**4. Interleukins**

well defined but from three of these circulating TNF it is possible to polymerize them forming one 51-kD polypeptide which facilitates the binding of the cytokine with three receptors

TNF have a lot of physiologic multifunction including immune and inflammatory roles and the survival and death of different cells. The main function of cytokine is to attract and activate immune cells to sites of infections and to destroy pathogens, such as bacteria and virus. In this context, TNF stimulate vascular endothelial cells to express adhesion molecules (e.g. selectins and ligands for leukocyte integrins) that allows immune system cells to connect the wall of blood vessels. Additionally, complementing the inflammatory response, TNF induces the production of chemokines that increase the affinity of leukocyte to their ligands, the expression of IL-1 and to activate microbicidal functions of immune system cells. For all TNF importance in the inflammatory reaction, if low quantities of this cytokine are presented in

TNF is also well known to act in inflammatory reaction of some autoimmune diseases, such as rheumatoid arthritis and inflammatory bowel disease. Errors in this production are responsible for a considerable number of autoimmune, neoplastic and other diseases. Under these conditions, the treatment of these diseases are based on biologic agents targeting TNF, and thus looking for reducing the number of available TNF molecules or to block it receptors

TNF also promotes necrosis of tumor cells by inducing programmed cell death, a cytolytic potential. The activation of apoptosis mechanism is mediated by TNFR1, by stimulating the recruitment of death signaling proteins, such as Fas-associated protein with death, TNFR-associated factor (TRAF)-1 and TNFR-associated death domain protein (TRADD). These intracellular proteins are responsible for the release of other proteins such as procaspase-8, which in it activated form activate caspase-3, caspase-6, caspase-7 and other cytosolic substrates. These proteins induce genomic DNA degradation and cell death through interacting with latent DNAse. Evidences also suggest that TNF have the capacity to induce carcinogenesis and to stabilize tumors, an event that it is opposite of the previous explained, by DNA mutations and it mechanism of repair (i.e. genotoxic potential). This is possible due to the activation of NF-κB in tumor cells and by promoting production of IL-6 (a tumor-promoting cytokine), both facilitate metastasis and cancer cells to escape from immune system

There are other biological events and actions caused by TNF. When this cytokine is produced in large scale, such as in severe infection, it may induce shock or decrease of blood pressure due to reducing vascular muscle tone and myocardial contractility. Additionally, in high concentrations TNF can reduce blood glucose concentration, and cause intravascular thrombosis (by decreasing anticoagulant capabilities of the endothelium). TNF is also known as an endogenous pyrogen because it promotes fever by stimulating hypothalamus cells to produce

the local, the containment of the infection may be impaired [33, 37, 41–43].

simultaneously [37, 41, 42].

72 Autoantibodies and Cytokines

[33, 35, 40].

defense [35, 40–42].

prostaglandins [33, 40].

**3.2. Tumor necrosis factor alpha biologic functions**

Interleukins (ILs) are a group of secreted proteins with diverse structures and functions. These proteins bind to receptors and are involved in the communication between leukocytes. They are intimately related with activation and suppression of the immune system and cell division. The interleukins are synthesized mostly by helper CD4+ T lymphocytes, monocytes, macrophages and endothelial cells [5, 44, 45].

Interleukins are named as IL plus a number. Previously, different names were used to refer to the same IL. For instance, IL-1 was called lymphocyte-activating factor, mitogenic protein or T cell replacing factor III. In order to standardize the nomenclature, in 1979, during the Second International Lymphokine Workshop, the term interleukin was introduced. After that, the interleukins started being named consecutively according to the date of their discovery [44, 46, 47].

There have been identified 40 interleukins so far and some of them are further divided into subtypes (e.g. IL-1α, IL-1β). These ILs are grouped in families based on sequence homology and receptor chain similarities or functional properties [5, 44, 48, 49].

In this section, a brief description of various ILs will be presented. Focus will be given to the families of interleukins 1 and 2.

### **4.1. The interleukin-1 family**

Interleukin-1 family is composed by 11 cytokines: 7 ligands with agonist activity (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β and IL-36γ), 3 receptor antagonists (IL-1Ra, IL-36Ra and IL-38) and 1 anti-inflammatory cytokine (IL-37) [44, 50].

The interleukin-1 family started with only two components: IL-1α, IL-1β. Over the years, new IL with similar behavior and/or structure were discovered and added to the family. All the agonists members of this family show pro-inflammatory activity. These cytokines share a common C-terminal three-dimensional structure with a typical β-trefoil fold consisting of 12-β-strands connected by 11 loops, and have identical positioning of certain introns. Considering that, it is plausible to affirm that they probably arose from the duplication of a common ancestral gene [45, 51, 52].

All members of the family except IL-18 and IL-33 have genes encoding on chromosome 2 in a 400 kb region in human species. Despite the fact that all the cytokines are extracellular, they are synthesized without a hydrophobic leader sequence and are not secreted via reticulum endoplasmic-Golgi pathway, with the exception of IL-1Ra. The secretion mechanism of the other members of the family is still not known. These cytokines bind to closely related receptors, and many of the encoding genes are clustered in a short region of chromosome 2. The receptors contain extracellular immunoglobulin domains and a toll/IL-1 receptor (TIR) domain in the cytoplasmic portion [45, 52].

In order to become active, both IL-1α and IL-1β bind to the ligand-binding chain type I (IL-1R1). Then, the co-receptor, termed the accessory protein (IL-1RAcP), is recruited, and together they form a heterodimeric complex. The signaling that will culminate in a variety of inflammatory activities is initiated by the recruitment of the adaptor protein MyD88 to the toll-IL-1 receptor (TIR), which is followed by the phosphorylation of kinases, the translocation of the nuclear factor kappa B (NF-κB) to the nucleus and the expression of inflammatory genes [50, 51].

IL-18 was first described as "IFN-γ-inducing factor", because it was discovered as an inducer of IFN-γ production. However, alone, IL-18 does not induce the production of considerable amounts of IFN-γ. For that to happen, it has to act in association with IL-12. IL-18 promotes

with IL-2. It also enhances NK toxicity by promoting the expression of Fas ligand in NK cells. IL-18 is involved in several autoimmune diseases, in myocardial infarction, metabolic syn-

IL-33 is an alarmin cytokine, rapidly released upon cellular damage. It is involved mainly

activated M2 polarized macrophages. This cytokine is expressed by keratinocytes, epithelial and endothelial cells, and monocytes. IL-33 is produced as a precursor, but, contrary to IL-1, caspase-1 transforms it in an inactive cytokine. The precursor is active and other proteases can

ing IL-1RacP. The activity of IL-33 is controlled essentially by the binding to soluble ST2 and soluble IL-1RAcP. Levels of increased soluble ST2 are present in various inflammatory dis-

IL-36α, IL-36β and IL-36γ are receptor agonists, while IL-36Ra is a receptor antagonist that blocks the activation of the receptor and competes with IL-36, acting as a regulator. These cytokines are included in the interleukin-1 family because they share homology to the first members of the family. Their homology to IL-Ra and IL-1β varies from 20 to 52%. Furthermore, IL-36β and IL-36γ have the core 12-fold, β-trefoil structure and lack a signal peptide, particular features of IL-1 family. All these cytokines need an N-terminal processing to become in the active form, but the enzyme responsible for this process is still not known. IL-36 cytokines are predominantly found in skin cells, and that is why they are related with several skin disorders, such as psoriasis. After binding to the receptor (IL-36R and IL-1RAcP co-receptor), dendritic cells are activated and participate in the polarizing of T-helper responses [50, 52, 57]. Different from the other members of the family, IL-37 is an anti-inflammatory cytokine, and reduces innate inflammation as well as acquired immune responses. Its presence has already been reported in skin, tonsils, esophagus, placenta, breast, prostate and colon. There are five different isoforms of IL-37: IL-37a, IL-37b, IL-37c, IL-37d and IL-37e, expressed in different locations of the human body. So far, IL-37b, which contains a 12β-strand trefoil, typical of the IL-1 family, appears to be the most biologically active, and therefore the object of the majority of studies. IL-37 suppresses the production of pro-inflammatory cytokines, such as IL-1A, IL-6, CC chemokine ligand (CCL-12), colony-stimulating factors (CSF-1 and CSF-2), chemokine ligand-13 (CXCL-13), IL-1β, IL23-A and IL1RA, and also inhibits dendritic cell

IL-38 is the most recent member of the Interleukin-1 family, identified in 2001. It binds to the same receptor that the IL-36 cytokines, IL-36R. However, it acts as an antagonist, similarly to IL-36Ra. Therefore, IL-38 acts reducing inflammatory response. IL-38 shares 41% homology with IL-1Ra and 43% with IL-36Ra. This cytokine is present in skin, tonsil, thymus, spleen, fetal liver and salivary glands. The properties and biological activities of IL-38 are still being

eases, such as systemic lupus erythematosus and rheumatoid arthritis [44, 50, 56].

cells responses, and also induces IL-13 production in T cells and NK cells together

, in innate lymphoid cell-2 (ILC2), and in

Cytokines and Interferons: Types and Functions http://dx.doi.org/10.5772/intechopen.74550 75

response binding to ST2 and next recruit-

TH1

and Th<sup>2</sup>

activation [58–60].

studied [52, 61, 62].

dromes and others [44, 55].

in type 2 immunity and inflammation. It acts in Th2

cleavage it in more potent forms. IL-33 induces Th2

Both IL-1α and IL-1β have precursor forms. The precursor of IL-1α is present in the epithelial layers of the gastrointestinal tract, lung, liver, kidney, endothelial cells and astrocytes; and it is capable of binding to the IL-1R1 and initiating the signaling cascade, essentially after cell death by necrosis (e.g. myocardial infarction and stroke). On the other hand, the precursor of IL-1β is not active and does not bind to the receptor. It requires a cleavage to become in the active form [44, 50, 51].

IL-1β is highly involved with autoimmune, infectious, degenerative and, especially, with autoinflammatory diseases. An important part of autoinflammatory diseases is caused by genetic defects in innate inflammatory pathways, and usually show their signals early in life. The first disease classified as autoinflammatory was tumor necrosis factor receptor associated periodic syndrome (TRAPS). Other examples are familial Mediterranean fever and adult and juvenile Still disease. This group of diseases promptly responds to the treatment with IL-1β blockade, with few exceptions. In many autoinflammatory diseases, there is a state of increased release of IL-1β. The precursor is converted to the active form through the action of Caspase-1. This enzyme is also found in the inactive form in tissue macrophages and dendritic cells, and requires conversion by autocatalysis to become active. However, it is in the active form in circulating human blood monocytes. The release of IL-1β from blood monocytes in highly controlled and takes several hours in healthy subjects. In patients with an autoinflammatory disease, more mature IL-1β is released when compared to healthy subjects, which leads to exacerbated inflammation. Despite of this group of diseases being characterized by severe inflammation, the amount of IL-1β released is not much greater than that released from healthy subjects. Currently, human anti-IL-1β monoclonal antibody is being developed to treat autoinflammatory diseases. Canakinumab was approved by Food and Drug Administration (FDA) in 2009 for the treatment of cryopyrin-associated periodic syndromes (CAPS). In 2016, Canakinumab was also approved for treating TRAPS, hyperimmunoglobulin D syndrome (HIDS)/mevalonate kinase deficiency (MKD) and familial Mediterranean fever (FMF) [50, 51].

IL-1Ra is a receptor antagonist. It is synthetized by the same cells that produce IL-1α and IL-1β (monocytes, macrophages, dendritic cells and others). The binding of IL-1Ra to the receptor does not involve conformational change and, hence, the co-receptor IL-1RAcP is not recruited. IL-1Ra regulates the activity of IL-1. However, to efficiently block the IL-1 response, it has to be in an amount approximately 100-folds greater than the agonists cytokines. Anakinra is a recombinant version of IL1-Ra used in the treatment of rheumatoid arthritis [44, 53].

IL-18 is synthetized as an inactive precursor, and, similarly to IL-1β, it needs cleavage by caspase-1 to become in the active form. The precursor form is present in almost all cells of the human body, likewise IL-1α. Usually diseases related to IL-18 appear when there is an imbalance between the amount of IL-18 and IL-18 binding protein, which is responsible for limiting the level of activity of IL-18. This cytokine is released usually from dying cells, once again like IL-1α [51, 54].

IL-18 was first described as "IFN-γ-inducing factor", because it was discovered as an inducer of IFN-γ production. However, alone, IL-18 does not induce the production of considerable amounts of IFN-γ. For that to happen, it has to act in association with IL-12. IL-18 promotes TH1 and Th<sup>2</sup> cells responses, and also induces IL-13 production in T cells and NK cells together with IL-2. It also enhances NK toxicity by promoting the expression of Fas ligand in NK cells. IL-18 is involved in several autoimmune diseases, in myocardial infarction, metabolic syndromes and others [44, 55].

form a heterodimeric complex. The signaling that will culminate in a variety of inflammatory activities is initiated by the recruitment of the adaptor protein MyD88 to the toll-IL-1 receptor (TIR), which is followed by the phosphorylation of kinases, the translocation of the nuclear factor kappa B (NF-κB) to the nucleus and the expression of inflammatory genes [50, 51].

Both IL-1α and IL-1β have precursor forms. The precursor of IL-1α is present in the epithelial layers of the gastrointestinal tract, lung, liver, kidney, endothelial cells and astrocytes; and it is capable of binding to the IL-1R1 and initiating the signaling cascade, essentially after cell death by necrosis (e.g. myocardial infarction and stroke). On the other hand, the precursor of IL-1β is not active and does not bind to the receptor. It requires a cleavage to become in the

IL-1β is highly involved with autoimmune, infectious, degenerative and, especially, with autoinflammatory diseases. An important part of autoinflammatory diseases is caused by genetic defects in innate inflammatory pathways, and usually show their signals early in life. The first disease classified as autoinflammatory was tumor necrosis factor receptor associated periodic syndrome (TRAPS). Other examples are familial Mediterranean fever and adult and juvenile Still disease. This group of diseases promptly responds to the treatment with IL-1β blockade, with few exceptions. In many autoinflammatory diseases, there is a state of increased release of IL-1β. The precursor is converted to the active form through the action of Caspase-1. This enzyme is also found in the inactive form in tissue macrophages and dendritic cells, and requires conversion by autocatalysis to become active. However, it is in the active form in circulating human blood monocytes. The release of IL-1β from blood monocytes in highly controlled and takes several hours in healthy subjects. In patients with an autoinflammatory disease, more mature IL-1β is released when compared to healthy subjects, which leads to exacerbated inflammation. Despite of this group of diseases being characterized by severe inflammation, the amount of IL-1β released is not much greater than that released from healthy subjects. Currently, human anti-IL-1β monoclonal antibody is being developed to treat autoinflammatory diseases. Canakinumab was approved by Food and Drug Administration (FDA) in 2009 for the treatment of cryopyrin-associated periodic syndromes (CAPS). In 2016, Canakinumab was also approved for treating TRAPS, hyperimmunoglobulin D syndrome (HIDS)/mevalonate kinase deficiency (MKD) and familial Mediterranean fever (FMF) [50, 51]. IL-1Ra is a receptor antagonist. It is synthetized by the same cells that produce IL-1α and IL-1β (monocytes, macrophages, dendritic cells and others). The binding of IL-1Ra to the receptor does not involve conformational change and, hence, the co-receptor IL-1RAcP is not recruited. IL-1Ra regulates the activity of IL-1. However, to efficiently block the IL-1 response, it has to be in an amount approximately 100-folds greater than the agonists cytokines. Anakinra is a

recombinant version of IL1-Ra used in the treatment of rheumatoid arthritis [44, 53].

IL-18 is synthetized as an inactive precursor, and, similarly to IL-1β, it needs cleavage by caspase-1 to become in the active form. The precursor form is present in almost all cells of the human body, likewise IL-1α. Usually diseases related to IL-18 appear when there is an imbalance between the amount of IL-18 and IL-18 binding protein, which is responsible for limiting the level of activity of IL-18. This cytokine is released usually from dying cells, once

active form [44, 50, 51].

74 Autoantibodies and Cytokines

again like IL-1α [51, 54].

IL-33 is an alarmin cytokine, rapidly released upon cellular damage. It is involved mainly in type 2 immunity and inflammation. It acts in Th2 , in innate lymphoid cell-2 (ILC2), and in activated M2 polarized macrophages. This cytokine is expressed by keratinocytes, epithelial and endothelial cells, and monocytes. IL-33 is produced as a precursor, but, contrary to IL-1, caspase-1 transforms it in an inactive cytokine. The precursor is active and other proteases can cleavage it in more potent forms. IL-33 induces Th2 response binding to ST2 and next recruiting IL-1RacP. The activity of IL-33 is controlled essentially by the binding to soluble ST2 and soluble IL-1RAcP. Levels of increased soluble ST2 are present in various inflammatory diseases, such as systemic lupus erythematosus and rheumatoid arthritis [44, 50, 56].

IL-36α, IL-36β and IL-36γ are receptor agonists, while IL-36Ra is a receptor antagonist that blocks the activation of the receptor and competes with IL-36, acting as a regulator. These cytokines are included in the interleukin-1 family because they share homology to the first members of the family. Their homology to IL-Ra and IL-1β varies from 20 to 52%. Furthermore, IL-36β and IL-36γ have the core 12-fold, β-trefoil structure and lack a signal peptide, particular features of IL-1 family. All these cytokines need an N-terminal processing to become in the active form, but the enzyme responsible for this process is still not known. IL-36 cytokines are predominantly found in skin cells, and that is why they are related with several skin disorders, such as psoriasis. After binding to the receptor (IL-36R and IL-1RAcP co-receptor), dendritic cells are activated and participate in the polarizing of T-helper responses [50, 52, 57].

Different from the other members of the family, IL-37 is an anti-inflammatory cytokine, and reduces innate inflammation as well as acquired immune responses. Its presence has already been reported in skin, tonsils, esophagus, placenta, breast, prostate and colon. There are five different isoforms of IL-37: IL-37a, IL-37b, IL-37c, IL-37d and IL-37e, expressed in different locations of the human body. So far, IL-37b, which contains a 12β-strand trefoil, typical of the IL-1 family, appears to be the most biologically active, and therefore the object of the majority of studies. IL-37 suppresses the production of pro-inflammatory cytokines, such as IL-1A, IL-6, CC chemokine ligand (CCL-12), colony-stimulating factors (CSF-1 and CSF-2), chemokine ligand-13 (CXCL-13), IL-1β, IL23-A and IL1RA, and also inhibits dendritic cell activation [58–60].

IL-38 is the most recent member of the Interleukin-1 family, identified in 2001. It binds to the same receptor that the IL-36 cytokines, IL-36R. However, it acts as an antagonist, similarly to IL-36Ra. Therefore, IL-38 acts reducing inflammatory response. IL-38 shares 41% homology with IL-1Ra and 43% with IL-36Ra. This cytokine is present in skin, tonsil, thymus, spleen, fetal liver and salivary glands. The properties and biological activities of IL-38 are still being studied [52, 61, 62].
