Cytokines and interferons

Cytokines are small, biologically highly active proteins that regulate the growth, function and differentiation of cells and help steer the immune response and inflammation. They are not antibodies and they are not classical hormones, although a few behave like hormones when they reach the circulation in large amounts. Most cytokine action is local. A cell that secretes a cytokine usually influences itself through autocrine signalling or a neighbouring cell through paracrine signalling. When cytokines spill into the blood in sufficient quantity, the result is a systemic phenotype: fever, malaise, muscle catabolism, hepatic acute-phase protein synthesis and, in the extreme, circulatory collapse.

Interleukins are the subset of cytokines first described as messengers between leukocytes. The name is historical. Many interleukins are now known to be produced by fibroblasts, endothelial cells, epithelial cells, adipocytes and hepatocytes, and to act on those same cells. IL-6 illustrates the point: it is involved both in immune responses and in inflammation, haematopoiesis, bone metabolism and embryonic development^[1]. A single molecule with such a wide range of target tissues is described as pleiotropic, and pleiotropy is the rule rather than the exception in cytokine biology.

Four principles help organise an otherwise confusing field. Redundancy means that several cytokines share the same action, so blocking one may be partially compensated by another. Pleiotropy means that one cytokine has many actions, which is why a drug that blocks it can produce unexpected benefits and unexpected harms. Synergy means that two cytokines together produce more than the sum of their separate effects. Antagonism means that one cytokine opposes another. IL-6 and IL-17 or TNFα synergistically enhance the production of various pro-inflammatory mediators from non-immune cells in a manner dependent on STAT3 and NF-κB^[1]. In the opposite direction, IL-2-activated STAT5 inhibits Th17 cell differentiation, while IL-6 reduces Foxp3 expression and opposes regulatory T-cell generation^[1]. The net behaviour of a tissue therefore depends on the mixture of cytokines present, not on any single molecule considered in isolation.

Most cytokine receptors signal through Janus kinases and signal transducers and activators of transcription, usually abbreviated to JAKs and STATs. The IL-6 receptor is a useful model: it is composed of an IL-6-binding receptor molecule, IL-6Rα, and a signal transducer, gp130, which is shared among the IL-6 family of cytokines^[1]. Other cytokines use different receptor families and different transcription factors, but the underlying logic is the same: ligand binding, receptor oligomerisation, kinase activation, transcription factor translocation and altered gene expression. NF-κB and STAT3 are central transcription factors in inflammation, and they act alongside IL-1β, IL-6 and TNFα as the core machinery of the inflammatory response^[1]. Synergistic interactions between NF-κB and STAT3 can hyper-activate NF-κB and drive production of inflammatory cytokines^[1].

This matters clinically because cytokines explain much of the phenotype of infection, autoimmunity and malignancy. Pro-inflammatory cytokines such as IL-1β, TNFα and IL-6 play crucial roles in inflammation, and IL-6 is a major player in chronic inflammatory diseases, autoimmune diseases, cancer and cytokine storm^[1]. Blocking a single cytokine can produce striking clinical improvement, which is the strongest practical argument that these molecules are not merely markers but drivers. Measuring cytokines is harder than blocking them. When cytokines are measured in serum, it is not possible to determine the origin of those cytokines, and serum concentrations are altered by production, tissue or cellular deposition, degradation and elimination. A serum level is therefore a snapshot of a dynamic system, not a direct readout of activity at the site of disease.

IL-1 is the prototypical endogenous pyrogen. It is the cytokine that taught immunology that a soluble mediator released at a site of injury can act on the brain to produce fever, on the liver to produce acute-phase proteins, and on the endothelium to make vessels leaky and adhesive. IL-1β is a pro-inflammatory cytokine that plays a crucial role in inflammation, and it sits alongside TNFα and IL-6 in the core inflammatory triad^[1]. It is also an activator of NF-κB, which places it upstream of a very large transcriptional programme^[1].

The family is larger than the name suggests. IL-1α and IL-1β are the two classical agonists. IL-1 receptor antagonist is a naturally occurring competitive blocker that binds the same receptor without triggering it. IL-18, IL-33 and the IL-36 subfamily are related molecules with overlapping but distinct roles. From a teaching perspective, the important structural point is that IL-1β is produced as an inactive precursor and requires cleavage to become fully active. That cleavage is carried out by caspase-1 after it is activated within a multiprotein complex called the inflammasome. The inflammasome is therefore a sensor that converts danger signals into biologically active IL-1.

This two-step arrangement, transcription first and cleavage second, gives the system a safety catch. A cell can accumulate precursor without causing inflammation until a second signal triggers assembly of the inflammasome. It also explains why diseases caused by inappropriate IL-1 release are autoinflammatory rather than autoimmune: the adaptive immune system is not necessarily driving the process. The classical clinical signatures of excess IL-1 are recurrent fever, serosal inflammation, neutrophilic skin lesions and a brisk acute-phase response. These are the features that distinguish IL-1-driven autoinflammation from autoantibody-driven disease.

IL-1 acts on the hypothalamus to produce fever, partly by inducing local prostaglandin synthesis. It acts on the liver to increase synthesis of C-reactive protein, serum amyloid A and other acute-phase proteins, while suppressing albumin and transferrin. It activates endothelium to express adhesion molecules and chemokines, recruiting neutrophils and monocytes. It acts on chondrocytes and synoviocytes to release matrix metalloproteinases, which helps explain cartilage damage in inflammatory arthritis. It also influences adaptive immunity by promoting Th17 responses. Human naive T cells, in the presence of IL-1β with IL-6 or IL-23, can differentiate to Th17 cells without TGFβ^[1]. This places IL-1 upstream of the IL-17 axis, not merely downstream of it.

IL-1 has a further role in the most dramatic systemic cytokine syndromes. IL-1 and IL-6 are involved in the lethal cytokine storm sometimes seen in CAR-T treatment^[1]. That single observation explains a great deal of modern supportive care in cellular therapy. If IL-1 and IL-6 together generate the storm, then blocking IL-6 receptor will not always be sufficient, and IL-1 blockade becomes a rational additional strategy in refractory cases.

The therapeutic logic follows directly. If IL-1 is produced in excess, then either block its receptor, neutralise the ligand, or remove the trigger that drives inflammasome activation. The first two approaches work rapidly and can produce dramatic defervescence. The third requires identifying the upstream cause, which may be a genetic variant in an inflammasome component, a crystal such as monosodium urate, or an infectious trigger. Understanding the two-step control of IL-1 release is what allows a clinician to choose between these strategies rather than reaching reflexively for broad immunosuppression.

IL-2 is the classic T-cell growth factor. It is produced mainly by activated CD4 T cells, with contributions from activated CD8 T cells, and it acts back on the cells that made it and on their neighbours. Its name describes its function well: it is a signal that a T cell clone has recognised antigen and should now expand. Without IL-2, an adaptive T-cell response cannot scale from a handful of specific cells to a population large enough to clear a pathogen.

The receptor explains the biology. IL-2 binds a complex of three chains: an alpha chain that confers high affinity, a beta chain and a common gamma chain shared with several other cytokines. Signalling proceeds through JAK kinases and STAT5. The same pathway that drives effector T-cell expansion also maintains regulatory T cells, because regulatory T cells express the high-affinity alpha chain constitutively. This dual role is the central paradox of IL-2 therapeutics: the same molecule that boosts effector immunity also boosts tolerance.

The IL-6 and IL-2 axes are therefore best understood as opposing arms of a single switch. TGFβ and IL-6 together induce Th17 cells, while TGFβ together with IL-2 induces regulatory T cells^[1]. IL-2-activated STAT5 inhibits Th17 cell differentiation, and IL-6 reduces Foxp3 expression, which inhibits RORγt-mediated activation of the IL-17A promoter^[1]. The reciprocal actions of STAT3 and STAT5 on the IL-17 locus mean that the balance between IL-6–STAT3 and IL-2–STAT5 regulates whether naive CD4 T cells become Th17 cells or regulatory T cells^[1]. When that balance fails, the result is either autoimmunity or immunodeficiency.

The same logic explains why low-dose IL-2 is an attractive therapeutic idea. Because regulatory T cells respond to lower concentrations than effector T cells, a low dose can preferentially expand the tolerogenic arm. Low-dose IL-2 therapy for autoimmune and inflammatory diseases is under development^[1]. This is a rare example of using the dose of a cytokine to select which cell population responds, rather than using a receptor-blocking antibody to remove a cytokine altogether. There is also a natural brake within the system: miR-125a-5p is induced by IL-2 and down-regulates the expression of IL-6R and STAT3 in regulatory T cells, which restrains the conversion of regulatory T cells into Th17 cells^[1].

The consequences of losing this pathway are severe. Defects of IL-2 signalling induce severe autoimmunity, and regulatory T-cell dysfunction due to Foxp3 mutation causes a lethal autoimmunity known as immunodysregulation, polyendocrinopathy, enteropathy and X-linked syndrome^[1]. Conversely, excessive IL-6–STAT3 signalling causes IL-17A-dependent autoimmune arthritis in mice, and inflammatory and autoimmune diseases including rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease show a bias for Th17 cells over regulatory T cells^[1]. IL-2 is therefore not simply an immune stimulant. It is the counterweight to IL-6 in the decision between tolerance and autoimmunity.

In practical immunology, IL-2 also activates natural killer cells and supports their cytotoxic function, which is why it has been used as an adjunct in tumour immunotherapy. Its systemic administration produces dose-limiting toxicity, chiefly increased vascular permeability with fluid retention and organ dysfunction. That toxicity is a direct consequence of the receptor being expressed on endothelial cells and on activated lymphocytes throughout the body, and it illustrates why local delivery or engineered variants with biased receptor binding are an active area of drug design.

IL-3 is a haematopoietic growth factor rather than a classical inflammatory cytokine. Its older name, multi-colony stimulating factor, describes its ability to support the formation of colonies of several different lineages from early progenitor cells in the bone marrow. It acts on multipotent progenitors and on committed precursors, keeping them alive and permitting them to divide before they respond to lineage-specific factors. It is produced by activated T cells and by mast cells, basophils and stromal cells, which means that a peripheral immune activation can send a signal back to the marrow.

The receptor is the key to understanding its redundancy. IL-3 binds a specific alpha chain, which confers ligand specificity, and then recruits a shared beta chain that is also used by granulocyte-macrophage colony-stimulating factor and by IL-5. Because the signalling module is shared, the three cytokines overlap substantially in their downstream effects. This is a textbook example of redundancy: blocking IL-5 removes eosinophil survival signals, but IL-3 and granulocyte-macrophage colony-stimulating factor can partially substitute, which is one reason anti-IL-5 therapy does not eliminate all eosinophils in every patient.

IL-3 acts on basophils and mast cells as a survival and priming factor. Basophils are rare in blood but potent sources of histamine and IL-4, and IL-3 can increase their numbers and their responsiveness to IgE-mediated stimulation. In this sense IL-3 is part of the type 2 inflammatory network even though it is not itself a type 2 cytokine. It supports the cellular infrastructure on which IL-4, IL-5 and IL-13 act. This is a recurring theme in cytokine biology: some molecules are effectors, and others are permissive factors that determine whether effectors have cells to act on.

In the marrow, IL-3 acts on myeloid, erythroid and megakaryocytic precursors, which is why it was once investigated as a way to accelerate haematopoietic recovery after chemotherapy or transplantation. The clinical results were modest, largely because endogenous colony-stimulating factors already provide strong signals and because the shared beta chain creates cross-talk that dilutes the effect of adding one ligand. The lesson is that a growth factor is rarely the sole determinant of a cell's fate.

Pathologically, IL-3 can contribute to myeloproliferative and allergic disease. Autonomous production of IL-3, or constitutive activation of the signalling pathway downstream of its receptor, can drive expansion of myeloid or basophilic populations. In clinical practice, IL-3 measurement is not part of routine workup, because serum levels do not reliably reflect marrow activity and because the cytokine acts locally within the marrow microenvironment. It is, however, an important conceptual bridge between inflammation and haematopoiesis, and a reminder that a cytokine produced in a lymph node can change the output of the bone marrow.

IL-4 is the signature type 2 cytokine. It is produced by Th2 cells, mast cells, basophils and eosinophils, and it drives the programme that characterises allergic inflammation: Th2 differentiation, immunoglobulin class switching to IgE in B cells, alternative activation of macrophages, goblet cell hyperplasia and mucus production, and increased contractility of smooth muscle. It also has important roles in tissue repair and in the expulsion of parasites, which is presumably why the programme evolved.

The receptor biology explains both the therapeutic promise and the toxicity. IL-4 signals through two receptor complexes. The type I receptor consists of IL-4Rα paired with the common gamma chain and is found mainly on haematopoietic cells. The type II receptor consists of IL-4Rα paired with IL-13Rα1 and is found on non-haematopoietic cells, where it is also used by IL-13. It is worth noting that mTOR-based approaches target the type I IL-4 receptor, that is IRS2 signalling, while many disadvantageous IL-4 effects, such as compromised epithelial barrier function, arise from IL-4 signalling via the type II IL-4 receptor. In other words, the beneficial immune effects and the harmful barrier effects are partly separable by receptor.

This separation has been pursued deliberately. IL-4 has been considered a therapeutic target for boosting and redirecting T and B cell functions, but the usage of IL-4 itself has been problematic, not least because of the harmful side effects of activating the type II IL-4 receptor in non-haematopoietic cells. In mice, IL-4, but not IL-13, induced weight loss and spontaneous erythrophagocytosis. Theoretically, an IL-4 that could activate only the type I receptor but not the type II receptor could be advantageous. Structural studies of human IL-4 receptor complexes showed that once IL-4 binds IL-4Rα, the D-helix of IL-4 faces the secondary receptor chain and forms the interacting surface of the complex with that second chain. That structural insight permitted mutation of the D-helix in a way that left the IL-4/IL-4Rα interaction intact but altered binding to the common gamma chain or to IL-13Rα1. Interestingly, a 1000-fold induction in the recruitment of the IL-4/IL-4Rα complex to the secondary chain had surprisingly little effect on the immediate STAT6 signalling induced by such a mutant. When the availability of the second chain was decreased with a blocking antibody, the difference between wild-type and the type I receptor-specific mutant became more evident. The practical implication is that receptor occupancy does not translate linearly into signal strength, and that signalling output depends on the availability of the second chain in a particular cell. ^[2]

Clinical translation has been substantial. A set of monoclonal antibodies blocking different aspects of the early events of IL-4 and IL-13 signalling is being considered for wider clinical use: dupilumab, a monoclonal blocking antibody for IL-4Rα; lebrikizumab, anrukinzumab and tralokinumab, blocking antibodies for IL-13; and pascolizumab, a blocking antibody for IL-4. Pitrakinra, an IL-4 receptor antagonist that blocks both type I and type II IL-4 receptors upon binding IL-4Rα, has shown initial efficacy in clinical trials. Beyond allergic disease, a STAT6 inhibitor has shown some potential in inhibiting prostate cancer cell growth, which opens new possibilities in targeting IL-4 and IL-13 signalling therapeutically even beyond allergic diseases.

The evolutionary logic is elegant. The notion of the effector function of IL-13 in parasite expulsion, combined with the known toxicity of IL-4, suggests that the system has evolved in a way that protects peripheral tissues from the toxicity of IL-4 by tuning the receptors in the periphery to be more responsive to IL-13 than to IL-4. That single idea explains why IL-13 blockade and IL-4 receptor blockade are not interchangeable, and why blocking the shared receptor produces broader effects than blocking either cytokine alone.

IL-5 is the eosinophil cytokine. It is produced by Th2 cells, by group 2 innate lymphoid cells, by mast cells and by eosinophils themselves, and it acts on eosinophil progenitors in the marrow, on maturing eosinophils and on mature tissue eosinophils. Its principal effects are to promote eosinophil growth, to drive terminal differentiation, and to prolong eosinophil survival by suppressing apoptosis. Because eosinophil death is the main mechanism by which eosinophilia resolves, a survival signal is a powerful lever: withdrawing IL-5 allows eosinophils to die on schedule.

The receptor follows the same architecture as IL-3. IL-5 binds a specific alpha chain, IL-5Rα, which then recruits the shared beta chain used by IL-3 and granulocyte-macrophage colony-stimulating factor. Signalling proceeds through JAK2 and STAT5, with additional contributions from RAS-MAPK and PI3K pathways. The shared beta chain explains the redundancy that frustrates simple therapeutic models. In a patient treated with an antibody against IL-5, residual eosinophils may persist because IL-3 and granulocyte-macrophage colony-stimulating factor continue to deliver survival signals through the same beta chain.

IL-5 also acts on B cells. It was originally described as a B-cell growth and differentiation factor, and it promotes immunoglobulin production, particularly class switching towards IgA. This dual identity, eosinophil growth factor and B-cell helper, is a useful reminder that interleukin names often capture only the first function discovered. In mucosal immunity, IL-5 may coordinate eosinophil recruitment with local antibody production, which is biologically sensible for defence against helminths.

The clinical importance of IL-5 lies in eosinophilic disease. Tissue eosinophilia is a defining feature of eosinophilic asthma, eosinophilic oesophagitis, eosinophilic granulomatosis with polyangiitis, chronic rhinosinusitis with nasal polyps and hypereosinophilic syndromes. In each of these, IL-5 is a plausible driver because it sustains the eosinophil population that causes the damage. Blocking IL-5, blocking its receptor, or blocking the shared beta chain are all rational strategies, and they differ in how much of the redundant signalling network they interrupt. Blocking the receptor removes signalling from IL-5 only, whereas blocking the shared chain also interrupts IL-3 and granulocyte-macrophage colony-stimulating factor.

Practically, IL-5 blockade reduces exacerbations and improves symptoms in selected patients with severe eosinophilic asthma, and it allows reduction of oral corticosteroid exposure, which is often the most important patient-centred outcome. Response is not universal, which is why biomarker selection matters: patients with higher eosinophil counts and with repeated exacerbations despite standard therapy are the most likely to benefit. IL-5 is therefore a good example of a cytokine that is neither a diagnostic marker nor a universal therapeutic target, but a mechanistic lever that works when the right pathway is dominant in the right patient.

IL-6 is the most instructive cytokine in clinical medicine because it sits at the junction of innate inflammation, adaptive immunity and malignancy. It was discovered in 1986 and is a pleiotropic cytokine involved not only in immune responses but also in inflammation, haematopoiesis, bone metabolism, embryonic development and other fundamental processes^[1]. It is the prototypical member of a family that is composed of 10 members, including IL-6, IL-11, IL-27, oncostatin M, leukaemia inhibitory factor, ciliary neurotrophic factor, cardiotrophin 1, cardiotrophin-like cytokine factor 1, IL-35 and IL-39^[1]. It was originally identified as B-cell stimulating factor-2, an inducer of immunoglobulin production, and several other proteins were subsequently found to be identical to it, including IFNβ2 and an IL-1-induced 26-kDa protein^[1]. That history explains its apparent redundancy: what looked like several different activities turned out to be one molecule acting on many tissues. ^[2]

Receptor biology determines which cells respond. The IL-6 receptor is composed of an IL-6-binding receptor molecule, IL-6Rα, and a signal transducer, gp130, which is shared among the IL-6 family of cytokines^[1]. For IL-6 activity, both IL-6Rα and gp130 must be expressed on the cell surface, which limits the number of target cells to lymphocytes, myeloid cells and hepatocytes^[1]. However, IL-6 complexed with the soluble form of IL-6Rα can act on cells expressing only gp130, such as heart muscle cells^[1]. This process, sometimes called trans-signalling, greatly widens the reach of IL-6 and explains why a cytokine best known for hepatic and lymphoid effects can also drive myocardial hypertrophy, as shown in double transgenic mice expressing both IL-6 and IL-6Rα^[1].

Transcription and messenger RNA handling add further layers of control. The IL-6 gene contains elements for NF-κB, activator protein 1, CCAAT/enhancer-binding protein beta, cyclic adenosine monophosphate-responsive elements and STAT3^[1]. IL-6 transcription is induced by Toll-like receptor ligands, IL-1, TNFα, reactive oxygen species and zinc, and also depends on epigenetic regulation^[1]. Post-transcriptional control matters too: Regnase-1 degrades IL-6 mRNA, while Arid5a protects IL-6 mRNA from Regnase-1-mediated degradation^[1]. Lin28, induced by NF-κB activation, prevents IL-6 mRNA degradation by inhibiting microRNA let-7, and STAT3-induced miR-21 and miR-181b-1 activate NF-κB, resulting in induction of IL-6 gene expression^[1].

The IL-6 amplifier is the mechanism that converts a local trigger into a sustained inflammatory state. Synergistic interactions between NF-κB and STAT3 induce hyper-activation of NF-κB followed by production of various inflammatory cytokines, and because IL-6 is an NF-κB target, simultaneous activation of NF-κB and STAT3 in non-immune cells triggers a positive feedback loop of NF-κB activation by the IL-6–STAT3 axis^[1]. That loop is a key player in the local initiation model, in which local initiators such as senescence, obesity, stressors, infection, injury and smoking trigger disease by promoting interactions between non-immune cells and immune cells^[1]. The amplifier is activated by a variety of local initiators, which is why the IL-6–STAT3 axis is a critical target for treating disease^[1]. Its output is broad: target molecules include EGFR, c-Myc, Pim1, VEGFα, hypoxia-inducible factor 1-alpha, matrix metalloproteinases and many pro-inflammatory cytokines and growth factors involved in cell survival, proliferation, angiogenesis and metastasis^[1].

In adaptive immunity, IL-6 shapes the T-cell decision. IL-6 together with TGFβ induces Th17 cells, with RORγt as the key transcription factor, while IL-6 inhibits the TGFβ-induced generation of regulatory T cells^[1]. IL-6 up-regulates IL-23R expression in a manner dependent on STAT3, and IL-23 further enhances its own receptor expression to increase the pathogenic potential of Th17 cells^[1]. IL-6 also directs differentiation of IL-4-producing Th2 cells while inhibiting IFNγ-producing Th1 cells^[1]. Myeloid-derived soluble IL-6R complexed with IL-6 inhibits Th1 responses through STAT3-induced c-Maf, which is considered one of the mechanisms by which IL-6 inhibits T-cell immunity against tumours^[1]. On the B-cell side, IL-6-induced STAT3 activation is critical for in vivo plasma cell survival and immunoglobulin secretion in humans^[1].

Disease associations follow directly. Synovial fluid from patients with rheumatoid arthritis contains high amounts of IL-6, and serum IL-6 levels correlate with clinical and laboratory indices of disease^[1]. Patients with cardiac myxoma produce IL-6 and show autoimmune features that resolve after resection^[1]. IL-6 is a growth factor for plasmacytoma and myeloma, and serum IL-6 levels correlate with poor prognosis in multiple myeloma^[1]. In COVID-19, the blood IL-6 level is highly correlated with disease mortality and predicts the need for mechanical ventilation, and IL-6 and TNFα serum levels are significant predictors of disease severity and death^[1]. In oncology, HER2 overexpression, which occurs in approximately 25% of breast cancers, activates an IL-6 autocrine signalling loop to activate STAT3, which is critical for tumorigenesis^[1]. ^[2]

Therapeutic blockade is now routine. A humanised monoclonal antibody against IL-6 receptor, tocilizumab, has proven successful for treating rheumatoid arthritis and Castleman's disease^[1]. The anti-IL-6R antibodies tocilizumab and sarilumab are used to treat rheumatoid arthritis, Castleman's disease and CAR-T-induced cytokine release syndrome, and have promise for treating severe COVID-19^[1]. One explanation for why inhibiting IL-6 works despite the many cytokines involved is that in inflammation IL-6 is the major stimulator of STAT3, which acts together with NF-κB, the factor that expresses IL-6 as a target^[1].

IL-8, also known as CXCL8, is a chemokine that was historically grouped with the interleukins. It is a potent chemoattractant for neutrophils and T lymphocytes, and it also prompts keratinocyte proliferation. This combination of leukocyte recruitment and epithelial growth helps explain why IL-8 is found at high levels in psoriatic lesions, where both neutrophilic inflammation and epidermal hyperproliferation are prominent. Elevated amounts of IL-8 have been detected in psoriatic lesional skin. Serum levels of IL-8 are also significantly higher in patients with psoriasis than in healthy controls. ^[4]

IL-8 is produced by a wide range of cells, including macrophages, epithelial cells, endothelial cells and fibroblasts. In senescent cells, IL-8 is a component of the senescence-associated secretory phenotype, a programme in which cells secrete more than 40 soluble factors. Conditioned medium from senescent fibroblasts is highly enriched in IL-6 and IL-8, two pleiotropic pro-inflammatory cytokines implicated in cancer progression. This places IL-8 in the interface between ageing, chronic inflammation and tumour promotion. ^[3]

In cancer biology, IL-8 has direct effects on tumour cells. Both IL-6 and IL-8 are essential for cell growth and anchorage-independent growth and colony formation in tumour cells from triple-negative breast cancers. Luminal A breast cancer cell lines do not express IL-6 or IL-8 constitutively, which suggests that the contribution of these cytokines varies between molecular subtypes. The presence of IL-8 in the tumour microenvironment may therefore be a marker of a more aggressive phenotype and a potential therapeutic target.

In viral infection and cytokine storm, IL-8 is one of the pro-inflammatory cytokines induced downstream of pattern-recognition receptors. SARS-CoV-1 and MERS-CoV activate NF-κB in a manner dependent on MyD88 through receptors such as RIG-I and MDA5, leading to activation of a variety of pro-inflammatory cytokines, including IL-6, TNFα, IL-8 and CCL2^[1]. This shared pathway explains why IL-8 rises alongside IL-6 and TNFα in severe viral illness.

Measurement of IL-8 in clinical samples is possible with sensitive immunoassays. In one laboratory, the minimum detectable concentrations included 1.9 pg/mL for IL-8. Such sensitivity allows detection of IL-8 in serum and supernatants, but interpretation requires caution because serum cytokine concentrations are altered by production, tissue deposition, degradation and elimination. A single measurement is a snapshot, not a direct measure of local chemokine activity. ^[4]

From a first-principles perspective, IL-8 illustrates the principle that chemokines and interleukins are not mutually exclusive categories. Its primary role is to direct the movement of cells that bear the appropriate chemokine receptors, chiefly CXCR1 and CXCR2 on neutrophils. That movement is beneficial in acute bacterial infection but harmful when it becomes chronic, as in psoriasis or the tumour microenvironment. Blocking IL-8 or its receptors is therefore a rational strategy in neutrophilic diseases, although the redundancy of the chemokine network means that such blockade may need to be combined with other approaches.

IL-10 is the archetypal anti-inflammatory cytokine. It is produced by M2 macrophages, regulatory T cells and other cell types, and it suppresses the production of pro-inflammatory mediators by macrophages and dendritic cells. In the M1/M2 framework, classically activated M1 macrophages produce nitric oxide synthase, IL-12 and TNFα and promote Th1 responses, while activated M2 macrophages produce arginase-1, IL-10 and TGFβ and relate with the Th2 response^[1]. M1 macrophages are pro-inflammatory, while M2 macrophages are anti-inflammatory^[1]. IL-10 is therefore a marker and mediator of the M2 programme.

Tumour-associated M2 macrophages not only promote tumour progression and metastasis by producing growth factors and angiogenic factors such as epidermal growth factor, VEGF and IL-6, but also inhibit tumour immunity by arginase-1, IL-10 and TGFβ^[1]. IL-6 and IL-10 are abundant in tumour microenvironments originating from myeloid cells, fibroblasts and tumour cells, and they further induce M2 macrophage differentiation through STAT3^[1]. This creates a positive feedback loop in which IL-10 reinforces the immunosuppressive environment that produced it.

Structurally, IL-10 belongs to the class II cytokine family. It forms intertwined dimers, a feature shared with IFN-γ but not with most other class II cytokines^[1]. The IL-10 receptor complex includes IL-10R2, a chain that is shared with several other cytokines. IFN-λ uses a heterodimeric receptor complex consisting of IFN-λR1 and IL-10R2, and IL-10R2 is shared with other members of the IL-10 family, namely IL-10, IL-22 and IL-26^[1]. This shared receptor chain is one reason why the type III interferons are structurally and functionally related to the IL-10 family. Type III IFNs have approximately 11–13% amino acid sequence homology with the IL-10 family^[1]. ^[3]

The anti-inflammatory action of IL-10 is essential for limiting tissue damage during infection. Without IL-10, inflammatory responses can become excessive and cause immunopathology. However, the same property can be exploited by tumours, which use IL-10 to suppress anti-tumour immunity. This duality is a recurring theme in cytokine biology: a molecule that protects the host in one context can promote disease in another. Therapeutic strategies that block IL-10 aim to restore anti-tumour immunity, while strategies that enhance IL-10 aim to treat inflammatory and autoimmune diseases.

IL-10 is also a useful biomarker. In psoriasis, serum levels of IL-10 were not among the cytokines that were significantly elevated in patients compared with controls in the study cited, although other cytokines such as TNF-α, IFN-γ, IL-6, IL-8, IL-12 and IL-18 were higher. This pattern is consistent with the idea that psoriasis is driven by pro-inflammatory cytokines rather than by a deficiency of IL-10. Measurement of IL-10 in serum requires sensitive assays, and the general principle of cytokine measurement applies. ^[4]

IL-12 is a heterodimeric cytokine composed of p40 and p35 subunits. The p40 chain is shared with IL-23, which means that antibodies against p40 block both IL-12 and IL-23. This shared subunit is clinically important because anti-IL-12p40 antibodies have shown significant effects in psoriatic patients. The p35 subunit is unique to IL-12. IL-12 is produced mainly by activated dendritic cells, macrophages and B cells, and it is a major driver of Th1-cell-mediated immune responses.

The principal action of IL-12 is to induce IFN-γ production by T cells and natural killer cells. It also upregulates the IL-18 receptor, and IL-18 acts on dendritic cells synergistically with IL-12 to greatly increase IFN-γ production. This synergy between IL-12 and IL-18 is a good example of how cytokines cooperate to shape the quality of an immune response. IL-12 provides the signal that commits naive CD4 T cells to the Th1 lineage, while IL-18 amplifies the effector output of those cells.

In psoriasis, IL-12 is thought to play a role in the induction of new skin lesions. Serum levels of IL-12 are significantly higher in patients with psoriasis than in healthy controls, and high levels of IFN-γ, IL-12 and IL-18 correlate with the Psoriasis Area and Severity Index. This correlation suggests that these cytokines may serve as objective parameters for disease activity and clinical severity. The measurement of IL-12 in serum is technically feasible; the minimum detectable concentrations included 2.9 pg/mL for IL-12. ^[4]

The IL-12 receptor is closely related to gp130, the signal transducer used by the IL-6 family. This structural similarity is a reminder that cytokine receptors often belong to larger families with shared signalling components. IL-12 signals through JAK2 and TYK2, leading to STAT4 activation, which drives IFN-γ transcription. The details of this pathway are textbook knowledge, but the principle is that IL-12 converts an innate recognition event into a Th1 adaptive response.

Therapeutically, blocking IL-12p40 reduces Th1-driven inflammation and has proven effective in psoriasis. This is one of the clearest examples of a cytokine target that was identified through an understanding of disease pathogenesis. However, blocking p40 also blocks IL-23, which is important for Th17 maintenance, so the clinical effect may be mediated by both Th1 and Th17 pathways. This illustrates the value and the complexity of targeting shared cytokine subunits.

IL-13 is a type 2 cytokine that is closely related to IL-4 in both structure and function. It is produced by Th2 cells, group 2 innate lymphoid cells, mast cells, basophils and eosinophils. IL-13 binds to two distinct receptor chains: IL-13Rα1 and IL-13Rα2. IL-13Rα2 binds IL-13 with higher affinity than IL-13Rα1. The role of IL-13Rα2 has been somewhat elusive; it has been considered merely a decoy receptor that binds free IL-13 strongly without eliciting signalling, and thus serves as a neutraliser of IL-13 by efficiently internalising it from extracellular spaces. Further studies have shown that IL-13Rα2 is not only a decoy receptor.

The functional receptor for IL-13 is a complex of IL-13Rα1 and IL-4Rα. IL-13 binds IL-13Rα1, and the IL-13/IL-13Rα1 complex then recruits IL-4Rα into the functional receptor complex. The fully assembled receptor complex activates the STAT6 transcription factor, but like IL-4 via the type II IL-4 receptor, IL-13 is a poor inducer of IRS activation through this receptor. The binding of IL-13 to IL-13Rα1 is relatively inefficient, indicating that once IL-13/IL-13Rα1 binding occurs, the ensuing formation of the functional receptor complex is likely. However, lowering the IL-13/IL-13Rα1-binding capability to IL-4Rα requires a substantial decrease in the second binding step to result in lowered STAT6 activation.

Depending on the cell type, IL-4 and IL-13 both can activate STAT6. As IRS2 is only weakly induced by type II IL-4 receptor, and thus by IL-13, intracellular signalling elicited by the two cytokines is somewhat different. By inducing IRS2, IL-4 subsequently activates various pathways including Sos/Ras, PI3K/Akt, PKB/mTOR, or PKC. Of these pathways, mTOR has recently been linked to CD4 Th2 cell differentiation as well as alternative macrophage activation. Unfortunately, experimental therapeutic efforts targeting mTOR in murine allergic disease models have failed. This is a cautionary tale about extrapolating from in vitro signalling to in vivo therapy.

IL-13 is a key effector cytokine in allergic inflammation, driving goblet cell hyperplasia, mucus production, airway hyperresponsiveness and alternative macrophage activation. In atopic individuals, the immune system appears to favour IL-13 over IL-4 in certain contexts. When peripheral blood mononuclear cells from atopic patients were stimulated with a mite allergen, the cells produced over 20 times more IL-13 than IL-4. This quantitative difference helps explain why IL-13 has emerged as a major therapeutic target in asthma and atopic dermatitis. ^[2]

IL-13 also has roles beyond allergy. ILC2-derived IL-13 acting on target cells via the type II IL-4 receptor is a mechanism for several physiological functions such as beige fat biogenesis or hepatic fibrosis. Whether ILC2-derived IL-13 can act in an autocrine manner has not been established. Future experiments will be warranted to reveal if IL-4Rα is differently expressed between ILC subtypes to tune the cells either to IL-4 or IL-13. This shows that the IL-4/IL-13 system is not a simple duplicate but a finely tuned network with context-dependent outputs.

TNF-alpha is one of the most potent pro-inflammatory cytokines. It is produced mainly by activated macrophages, but also by T cells, mast cells and other cell types. In the M1/M2 framework, classically activated M1 macrophages produce TNFα along with IL-12 and nitric oxide synthase, promoting Th1 responses^[1]. TNF-α acts on endothelium to increase adhesion molecule expression, on hepatocytes to induce acute-phase proteins, on adipocytes to stimulate lipolysis, and on the hypothalamus to produce fever. It also promotes cachexia by increasing muscle protein breakdown.

In psoriasis, TNF-α is centrally involved. Serum levels of TNF-α are significantly higher in patients with psoriasis than in healthy controls. The exact role of TNF-α in the pathomechanism of psoriasis is still unclear, but anti-TNF-α therapy is highly effective in psoriasis, indicating that this cytokine has, together with IFN-γ, a central role in the pathogenesis. IFN-γ and TNF-α induce IL-6, IL-8, IL-12 and IL-18, and constitute an important link in the cytokine network in the pathogenesis of psoriasis. This places TNF-α upstream of several other inflammatory mediators, which explains why blocking it can have broad effects.

In viral infection and cytokine storm, TNF-α is one of the cytokines induced by pattern-recognition receptor signalling. SARS-CoV-1 and MERS-CoV activate NF-κB through MyD88-dependent pathways, leading to activation of pro-inflammatory cytokines including IL-6, TNFα, IL-8 and CCL2^[1]. The massive cell death commonly observed in cytokine storm activates damage-associated molecular pattern receptors and Toll-like receptors, which further induce pro-inflammatory cytokines including IL-6 through NF-κB activation^[1]. TNF-α is therefore part of a self-amplifying inflammatory loop.

Measurement of TNF-α in serum is routine in research but less so in clinical practice. The minimum detectable concentrations in one laboratory were 4.8 pg/mL for TNF-α. Like other cytokines, serum TNF-α levels are affected by production, tissue binding, degradation and elimination, so a single value must be interpreted in context. In healthy controls, there were no correlations among the serum levels of TNF-α, IFN-γ, IL-6, IL-8, IL-12, IL-17 and IL-18, which suggests that each cytokine is regulated independently in the steady state. ^[4]

Therapeutic blockade of TNF-α is one of the most successful stories in cytokine-targeted therapy. Anti-TNF-α agents are effective in psoriasis and in other inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease. The principle is straightforward: remove a key upstream mediator and the downstream inflammatory cascade contracts. The practice is more complex because TNF-α has protective roles in host defence against mycobacteria and other intracellular pathogens, which is why screening for latent tuberculosis is mandatory before starting anti-TNF therapy. This trade-off between efficacy and infection risk is a direct consequence of the cytokine's biology.

Interferon-gamma is the signature type II interferon. It is produced by Th1 cells, CD8 cytotoxic T cells and natural killer cells, and it is the principal cytokine that activates macrophages. IFN-γ increases MHC class I and class II expression, enhances antigen presentation, promotes IgG class switching, and activates antimicrobial mechanisms such as reactive oxygen and nitrogen species. It is therefore a critical link between innate and adaptive immunity.

In psoriasis, IFN-γ is one of the key pro-inflammatory cytokines. Serum levels of IFN-γ are significantly higher in patients with psoriasis than in healthy controls. IFN-γ and TNF-α induce IL-6, IL-8, IL-12 and IL-18, and constitute an important link in the cytokine network in the pathogenesis of psoriasis. Moreover, the intradermal administration of IFN-γ into nonlesional skin of psoriatic patients causes the appearance of lesions at the inoculation site. This is strong evidence that IFN-γ is not merely a marker but a driver of disease.

Measurement of IFN-γ is possible with sensitive assays. In one laboratory, the minimum detectable concentrations included 0.2 IU/mL for IFN-γ. In healthy controls, there were no correlations among serum levels of TNF-α, IFN-γ, IL-6, IL-8, IL-12, IL-17 and IL-18. This independence suggests that each cytokine has its own regulatory logic, even though they cooperate during an immune response. ^[4]

Type I interferons are a family of cytokines that includes IFN-α and IFN-β. They are produced by almost all nucleated cells in response to viral infection, and they induce a potent antiviral state. Type I interferons bind a ubiquitously expressed receptor, in contrast to the type III interferon receptor, which is restricted to a subset of cells^[1]. This broad receptor distribution explains why type I interferons can cause systemic side effects.

The signalling pathway is well defined. Type I interferon binds its receptor, activates JAK kinases, and leads to the formation of a transcription factor complex composed of STAT1, STAT2 and IRF9, known as interferon-stimulated gene factor 3^[1]. The same complex is activated by type III interferons, so despite using distinct receptor complexes, type I and type III interferon signalling converge at ISGF3^[1]. The genes induced by type III interferons are a subset of those induced by type I interferons, typically with a delayed peak and longer duration.

Type I interferons have important clinical uses. They have been approved for cancer treatment against, for instance, hairy cell leukaemia, malignant melanoma, and AIDS-related Kaposi's sarcoma^[1]. The side effects of type I interferon treatment are, however, severe, with flu-like symptoms, haematopoietic toxicity, anorexia and depression among others^[1]. These side effects are a direct consequence of the ubiquitous receptor and the broad range of genes induced.

The contrast with type III interferons is instructive. Type III interferons have a restricted receptor distribution, being expressed particularly on epithelial cells, keratinocytes and differentiated dendritic cells^[1]. This means that frequent activation of the type III system is better tolerated by the organism, as only certain specific tissues will respond^[1]. In a phase I study of type III interferon against hepatitis C virus, some patients treated with type I interferon developed neutropenia or anaemia, while patients treated with type III interferon remained symptom-free^[1]. This favourable side-effect profile has led to interest in type III interferons as novel therapeutics^[1]. ^[3]

The clinical importance of interferons extends beyond antiviral defence. IFN-γ is central to granulomatous inflammation and macrophage activation, and defects in the IFN-γ pathway cause susceptibility to mycobacterial infections. Type I interferons are implicated in systemic lupus erythematosus and other autoimmune diseases, where inappropriate interferon signalling drives tissue damage. Understanding the differences between type I, type II and type III interferons is therefore essential for interpreting disease phenotypes and for choosing the right therapeutic target.

The respiratory burst is the rapid, oxygen-dependent release of reactive oxygen species by phagocytes, especially neutrophils, after engagement of pattern-recognition or opsonic receptors. From first principles, the cell must first recognise a microbial target, assemble the nicotinamide adenine dinucleotide phosphate oxidase complex at the phagosomal membrane, transfer electrons across that membrane, and convert molecular oxygen into superoxide and downstream oxidants. The purpose is not simply to “burn” the pathogen: the burst acidifies and damages the phagosome, supports protease activity, generates signals that amplify or terminate inflammation, and helps antigen-presenting cells process material taken up by neutrophils and macrophages. In teaching terms, think of it as a controlled explosion at the interface between innate sensing and tissue repair; powerful enough to clear organisms, but harmful when it leaks into endothelium and alveoli.

In the lung this matters because antiviral immunity depends on a balance between rapid pathogen restriction and restraint of collateral damage. Respiratory epithelial cells express receptors for both type I and type III interferons, and nose, bronchial and alveolar epithelial cells express high levels of IFNλs after infection. Neutrophils recruited to infected airways are therefore operating in an interferon-rich environment. Type I interferons can promote antiviral states broadly, whereas IFNλs appear especially important in limiting innate pro-inflammatory activation: during influenza infection they restrain production of TNF and IL-1β from neutrophils while antiviral responses still develop. The clinical lesson is that an effective immune response is not the same as a maximal inflammatory response. A patient can have viral clearance and still deteriorate if neutrophil oxidants, proteases and cytokines injure the alveolar-capillary barrier. ^[1]

This distinction becomes critical in severe viral pneumonia. COVID-19 is caused by SARS-CoV-2, and its life-threatening phenotype is severe acute respiratory distress syndrome, a lethal syndrome induced by mechanical injury and infection such as pneumonia, sepsis and trauma^[1]. In COVID-19 the ARDS is considered a consequence of cytokine storm, resembling disease caused by SARS-CoV-1 and MERS-CoV, with uncontrolled and rapidly progressing inflammation accompanied by massive production of pro-inflammatory cytokines, chemokines and growth factors^[1]. The respiratory burst is one cellular engine inside that storm, but it is not the whole storm. Superoxide and hydrogen peroxide can recruit and activate further leucocytes; epithelial injury exposes basement membrane; endothelium becomes leakier; hyaline membranes and oedema reduce compliance. At the same time, renin-angiotensin signalling can amplify inflammatory circuits: the AngII-AT1R axis activates the NF-κB pathway through growth factors and pro-inflammatory cytokines, activates ADAM17, generates mature EGFR ligand and TNFα, and increases soluble IL-6 receptor α that activates STAT3 through gp130 on fibroblasts, endothelial cells and epithelial cells^[1]. ^[2]

Rational cytokine therapy aims to interrupt amplification loops without abolishing host defence. The best examples come from pathways in which one cytokine has both local and systemic reach. IL-6 usually acts on lymphoid and myeloid immune cells expressing IL-6Rα and gp130, but during inflammation it also acts on non-immune cells such as fibroblasts and endothelial cells that express gp130; for this action soluble IL-6 receptor α is required^[1]. This is the logic behind blocking IL-6 receptor rather than merely hoping to reduce ligand production. Tocilizumab, an anti-IL-6R antibody, is effective in treating patients with CAR-T-induced cytokine release syndrome and has been considered for treating ARDS in COVID-19 patients^[1]. Its appeal in cytokine storm is that IL-6 sits at the meeting point of innate activation, Th17 biology, acute-phase responses, endothelial activation and STAT3-dependent survival programmes. ^[2]

A second approach is to inhibit the kinases that convert receptor engagement into transcription. Baricitinib, an inhibitor of JAK1 and JAK2, is reported to prevent COVID-19 severity followed by lowering IL-6 and TNFα levels^[1]. In a pathway diagram this seems tidy: receptor proximal JAKs feed STATs, so blockade should reduce several cytokine outputs together. In practice the trade-off is breadth. Because JAK-STAT signalling supports antiviral interferon responses, haematopoietic homeostasis and growth-factor signalling, the therapeutic question is whether disease cytokines dominate the signalling balance at that moment. Dexamethasone reduces the death rate of patients with severe COVID-19^[1]; it is less cytokine-specific than biologics, but illustrates that dampening the inflammatory host response can be beneficial once severe disease is driven by immune-mediated lung injury. ^[2]

Type III interferons offer a different lesson in targeting. IFN-λ has substantial therapeutic potential largely because of a favourable side-effect profile compared with type I IFN^[1]. Its biology is attractive: IFN-λR1 is expressed in a narrow range of cell types, particularly epithelial cells and keratinocytes in murine models, with pronounced expression in stomach, intestine, lung and skin; epithelial cells are largely responsible for the response^[1]. Recombinant IFN-λ has shown antiviral activity against a very diverse range of viruses in infected cells in culture or experimental animals. Zymogenetic, in cooperation with Bristol-Myers, completed phase I trials and is undertaking phase II trials for use of IFN-λ against hepatitis C virus infection, with hope that it may be tested against other viruses replicating in IFN-λ responsive tissues such as highly pathogenic influenza A^[1]. ^[3]

The first pitfall is over-reading redundancy from knockout phenotypes. In the respiratory tract, redundancy between type I and type III interferon systems is suggested by work in mice deficient in both IFNLR1 and IFNAR1 showing increased susceptibility to respiratory viral infections, while mice deficient only in IFNLR1 did not. That sounds like redundancy, but later work uncovered major and unique roles of IFNλs in antiviral immunity in the lung. For students the rule is: absence of an early phenotype does not prove absence of function; it may reflect compensation, timing, dose, route or cell-specific receptor distribution.

A second pitfall is treating STAT3 as simply “pro-inflammatory” or “anti-inflammatory”. In CD4-positive T cells, the IL-6-STAT3 pathway is essential for Th17 development and RORγt expression, whereas IL-27, an IL-6 family cytokine and STAT3 activator, inhibits Th17 differentiation in a STAT1-dependent manner; Th17 differentiation depends on the ratio of activated STAT3 to STAT1, which is more than 1 for IL-6 and less than 1 for IL-27^[1]. Both STAT3 loss-of-function and STAT1 gain-of-function mutations in patients show defects in Th17 differentiation^[1]. The same transcription factor can therefore drive, restrain or redirect a lineage depending on partner factors, receptor dose and feedback. ^[2]

Third, negative feedback is not decorative. Mice expressing human gp130 with Y759F mutation display splenomegaly, lymphadenopathy, an enhanced acute-phase reaction and prolonged STAT3 activation^[1]. The site recruits SHP-2 and SOCS3, and F759/F757-dependent negative regulation is mainly dependent on recruitment of SOCS3; SOCS3 is induced by STAT3 and regulates a major negative feedback loop of gp130-mediated signalling^[1]. If a pathway lacks its brake, a normal ligand dose can produce chronic activation. This is why cell signalling should be learned as receptor plus kinase plus transcription factor plus feedback, not as a one-way arrow.

Fourth, mouse models can mislead if induction is artificial. Experimental autoimmune models usually use autoantigens with adjuvants, making it difficult to interpret results in relation to naturally occurring diseases; F759 mice are regarded as an ideal murine model of rheumatoid arthritis because they show increased STAT3 activation through gp130 from lack of SOCS3-mediated negative regulation^[1]. Yet F759 mice take about one year to develop arthritis, while gp130Y757F/Y757F mice show hyperproliferative lesions in stomach mucosa as early as six weeks^[1]. The lesson is not to discard models, but to ask what initiator, cell type and time frame each model represents.

Fifth, do not conflate cytokine presence with cytokine causality. Senescent cells secrete inflammatory cytokines, chemokines, growth factors and proteases, and IL6 and IL8 are consistently present and known to maintain and propagate the senescence-associated secretory phenotype response in the microenvironment. Elevated serum levels of both cytokines have been used independently as prognostic markers for breast cancer. A biomarker can be prognostic without being the best therapeutic target, especially where SASP composition varies by cell and tissue of origin and by trigger.

In intensive care and infectious diseases, cytokine targeting is most established when immune amplification has become the threat. The life-threatening phenotype of COVID-19 is severe ARDS, and ARDS in COVID-19 is considered the result of cytokine storm^[1]. Dexamethasone reduces death in severe COVID-19^[1]; tocilizumab is effective for CAR-T-induced cytokine release syndrome and has been considered for COVID-19 ARDS^[1]; baricitinib is reported to prevent COVID-19 severity with lower IL-6 and TNFα^[1]. These are not interchangeable. Glucocorticoids broadly restrain inflammatory transcription; IL-6R blockade interrupts IL-6 cis and trans effects; JAK1/JAK2 inhibition reduces cytokine signal transduction. The bedside skill is recognising the phase of disease: antiviral support and source control early, immunomodulation when hypoxaemia and shock reflect dysregulated host response rather than unreplicated virus alone.

In oncology and cell therapy, cytokines are both danger and lever. CAR-T therapy can provoke cytokine release syndrome severe enough to require IL-6 receptor blockade^[1]. Chronic inflammation also creates a soil for tumour progression: STAT3 and NF-κB are involved in a wide range of inflammatory diseases and cancer development, and their interaction is central to activation of the IL-6 amplifier^[1]. Senescent stroma, pre-neoplastic cells and K-RAS-mutant tumours can supply local initiators. Mutated K-RAS activates NF-κB in a mouse model of lung cancer, K-RAS-induced NF-κB activation is required for pancreatic cancer development, and that dependence involves IL-6 trans-signalling-mediated STAT3 activation^[1]. In practice this means “anti-inflammatory” and “anti-cancer” strategies often overlap conceptually, even when the approved drug indications differ.

In rheumatology and mucosal inflammation, IL-6 biology explains why one pathway can generate arthritis, acute-phase responses and epithelial repair. IL-6 is required in models of type II collagen-induced arthritis, antigen-induced arthritis, EAE, pristane-induced autoantibody production and plasmacytoma^[1]. F759 arthritis development is dependent on Th17 cells, yet the F759 mutation is required in non-immune cells rather than immune cells; mutated gp130-driven cytokine and chemokine production by non-immune cells supports CD4-positive T-cell proliferation and Th17 accumulation in the lesion^[1]. This is directly relevant to biologic choice: blocking IL-6R can reduce systemic and joint inflammation, but the deeper aim is to interrupt a circuit in which stromal cells educate adaptive immunity.

In respiratory medicine, the promise of IFN-λ is tissue-biased antiviral therapy. Type III IFNs are potent antiviral cytokines in vivo and target epithelial cells to induce antiviral activity^[1]. Decreased expression of IFN-λ in the airways is associated with rhinovirus-induced asthma exacerbation^[1]. In the gastrointestinal tract, IFNλ responsiveness is largely compartmentalised: IFNλs act on intestinal epithelial cells expressing high IFNLR1 with minimal effects on lamina propria cells, whereas type I IFNs show the opposite pattern. In the respiratory tract compartmentalisation is less obvious because respiratory epithelial cells respond to both IFN types. Clinically, that breadth may broaden antiviral reach while removing some epithelial specificity; development therefore depends on showing where IFN-λ adds benefit beyond type I interferon.