Innate immune cells, particularly macrophages and epithelial cells, play a key role in multiple layers of immune responses. Alarmins and pro-inflammatory cytokines from the IL (interleukin)-1 and TNF (tumour necrosis factor) families initiate the cascade of events by inducing chemokine release from bystander cells and by the up-regulation of adhesion molecules required for transendothelial trafficking of immune cells. Furthermore, innate cytokines produced by dendritic cells, macrophages, epithelial cells and innate lymphoid cells seem to play a critical role in polarization of helper T-cell cytokine profiles into specific subsets of Th1/Th2/Th17 effector cells or regulatory T-cells. Lastly, the innate immune system down-regulates effector mechanisms and restores homoeostasis in injured tissue via cytokines from the IL-10 and TGF (transforming growth factor) families mainly released from macrophages, preferentially the M2 subset, which have a capacity to induce regulatory T-cells, inhibit the production of pro-inflammatory cytokines and induce healing of the tissue by regulating extracellular matrix protein deposition and angiogenesis. Cytokines produced by innate immune cells represent an attractive target for therapeutic intervention, and multiple molecules are currently being tested clinically in patients with inflammatory bowel disease, rheumatoid arthritis, systemic diseases, autoinflammatory syndromes, fibrosing processes or malignancies. In addition to the already widely used blockers of TNFα and the tested inhibitors of IL-1 and IL-6, multiple therapeutic molecules are currently in clinical trials targeting TNF-related molecules [APRIL (a proliferation-inducing ligand) and BAFF (B-cell-activating factor belonging to the TNF family)], chemokine receptors, IL-17, TGFβ and other cytokines.
Innate immunity cells represent extremely powerful host defence mechanisms with fine-tuning ability to limit inappropriate damage to affected tissues in which such responses take place. There are no doubts about an extensive cross-talk between the innate and adaptive immune mechanisms and current data presenting new innate lymphoid populations support the hypothesis that innate and adaptive systems do not represent completely separate entities (Figure 1). Most of the cell–cell interactions between immune cells are mediated by cytokines released from cells in response to different stimuli. Cytokines are involved in both the activation and inhibition of cell functions. They regulate their differentiation, normal turnover or fast migration into the injury site and reparation processes. In this respect, defective regulation of the cytokine network seems to play an important role in the pathogenesis of many diseases and clinical settings. Modern cytokine-targeted therapies with monoclonal antibodies, soluble receptors or small-molecule inhibitors promise new possibilities for patients resistant to standard pharmacological regimes.
Nomenclature of immune cells
COMPONENTS OF INNATE IMMUNITY
Mucosal surfaces covering approximately 300 m2 of the body/environmental interface are not only a mechanical barrier, but regulate many of the functions of ‘professional’ immune cells . Epithelial cells represent an important source of cytokines used for the amplification or inhibition of immune or inflammatory reactions . By the release of multiple chemokines, epithelial cells regulate the influx of immune cells to the site of injury . In vitro data suggest that unstimulated epithelial cells constitutively express chemokine mRNA attracting neutrophils, whereas, for mononuclear phagocytes and lymphocytes, stimulation with pro-inflammatory cytokines is required . Furthermore, cell–cell interactions of recruited monocytes with the epithelium seem to be critical for their final differentiation into tissue macrophages . Under steady-state conditions, epithelial cells considerably down-regulate immune mechanisms and dampen inflammatory responses . The plasticity of epithelial cells enabling their transition into myofibroblasts, the epithelial–mesenchymal transition, is important for the reparation processes but is also involved in fibrosing mechanisms associated with immunopathological reactions . Defective regulation of mucosal responses seems to play a key role in the pathogenesis of IBD (inflammatory bowel disease) , bronchial asthma , interstitial lung diseases or rejection of kidney allografts .
Monocytes, macrophages and dendritic cells
Peripheral blood monocytes and their tissue-differentiated forms, macrophages and DCs (dendritic cells), represent variations of maturation/activation stages of mononuclear phagocytes. In the peripheral blood, different subsets might be distinguished based on the membrane expression of CD14 and CD16 antigens. Although ‘traditional’ monocytes are characterized by a very high expression of CD14 and the absence of CD16 on their surface, a limited subpopulation of CD14+CD16+ monocytes can be identified. These monocytes are smaller and express HLA-DR and CD43 at a higher density . Similarities of gene expression profiles with macrophages and DCs , together with shortened telomere length , indicate that these might be senescent repeatedly activated monocytes. Furthermore, the capacity of CD14+CD16+ monocytes to preferentially produce the pro-inflammatory cytokine TNFα (tumour necrosis factor α) with a limited capability to release the anti-inflammatory cytokine IL (interleukin)-10  suggest that these cells represent pro-inflammatory monocytes. Differentiation of human monocytes into macrophages in vitro has been found to be associated with the modulation of more than 800 different genes  and might be modulated by direct interactions of mononuclear phagocytes with other cells  or by soluble factors, particularly cytokines. Typically, tissue macrophages can polarize into the inflammatory M1 subset, with potent antimicrobial activity and the ability to produce the pro-inflammatory cytokines TNFα, IL-12, and IL-1 β, or the M2 subset , which is involved in tissue surveillance and reparative processes by the production of IL-10 and/or TGFβ (transforming growth factor β), down-regulating immune responses (Figure 2). DCs are the most potent antigen-presenting cells. The two subsets, myeloid DCs and plasmocytoid DCs, differ in their origin and the expression of PRRs (pattern recognition receptors), e.g. TLRs (Toll-like receptors), but both are able to induce antigen-specific proliferation of naive T-cells. In humans, plasmocytoid DCs, producing high levels of IFNα (interferon α), and two types of myeloid DCs (CD1c compared with CD141+) can be distinguished . In addition to the role of DCs in the induction of adaptive responses, these cells also activate NK (natural killer) cells and NKT cells (natural killer T-cells) . The CD103+ subpopulation of DCs with a capacity to induce the expansion of Treg-cells (regulatory T-cells) has been described in intestinal mucosa and might play a role in the mechanism of oral tolerance .
Subpopulations of macrophages
Neutrophils are potent phagocytic cells protecting the host from invading micro-organisms by multiple antimicrobial substances present in cytotoxic granules  and by neutrophil extracellular traps . The failure of neutrophils to clear pathogens from mucosal surfaces leads to bacterial colonization and also contributes to the persistence of inflammation in affected tissue. Dysfunction of neutrophils at multiple stages during sepsis leads to the overexpression of pro-inflammatory cytokines and mediators involved in tissue damage and multiple organ failure . In addition to their purely effector functions, neutrophils regulate the influx of monocytes and DCs by the release of chemokines . Neutrophils are also associated with the adaptive Th17 immune response by playing a role in either the effector or regulatory phases .
Eosinophils are multifunctional cells required for defence against helminth parasitic infections  and are involved in the pathogenesis of allergic diseases . It has been suggested that tissue eosinophilia may result in fibrotic changes in the parenchyma by induction of fibroblast proliferation  and collagen deposition . These changes may play a role in the pathogenesis of cardiac fibrosis as a complication of hypereosinophilic syndromes , strictures accompanying eosinophilic esophagitis , or thickening of basal membrane in patients with bronchial asthma . Furthermore, eosinophils and their mediators ECP (eosinophilic cationic protein) and MBP (major basic protein) are associated with the ability to promote the coagulation of plasma, and tissue factor production might be relevant to the occurrence of thrombotic complications in patients with blood eosinophilia .
Basophils and mast cells
Peripheral blood basophils and tissue-resident mast cells share many characteristics, including the membrane expression of high-affinity IgE receptor [FceRI (IgE Fc receptor I)], TLR2 and TLR4, and the ability to release aggressive mediators immediately upon activation, but differ in several specific membrane markers . Nevertheless, both cell types are thought to originate from a common progenitor . The function of mast cells and basophils has always been associated with their role in the pathogenesis of allergic disorders, but these cells might also be involved in combating parasitic, bacterial or viral infections . Furthermore, experimental data suggest the involvement of mast cells in the pathogenesis of autoimmune diseases , cancer  and transplantation tolerance . Although mast cells represent the first line of immune response and their subepithelial location predisposes them to immediate activation, basophils seem to be recruited in the course of allergic reaction .
Natural killer cells and natural killer T-cells
NK cells are innate immune cells involved in the defence against cancer cells and infectious agents by killing the target without prior sensitization after the recognition through unique NK receptors recognizing self-MHC class I or II molecules, which can either inhibit or induce the effector functions of NK cells . Previous findings have suggested that NK cells share some features of the adaptive immune system, particularly with CD8 cytotoxic cells, since they express antigen-specific receptors, proliferate after their induction and have a capability to differentiate into memory cells . NKT cells are a subset of T-cell that share receptor structures and functions with conventional T-cells (CD3) and NK cells (CD16 and CD56). NKT cells preferentially recognize lipid antigens in the context of the CD1d molecule (MHC class I-like) and consists of two subsets: (i) type I cells expressing invariant TCR (T-cell receptor), which are involved in tumour surveillance; and (ii) type II cells utilizing diverse TCR gene segments, which suppress anti-tumour immune responses .
Innate lymphoid cells
ILCs (innate lymphoid cells) have been described in addition to the already well-characterized NK and NKT cells and are new populations of innate cells originating from lymphoid linkage. These cells, identified by different laboratories as natural helper cells , nuocytes , innate type 2 helper cells  or multipotent progenitor type 2 cells , are activated by the epithelial-cell-derived cytokines IL-23 or IL-25 and promote Th2 cytokine responses. All four populations do not express lineage-specific markers of T- and B-cells, macrophages, DCs, NK cells or neutrophils, and their original physiological role might be to protect the host against helminth parasites. Alternatively, a classification based on cytokine profiles has been proposed, including the ILC1 subset of IFNγ-producing NK cells, the ILC2 subset (nuocytes and NK cells), which produce IL-13 to eliminate extracellular parasites, the ILC17 subset producing IL-17, IL-22 and TNFα and restricted to mucosal tissues, together with the ILC22 subset preferentially releasing IL-22 .
CYTOKINES OF INNATE IMMUNE CELLS RESPONDING TO INJURY: ALARMINS, PRO-INFLAMMATORY CYTOKINES AND CHEMOKINES
Epithelial cells and mucosal macrophages, the main source of pro-inflammatory cytokines, represent the first line of host defence against invading micro-organisms and other stimuli leading to tissue injury. These cells are able to recognize conserved molecules on the surface of micro-organisms by various families of PRRs, such as membrane TLRs or intracellular NLRs [NOD (nucleotide oligomerization domain)-like receptors], and initiate recruitment and activation of neutrophils and other immune cells at the site of injury. Alarmins and pro-inflammatory cytokines initiate the cascade of events by inducing the release of chemokines from bystander cells and by the up-regulation of adhesion molecules required for transendothelial trafficking of immune cells (Figure 3).
Inflammatory reaction and initiation of immune responses
Alarmins are structurally heterogeneous peptides or protein mediators released immediately by degranulation or cell injury/death from phagocytes  and epithelial cells . Currently identified alarmins include defensins , cathelicidins , eosinophil-associated ribonucleases , HMGB (high-mobility group box) proteins (e.g. HMGB1) , granulysin  and iron-binding proteins (e.g. lactoferrin) . Alarmins are involved mainly in the recruitment and activation of phagocytic cells and maturation of DCs, and are considered to be one form of DAMPs (damage-associated molecular patterns) . In addition to their pro-inflammatory role , alarmins may provide direct antibacterial activity. HMGB1 production seems to be critical for DC maturation, migration to lymphoid tissues and T-cell polarization into the Th1 phenotype .
TNFα is a pleiotrophic pro-inflammatory cytokine and the most extensively studied member of the family involving 50 structurally related soluble and membrane proteins: the TNF superfamily . It is produced mainly by activated macrophages , but can also be induced in a broad variety of other cells types. The primary function of TNFα is the up-regulation of multiple pro-inflammatory proteins (chemokines, cytokines, adhesion molecules, growth factors, etc.) by activation of transcription factor NF-κB (nuclear factor κB) and MAPK (mitogen-activated protein kinase) pathways . In addition to its local effects, TNFα is directly linked to the induction of cachexia , is associated with chronic diseases, and affects thermoregulation , lipid metabolism , blood flow , coagulation  and insulin resistance . Up-regulation of TNFα is implicated in the pathogenesis of rheumatoid arthritis , IBD , psoriasis , COPD (chronic obstructive pulmonary disease)  and other chronic inflammatory disorders. Furthermore, the immediate systemic generation of TNFα at high levels might lead to the development of a shock reaction . Anti-TNF therapy using either chimaeric (infliximab) or human (adalimumab or golimumab) monoclonal antibodies is highly effective in the treatment of IBD [72,73] and, together with the recombinant soluble TNF receptor etanercept, is also used to treat patients with rheumatoid arthritis  and different autoimmune diseases [75,76]. Another prospective TNF blocker for the treatment of rheumatoid arthritis is certolizumab pegol, a recombinant, PEGylated antigen-binding fragment of a humanized monoclonal antibody that selectively targets and neutralizes TNFα .
TRAIL (TNF-related apoptosis-inducing ligand; TNSF10) is a member of TNF family expressed by most cells as CD253 which, after proteolytic cleavage from the membrane, forms a soluble cytokine selectively cytotoxic in inducing apoptosis in tumour cells with minimal or no toxicity in normal tissues . In this respect, the molecule and agonistic anti-TRAIL antibodies are among the potential targets of cancer therapy .
RANKL (receptor activator of NF-κB ligand; TNSF11), also known as TRANCE (TNF-related activation-induced cytokine), is expressed by osteoblasts  as the membrane molecule CD254 to activate osteoclasts and, in its soluble form, seems to be important for the differentiation of DCs . RANKL inhibitors, such as denosumab, are currently tested in patients with osteoporosis, multiple myeloma and bone metastases .
TWEAK (TNF-like weak inducer of apoptosis; TNSF12) is a multifunctional cytokine with mainly pro-inflammatory and pro-angiogenic properties, which regulates the migration of endothelial cells and provides cytotoxicity against some cells . TWEAK induces the expression of ICAM-1 (intercellular adhesion molecule-1) and E-selectin on endothelial cells  and promotes fibroblast proliferation . Up-regulation of IL-17 by TWEAK has been suggested to play a role in the pathogenesis of rheumatoid arthritis , and a Phase II trial with a neutralizing anti-TWEAK antibody is currently in progress in patients with lupus nephritis .
APRIL (a proliferation-inducing ligand; TNSF13) is expressed in monocytes, macrophages, DCs and T-cells, but may also be secreted from non-immune cells, especially tumour cell lines, and play a critical role in differentiation and activation of B-cells . In experimental models, APRIL was found to suppresses both allergic lung inflammation with the production of Th2 cytokines  and arthritis mediated by collagen-specific antibodies . A humanized anti-APRIL antibody is currently being tested in pre-clinical studies , including a primate model of multiple sclerosis .
BAFF (B-cell-activating factor belonging to the TNF family; TNSF20) is a TNF-related cytokine produced and secreted mainly by mononuclear phagocytes (monocytes, macrophages and DCs), but may also be released from epithelial cells, including airway or gut epithelial cells . In addition to the role of BAFF in activating B-cells and immunoglobulin isotope switching, the cytokine drives the expansion of Th1 and Th17 pathways while inhibiting Th2 responses . BAFF, together with APRIL, are two innate cytokines of critical importance for the regulation of antibody production during the adaptive immune response, and the monoclonal anti-BAFF antibodies belimumab and tabalumab are being clinically tested in patients with autoimmune diseases  and are being considered for use in kidney transplantation .
LIGHT (an acronym derived from: homologous to lymphotoxins, inducible expression, competes with HSV glycoprotein D for HVEM, a receptor expressed on T-lymphocytes; TNSF14/CD258) is inducible on the membrane of most immune cells, but may also be released from platelets, vascular smooth muscle cells and endothelial cells . This cytokine activates STAT3 (signal transducer and activator of transcription 3) signalling  and seems to be important for memory T-helper-cell-mediated activation of DCs .
IL-1 family members are among the most potent cytokines produced by innate immune cells. IL-1, a multifunctional pro-inflammatory cytokine, was the first interleukin to be described . Both IL-1α (IL-1F1) and IL-1β (IL-1F2) are synthesized as precursors (pro-IL-1α and pro-IL-1β). The mature form of IL-1α, cleaved by calpain  from pro-IL-1α, remains mostly bound to a plasma membrane, whereas IL-1 β, processed by caspase 1 (IL-1β-converting enzyme) , is released in its biologically active form. The intracellular signalling involves adaptor proteins MyD88 (myeloid differentiation factor 88), IRAK (IL-1-receptor-associated kinase) and TRAF6 (TNF-receptor-associated factor 6), and leads to the activation of NF-κB, JNK (c-Jun N-terminal kinase) and MAPK . IL-1β thus shares multiple pro-inflammatory effects (induction of chemokines, adhesion proteins, etc.) with TNFα and both cytokines potentiate each other . Therapeutic anti-(IL-1) antibodies use different mechanisms to block the cytokine: canakinumab competes for binding to IL-1R (IL-1 receptor), whereas gevokizumab is claimed to modulate IL-1β bioactivity and receptor signalling . Canakinumab has a promising preliminary safety and efficacy profile in patients with rheumatic diseases  and urticarial vasculitis , and is also being evaluated as an anti-inflammatory agent in cardiovascular diseases  and Type 1 diabetes . Furthermore, rilonacept, an anti-(IL-1) fusion protein, is currently tested in patients with active systemic juvenile idiopathic arthritis , and a new monoclonal antibody directed against the IL-1R and a neutralizing anti-(IL-1α) antibody are in clinical trials .
IL-1Ra (IL-1 R antagonist; IL-1F3), another IL-1 family member, binds with high avidity to IL-1Rs, but fails to activate cells . In this respect, IL-1Ra is a potent competitive inhibitor of the pro-inflammatory effects of IL-1β, and the recombinant protein anakinra has been successfully used to treat autoinflammatory diseases such as Schnitzler's syndrome  or Mediterranean fever .
IL-18 (IL-1F4), originally described as an IFNγ-inducing factor, is produced mainly by epithelial cells  and mononuclear phagocytes . Similarly to IL-1β, an inactive IL-18 precursor peptide is cleaved by caspase 1 or granzyme B  to gain biological activity and the signalling pathways resemble those of IL-1 . Serum levels of IL-18 are up-regulated in Th1-predominant diseases such as pulmonary  or kidney allograft rejection , as well as in Th2-predominant diseases including bronchial asthma . Increased serum concentrations of IL-18 are common in active coeliac disease, , ANCA (anti-neutrophil cytoplasmic antibody)-positive vasculitis  or Type 1 diabetes . The overproduction of IL-18 may also be induced by non-specific factors such as haemodialysis . In patients with metastatic melanoma, intravenously administered recombinant human IL-18 was well-tolerated, but had limited activity when used as a single agent , and further studies are in progress. IL-18BP (IL-18-binding protein), a natural inhibitor of IL-18, binds the cytokine with high affinity and prevents its interaction with cell-membrane receptors. In a model of renal ischaemia/reperfusion injury, exogenous IL-18BP inhibited inflammation and protected the tubular epithelium and endothelium . On the other hand, up-regulated IL-18BP secretion from prostate cancer cells might be one of the mechanisms used by tumours to escape immune surveillance .
IL-33 (IL-1F11), a cytokine associated with Th2 responses , is preferentially expressed in vascular endothelial cells , with constitutive expression also in epithelial surfaces and lymphoid organs . The mechanisms of IL-33 release are not completely elucidated, but current findings support the hypothesis that most of the soluble IL-33 originates from destroyed cells, and the cytokine thus might be seen functionally as a member of the alarmin family intended for signalling of tissue injury . In addition, experimental data with recombinant IL-33 suggested Th2-polarizing effects by activation of basophils , mast cells , eosinophils , Th2 lymphocytes and ILCs with potent IL-5 induction . IL-33-activated DCs prime naive T-cells to release the Th2 cytokines IL-5 and IL-13, but not IL-4 . A recent study also showed the potentiation of IgE production by IL-33 . In contrast with the Th2-inducing effects, IL-33 may also potentiate Th1 responses, since it directly interacts with invariant NKT cells and NK cells to induce IFNγ production  and stimulates effector activity of CD8 cytotoxic T-cells . Anti-(IL-33) antibodies have been shown to have a therapeutic effect by suppressing mucosal inflammation in murine models of bronchial asthma  and allergic rhinitis .
IL-36α (IL-1F6), IL-36 β (IL-1F8), IL-36γ (IL-1F9) and IL-36Ra (IL-36 receptor antagonist; IL-1F5) are structurally and functionally related cytokines induced in keratinocytes by TNFα and are capable of promoting the expression of antimicrobial peptides and matrix metalloproteinases . A recent study indicated IL-36R (interleukin 36 receptor)-deficient mice to be resistant in a model of experimental psoriasis . In a myeloid cell line, IL-36β and IL-36γ were found to be induced by interaction with epithelial cells . IL-37 (IL-1F7) is an anti-inflammatory cytokine  produced in adipose and liver tissue . Its ability to suppress the production of pro-inflammatory cytokines from macrophages is dependent on Smad3 activation .
IL-38 (IL-1F10) is another anti-inflammatory cytokine inhibiting the release of IL-17 and IL-22 from T-cells and shares the receptor with IL-36 and IL-36Ra .
IL-6 is a pleiotropic cytokine produced mainly by monocytes/macrophages  and epithelial cells , but may also be induced in endothelial cells and fibroblasts , bone marrow cells , neutrophils , mast cells  or T- and B-cells [156,157]. IL-6 regulates physiological functions of multiple immune and non-immune cell types and represents a critical interphase between immune, endocrine and neural systems. In addition to differentiation of myeloid cells  and activation of NK cells , IL-6 affects adaptive immune responses when it stimulates the differentiation of T-cells  and B-cells and promotes imunoglobulin production . The systemic effects of IL-6 involve the up-regulation of body temperature  and production of acute-phase proteins in hepatocytes . An anti-(IL-6) antibody, tocilizumab, has been found to inhibit structural joint damage and improve physical function in patients with rheumatoid arthritis [163,164] with a good chance of long-term clinical remission . Tocilizumab was also effective in severe persistent systemic juvenile idiopathic arthritis , giant cell arteritis  and neuromyelitis optica , and down-regulated inflammation in amyotrophic lateral sclerosis . Sirukumab, another human anti-(IL-6) monoclonal antibody, was well-tolerated in a Phase I study in patients with cutaneous or systemic lupus erythematosus . Sarilumab, an anti-[IL-6R (IL-6 receptor)] antibody, is currently in two Phase III studies as a treatment for rheumatoid arthritis .
Other IL-6-related cytokines, IL-11, LIF (leukaemia inhibitory factor), oncostatin M, CNTF (ciliary neurotrophic factor) and cardiotrophin-1, are similarly multifunctional and exhibit overlapping biological functions  in immune and non-immune cells. IL-31, a recent member of the IL-6 cytokine family, is expressed mainly by mast cells and Th2 lymphocytes during the late phase of allergic inflammation, and provides a direct pruritogenic effect in the skin and induction of pro-inflammatory cytokines in mononuclear phagocytes .
IFNs, named after their ability to ‘interfere’ with viral replication, are cytokines with potent immunomodulatory effects. According to their structure and chromosomal location, three types of IFNs have been described. Type I IFNs (IFNα and IFNβ) are produced by almost all cell types in response to PRR signalling, with macrophages and epithelial cells being the main source . These cytokines promote the activation of DCs, activate T-cells contributing to Th1 differentiation, activate effector functions of CD8+ cytotoxic T-cells and regulate the recruitment of immune cells by the induction of chemokines . Simultaneously, there are data supporting the concept that Type I IFNs may also have an anti-inflammatory capacity  and inhibit some of the effects of IFNγ . IFNα treatment is beneficial in patients with viral hepatitis B and C , and multiple subtypes of IFNα (13 subtype genes on chromosome 9) with different receptor affinities and biological effects have been extensively studied with respect to their potential anti-viral or immunomodulatory activity . Some concerns about its use need to be addressed in patients with depressive syndromes, which might worsen in some subjects after IFNα therapy . Type II IFNs are constituted by a single cytokine, IFNγ, which is produced preferentially by NK cells, NKT-cells and activated Th1 cells.
IFNγ is a potent activator of effector functions in phagocytic cells by increasing their lysosomal enzymatic activity and oxidative burst . Furthermore, this cytokine plays an important role in the induction of adaptive responses  by promoting antigen processing and presentation of DCs, partially by an up-regulation of MHC molecules . In this respect, recombinant IFNγ has been tested as an adjunct therapy in opportunistic infections . Blocking IFNγ has been found to be effective in Crohn's disease .
Type III IFNs can be produced by most cell types stimulated by infectious agents through TLR signalling , and the signalling pathways are reminiscent of the responses by type I IFNs . This class of IFNs includes IFN-λ1 (IL-28A), IFN-λ2 (IL-28B) and IFN-λ3 (IL-29), cytokines structurally related to IL-10 .
Granulocyte/macrophage colony-stimulating factor and Granulocyte colony-stimulating factor
GM-CSF (granulocyte/macrophage colony-stimulating factor) is synthesized by multiple cell populations (macrophages, T-cells, NK cells, NKT cells, endothelial and epithelial cells, fibroblasts, etc.) in response to inflammatory stimuli. It increases the phagocytic and microbicidal activity  of neutrophils and macrophages and induces their production of pro-inflammatory cytokines . Furthermore, GM-CSF induces the differentiation of DCs and promotes the survival of eosinophils . In an experimental model, neutralizing GM-CSF suppressed LPS (lipopolysaccharide)-induced lung inflammation through NF-κB- and AP-1- (activator protein-1) dependent mechanisms . The pro-inflammatory role of GM-CSF is also supported by the protective effect of an anti-(GM-CSF) antibody in a model of cigarette smoke-induced inflammation . Administra-tion of GM-CSF accelerates myeloid cell and platelet recovery in patients with haematological malignancies after receiving intensive chemotherapy . On the other hand, a human anti-(GM-CSF receptor-α) monoclonal antibody, mavrilimumab, is currently being tested to target mononuclear phagocytes in patients with moderate rheumatoid arthritis .
G-CSF (granulocyte colony-stimulating factor) is produced by macrophages and a variety of non-immune cells, such as fibroblasts, endothelial cells or bone marrow stromal cells . This cytokine is critical for the proliferation and maturation of granulocytic progenitors in the bone marrow, but also seems to support effector functions and survival of mature neutrophils in peripheral blood . Patients with acute lymphoblastic leukaemia after myelotoxic chemotherapy treated with G-CSF recover faster from the neutropenia, have less infectious complications and require less anti-infection medication . Therapy with G-CSF to regulate the homing and engraftment of bone marrow stem cells has been tested in patients with myocardial infarction . Recently, G-CSF therapy was found to be effective in chronic mucocutaneous candidiasis, possibly by inducing an IL-6-mediated IL-17 pathway .
Chemokines are low-molecular-mass proteins regulating the recruitment of inflammatory cells to the site of injury or immune response. There are approximately 50 different chemokines identified and these are grouped into one of four traditional families (CXC, CC, CX3X, and XC) on the basis of the arrangements of the two N-terminal cysteine residues . In CXC and CX3C chemokines, one and three amino acids respectively separate the first and second cysteine residues, whereas, in the CC chemokines, the two cysteine residues are adjacent to each other. In XC chemokines, the first and third cysteine residues are absent. The CXC chemokines can be subdivided further into those having an ELR (glutamate/leucine/arginine) motif before the CXC motif (ELR-CXC chemokines are selective for neutrophil recruitment) and those without the ELR motif . In experimental models, such as a transplant model, early chemokine expression appears to occur in two phases . There is an immediate induction of the ELR-CXC chemokines attracting neutrophils, such as IL-8/CXCL8 (CXC chemokine ligand 8), MIP-2 (macrophage inflammatory protein-2)/CXCL2 and Gro-α/CXCL1, which is evident within 1–3 h following transplantation. Early CXC chemokine expression is transient, with the levels falling substantially by 24 h and returning to near-basal levels by 48 h. The next chemokines expressed subsequently [RANTES (regulated upon activation, normal T-cell expressed and secreted)/CCL5 (CC chemokine ligand 5), MCP-1 (monocyte chemoattractant protein-1)/CCL2, MIP-1/CCL3, MIG (monokine induced by IFNγ)/CXCL9, IP-10 (IFN-inducible protein of 10 kDa)/CXCL10 and I-TAC (IFN-inducible protein of 10 kDa)/CXCL11] serve to start monocyte, NK cell and T-cell recruitment. MIG/CXCL9 appears to be a key chemokine in the recruitment of allogeneic T-cells into allografts, and its neutralization may significantly prolong allograft survival . This second stage of chemokine expression is at least partially dependent on the first and seems to be regulated by pro-inflammatory cytokines. It has been shown that IL-6 in complex with soluble IL-6Rs induces the transition from a neutrophil to a monocyte type of inflammation by changing the chemokine profile in endothelial and parenchymal cells . In addition to the regulation of cell trafficking, several chemokines are involved in the regulation of angiogenesis  or deposition of cellular matrix in fibrosing disorders . In addition, chemokines are intensively studied in tumour immunology since they not only regulate the recruitment of immune cells, but some of them also promote tumour progression and metastasis . To date, a number of chemokine receptor inhibitors, mostly peptide inhibitors or neutralizing anti-receptor antibodies, have been tested in clinical studies targeting either CXC (CXCR1–CXCR4 and CXCR7) or CC (CCR1–CCR5 and CCR9) receptors with rather disappointing results, especially in autoimmune diseases . Since two of the chemokine receptors also serve as co-receptors required for the entry of the HIV virus into cells, antagonists of CCR5 (aplaviroc, maraviroc and vicriviroc) are currently being tested in clinical trials , and a CXCR4 inhibitor (plerixafor) represents another possible target, which is also used for the mobilization of haematopoietic cells during high-dose chemotherapy protocols . Recently, CCX282-B (vercirnon), an orally administered CCR9 antagonist regulating the migration and activation of inflammatory cells in the intestine, has shown promising preliminary results and is being tested in Phase III in patients with Crohn's disease .
CYTOKINES REGULATING THE INITIATION AND MODULATION OF ADAPTIVE IMMUNE RESPONSES: POLARIZATION OF Th1/Th2/Th17 PATHWAYS
A large body of evidence suggests the existence of functionally polarized Th cell responses based on their profile of cytokine secretion. Human Th1 (production of IL-2 and IFNγ), Th2 (production of IL-4, IL-5, IL-3 and IL-13) and Th17 (production of IL-17 and IL-22) cells not only produce a different set of cytokines, but also exhibit distinct functional properties and play different roles in protection . Th1 lymphocytes activate macrophages and are responsible for cell-mediated immunity and phagocyte-dependent protective responses. In contrast, Th2 cells are involved in regulating antibody production, eosinophil activation and inhibition of macrophage functions. Th17 cells stimulate epithelial cells and fibroblasts to release CXCL8/IL-8, attracting neutrophils to the site of immune response. Th1 cells mainly develop following infections by intracellular bacteria and viruses, whereas Th2 cells predominate in responses to parasites and Th17 cells fight extracellular microbes . Other subsets of effector Th cells, Th9 (production of IL-9) and Th22 (production of IL-22), have been described to play a role in allergic inflammation, although their physiological role in defence mechanisms has not been completely characterized yet . Cytokines of innate immune cells, particularly macrophages, epithelial cells and ILCs, seem to play a critical role in the polarization of Th cytokine profiles into specific subsets of effector cells or Treg-cells (Figure 4).
Cytokine polarization of Th cell responses
The IL-12 family share the unique heterodimeric molecular structure composed of two covalently linked chains, and these cytokines play a critical role in the differentiation and regulation of Th cells .
IL-12 (p40/p35 complex) produced mainly by DCs and macrophages triggers CD4+ Th cells to differentiate into Th1 cells secreting IL-2 and IFNγ  and also activates NK cells and CD8+ cytotoxic T-cells . Furthermore, IL-12 stimulates the antimicrobial activity of macrophages . In this respect, IL-12 bridges the early innate immune mechanisms with subsequent cell-mediated adaptive immune responses regulated by Th1 lymphocytes. When rhIL-12 (recombinant human IL-12) was administered to patients with bronchial asthma, the number of blood and sputum eosinophils fell significantly, but without any significant effects on airway hyper-responsiveness or the late asthmatic reaction. Furthermore, serious adverse effects, including cardiac arrhythmias and abnormal liver function, were reported . However, there are still on-going Phase I and Phase II clinical trials with rhIL-12 related to cancer, viral infections and haematopoietic stem cell transplantation .
IL-23 (p40/p19 complex) preferentially induces Th17 polarization and the production of IL-17, but may play a role also in induction of memory T-cells . Ustekinumab, a human monoclonal antibody to the p40 subunit shared by IL-12 and IL-23, significantly improved active psoriatic arthritis in a placebo-controlled study . Furthermore, blocking IL-12/IL-23 with ustekinumab holds promise as a therapy for patients with moderate-to-severe Crohn's disease unresponsive to previous anti-TNF therapy .
IL-27 (p28/EBi3 complex), produced by activated mononuclear phagocytes, seems to be a cytokine with predominantly immunosuppressive effects by promoting the development of Treg-cells specialized to control Th1 cell-mediated immunity . In addition to further inhibitory effects on Th2 and Th17 responses, IL-27 was found to induce anti-inflammatory IL-10 production in T-cells .
IL-35 (p35/EBi3 complex) is the only IL-12 family member not produced by macrophages or DCs. Treg-cells release IL-35 to inhibit T-cell proliferation  and suppress Th17-mediated inflammation .
The IL-17 cytokine family consists of six members: IL-17A (the most intensively studied, referred to as IL-17), IL-17B, IL-17C, IL-17D, IL-17E (IL-25) and IL-17F. Originally, IL-17 was thought to be secreted exclusively by a Th17 subpopulation of Th cells, mostly CD4+CDD161+ cells , but it is also clearly produced by innate immune cells such as macrophages, DCs, NK and NKT cells  or the recently described ILCs . IL-17 provides numerous inflammatory effects through NF-κB, MAPKs and C/EBP (CCAAT/enhancer-binding protein) signalling, leading to the induction of genes for matrix metalloproteinases, growth factors, pro-inflammatory cytokines and chemokines preferentially attracting neutrophils . IL-17 is involved in host defence against bacteria and fungi and plays a role in the pathogenesis of inflammatory and autoimmune disorders . Secukinumab (AIN457), a recombinant human monoclonal anti-(IL-17A) antibody, had sustained symptom reductions in psoriasis, rheumatoid arthritis and ankylosing spondylitis with no alarming safety issues . There is also another anti-(IL-17) antibody ixekizumab (LY2439821) , anti-(IL-17RA) antibody brodalumab (AMG 827), and two small-molecule drugs, vidofludimus and tofacitinib, which inhibit IL-17 as part of their overall pharmacological profiles .
IL-17C released from epithelial cells stimulates inflammatory responses in an autocrine manner and up-regulates the release of pro-inflammatory cytokines, chemokines and antimicrobial peptides in response to bacterial challenge or inflammatory conditions . IL-17D does not seem to directly affect immune cells, but might amplify local inflammatory reactions by the induction of IL-6, IL-8 and GM-CSF from parenchymal cells, and may also be important for growth and repair of structural damage . IL-17E (IL-25) is produced by epithelial cells and plays an important role in the Th2 response in a mouse model of bronchial asthma . It binds to IL-17RB and the biological effects are entirely dependent on adaptor protein CIKS (connection to IκB kinase and stress-activated protein kinase; Act-1) signalling . The main targets of IL-25 are Th2 cells, NKT cells and macrophages. IL-17F up-regulates pro-inflammatory genes in vitro, and overexpression of this cytokine in lung epithelium is associated with infiltration of lymphocytes and macrophages and mucus hyperplasia .
IL-7, TSLP (thymic stromal lymphopoietin) and IL-15 are members of the receptor γ-chain cytokine family (together with IL-2, IL-4, IL-9 and IL-21, which are produced mostly by lymphocytes), which are critical for the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immunity . IL-7, produced mainly by epithelial cells, keratinocytes and macrophages is a crucial survival factor for mature T-cells, but also regulates homoeostasis of epithelium .
TSLP of epithelial origin induced by allergens, viruses, helminths, diesel exhaust or cigarette smoke seems to be a critical factor in the development of Th2 responses at body/environment interfaces . Bronchial epithelial cells from asthmatic patients have a higher capacity to release TSLP in response to dsRNA, which might explain the Th2 polarization of immune reactivity in viral infections of these patients . In a murine model of allergic asthma induced by chronic exposure to the house dust mite, neutralization of TSLP inhibits airway remodelling .
IL-15 is a pleiotropic cytokine with widespread mRNA expression in most tissues, which is produced mainly by macrophages, DC, and epithelial cells. IL-15 is critical for the development of NK and NKT cells, pro-inflammatory cytokine and chemokine release from monocytes, differentiation of DC, activation of neutrophils, migration of mast cells, survival of CD8+ cytotoxic T-cells and induction of the co-stimulatory molecule CD154 [CD40L (CD40 ligand)] on CD4+ Th cells .
CYTOKINES FOR THE INHIBITION OF IMMUNE RESPONSES AND REGULATION OF REPARATIVE PROCESSES
Much progress has been made in recent years in defining the cellular and molecular mechanisms by which the immune system down-regulates effector mechanisms and restores homoeostasis in injured tissue. Under normal circumstances, cytokines released mainly from macrophages, preferentially the M2 subset, have a capacity to inhibit the production of pro-inflammatory cytokines and induce healing of the tissue by regulation of extracellular matrix protein deposition and angiogenesis. Understanding these events is also of particular importance in numerous immunopathological conditions associated with inappropriate tissue damage or an exaggerated fibrotic response associated with chronic inflammation. In some diseases, such as idiopathic pulmonary fibrosis  or rheumatoid arthritis , genetic factors, including cytokine gene polymorphisms, might play an important role. Most of the cytokines regulating reparative processes are released from innate immune cells, which also potentiate the immunosuppressive activity of Treg-cells (Figure 5).
Inhibition of immune responses and reparation
The IL-10 family consists of nine cytokines structurally related to IFNs, which can be subdivided into three functionally distinct groups: (i) the anti-inflammatory cytokine IL-10 itself, (ii) the IL-20 subfamily with direct antibacterial effects (IL-19, IL-20, IL-22, IL-24 and IL-26), and (iii) type III IFNs with antiviral activities (IL-28A, IL-28B and IL-29). Most of the IL-10 family members are produced preferentially by myeloid cells or lymphocytes, but may also be secreted by non-immune cells such as epithelial cells, keratinocytes and fibroblasts.
The structure of IL-10 is quite similar to IFNγ , but the effects of these two cytokines are completely different. In contrast with the strong pro-inflammatory effects of IFNγ, IL-10 is probably the most important anti-inflammatory cytokine used for limiting the overwhelming immune reactions leading to tissue injury. Mononuclear phagocytes and lymphocytes (not only Th2 and Treg-cells, but also Th1, Th17, Th22 and B-cells) represent the main source of IL-10, but limited amounts of this cytokine might also be produced by granulocytes, NK cells or keratinocytes . IL-10 inhibits the production of pro-inflammatory cytokines and chemokines and reduces the antigen-presenting capacity of DCs by down-regulating MHC class II molecules . On the other hand, IL-10 may also play a pro-inflammatory function in antibody-mediated immunopathologies by contributing to B-cell activation . In patients with rheumatoid arthritis, recombinant human IL-10, SCH52000 , did not have any significant clinical effects in a placebo-controlled study .
IL-19 is produced by macrophages, epithelial cells, B-cells and keratinocytes and appears to be critical in controlling the release of pro-inflammatory cytokines in different clinical settings related to both Th1 and Th2 responses. Induction of IL-19 transcription depends on MyD88-dependent TLR signalling and might be crucial for endotoxin tolerance to prevent lethality from septic shock . Although an anti-inflammatory function of IL-19 has been suggested in IBD or coronary procedures, data also support a potential pro-inflammatory role in rheumatoid arthritis  or a proliferative effect in epithelial remodelling .
Mononuclear phagocytes and keratinocytes are the main source of IL-20, and the cytokine is up-regulated in psoriatic arthritis , as well as in the plasma of patients with rheumatoid arthritis . Biological functions of IL-20 include a role in wound healing and remodelling, together with the induction of pro-inflammatory chemokines and cytokines .
In innate immunity, IL-22 is produced by subsets of ILCs (ILC22; NK-22) and not by mononuclear phagocytes . Among T-cells, IL-22 is produced not only by Th22 cells, but may also be released from Th0 and Th17 cells . IL-22 is a cytokine important for antibacterial defence by inducing defensins, calgranulins and other antibacterial peptides in keratinocytes, which are probably a major target for this cytokine, although elevated levels of IL-22 have also been found in patients with IBD . With respect to experimental data, IL-22 appears to play a beneficial role in inflammation by counteracting the destructive effects of immune mechanisms and supporting wound healing . Previous findings suggest that IL-22 produced by ILCs regulates the containment of commensal bacteria in the gut . IL-24 produced by activated monocytes and Th2 cells  is involved in wound repair processes by down-regulating keratinocyte migration and proliferation , and has been reported to induce the death of malignant cells . IL-26 can be produced by monocytes, Th17 cells  and NK-22 cells  and may play a role in modulation of epithelial homoeostasis and integrity, although its physiological role has not been sufficiently elucidated yet.
All five isoforms of TGFβ (β1, β2, β3, β4 and β5) inhibit the proliferation of most cell types, but simultaneously stimulate the proliferation of some mesenchymal cells, e.g. fibroblasts, and induce their production of extracellular matrix proteins (fibronectin, collagens, tenascins, proteoglycans and glycosamidoglycans) and mediate epithelial-to-mesenchymal transition . Furthermore, direct anti-proliferative effects on T-lymphocytes, B-lymphocytes and NK cells result in the immunosuppressive properties of this cytokine, together with inhibition of macrophage activation . The activity of TGFβ requires processing from a latent molecule , and the effect is dependent on the cell differentiation state . TGFβ1 is widely expressed and most intensively studied compared with other isoforms. The multifunctional nature of TGFβ  suggests an important role in wound healing, down-regulation of immune responses and modulation of cell growth and differentiation being critical in the development of regulatory T-cells , but it also plays an important role in the immunopathogenesis of fibrosing disorders . In addition to high levels of TGFβ produced by platelets, macrophages, lymphocytes, endothelial cells and epithelial cells, eosinophils represent other cellular sources of the cytokine . TGFβ activation is also regulated by its binding to integrins . Fresolimumab, a human anti-TGFβ antibody, has been tested in primary focal segmental glomerulosclerosis  and there are also on-going studies in cancer patients .
The activin/inhibin family and BMP (bone morphogenetic proteins) are other cytokines structurally related to TGFβ and are involved in tissue reparation and remodelling, but might also play a role in differentiation of T-lymphocytes . Activin A can also be classified as a pro-inflammatory cytokine by induction of macrophage polarization towards the acquisition of an M1 phenotype .
FGFs (fibroblast growth factors) are a family of 23 proteins which play an important role in reparative processes by induction of wound healing by promoting fibroblast proliferation and angiogenesis. FGF1 (acidic FGF) and FGF2 (basic FGF) appears to be the most potent in this respect, whereas FGF7 and FGF10 (also known as keratinocyte growth factors) regulate the proliferation and differentiation of epithelial cells. In addition to these effects, FGFs also play a role in the regulation and differentiation of multiple cells and tissues during embryogenesis . Elevated levels of FGF23 are associated with increased cardiovascular mortality in patients with chronic kidney diseases .
PDGF (platelet-derived growth factor), produced not only by platelets but macrophages, represents another important local source of this cytokine. In addition to its role in reparative processes by induction of proliferative response in fibroblasts and muscle cells, PDGF serves as a chemotactic factor for monocytes, neutrophils, and fibroblasts and is involved in the pathogenesis of fibrotic diseases .
HGF (hepatocyte growth factor) is ubiquitously expressed in a variety of cell types (most abundant in the liver and placenta) and plays a significant role in tissue regeneration by inducing cell proliferation and migration . On the other hand, angiogenic activity of HGF may also contribute to the neovascularization of tumours .
VEGF (vascular endothelial growth factor) is an important inducer of angiogenesis produced by a number of innate immune cells including mast cells . In addition to its physiological role in the development of blood vessels, VEGF also regulates angiogenesis of vessels in tumours and might directly stimulate the growth of cancer cells through an autocrine mechanisms . Targeting VEGF using the monoclonal antibody bevacimumab, an anti-VEGF receptor antibody ramucirumab, or a recombinant fusion protein aflibercept is currently being tested in cancer patients , where, for several years, the field has been very active with molecules blocking EGFRs (epidermal growth factor receptors) with monoclonal antibodies  or tyrosine kinase inhibitors .
Cytokines released from innate immune cells are involved in the precise regulation of all phases of the immune response and, under physiological conditions, provide important signals for the turnover of multiple cell populations. With respect to the role of pro-inflammatory cytokines released from macrophages in the pathogenesis of chronic diseases, blocking substances based on monoclonal antibodies or recombinant proteins are currently being tested in clinical studies to target cell–cell interactions in chronic inflammatory diseases, and inhibitors of angiogenic cytokines represent attractive targets in malignancies. A better understanding of cytokine networking between innate and adaptive immune cells is essential to increase the effectiveness of biological therapy, reduce the risk of associated adverse effects and, furthermore, might be helpful in finding new potential targets.
a proliferation-inducing ligand
B-cell-activating factor belonging to the TNF family
CC chemokine ligand
CC chemokine receptor
CXC chemokine ligand
CXC chemokine receptor
- ELR motif
fibroblast growth factor
granulocyte colony-stimulating factor
granulocyte/macrophage colony-stimulating factor
hepatocyte growth factor
high-mobility group box
inflammatory bowel disease
- IL-1R etc.
IL-1 receptor etc
- IL-1Ra etc.
IL-1R antagonist etc
innate lymphoid cell
mitogen-activated protein kinase
monokine induced by IFNγ
macrophage inflammatory protein
myeloid differentiation factor 88
nuclear factor κB
- NKT cell
natural killer T-cell
platelet-derived growth factor
pattern recognition receptor
receptor activator of NF-κB ligand
recombinant human IL-12
transforming growth factor
tumour necrosis factor
TNF-related apoptosis-inducing ligand
thymic stromal lymphopoietin
TNF-like weak inducer of apoptosis
vascular endothelial growth factor
Our work is supported by MH CZ–DRO (Institute for Clinical and Experimental Medicine–IKEM) [grant number IN 00023001].