Cytokines are small secreted proteins that act as the primary messengers of the immune system, mediating communication between cells to coordinate inflammatory and immune responses. Chemokines are a specialized subclass of cytokines that direct the migration of leukocytes to sites of infection and inflammation.
Cytokine Families and Classification
Cytokines can be classified based on structure, receptor usage, or function. The hematopoietin family includes interleukins such as IL-2, IL-4, IL-6, IL-7, and IL-15, which share a four-helix bundle structure and signal through receptors that associate with the common gamma chain. The tumor necrosis factor (TNF) family includes TNF-α, lymphotoxin, and Fas ligand, which are trimeric transmembrane proteins that signal through TNFR family members. The interleukin-1 family comprises IL-1α, IL-1β, IL-18, and IL-33, which are generated as proforms cleaved by inflammasome-activated caspases. The type I and type II interferon families include IFN-α subtypes, IFN-β, IFN-γ, and IFN-λ, which are central to antiviral defense. The transforming growth factor-β (TGF-β) family includes TGF-β1, TGF-β2, TGF-β3, and activins, which regulate cell growth, differentiation, and immune suppression.
Cytokine Receptors and Signaling
Cytokine receptors are classified into several families based on structural homology. Type I cytokine receptors (hematopoietin receptors) bind interleukins and colony-stimulating factors and signal through JAK-STAT pathways, with receptor dimerization bringing JAK kinases into proximity to phosphorylate receptor cytoplasmic tails and STAT transcription factors. Type II cytokine receptors bind interferons and IL-10 family members, also signaling through JAK-STAT pathways. TNF receptor family members contain death domains or TRAF-interacting domains and activate NF-κB, MAP kinase, and caspase cascades. IL-1 receptor family members signal through MyD88 and IRAK kinases to activate NF-κB and AP-1. Receptor antagonists such as IL-1Ra competitively block IL-1 binding and provide natural regulation.
Proinflammatory Cytokines
TNF-α is produced primarily by activated macrophages and is a master regulator of the inflammatory response, inducing fever, activating endothelial cells to express adhesion molecules, promoting neutrophil recruitment, and causing cachexia in chronic inflammation. IL-1β is produced by macrophages and epithelial cells after inflammasome activation, inducing fever as an endogenous pyrogen, promoting T cell and B cell activation, and enhancing adhesion molecule expression. IL-6 is produced by macrophages, T cells, and fibroblasts, with pleiotropic effects including stimulation of acute-phase protein synthesis in the liver, B cell differentiation into plasma cells, and T cell activation.
Anti-inflammatory and Regulatory Cytokines
IL-10 is a potent anti-inflammatory cytokine produced by regulatory T cells, macrophages, and B cells, inhibiting the production of proinflammatory cytokines such as TNF-α, IL-1, and IL-6 while promoting B cell class switching to IgG4. TGF-β is produced by regulatory T cells, macrophages, and platelets, suppressing T cell proliferation, inhibiting NK cell activity, promoting regulatory T cell differentiation, and stimulating wound healing and fibrosis. IL-4 and IL-13 promote alternative macrophage activation, B cell class switching to IgE, and Th2 differentiation, and are central to allergic responses and helminth immunity.
Chemokines and Leukocyte Migration
Chemokines are small chemotactic cytokines classified into four families based on the arrangement of conserved cysteine residues: CXC chemokines, CC chemokines, C chemokines, and CX3C chemokine. Inflammatory chemokines such as CXCL8 (IL-8) and CCL2 (MCP-1) are induced by proinflammatory stimuli and recruit neutrophils, monocytes, and effector T cells to infection sites. Homeostatic chemokines such as CXCL12 (SDF-1) and CCL19/CCL21 are constitutively expressed and regulate lymphocyte trafficking through lymphoid organs, bone marrow homing, and dendritic cell migration to lymph nodes. Chemokine receptors are seven-transmembrane G protein-coupled receptors, with HIV using CCR5 and CXCR4 as co-receptors for entry into CD4+ T cells.
Cytokine Networks in T Helper Cell Polarization
CD4+ T cell differentiation into distinct subsets is directed by the cytokine milieu. Th1 polarization is driven by IL-12 and IFN-γ, leading to the transcription factor T-bet and production of IFN-γ, TNF-α, and IL-2, which are essential for immunity against intracellular pathogens. Th2 polarization is driven by IL-4, leading to GATA3 expression and production of IL-4, IL-5, and IL-13, which mediate immunity against helminths and promote allergic inflammation. Th17 polarization is driven by TGF-β and IL-6, leading to RORγt expression and production of IL-17A, IL-17F, and IL-22, which recruit neutrophils and are critical for antifungal immunity. Regulatory T cell differentiation is driven by TGF-β, leading to FoxP3 expression and production of IL-10 and TGF-β for immune suppression.
Cytokines in Clinical Medicine
Cytokine-based therapies are widely used in clinical medicine. Recombinant cytokines include erythropoietin for anemia, granulocyte colony-stimulating factor for neutropenia, type I interferons for hepatitis C and multiple sclerosis, and IL-2 for metastatic renal cell carcinoma. Cytokine inhibitors are among the most successful biologic drugs: TNF inhibitors such as infliximab and adalimumab for rheumatoid arthritis and inflammatory bowel disease, IL-1 receptor antagonist (anakinra) for rheumatoid arthritis, anti-IL-6 receptor (tocilizumab) for rheumatoid arthritis and cytokine release syndrome, and anti-IL-17 (secukinumab) for psoriasis. Cytokine release syndrome, a severe systemic inflammatory response triggered by excessive cytokine release following CAR-T cell therapy or certain infections, is managed with tocilizumab and corticosteroids.
