IL-1 Superfamily Members and Periodontal Diseases

Abstract

Periodontitis is a complex, multifactorial chronic disease involving continuous interactions among bacteria, host immune/inflammatory responses, and modifying genetic and environmental factors. More than any other cytokine family, the interleukin (IL)–1 family includes key signaling molecules that trigger and perpetuate periodontal inflammation. Over the years, the IL-1 family expanded to include 11 members of cytokines, some with agonist activity (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β, and IL-36γ), receptor antagonists (IL-1Ra, IL-36Ra), and 2 anti-inflammatory cytokines (IL-37, IL-38). The IL-1 receptor antagonist (IL-1Ra) has emerged as a pivotal player in the defense against periodontitis. IL-33 primarily induces the production of Th2-associated cytokines but acts as an “alarmin” via stimulation of mast cells. The IL-36 subclass of cytokines may be important in regulating mucosal inflammation and homeostasis. IL-37 suppresses innate and acquired immune responses. IL-38 is the most recent member of the IL-1 superfamily and has anti-inflammatory properties similar to those of IL-37 but through different receptors. However, limited evidence exists regarding the role of IL-37 and IL-38 in periodontitis. Despite the development of IL-1 blocking agents, therapeutic blockade of select IL-1 family members for periodontitis has only been partially investigated in preclinical and clinical research, while the development of IL-37 and IL-38 as novel anti-inflammatory drugs has not been considered adequately. Here, we review the key properties of the IL-1 family members and provide insights into targeting or promoting select cytokines as new therapeutic agents.

Introduction

Periodontitis is a widespread health problem and the primary cause for tooth loss in adults. In the United States, at least 47% of adults aged 30 y and older have periodontitis, consisting of 7.8% of adults with severe periodontitis (Eke et al. 2020). Periodontitis is an infectious-inflammatory disease of the gingiva and of the supporting structures of the teeth with a multifactorial etiology and pathogenesis. Although the presence of pathogens per se is required, it is not sufficient for periodontal disease initiation and progression (Graves 2008). It is ultimately the host immune response that largely drives the pathological process (Kinane et al. 2017). A dysregulated host immune response or an imbalance within the microbial community, known as dysbiosis, can cause a disruption of homeostasis in the periodontium, leading to extracellular matrix degradation and alveolar bone loss (Darveau 2010).

Research using animal models has shown that cytokines play a critical role in the pathogenesis of periodontitis (Graves et al. 1998). Elevated levels of inflammatory mediators, such as interleukin (IL)–1, IL-6, and tumor necrosis factor (TNF)–α have been detected in gingival tissues and gingival crevicular fluid (GCF) of patients diagnosed with periodontitis (Gupta 2013; Papathanasiou et al. 2014).

IL-1 has been the most widely studied cytokine in periodontal research to date. IL-1 exhibits numerous biologic effects on different cells and is a crucial biological mediator in many diseases such as autoimmunity and inflammatory disorders (Dinarello 1984; Dinarello et al. 1986). The IL-1 family is primarily responsible for innate immunity but also plays an important role in adaptive immunity (Dinarello 2018). The IL-1 family now includes cytokines with both proinflammatory (IL-18, IL-33, IL-36) and anti-inflammatory properties such as the IL-1 receptor antagonist (IL-1Ra), IL-36Ra, IL-37, and IL-38 (Table 1).

Table 1.

Characteristics of IL-1 Family Members and Their Role in Periodontal Diseases.

IL-1 Member	Receptor	Co-receptor	Role
Proinflammatory
IL-1α, IL-1β	IL-1R1	IL-1R3 (or IL-1RAcP)	Upregulated expression and secretion in periodontal diseases. Trigger cell chemotaxis, collagen destruction, and bone resorption.
IL-18	IL-1R5 (or IL-18Rα)	IL-1R7 (or IL-18Rα)	Elevated levels in saliva, GCF, and serum of periodontitis patients and may participate in disease pathogenesis.
IL-33	IL-1R4 (or ST2)	IL-1RAcP (or IL-1R3)	Increased expression in gingival tissue biopsies associated with bone loss via upregulating RANKL.
IL-36α, β, γ	IL-36R, IL-1R6	IL-1RAcP (or IL-1R3)	Regulate mucosal inflammation and homeostasis via secretion of cytokines and antimicrobial peptides.
Anti-inflammatory
IL-1Ra	IL-1R1	-	Aims to “counteract” the detrimental effects of IL-1β in periodontium. Reduced ratios of IL-1Ra/IL-1β in GCF of periodontitis patients compared to healthy subjects.
IL-36Ra	IL-36R, IL-1R6	IL-1RAcP (or IL-1R3)	Competes with IL-36 agonists. Decreased expression ratios of IL-36Ra/IL-36 agonists in gingival samples of periodontitis patients.
IL-37	IL-1R5 (or IL-18Rα)	IL-1R8	Increased expression in gingival biopsies of periodontitis patients, suggesting an effort to restrain inflammation.
IL-38	IL-36R, IL-1R6	IL-1R9	Unknown in periodontal diseases.

GCF, gingival crevicular fluid; IL, interleukin; RANKL, receptor activator of nuclear factor kappa-β ligand.

In this review, we explore the roles of the IL-1 family members in inflammatory disorders, including periodontal disease, and provide further insights into novel therapeutic strategies. This review is timely given the emerging importance of cytokines and mast cells (MCs) in COVID-19 in general (Theoharides 2020) and in periodontal disease in particular (Chompunud Na Ayudhya et al. 2020; Sahni and Gupta 2020).

IL-1

There are 11 known members of the IL-1 family that include secreted molecules with agonist activity (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β, and IL-36γ), receptor antagonists (IL-1Ra, IL-36Ra), and 2 anti-inflammatory cytokines (IL-37, IL-38) (Table 1) (Dinarello 2018). The first members of the IL-1 family to be identified are IL-1α and IL-1β, which are encoded by different genes and are synthesized as precursor (proform) proteins (about 31 kDa) but cleaved to smaller mature forms (17 kDa) that bind to the cell surface receptor IL-1R1, triggering somewhat similar biological responses (Di Paolo and Shayakhmetov 2016; Dinarello 2018).

Different stimuli (e.g., oxidative stress, fatty acids, cytokines, hormones, trauma) can trigger translocation of pro-IL-1α onto the plasma membrane of living cells and the appearance of membrane-bound pro-IL-1α that activates IL-1R1-dependent cytokine production from neighboring nonhematopoietic cells or tissue-resident macrophages. This initial IL-1α-dependent chemokine production leads to a recruitment of myeloid cells to the site of stress and increased production of IL-1α and IL-1β upregulating and sustaining an inflammatory loop (Di Paolo and Shayakhmetov 2016; Dinarello 2018). Upon nonapoptotic cell death due to damage, stress, or infection, the loss of plasma membrane integrity allows the passive release of pro-IL-1α into the surrounding milieu where IL-1α functions as an “alarmin” activating the inflammatory cascade as described above (Di Paolo and Shayakhmetov 2016; Dinarello 2018). Thus, IL-1α plays an important role in both the initiation and maintenance of inflammation.

IL-1β is a key proinflammatory cytokine secreted mostly by monocytes/macrophages and dendritic cells (DCs) in response to several pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS), damage-associated molecular patterns (DAMPs), and other proinflammatory cytokines such as TNF-α. Activation of IL-1RI by IL-1β leads to recruitment of adaptor molecules such as MyD88 and activation of IL-1R-associated kinases (IRAKs), resulting in translocation of nuclear factor (NF)–κB to the nucleus and mitogen-activated protein kinase (MAPK)–regulated transcription factors such as c-jun n-terminal kinase (JNK) and p38 that ultimately trigger the transcription of a large portfolio of inflammatory genes (Akira et al. 2006; Dinarello 2011; 2018).

Pro-IL-1β requires activation via proteolytic cleavage by caspase 1 (Dinarello 2018). This protease is also present in the cytoplasm in a proform (pro–caspase I) and is activated by the multiprotein complex known as inflammasome (nod-like receptor pyrin domain containing protein 3 [NLRP3] and apoptosis-associated speck-like protein containing CARD [ASC]) (Vanaja et al. 2015). NLRP3 activation leads to activated caspase 1; caspase 1 then cleaves pro-IL-1β into the active mature IL-1β, which is then secreted extracellularly perpetuating inflammation (Schroder et al. 2010; Franchi and Nunez 2012; Dinarello 2018).

IL-1β plays a key role in the regulation of the innate immune response and is fundamental in autoinflammatory diseases such as cryopyrin-associated periodic syndromes (CAPS) and familial Mediterranean fever (FMF) (Dinarello 2011). In general, these diseases are poorly controlled with immunosuppressive drugs but are usually responsive to IL-1β blockade (Hoffman et al. 2004; Dinarello 2011). IL-1β may also participate in diseases that involve MCs, including asthma, atopic dermatitis, rheumatoid arthritis (RA), multiple sclerosis, and psoriasis (Theoharides et al. 2004; Gallenga et al. 2019). The fact that unstimulated MCs contain pro-IL-1β and active caspase 1 implies that MCs can respond rapidly to danger signals without requiring induction of their respective genes (Theoharides 2016).

Bacterial virulence factors (e.g., LPS, leukotoxin) trigger the release of IL-1α and IL-1β from oral epithelial cells that contribute to periodontal destruction. Recent data showed that host defense peptides (HDPs) produced from gingival epithelium in response to periodontal pathogens lead to recruitment of MCs that release more proinflammatory cytokines such as IL-1β, IL-6, and TNF-α, resulting in additional periodontal tissue breakdown (Chompunud Na Ayudhya et al. 2020). IL-1β amplifies vasodilation and inflammatory cell chemotaxis, collagen degradation via upregulating the secretion of matrix metalloproteinases (MMPs), and bone resorption by increasing osteoclastogenesis. IL-1β is a robust GCF biomarker strongly associated with periodontitis severity, progression, tooth loss, disease recurrence, and success of periodontal therapy (Graves and Cochran 2003). Analysis of gingival biopsies from patients with active periodontal disease showed downregulation of inflammasome regulators and increased messenger RNA (mRNA) expression of NLRP3 and IL-1β that trigger periodontal tissue destruction (Aral et al. 2020).

Recent data highlighted the promising use of salivary IL-1β as a biomarker for distinguishing gingivitis from periodontitis (Kc et al. 2020). The excellent diagnostic capability of salivary IL-1β to distinguish patients with untreated periodontitis from periodontal healthy patients using predictive models is reduced in smokers (Arias-Bujanda et al. 2020). IL-1β may serve as a link between periodontitis and diabetes since elevated levels of IL-1β have been detected in gingival tissues of human and animal models with diabetes (Sanz et al. 2018). Periodontal treatment resulted in reduction of systemic and local inflammation, including a decrease in serum and GCF IL-1β levels in periodontitis patients with and without type 2 diabetes (Preshaw et al. 2020). Blocking IL-1R or knocking down IL-1R in animal models of periodontitis resulted in reduced periodontal tissue destruction compared to control animals, supporting the role of IL-1 in promoting periodontitis (Graves 2008). Excessive mechanical stress, including orthodontic movement and occlusal trauma, causes hyaline degeneration and can activate NLRP3 inflammasomes, increasing IL-1β expression in the periodontal apparatus. Glyburide, an NLRP3 inflammasome inhibitor, inhibited bone resorption by traumatic occlusion in rats by decreasing IL-1β tissue levels (Arita et al. 2020).

IL-1Ra

IL-1Ra is the first described naturally occurring specific receptor antagonist of any cytokine. IL-1Ra is encoded by the IL1RN gene and can occupy the binding pocket of IL-1RI, preventing the activity of IL-1α and IL-1β without eliciting any downstream signaling (Eisenberg et al. 1990). IL-1Ra is the sole IL-1 family member that does not need processing to be active (Dinarello 2011).

Several animal models deficient in IL-1Ra showed higher susceptibility to inflammatory diseases, including arthritis and colitis (Dinarello 2011). In fact, patients with deficiency of the IL-1Ra (DIRA), an autosomal recessive genetic autoinflammatory syndrome resulting from mutations in the IL1RN gene, suffer from birth from sterile osteomyelitis, periostitis, and pustulosis due to skin inflammation (Reddy et al. 2009). The molar “ratio” of endogenous IL-1Ra to IL-1β levels in body fluids from patients with infectious, inflammatory, or autoimmune diseases is often 10- to 100-fold more IL-1Ra than IL-1β in an apparent attempt to contain IL-1-driven inflammation (Dinarello 2011).

IL-1Ra plays a defense role in periodontitis. Salivary IL-1Ra was reported as a potential indicator of probing depth changes during experimental gingival inflammation in patients (Morelli et al. 2014). Inadequate or even increased secretion of IL-1Ra in periodontitis apparently is not sufficient to “counteract” the detrimental effects of IL-1β. In fact, the ratios of IL-1Ra/IL-1β in GCF were reported to be 1.4 in healthy subjects and only 0.2 in periodontitis patients (Gilowski et al. 2014). Nonsurgical treatment of patients with periodontitis and FMF resulted in reduction of GCF IL-1β and IL-1Ra (Bostanci et al. 2017). IL-1Ra deficiency contributed to more severe periodontal tissue destruction in an experimental murine model of periodontitis (Izawa et al. 2014). Induction of M2 macrophages in a murine model of periodontitis resulted in increased mRNA expression of IL-1Ra and decreased alveolar bone loss (Zhuang et al. 2019). Loading IL-1Ra in a hyaluronic acid hydrogel synthetic extracellular matrix improved the regenerative potential of gingival margin-derived stem/progenitor cells in a miniature pig periodontitis model (Fawzy El-Sayed et al. 2015).

IL-18

IL-18 is best known for its ability to trigger interferon-γ (IFN-γ) production from T cells and NK cells and is mainly expressed by macrophages, DCs, and epithelial cells (Sims and Smith 2010; Dinarello 2018). IL-18 is first synthesized as an inactive precursor requiring caspase 1–mediated cleavage to become active. IL-18 forms a signaling complex by binding with low affinity to the IL-18 receptor α chain (IL-18Rα) and with high affinity to the co-receptor IL-18 receptor β chain (IL-18Rβ), triggering intracellular signal transduction (Dinarello 2018; Kaplanski 2018). IL-18, together with IL-18Rα and IL-18Rβ, forms a complex with the co-receptor IL-1R accessory chain IL-1RAcP (also termed IL-1R3). Following the formation of the heterodimer, the Toll-IL-1 receptor (TIR) domains approximate and allow the binding of MyD88, resulting in NF-κB activation. IL-18 can be suppressed by extracellular IL-18 binding protein (IL-18BP), which binds to soluble IL-18 with higher affinity than IL-18Rα, thus preventing IL-18 binding to the IL-18 receptor (Kaplanski 2018).

Several autoimmune diseases such as systemic lupus erythematosus (SLE), RA, type 1 diabetes, Crohn disease, and graft-versus-host disease are thought to be mediated, in part, by IL-18 (Dinarello 2018). IL-18 concentrations in GCF were increased in periodontal disease and positively correlated with the severity of disease (Pradeep et al. 2009). Increased mRNA expression of IL-18 and NLPR3 inflammasome was found in human inflamed periodontal tissues compared to healthy ones (Bostanci et al. 2009; Song et al. 2018). Salivary levels of IL-18 were found to be 5-fold higher in patients with periodontitis than healthy ones (Banu et al. 2015). In another recent study, salivary IL-18 levels were not different between patients with and without periodontitis; however, salivary IL-18 was positively associated with fasting plasma glucose, and serum IL-18 levels were correlated with HbA_1C (Techatanawat et al. 2020).

IL-18 could be responsible for the elevated IFN-γ in GCF of inflamed sites in patients with periodontal disease (Papathanasiou et al. 2014). IL-18 is mainly released from DCs that can trigger IFN-γ production from Th1 cells and IL-17 from Th17 cells, as well as upregulate the release of IL-17, TNF-α, and IL-1β, thus promoting more osteoclast formation and bone loss (Song et al. 2018). Periodontal diseases may increase the systemic circulation of IL-18. For instance, higher IL-18 levels were found in serum but not in GCF from periodontitis patients with Crohn disease and ulcerative colitis compared with controls (Figueredo et al. 2011). In addition, patients with RA and periodontitis had higher IL-18 levels in serum than patients with RA without periodontitis (Panezai et al. 2020). Interestingly, initiation and progression of ligature-induced periodontitis in monkeys were characterized by a significant reduction of IL-18 mRNA levels in gingival samples (Ebersole et al. 2014). The role of IL-18 in periodontitis is still inconclusive, and further studies are needed.

IL-33

IL-33 is a newer member of the IL-1 family and has emerged as an “alarmin” for injury-induced cell damage, leading to activation of local immune cells (Cayrol and Girard 2018). IL-33 was identified as the ligand for the then “orphan” receptor ST2 of the IL-1 receptor superfamily (Schmitz et al. 2005). IL-33 binding recruits the IL-1R AcP co-receptor, the adaptor protein MyD88, along with the associated IRAK. ST2 activation also leads to stimulation of MAPK via TNF receptor-associated factor 6 (TRAF6), which can signal the activator protein 1 (AP-1) via JNKs. TRAF6 can also activate NF-κB and proinflammatory gene transcription (Kakkar and Lee 2008).

IL-33 is widely expressed by several cell types, including smooth muscle cells, epithelial cells, fibroblasts, DCs, and macrophages. Major targets of IL-33 that express ST2 include MCs, group 2 innate lymphoid cells (ILC2s), and tissue regulatory T cells (Tregs) (Cayrol and Girard 2018). IL-33 primarily induces the production of Th2-associated cytokines but can also activate NK cells. After stimulation, IL-33 is released from the nuclei of producing cells as a full-length protein (IL-33_FL) that can be cleaved extracellularly by proteases from inflammatory cells such as neutrophils or MCs. Thus, highly active mature forms of IL-33 are generated that exhibit 10- to 30-fold higher activity than IL-33_FL (Cayrol and Girard 2018). Conversely to IL-1β, in cells undergoing apoptosis, endogenous caspases such as caspase 1 were found to cleave and inactivate IL-33 (Cayrol and Girard 2018).

IL-33 is implicated in a growing number of allergic and autoimmune disorders, including RA, inflammatory bowel disease, asthma, psoriasis, atopic dermatitis, and Alzheimer disease (Cayrol and Girard 2018). IL-33 contributes to the pathology of these diseases by activating MCs that have therefore been considered “sensors of cell injury” (Enoksson et al. 2011). In fact, IL-33 is involved in airway inflammation as it modulates crosstalk between MCs and smooth muscle cells in airways (Kaur et al. 2015). IL-33 also stimulates MCs by triggering selective secretion of TNF-α (Taracanova et al. 2017) and IL-1β (Taracanova et al. 2018).

In human gingival samples, overexpression of IL-33 was associated with periodontitis and triggered osteoclastogenesis and bone loss via increased receptor activator of nuclear factor kappa-β ligand (RANKL) (Laperine et al. 2016). Salivary IL-33 levels were higher in patients with SLE and periodontitis compared to healthy ones (Mendonca et al. 2019). In another study, higher IL-33 levels in saliva could be responsible for the microbial dysbiosis and higher bacterial loads observed in patients with SLE and periodontitis compared to a control group with periodontitis (Correa et al. 2017). IL-33 can be detected in GCF, but the levels are not known (Papathanasiou et al. 2014; Kursunlu et al. 2015; Malcolm et al. 2015). However, the gene expression of IL-33 and ST2 was increased in gingival biopsies from patients with periodontitis compared to healthy ones (Malcolm et al. 2015). In an experimental murine model of periodontitis, exogenous administration of IL-33 exacerbated Porphyromonas gingivalis infection-induced alveolar bone loss by triggering increased expression of RANKL (Malcolm et al. 2015). Stimulation of human and murine immune cells with periodontal pathogens led to increased expression of IL-33 in vitro (Malcolm et al. 2015).

The roles of IL-33 in periodontal disease pathogenesis but also in the maintenance of tissue homeostasis and healing repair need to be investigated further.

IL-36

The IL-36 subfamily includes IL-36α, IL-36β, IL-36γ, and IL-36Ra. They are primarily found in the lungs and skin, where they are mainly expressed by epithelial cells, fibroblasts, and keratinocytes, respectively. Upon binding to IL-36R and IL-1RAcP, the IL-36α, IL-36β, and IL-36γ agonists generate an intracellular signaling cascade leading to the activation of NF-κB and MAPK, inducing inflammatory responses (Bassoy et al. 2018).

Neutrophil-derived proteases present in psoriatic skin can remove 9 amino acids from the N-terminus of IL-36 cytokines, thus increasing their proinflammatory activity by 500-fold (Henry et al. 2016). The expression of IL-36 cytokines has been shown to be enhanced in humans and mice with colitis, indicating that they regulate intestinal immune responses (Bassoy et al. 2018). IL-36Ra competes with IL-36 agonists for binding to IL-36R, thereby inhibiting their proinflammatory activities. Several types of psoriasis have been linked to mutations of the IL36RN gene, resulting in reduced production or deficient IL-36Ra (Bassoy et al. 2018).

IL-36 cytokines may be important in mucosal inflammation and homeostasis, possibly through modulation of γδ⁺ T-cell immune functions (Ciccia et al. 2015). TLR2 activation of human oral epithelial cells by P. gingivalis promoted the expression of IL-36γ and triggered chemotaxis of DCs and macrophages, amplifying the secretion of inflammatory cytokines, including IL-17 (Heath et al. 2019), but also the antimicrobial peptide PGLYRP2, regulating oral mucosal homeostasis (Scholz et al. 2018). Moreover, Candida albicans infection of the oral mucosa in mice induced a production of IL-36 as an innate protective response to this fungus (Verma et al. 2018). IL-36α was overexpressed in the serum and salivary glands of patients with primary Sjögren syndrome (pSS) (Ciccia et al. 2015). IL-36β and IL-36γ have been detected in the GCF, showing higher IL-36β levels in aggressive periodontitis than chronic periodontitis (Kursunlu et al. 2015). Moreover, IL-36γ gingival expression levels were reported to be increased and positively correlated with the RANKL/osteoprotegerin ratio in periodontitis patients; in contrast, IL-36β and IL-36Ra expression was lower compared to healthy controls (Cloitre et al. 2019). IL-36 can promote the release of IL-17 by γδ⁺ T cells in the skin of patients with psoriasis (Tortola et al. 2012). Higher expression of IL-36α and IL-17 was shown from γδ⁺ T cells isolated from pSS compared to controls (Ciccia et al. 2015). γδ⁺ T cells represent the major source of IL-17 in oral mucosa (Hovav et al. 2020). IL-17 was also found to increase secretion of IL-36γ by oral epithelial cells, supporting an inflammatory axis between IL-36 and IL-17 (Cloitre et al. 2019). More studies are needed to investigate the role of IL-36 cytokines in the pathogenesis of periodontitis and other oral pathologies.

IL-37

IL-37 is produced mainly by macrophages in response to TLR activation, by IL-1β and transforming growth factor–β (TGF-β) (Dinarello 2018). An IL-37 precursor (pro-IL-37) is cleaved like IL-1β by caspase 1 into mature IL-37, some of which (~20%) enters the nucleus and the rest is released extracellularly along with the pro-IL-37, where both are biologically active (Cavalli and Dinarello 2018). The nuclear translocation and anti-inflammatory properties of IL-37 appear to be linked to binding to Smad3, a transcription factor that mediates the anti-inflammatory properties of TGF-β (Grimsby et al. 2004; Dinarello 2018).

IL-37 mainly suppresses innate but also acquired immune responses. An instability sequence in the IL37 gene limits mRNA half-life of IL-37. IL-37 is structurally similar to IL-18, suggesting a similar evolutionary process (Kaplanski 2018). IL-37 is a natural inhibitor of IL-18, as extracellular IL-37 binds to IL-18Rα with lower affinity than IL-18 and does not act as a classical receptor antagonist solely for IL-18 (Bufler et al. 2002). Free IL-37 binds to IL-18Rα, inducing the recruitment of IL-1R8 to form a high-affinity receptor, which does not bind MyD88 but induces an anti-inflammatory signal intracellularly (Kaplanski 2018). IL-18BP may also bind IL-37, preventing its binding to IL-18Rα (Cavalli and Dinarello 2018).

The anti-inflammatory effects of IL-37 have been demonstrated in several in vitro and in vivo studies (Li et al. 2015; Cavalli and Dinarello 2018). Transfection of murine macrophages with IL-37 resulted in reduced expression of proinflammatory cytokines, while silencing of IL-37 in human monocytes led to increased generation of inflammatory markers (Cavalli and Dinarello 2018). IL-37 is elevated in patients with inflammatory diseases, including inflammatory bowel disease, SLE, psoriasis, atherosclerosis, pSS, and RA (Table 2), while reduced levels of IL-37 may be associated with disease severity in asthma, insulin resistance, and allergic rhinitis (Dinarello 2018). IL-37 and IL-18R gene expression was increased in brain samples from children with autism spectrum disorder (ASD) compared to non-ASD controls (Tsilioni et al. 2019), suggesting a possible attempt to restrain inflammation.

Table 2.

Diseases with Elevated Levels of IL-37.

Disease	Tissue Examined and Method of Detection
Atherosclerosis	Serum (ELISA)
Autism spectrum disorder	Brain samples (RT-PCR)
Inflammatory bowel disease	Colonic biopsies (RT-PCR) Colonic biopsies (immunohistochemistry)
Oral leukoplakia	Biopsies (immunohistochemistry)
Oral squamous cell carcinoma	Biopsies (immunohistochemistry)
Periodontitis	Gingival samples (immunohistochemistry)
Primary Sjögren syndrome	Serum (ELISA)
Psoriasis	Serum (ELISA)
Rheumatoid arthritis	Serum (ELISA) Synovial fluid (ELISA) Peripheral blood mononuclear cells (RT-PCR)
Systemic lupus erythematosus	Serum (ELISA) Peripheral blood mononuclear cells (RT-PCR)

ELISA, enzyme-linked immunosorbent assay; RT-PCR, reverse transcription–polymerase chain reaction.

Administration of recombinant forms of IL-37 has been reported to reduce inflammation in various animal models of acute lung injury, asthma, colitis, and arthritis (Cavalli and Dinarello 2018). The protective effects of IL-37 against joint inflammation in collagen-induced arthritis were via inhibition of IL-17 and Th17 cell proliferation (Ye et al. 2015). MCs could regulate the anti-inflammatory activity of IL-37 via promoting its dimerization and loss of biological activity in the presence of heparin or activated by tryptases (Theoharides et al. 2019).

Limited evidence exists regarding the role of IL-37 in periodontitis. The expression of IL-37 increased in oral leukoplakia and oral squamous cell carcinoma as compared to normal control, suggesting a role of IL-37 in oral mucosal carcinogenesis (Lin et al. 2016). Analysis of gingival biopsies showed that IL-37 expression levels were higher in epithelial cells, endothelial cells, and infiltrating immune cells of the connective tissue in periodontitis patients (Jing et al. 2019). Moreover, anti-inflammatory cytokine-producing plasma cells (P_IL-37) were discovered to infiltrate periodontal tissues. In addition, recombinant human IL-37 inhibited osteoclastogenesis in vitro (Jing et al. 2019) and in vivo (Saeed et al. 2016). Another study detected IL-37 in all biofluids (GCF, saliva, serum) but could not distinguish periodontitis patients from healthy controls (Saglam et al. 2015). Interestingly, a recent genome-wide association study (GWAS) demonstrated that IL-37 variants with functional roles in decreased expression of IL-37 led to upregulation of IL-1β levels in GCF, contributing to a hyperinflammatory periodontal environment (Offenbacher et al. 2018).

IL-38

IL-38 is the most recent member of the IL-1 superfamily. IL-38 belongs to the IL-36 subfamily and is emerging as a key anti-inflammatory cytokine with properties similar to those of IL-37 (Dinarello 2018). IL-38 is expressed in several tissues but mainly in skin and proliferating B cells of the tonsils (Dinarello 2018). IL-38 exists intracellularly as a precursor full-length form called IL-38 (aa1-152) and must be cleaved at the N-terminus before it is secreted extracellularly as an active form (Xie et al. 2019). Unlike other members of the IL-1 family, IL-38 is not processed by caspase 1, but neither the processing enzyme nor the active form are presently known (Xie et al. 2019). The main cellular targets of IL-38 and its receptors remain obscure. The main receptor IL-36R, the cofactor IL1-R9, and IL-1 receptor accessory protein-like 1 (IL-1RAPL1) seem to be involved in the inhibitory action of IL-38 (Xie et al. 2019). IL-38 binds to IL-1R6 that results in the recruitment of IL-1R9, the formation of a trimeric signaling complex, and the triggering of downstream anti-inflammatory signaling pathways (van de Veerdonk et al. 2012). IL-38 also appears to act as a partial receptor antagonist of IL-36R, inhibiting the proinflammatory activities of IL-36, similar to IL-36Ra. However, the exact signaling and functional pathway of IL-38 remains enigmatic.

A large GWAS in 66,185 subjects with elevated C-reactive protein (CRP) implicated the loci for IL-38 for higher CRP levels and the regulation of chronic inflammation such as cardiovascular disease (Dehghan et al. 2011). Serum concentrations of IL-38 were found higher in patients with SLE compared to healthy ones (Rudloff et al. 2015). However, IL-38 levels in patients with psoriasis were decreased (Mercurio et al. 2018). IL-38 was elevated in mice with collagen-induced arthritis (CIA) and in the synovium of patients with RA, with high correlation with IL-1β concentrations (van de Veerdonk et al. 2018). Silencing of IL-38 resulted in increased in vitro production of proinflammatory cytokines, including IL-6. Treating mice with recombinant IL-38 resulted in reduction of IL-17 and improved the clinical symptoms of several inflammatory disorders (Mercurio et al. 2018; van de Veerdonk et al. 2018). Psoriatic IL-38-KO mice showed exacerbated IL-17-mediated inflammation and delayed skin regeneration, which was reversed by the administration of mature IL-38 that limited IL-17 production from γδ T cells (Song et al. 2018). It was recently reported that IL-38 is a potent inhibitor of activation of cultured human microglia, but its gene expression is decreased in the amygdala of children with ASD, implying the presence of focal brain inflammation (Tsilioni et al. 2020).

There are no studies investigating the role of IL-38 in periodontal diseases yet.

Therapeutic Strategies

Earlier studies in primate experimental periodontitis recognized IL-1 as a therapeutic target and showed that blocking IL-1 activity with intrapapillary injections of IL-1Ra resulted in significant (50%) reduction of radiographic bone loss (Oates et al. 2002). Currently, there are 3 Food and Drug Administration (FDA)–approved biologic drugs (anakinra, rilonacept, canakinumab) that reduce the activity of IL-1α and IL-1β, with more in clinical trials (Table 3). However, their high costs and potential risks have limited their use.

Table 3.

Food and Drug Administration–Approved Biologics That Target IL-1 Family Members.

Drug Name	Target	Type of Agent	Approved
Anakinra	IL-1R1 (IL-1α and IL-1β)	Recombinant version of IL-1Ra	RA, CAPS
Rilonacept	IL-1β, IL-1α, and IL-1Ra	IL-1R1 fusion protein	CAPS
Canakinumab	IL-1β	Anti-IL-1β monoclonal antibody (mAb)	CAPS, FMF, AoSD, gout, and sJIA

AoSD, adult-onset Still disease; CAPS, cryopyrin-associated periodic syndromes; FMF, familial Mediterranean fever; IL, interleukin; RA, rheumatoid arthritis; sJIA, systemic-onset juvenile idiopathic arthritis.

Anakinra, the recombinant version of IL-1Ra, is FDA approved for RA and CAPS, but is also used for off-label indications, like gout and type 2 diabetes (Mantovani et al. 2019). Rilonacept, an IL-1β blocker, is currently FDA approved for CAPS and mainly acts as a decoy receptor that binds IL-1β and prevents its interaction with cell surface receptors. Canakinumab is a human monoclonal antibody targeting IL-1β and is FDA approved for CAPS, FMF, gout, and systemic-onset juvenile idiopathic arthritis. Recent results of the CANTOS (Canakinumab Anti-inflammatory Thrombosis Outcomes Study) trial showed that blocking IL-1β with canakinumab was effective at preventing major adverse cardiac events but also lung cancer (Ridker et al. 2019). A new clinical trial (CAN-COVID) that examines the efficacy of canakinumab on cytokine release syndrome in people with COVID-19 pneumonia is in progress (ClinicalTrials.gov: NCT04348448).

Several antibodies targeting IL-1β, IL-1R1, IL-1α, IL-18, IL-33, and IL-36 are currently under clinical trials for several inflammatory diseases, including arthritis, asthma, psoriasis, diabetes, and Behçet disease (Mantovani et al. 2019). NLRP3 inflammasome inhibitors that are under preclinical and clinical investigation are also promising since they can inhibit other members of the IL-1 family, such as IL-18 (Zahid et al. 2019). However, the use of NLRP3 inhibitors might prevent the synthesis of anti-inflammatory IL-37. The potential suppression of innate inflammatory responses by recombinant IL-37 would make it an attractive therapeutic candidate for inflammatory disorders and requires further investigation (Theoharides et al. 2019). Finally, IL-38 may also be developed as a novel anti-inflammatory drug.

Conclusion

The available evidence reviewed supports the involvement of IL-1 family members in triggering and perpetuating periodontal inflammation (Fig.). Therapeutic blockade of select IL-1 family members for periodontitis in preclinical and clinical research has been limited to date. This strategy is a promising approach for the treatment of periodontal diseases and should also include investigation of IL-37 and IL-38.

Figure.

Diagrammatic representation of the key immune cells and interleukin (IL)–1 family members involved in periodontal inflammation and alveolar bone loss. The development of a dysbiotic dental biofilm triggers the release of IL-1 proinflammatory cytokines (e.g., IL-1α, IL-1β, IL-18, IL-36) from oral junctional epithelial cells (1). Proinflammatory signals combined with translocated bacteria or bacterial products (e.g., LPS) in periodontal tissues activate both innate (PMNs, mast cells, macrophages, dendritic cells) and adaptive (T cells, B cells) immune cells to release inflammatory mediators (2). IL-1 proinflammatory cytokines stimulate vasodilation and increase vascular permeability, amplifying the recruitment of inflammatory cells in the periodontium (chemotaxis) (3). Macrophages are the main sources of IL-1 proinflammatory cytokines that magnify periodontal inflammation. Macrophages and PMNs also release MMPs that result in the degradation of extracellular matrix (4). IL-33 released from oral epithelial cells activates mast cells that secrete further IL-33, triggering osteoclastogenesis and alveolar bone loss via increased RANKL (5). IL-18 is mainly released from DCs that can trigger differentiation of Th1 and Th17 cells and upregulate the release of IL-17, TNF-α, and IL-1β, promoting more periodontal tissue destruction (6). Increased levels of IL-1 proinflammatory cytokines (IL-1β, IL-33, and IL-36) are positively correlated with increased RANKL release; both stimulate recruitment of osteoclast progenitors and formation of osteoclast precursors (7) that turn into activated osteoclasts, responsible for alveolar bone resorption that occurs in periodontitis (8). IL-1 family members with anti-inflammatory properties (IL-1Ra, IL-36Ra, IL-37, and IL-38) aim to restrain the magnitude of periodontal inflammation stimulated by IL-1 proinflammatory cytokines. DC, dendritic cell; LPS, lipopolysaccharide, MMPs, matrix metalloproteinases; PMN, polymorphonuclear neutrophil; RANKL, receptor activator of nuclear factor kappa-β ligand. This figure was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com.

Author Contributions

E. Papathanasiou, contributed to conception, design, and data interpretation, drafted and critically revised the manuscript; P. Conti, F. Carinci, D. Lauritano, T.C. Theoharides, contributed to conception, design, and data interpretation, critically revised the manuscript. All authors gave final approval and agree to be accountable for all aspects of the work.