🛡️Immunobiology Unit 7 – Cytokines & Chemokines: Immune Regulators

What Are Cytokines and Chemokines?

• Cytokines are small proteins secreted by cells that act as signaling molecules to regulate immune responses and mediate communication between cells
• Chemokines are a specific type of cytokine that attract and guide the movement of immune cells (chemotaxis) to sites of inflammation or infection
• Play crucial roles in coordinating the development, differentiation, and activation of various immune cells (T cells, B cells, macrophages, and dendritic cells)
• Regulate the balance between pro-inflammatory and anti-inflammatory responses to maintain homeostasis and prevent excessive tissue damage
• Cytokines bind to specific receptors on target cells, triggering intracellular signaling cascades that alter gene expression and cellular functions
• Exhibit pleiotropic effects, meaning a single cytokine can have multiple functions depending on the target cell and the presence of other cytokines
• Cytokines act in a paracrine manner (affecting nearby cells), autocrine manner (affecting the cell that secreted them), or endocrine manner (affecting distant cells through circulation)
• Examples of cytokines include interleukins (IL-1, IL-6), interferons (IFN-α, IFN-γ), tumor necrosis factors (TNF-α), and transforming growth factors (TGF-β)

Types and Classification

• Cytokines can be classified based on their structural similarities, receptor usage, or functional properties
• Interleukins (IL) are a large group of cytokines primarily involved in regulating the growth, differentiation, and activation of immune cells
  • IL-1 family (IL-1α, IL-1β, IL-18) promotes inflammation and activates innate immune responses
  • IL-2 stimulates the proliferation and differentiation of T cells and NK cells
  • IL-4, IL-5, and IL-13 are associated with Type 2 immune responses and allergic reactions
• Interferons (IFNs) are cytokines with antiviral, antiproliferative, and immunomodulatory effects
  • Type I IFNs (IFN-α, IFN-β) are produced by various cells in response to viral infections and stimulate antiviral immune responses
  • Type II IFN (IFN-γ) is produced by activated T cells and NK cells and enhances macrophage activation and antigen presentation
• Tumor necrosis factors (TNFs) are pro-inflammatory cytokines that play a role in host defense against pathogens and in the development of autoimmune diseases
  • TNF-α is produced by macrophages and promotes inflammation, fever, and apoptosis
• Chemokines are classified into four subfamilies based on the arrangement of conserved cysteine residues: CC, CXC, C, and CX3C
  • CC chemokines (CCL2, CCL5) attract monocytes, T cells, and dendritic cells
  • CXC chemokines (CXCL8, CXCL10) attract neutrophils and activate their functions

Production and Secretion

• Cytokines and chemokines are produced by a wide range of immune and non-immune cells, including macrophages, dendritic cells, T cells, B cells, NK cells, endothelial cells, and fibroblasts
• The production of cytokines is tightly regulated and induced in response to specific stimuli, such as pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs)
• Toll-like receptors (TLRs) on innate immune cells recognize PAMPs and trigger the production of pro-inflammatory cytokines (IL-1, IL-6, TNF-α) and type I interferons
• Antigen-presenting cells (APCs) secrete cytokines (IL-12, IL-23) that shape the differentiation of naive T cells into specific effector subsets (Th1, Th2, Th17)
• Activated T cells produce cytokines (IL-2, IFN-γ, IL-4, IL-17) that further amplify immune responses and regulate the functions of other immune cells
• The secretion of cytokines involves the conventional secretory pathway, where newly synthesized proteins are transported through the endoplasmic reticulum and Golgi apparatus before being released extracellularly
• Some cytokines (IL-1β, IL-18) require proteolytic processing by inflammasomes to become biologically active before secretion
• The local concentration and duration of cytokine secretion are critical factors in determining their biological effects and the overall immune response

Receptors and Signaling Pathways

• Cytokines and chemokines exert their effects by binding to specific cell surface receptors on target cells
• Cytokine receptors are typically composed of two or more subunits that form a heteromeric complex upon ligand binding
• The cytokine-receptor interaction triggers conformational changes in the receptor, leading to the activation of intracellular signaling cascades
• Janus kinases (JAKs) are associated with the cytoplasmic domains of many cytokine receptors and become activated upon receptor engagement
  • Activated JAKs phosphorylate specific tyrosine residues on the receptor, creating docking sites for signal transducers and activators of transcription (STATs)
  • Phosphorylated STATs dimerize and translocate to the nucleus, where they bind to specific DNA sequences and regulate gene transcription
• The JAK-STAT pathway is a common signaling mechanism used by many cytokines (interferons, interleukins) to mediate their effects on cell proliferation, differentiation, and survival
• Other signaling pathways activated by cytokine receptors include the mitogen-activated protein kinase (MAPK) pathway and the phosphoinositide 3-kinase (PI3K) pathway
• Chemokine receptors are G protein-coupled receptors (GPCRs) that signal through the activation of heterotrimeric G proteins and downstream effectors (adenylyl cyclase, phospholipase C)
• The activation of chemokine receptors leads to changes in cell motility, adhesion, and gene expression, facilitating the migration of immune cells to sites of inflammation

Functions in Immune Regulation

• Cytokines and chemokines play pivotal roles in regulating the initiation, amplification, and resolution of immune responses
• Pro-inflammatory cytokines (IL-1, IL-6, TNF-α) are released early during an immune response and promote the recruitment and activation of immune cells at the site of infection or tissue damage
  • IL-1 and TNF-α activate endothelial cells to express adhesion molecules and chemokines, facilitating the extravasation of leukocytes into the affected tissue
  • IL-6 stimulates the production of acute-phase proteins by the liver and contributes to the development of fever and systemic inflammation
• Chemokines establish chemotactic gradients that guide the migration of immune cells to the site of inflammation
  • CXCL8 (IL-8) attracts neutrophils, which are essential for the initial control of bacterial and fungal infections
  • CCL2 (MCP-1) recruits monocytes and macrophages, which play a role in phagocytosis and antigen presentation
• Cytokines shape the differentiation and effector functions of T cell subsets
  • IL-12 and IFN-γ promote the development of Th1 cells, which are important for cell-mediated immunity against intracellular pathogens
  • IL-4 and IL-13 drive the differentiation of Th2 cells, which mediate humoral immunity and allergic responses
  • IL-6, TGF-β, and IL-23 contribute to the generation of Th17 cells, which are involved in the clearance of extracellular bacteria and fungi
• Anti-inflammatory cytokines (IL-10, TGF-β) help to control excessive inflammation and prevent tissue damage
  • IL-10 inhibits the production of pro-inflammatory cytokines by macrophages and dendritic cells and promotes the development of regulatory T cells (Tregs)
  • TGF-β suppresses the proliferation and effector functions of T cells and contributes to the maintenance of immune tolerance

Role in Inflammation and Disease

• Dysregulation of cytokine and chemokine production or signaling can contribute to the development of various inflammatory and autoimmune diseases
• Chronic inflammation is characterized by the persistent activation of immune cells and the overproduction of pro-inflammatory cytokines (TNF-α, IL-1, IL-6)
  • In rheumatoid arthritis, TNF-α and IL-6 drive the inflammation and destruction of synovial joints, leading to pain and disability
  • In inflammatory bowel diseases (Crohn's disease, ulcerative colitis), IL-23 and Th17-associated cytokines (IL-17, IL-22) contribute to the inflammation and damage of the gastrointestinal tract
• Cytokine imbalances can also contribute to the pathogenesis of allergic diseases and asthma
  • Overproduction of Th2 cytokines (IL-4, IL-5, IL-13) leads to the activation of eosinophils, mast cells, and IgE production, resulting in airway inflammation and hyperresponsiveness
• Interferons play a critical role in antiviral immune responses, but their excessive or prolonged production can lead to autoimmune disorders
  • In systemic lupus erythematosus (SLE), type I interferons contribute to the activation of autoreactive B cells and the production of autoantibodies
• Chemokines and their receptors are involved in the migration and infiltration of immune cells into inflamed tissues, perpetuating the inflammatory response
  • In multiple sclerosis, the expression of CCL2 and CCL5 attracts T cells and macrophages into the central nervous system, leading to demyelination and neurological deficits

Therapeutic Applications

• Modulating cytokine and chemokine signaling has become a promising approach for the treatment of inflammatory and autoimmune diseases
• Monoclonal antibodies targeting specific cytokines or their receptors have been developed to inhibit the action of pro-inflammatory cytokines
  • Anti-TNF-α antibodies (infliximab, adalimumab) are used to treat rheumatoid arthritis, psoriasis, and inflammatory bowel diseases by neutralizing the effects of TNF-α
  • Anti-IL-6 receptor antibodies (tocilizumab) are used to treat rheumatoid arthritis and cytokine release syndrome by blocking IL-6 signaling
• Small molecule inhibitors targeting cytokine signaling pathways have also been developed
  • JAK inhibitors (tofacitinib, baricitinib) are used to treat rheumatoid arthritis and other autoimmune diseases by blocking the JAK-STAT pathway downstream of cytokine receptors
• Recombinant cytokines and growth factors are used to boost immune responses or promote tissue repair
  • Recombinant IL-2 (aldesleukin) is used to treat metastatic renal cell carcinoma and melanoma by enhancing the proliferation and activity of T cells and NK cells
  • Recombinant G-CSF (filgrastim) is used to stimulate the production of neutrophils in patients with chemotherapy-induced neutropenia
• Chemokine receptor antagonists are being developed to inhibit the migration of immune cells to sites of inflammation
  • CCR5 antagonists (maraviroc) are used to treat HIV infection by blocking the entry of the virus into CD4+ T cells
• Cytokine-based immunotherapies, such as chimeric antigen receptor (CAR) T cell therapy, harness the power of cytokines to enhance the antitumor activity of T cells
  • CAR T cells are genetically engineered to express a chimeric receptor that recognizes a specific tumor antigen and an intracellular signaling domain that includes a costimulatory molecule (CD28 or 4-1BB) and a cytokine receptor (IL-2 receptor)
  • The engagement of the CAR by the tumor antigen triggers T cell activation, proliferation, and cytokine production, leading to the elimination of tumor cells

Key Players and Their Effects

• IL-1: A pro-inflammatory cytokine that induces fever, stimulates the production of acute-phase proteins, and activates T cells and macrophages
• IL-2: A growth factor for T cells that promotes their proliferation, differentiation, and survival; also stimulates the activation of NK cells
• IL-4: A key cytokine in Type 2 immune responses that promotes the differentiation of Th2 cells, IgE production, and the activation of mast cells and eosinophils
• IL-6: A pleiotropic cytokine that induces fever, stimulates the production of acute-phase proteins, and promotes the differentiation of Th17 cells and plasma cells
• IL-10: An anti-inflammatory cytokine that inhibits the production of pro-inflammatory cytokines by macrophages and dendritic cells and promotes the development of regulatory T cells
• IL-12: A cytokine that promotes the differentiation of Th1 cells and enhances the cytotoxic activity of CD8+ T cells and NK cells
• IL-17: A pro-inflammatory cytokine produced by Th17 cells that stimulates the production of chemokines and other cytokines, leading to the recruitment of neutrophils and the clearance of extracellular pathogens
• IFN-α/β: Type I interferons that induce an antiviral state in cells, activate NK cells, and promote the maturation of dendritic cells
• IFN-γ: A type II interferon produced by Th1 cells and NK cells that activates macrophages, enhances antigen presentation, and promotes cell-mediated immunity
• TNF-α: A pro-inflammatory cytokine that induces fever, stimulates the production of acute-phase proteins, and activates endothelial cells to express adhesion molecules and chemokines
• TGF-β: A pleiotropic cytokine that has both pro-inflammatory and anti-inflammatory effects; promotes wound healing, fibrosis, and the differentiation of regulatory T cells
• CXCL8 (IL-8): A chemokine that attracts neutrophils to sites of inflammation and activates their effector functions
• CCL2 (MCP-1): A chemokine that recruits monocytes, T cells, and dendritic cells to sites of inflammation and tissue damage
• CCL5 (RANTES): A chemokine that attracts T cells, eosinophils, and basophils to sites of inflammation and is involved in the pathogenesis of allergic diseases and asthma