better than IL-22BPi3, suggesting that the isoforms could have distinct temporal and spatial roles in tissues (31). In contrast, the one mouse isoform produces one form of the protein, which is able to bind IL-22 (32). Furthermore, differences in binding concentrations between mouse and human IL-22-IL-22BP complexes, mouse IL-22BP binds IL-22 at high nanomolar concentrations whereas human IL-22BP-i2 can bind IL-22 at pM concentrations (20), may also impact our elucidation of IL-22-IL-22BP biology through mouse models. In vitro human and translational studies are undoubtedly necessary to complement mouse models of disease to better understand IL-22BP biology.

Environmental signals that control IL-22BP production include factors that induce the protein, such as retinoic acid, and factors that downregulate IL-22BP production, such as prostaglandin E2 (PGE₂) (23, 33). In DCs, retinoic acid induces Il22ra2, suggesting that this active metabolite of vitamin A has further functions in addition to its well-described role in tolerizing GI tract CD103+ DCs (34, 35). PGE₂ is a suppressor of Il22ra2. In skin inflammation models where Il22ra2 is downregulated during inflammation, inhibition of PGE₂ inhibits inflammation-mediated decreases in Il22ra2 levels in the skin (33). Unlike IL-22, which is highly inducible in the mouse GI tract by colonization of the microbiome, colonic levels of IL-22BP are not affected by the presence or absence of commensal bacteria (36). Other environmental signals found in an in vivo niche are likely important, as in vitro differentiated DCs produce low levels of IL-22BP. The transcription factors that control IL22RA2 and Il22ra2 expression have not been well elucidated. There is great potential to mine databases such as the Encyclopedia of DNA Elements (ENCODE) to identify functional elements in the human genome that regulate genes involved in IL-22-IL-22BP biology.

Most studies examining in vivo production of IL-22BP have focused on the GI tract. Conventional DCs in the GI tract, including CD103+ DCs, which are a source of IL-22-inducing IL-23, can secrete IL-22BP (23). CD4 T cells are also a source of IL-22BP in mouse IBD models and in Crohn’s disease (CD) and ulcerative colitis (UC) patients (28). Upon inflammasome activation, Il22ra2 levels are downregulated in the inflamed mouse colon (25). In addition to proteins involved in inflammasome activation, such as nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain (NLR) family pyrin domain containing 3 (NLRP3), apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD) domain (ASC), and caspase 1, IL-18 is also an important factor in mediating IL-22BP downregulation (25). It is not clear whether inflammasome activation downregulates Il22ra2 expression, or if it leads to death and/or trafficking of the producing cells.

IL-22 and IL-22BP frequently have inverse levels in the healthy or inflamed GI tract (25). IL-22BP levels are highest during immune homeostasis and then rapidly downregulated during inflammation. In contrast, IL-22 levels are low in healthy tissue and are then rapidly induced in ILC3s and CD4 T cells by inflammation. Although this inverse expression was observed in a DSS-mediated model of colitis and mechanical injury in mice (25), IBD patients can have elevated levels of both IL-22 and IL-22BP (28).

Many tissues can be the target of IL-22, but the best studied are the skin, liver, lung, GI tract, and central nervous system (CNS). These are also the tissues for which the role of IL-22BP has been elucidated. IL-22 is a dual-natured cytokine. As such, the role of IL-22BP in inflammation can also be pro-inflammatory or protective. All known functions attributed to IL-22BP occur through its inhibition of IL-22. Many studies, many of which are cited in this review, to elucidate the role of IL-22BP have used gene-deficient mice to determine a phenotype. Under specific pathogen free conditions these mice appear healthy, but in some, but not all, inflammation models where IL-22 is important, they have observable phenotypes. When Il22ra2^-/- Il22^-/- double-deficient mice are studied, the phenotype is ablated and the mice return to a wild-type phenotype, suggesting that the sole function of IL-22BP is to modulate IL-22 biology (25, 37).

IL-22 biology is of interest to the pharmaceutical industry for development of therapeutics to combat chronic inflammatory diseases (68). Both the protective and pathogenic natures of IL-22 are being targeted. Recombinant IL-22-Fc is under trials for wound repair in ulcerative colitis flares (NCT02749630, NCT03650413, NCT03558152), acute graft-versus-host disease (NTC04539470) and COVID-19 (NCT04386616) (69). In contrast, neutralizing Abs to IL-22 are being investigated for treatment of psoriasis (70). IL-22BP has received less scrutiny as a potential drug or drug target. Studies using experimental models of inflammation provide some insight into the potential for IL-22BP as a therapeutic. As for IL-22, there may be promise in increasing IL-22BP for some diseases and blocking IL-22BP activity in other diseases. For example, in a mouse model of sepsis, injection of IL-22BP-Fc reduced disease severity (71). In contrast, pharmacological modification of IL-22BP may be an effective strategy to limit liver cirrhosis (51). Immunodepletion of IL-22BP may be possible although targeting IL-22BP to increase bioactive IL-22 levels would need to be carefully performed as depleting IL-22BP may also deplete bound IL-22. Design of a small molecule to block the interaction of IL-22BP with IL-22 may be possible as the IL-22BP-IL-22 crystal structure has been elucidated to 2.75 Å resolution with identification of critical binding residues (22).

As preventative treatments and therapeutics targeting IL-22 biology are developed, it is important to also use our knowledge to predict potential issues. Excess IL-22BP beyond physiological levels could have undesirable side effects. Reductions in bioactive IL-22 could lead to issues such as increased susceptibility to GI or pulmonary infections. However, this is somewhat unlikely, as the only described IL-22 immunodeficiency in humans is autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) (72, 73). In this rare, inherited disorder, patients generate auto-antibodies to IL-22 and/or IL-17 that predispose them to chronic mucosal fungal infections, particularly C. albicans. Decreased levels of IL-22BP could have long-term effects on development of cancers, particularly colorectal cancer, due to long-term higher levels of bioactive IL-22 (25).

IL-22BP may have potential as a biomarker. IL-22BP levels could be used for a prognostic or predictive capacity of different inflammatory or infectious diseases. For example, patients with active IBD on anti-TNFα therapy that responded to that therapy had undetectable levels of IL22RA2 in T cells and DCs isolated from biopsies, compared with the patients that did not respond to anti-TNFα therapy (28). IL-22BP has been discovered and validated to predict the prognosis of colorectal cancer patients; patients with high IL-22BP have greater survival than do patients with low IL-22BP (47). IL-22 biology certainly holds promise for therapeutic targeting. IL-22BP is an important target itself, or at a minimum, should be considered when targeting IL-22.

IL-22BP is a fundamental example of a protein that is downregulated during inflammation, but nevertheless has a strong effect on the course of inflammation. By binding to IL-22, IL-22BP maintains immune homeostasis and limits the activity of a potent cytokine. Many tissues, especially those at the interface with the environment, respond to IL-22 stimulation to protect themselves and the host. There are still many gaps in our knowledge on IL-22BP. Understanding how IL-22BP levels are transcriptionally and post-transcriptionally regulated and the microenvironmental interactions between IL-22BP and IL-22 are two important research directions. Ongoing and future work will help elucidate these unknowns. IL-22BP allows for more exquisite regulation of a cytokine and may be key to harnessing IL-22 biology to combat chronic inflammatory and infectious diseases.

The author confirms being the sole contributor of this work and has approved it for publication.

Research in the Zenewicz laboratory has been supported by an OUHSC College of Medicine Alumni grant, an American Heart Association Scientist Development grant (14SDG18700043), Oklahoma Center for Advancement of Science and Technology grant (HR13-003), the National Institute of General Medical Sciences of the National Institutes of Health (P20GM103447 and P20 GM134973), and funding from the Oklahoma Center for Adult Stem Cell Research (a program of the Oklahoma Tobacco Settlement Endowment Trust), Presbyterian Health Foundation, and the Stephenson Cancer Center.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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