IL-1 Family Cytokines

IL-1 Family Cytokines, Receptors, and Intracellular Signaling Pathways

IL-1 Family Cytokines

The IL-1 cytokine family consists of eleven members that share a conserved beta-trefoil structure and have either pro- or anti-inflammatory effects. Pro-inflammatory members of the IL-1 cytokine family include IL-1 alpha, IL-1 beta, IL-18, IL-33, IL-36 alpha, IL-36 beta, and IL-36 gamma, while anti-inflammatory members include IL-37, IL-38, and two specific receptor antagonists, IL-1ra and IL-36Ra. Although the full-length forms of IL-1 alpha and IL-33 are biologically active, other pro-inflammatory IL-1 family cytokines need to undergo processing at their N-terminal ends to be active. IL-1 beta and IL-18 are synthesized as inactive precursor proteins that require inflammasome/Caspase-1-mediated cleavage for their maturation and secretion. IL-36 alpha, IL-36 beta, IL-36 gamma, and IL-36Ra are secreted as inactive precursors that require N-terminal cleavage by different proteases to be activated. IL-36 alpha is activated by cathepsin G and elastase, IL-36 beta is activated by cathepsin G, IL-36 gamma is activated by elastase and proteinase-3, and IL-36Ra is activated by neutrophil elastase. Additionally, although IL-33 is active in its full-length form, N-terminal processing has been shown to increase its potency.

IL-1 Receptor Family

IL-1 family cytokines signal through members of the IL-1 receptor family. This family of receptors consists of ten structurally-related proteins (IL-1 R1-IL-1 R10), and the more distantly-related, soluble IL-18 binding protein (IL-18 BP), which inhibits IL-18 activity and thus, functions similarly to the other soluble IL-1 family receptors. Most IL-1 family receptors have three extracellular immunoglobulin-like domains and a cytoplasmic Toll/IL-1 receptor (TIR) domain, with the exception of SIGIRR/IL-1 R8 and IL-18 BP, which have only one Ig-like domain, and IL-1 R2, which lacks an intracellular TIR domain. The intracellular TIR domain found in the IL-1 family receptors is shared with proteins belonging to the toll-like receptor (TLR) family of pattern recognition receptors, and with several intracellular adaptor proteins that mediate signaling by either the IL-1 receptor family or TLRs. One notable difference among the IL-1 family receptors is that SIGIRR/IL-1 R8, TIGIRR-1/IL-1 R9, and TIGIRR-2/IL-1 R10 contain an intracellular C-terminal extension that is not found in other receptors belonging to this family. The significance of this difference is still being investigated as the ligands and functions of TIGIRR-1/IL-1 R9 and TIGIRR-2/IL-1 R10 are not yet fully understood.

IL-1 alpha, IL-1 beta, and IL-1ra

Following secretion, IL-1 family cytokines initiate intracellular signaling by binding to their primary receptor subunit, which subsequently recruits an accessory receptor. IL-1 alpha or IL-1 beta bind to IL-1 R1, which then recruits the accessory receptor, IL-1 RAcP/IL-1 R3, to form a ternary ligand-receptor complex. Formation of this complex leads to the recruitment of the MyD88 adaptor protein and activation of downstream signaling pathways, including the PI 3-kinase/Akt pathway, the NF-kappa B pathway, and the JNK/p38 MAPK pathways, resulting in the activation of transcription factors that promote the expression of pro-inflammatory cytokines, chemokines, and secondary mediators of the inflammatory response. In addition to mediating pro-inflammatory signaling, IL-1 alpha also contains a nuclear localization signal and functions as a transcription factor. Chromatin binding of IL-1 alpha is thought to act as a sink to limit inflammation. IL-1 alpha also differs from IL-1 beta in that it is constitutively expressed in epithelial and mesenchymal cell types under normal physiological conditions, while IL-1 beta expression is primarily induced under disease conditions. Furthermore, a biologically active membrane form of IL-1 alpha has been identified that is involved in regulating the activities of IFN-gamma. There are several intrinsic inhibitors of IL-1 activity. IL-1ra is an IL-1 family cytokine that binds to IL-1 R1 and prevents binding of IL-1 alpha or IL-1 beta to its primary receptor. Binding of IL-1ra to IL-1 R1 inhibits the recruitment of IL-1 RAcP/IL-1 R3 and downstream signaling. Another intrinsic inhibitor of IL-1 signaling is the IL-1 R2 decoy receptor. This receptor has an extracellular domain that is similar to IL-1 R1, but it contains a short cytoplasmic domain that is incapable of transducing an IL-1 signal. Other intrinsic inhibitors of IL-1 signaling include soluble forms of IL-1 R1, IL-1 R2, and IL-1 RAcP/IL-1 R3, all of which are incapable of propagating a downstream signal. One final inhibitor of IL-1 signaling that has been identified is SIGIRR/IL-1 R8. This receptor contains a single immunoglobulin-like domain and may act as a decoy receptor or inhibit IL-1 signaling in a context-dependent manner by competing with activated receptor complexes for association with intracellular regulators of IL-1 family signaling. Additionally, SIGIRR/IL-1 R8 has also been suggested to act as a co-receptor for IL-37-mediated anti-inflammatory signaling.

IL-18

IL-18 is another pro-inflammatory member of the IL-1 cytokine family. Like IL-1 beta, it requires inflammasome/Caspase-1-mediated processing for its maturation and secretion, but in contrast to IL-1 beta, the IL-18 precursor protein is constitutively expressed in almost all cell types under normal physiological conditions. The active form of IL-18 is primarily secreted by dendritic cells and macrophages, and like IL-1 alpha and IL-33, the precursor form of IL-18 is released from dying cells and processed extracellularly. IL-18 initially binds to its primary receptor, IL-18 R alpha/IL-1 R5, which subsequently recruits IL-18 beta/IL-1 R7 as an accessory receptor to form a ternary ligand-receptor complex. Following formation of the receptor complex, IL-18 mediates pro-inflammatory signaling through activation of pathways similar to those activated by IL-1. In the presence of IL-12, IL-18 promotes IFN-gamma production by Th1 cells, while it promotes the activation and proliferation of natural killer cells in the presence of IL-15. Similar to IL-1, there are several intrinsic inhibitors of IL-18 signaling. IL-18 BP is a constitutively secreted protein with a single immunoglobulin-like domain that binds to IL-18 with higher affinity than either the cell bound or soluble forms of IL-18 R and prevents IL-18 signaling. Other IL-18 inhibitors include soluble IL-18 R alpha/IL-1 R5, which can bind to IL-18, but is incapable of propagating a signal, and SIGIRR/IL-1 R8.

IL-33

Similar to IL-1 alpha, IL-33 is biologically active as a full-length molecule and functions as an alarmin following cell damage. While the precursor form of IL-33 is active, processing by extracellular enzymes significantly increases its potency. IL-33 is constitutively produced by epithelial cells, endothelial cells, and fibroblasts in the skin, lungs, and gastrointestinal tract. It binds to ST2/IL-1 R4, which subsequently undergoes a conformational change that then allows it to interact with IL-1 RAcP/IL-1 R3. Formation of this receptor complex leads to the activation of pro-inflammatory signaling pathways similar to those activated by IL-1 and IL-18. IL-33 regulates a select group of cells that stably express the ST2/IL-1 R4 receptor, including Th2 cells, group 2 innate lymphoid cells (ILC2s), basophils, eosinophils, mast cells, macrophages, and dendritic cells, and as a result, it has been suggested to play a central role in the pathogenesis of allergic inflammation. Unlike IL-1 and IL-36, no specific ST2/IL-1 R4 receptor antagonist has been identified. IL-33 activity is primarily thought to be regulated by the soluble form of ST2/IL-1 R4, which is incapable of propagating a downstream signal and potentially by SIGIRR/IL-1 R8. Additionally, IL-33 contains a nuclear localization signal and chromatin binding is thought to sequester IL-33 and thereby limit inflammation.

IL-36 alpha, IL-36 beta, IL-36 gamma, and IL-36Ra

IL-36 alpha, IL-36 beta, and IL-36 gamma are all pro-inflammatory members of the IL-1 cytokine family. They are primarily produced by keratinocytes in the skin, as is the specific receptor antagonist, IL-36Ra. Additionally, IL-36 cytokines can be produced by immune cells such as dendritic cells, macrophages, and T cells, under certain pathological conditions. As previously mentioned, all IL-36 cytokines are secreted as inactive precursor proteins that are activated following N-terminal cleavage by neutrophil-derived proteases. All of the IL-36 cytokines signal through a receptor complex consisting of IL-1 Rrp2/IL-1 R6 and the accessory receptor, IL-1 RAcP/IL-1 R3. Formation of this complex leads to the production of pro-inflammatory mediators by keratinocytes, Langerhans cells, and macrophages, as well as increased production of anti-microbial peptides, and leukocyte recruitment. IL-36Ra is an intrinsic inhibitor of the IL-36 cytokines. Similar to IL-1ra binding to IL-1 R1, IL-36Ra binds to IL-1 Rrp2/IL-1 R6 and prevents its ability to interact with IL-36 alpha, IL-36 beta, and IL-36 gamma. Binding of IL-36Ra to IL-1 Rrp2/IL-1 R6 inhibits recruitment of IL-1 RAcP/IL-1 R3 and downstream signaling. The anti-inflammatory cytokine, IL-38, also acts as an IL-36 inhibitor. It too can bind to IL-1 Rrp2/IL-1 R6 and prevent its ability to bind to the IL-36 cytokines. Similar to IL-36Ra, IL-38 binding to IL-1 Rrp2/IL-1 R6 also prevents the recruitment of the IL-1 RAcP/IL-1 R3 accessory receptor and downstream signaling. SIGIRR/IL-1 R8 may also act as another inhibitor of IL-36 signaling, but this requires further investigation.

IL-37

Although less is currently known about IL-37 and IL-38, both of these IL-1 family cytokines are thought to have anti-inflammatory effects. Five different splice variants of IL-37 (IL-37a-e) have been identified, with IL-37b being the longest isoform with the most potent anti-inflammatory effects. IL-37 has been found to be expressed in both normal tissues, primarily by epithelial cells, and by immune cell types such as monocytes, macrophages, dendritic cells, and T cells. It is frequently elevated in patients with inflammatory and autoimmune diseases. Both the precursor and the mature forms of IL-37 are secreted and are biologically active. Following secretion, IL-37 binds to IL-18 R alpha/IL-1 R5, which then recruits SIGIRR/IL-1 R8 to form a tripartite complex. The TIR domain of the SIGIRR/IL-1 R8 receptor subunit is mutated and as a result, it functions as a sink for the intracellular adaptor protein, MyD88, limiting its availability for recruitment to pro-inflammatory receptor complexes. IL-37 was also shown to bind to IL-18 BP and it has been suggested that high concentrations of IL-18 BP may suppress the anti-inflammatory effects of IL-37 by limiting its availability to form a complex with IL-18 R alpha/IL-1 R5 and SIGIRR/IL-1 R8. In addition to its receptor binding effects, the precursor form of IL-37 is cleaved by Caspase-1 and subsequently translocates to the nucleus, where it interacts with Smad3 and can affect the expression of pro-inflammatory genes.

IL-38

Similar to IL-36Ra, IL-38 binds to IL-1 Rrp2/IL-1 R6 and functions as a partial receptor antagonist. The interaction of IL-38 with IL-1 Rrp2/IL-1 R6 prevents the IL-36 cytokines from binding to their primary receptor subunit and blocks the recruitment of IL-1 RAcP/IL-1 R3, thereby inhibiting the initiation of downstream signaling. IL-38 is primarily expressed in the skin and has been shown to inhibit Th17 responses induced by Candida albicans in cultures of PBMCs. Additionally, both the full-length and truncated forms of IL-38 have been shown to bind TIGIRR-2/IL-1 R10. Abnormal expression of IL-38 has been found in patients with many different types of inflammatory autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis, psoriasis, Sjogren’s syndrome, and ulcerative colitis, but its precise role in the pathogenesis of these diseases requires further investigation.

Bio-Techne offers a wide selection of bioactive R&D Systems™ recombinant human and mouse IL-1 family proteins for studying the functions of these molecules, along with single and multianalyte immunoassays for monitoring immune responses. Additionally, we offer antibodies against IL-1 family cytokines, receptors, and intracellular signaling molecules that are validated for one or more of the following applications: blocking/neutralization, Western blot, flow cytometry, ELISA, immunocytochemistry, and immunohistochemistry.