Cytokines and Their Functions

What are cytokines and what do they do?

• Cytokines are low molecular weight proteins produced by one cell that act on itself or another cell by binding to cell-surface receptors.
• They can deliver positive or negative signals for immune activation.
• Cytokines are messengers that regulate immune responses.
• Cytokines are not usually stored as preformed molecules; they are synthesized and released rapidly after cellular stimulation.
• Cytokine gene transcriptional activation is transient, and most cytokine mRNAs are unstable to ensure brief and self-limiting synthesis.
• One cell can simultaneously produce multiple cytokines.

How do cytokines exert their effects?

• Cytokines act on their target cells by binding to specific membrane receptors.
• Receptors consist of one or more transmembrane proteins (receptor sub-unit or chain).
  • Extracellular domains bind cytokines.
  • Cytoplasmic tails initiate signaling pathways.

Types of Cytokines:

• Include Interleukins (IL, from IL-1 to IL-37), Interferons (IFN), Tumor Necrosis Factors (TNF), and growth factors (GM-CSF).

Biological Activity:

• Interferons (type I, II, and III).
• Pro-inflammatory cytokines (IL-1, IL-6, TNF).
• Anti-inflammatory cytokines (IL-10, TGF-b).
• T helper cytokines (IFN-g, IL-4, IL-21).

Grouped Based On:

• Receptor structure: Type I vs. type II cytokines.
• Cellular source:
  • Lymphokines (produced by lymphocytes, IL-2, IFN-g).
  • Monokines (monocytes/macrophages, IL-1, IL-12).
  • Innate vs. adaptive cytokines.

Cytokine Receptor Families

• Type I cytokine (hemopoietin) receptors
  • Ligands: IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, IL-11, IL-12, IL-13, IL-15, GM-CSF, G-CSF
• Type II cytokine receptors
  • Ligands: IFN-$\alpha/\beta$, IFN-$\gamma$, IFN-I, IL-10, IL-20, IL-24, IL-26
• TNF receptor family
  • Ligands: TNF-$\alpha$, TNF-$\beta$, LT, CD40, FasL, BAFF, April, Ox40, GITR, NGF
• IL-1 receptor family
  • Ligands: IL-1, IL-18
• G-protein-coupled receptors
  • Ligands: Chemokines

Regulation of Cytokine Activity

• Three mechanisms prevent cytokines from activating cells non-specifically:
  • Regulated cytokine receptor expression
  • Short half-lives of most cytokines in body fluids
  • Cytokines and target cells need to be in close proximity to establish effective concentration gradients

Common Properties of Cytokines

• Pleiotropic: act on different target cells
• Redundant: multiple cytokines have the same effect
• Synergistic: cooperative effect of multiple cytokines
• Antagonistic: inhibition of one cytokine's effects by another

What do Cytokines do?

• Can classify cytokines into 3 major functional categories
  1. Mediators and regulators of innate immunity
  2. Mediators and regulators of adaptive immunity
  3. Stimulators of hematopoiesis

Key Cytokines that Mediate Immune Responses

• Important component of the early innate immune response to danger, viruses, and bacteria.
• Major pro-inflammatory cytokines of innate immunity include IL-12, TNF, IL-1, IL-6, and type I interferons (IFNs).
• Major effector cytokines of adaptive immunity include IFN-g (type II IFN) and IL-4.

Interferons (IFNs)

• Discovered based on their ability to interfere with viral infection.
• Now recognized as a family of pleiotropic cytokines that play an essential role in controlling viral, intracellular bacterial, and parasitic pathogens.

Three Major IFN Families:

• Type I ($\alpha$, $\beta$, $\epsilon$, $\kappa$, and $\omega$)
• Type II ($\gamma$)
• Type III ($\lambda$)
• Type I and III IFNs: produced by tissue cells and innate immune cells; essential for anti-viral immunity.
• Type II IFN (IFN-g): produced predominately by activated lymphocytes; important for control of intracellular bacterial and parasitic infection.

Biologic Actions of Type I Interferons (IFN)

• A large family of structurally related cytokines (single IFN-b and 13 IFN-a) that mediate the early innate immune response to viral infections.
• The most potent stimuli for type I IFNs synthesis are viral nucleic acids.
• The antiviral action of type I IFN is primarily a paracrine action, where a virally infected cell secretes IFNs to act on and protect neighboring cells that are not yet infected.

Biologic Actions of Type II Interferons (IFN)

• IFN-g is the only member of type II IFN.
• Mainly produced by activated lymphocytes.
• A major phagocyte activating cytokine.
• Upregulates MHC class I and II molecules.
• Induces antimicrobial substances, such as nitric oxide.

Tumour Necrosis Factor (TNF)

• Principle mediator of acute inflammation produced in response to bacteria and other infectious agents.
• Large amounts of TNF can be produced in response to gram+ and gram- bacteria.
• LPS is the most potent stimulus for eliciting TNF production.
• TNF is produced by macrophages, DCs, Th1 cells, and other cell types.
• Preformed TNF is quickly released from mast cells.
• IFN-g produced by T cells and NK cells augments TNF production by LPS activated macrophages.

Functions of TNF: Local Inflammation

• Stimulates the recruitment of neutrophils and monocytes to sites of infection and activates these cells to eradicate microbes.
• Induces endothelial cells (ECs) to express adhesion molecules, making them more adhesive for leukocytes.
• Stimulates ECs and macrophages to secrete chemokines, enhancing the affinity of leukocyte integrins for their ligands.
• Acts on mononuclear phagocytes to stimulate IL-1, which functions much like TNF.
• Induces DC migration to lymph nodes.

Functions of TNF: Systemic Protective Effects

• In severe infections, TNF is produced in large amounts and causes systemic effects.
• Large amounts of TNF can enter the bloodstream and act at distant sites as an endocrine hormone.
• Can act on the hypothalamus (brain) to induce fever.
• Acts on hepatocytes (liver) to increase the synthesis of serum proteins (e.g., serum amyloid A & fibrinogen).
• Prolonged production of TNF causes wasting of muscle and fat cells (cachexia).

Functions of TNF: Pathogenic Effect (Septic Shock)

• High concentrations of TNF cause the pathologic abnormalities of septic shock.
• Reduction in myocardial contractility.
• Disseminated intravascular thrombosis leading to organ failure.
• Causes severe metabolic disturbances, including a fall in blood glucose (hypoglycemia).

Cytokines Involved in Hematopoiesis

• Stem cell factor
• IL-3, GM-CSF, IL-1, IL-6
• Erythropoietin
• Thrombopoietin; IL-11
• IL-7
• IL-5
• G-CSF
• M-CSF

Chemokines (chemotactic cytokine)

• Divided into 4 families (CC, CXC, C, and CX3C) based on the number and location of two of four conserved cysteine residues.
• The chemokines of the CC and CXC subfamilies are produced by leukocytes and several types of tissue cells, such as endothelial cells and epithelial cells.
• Induced by microbial products and cytokines (IL-1, IL-17, and TNF).
• Chemokine receptors are expressed on all leukocytes. The receptors exhibit overlapping specificity for chemokines within each family.
• The pattern of cellular expression of the receptors determines which cell types respond to which chemokines.

Biological Functions of Chemokines

• Interaction of chemokine and chemokine receptor stimulate cytoskeletal changes, leading to increased cell motility.
• Critical in the development of lymphoid organs.
• Required for the migration of dendritic cells from the sites of infection into draining lymph nodes.
• CCR5 and CXCR4 are co-receptors for HIV.
• Essential for recruiting circulating leukocytes from blood vessels into extravascular sites, partly through enhancing integrin affinity.

Integrin Activation by Chemokines

• Integrins: heterodimeric cell surface protein composed of 2 polypeptides.
• Respond to intracellular signals (e.g., chemokines) by rapidly increasing their affinity to their ligands.
• Mediate adhesion of cells to other cells or the extracellular matrix.
• Essential for the recruitment of leukocytes into the sites of infection or injury.

Leukocyte-Endothelial Interactions and Leukocyte Recruitment into Tissues

• Leukocyte recruitment from the blood into tissues requires adhesion of the leukocytes to the endothelial lining of postcapillary venules and then movement through the endothelium and vessel wall into the extravascular tissue.
• This is a multistep process in which each step is orchestrated by different types of adhesion molecules and chemokines:
  • Selectin-mediated rolling of leukocytes on endothelium.
  • Chemokine-mediated increase in the affinity of integrins.
  • Stable integrin-mediated arrest of leukocytes on endothelium.
  • Transmigration of leukocytes through the endothelium.

Positioning of Lymphocyte Populations by Chemokines in Lymphoid Tissues

• CCL21 is a ligand of CCR7 expressed on naïve T cells

Summary

– Main types and functions of cytokines
– Major functional categories based on their principle biological functions
– Mediators of innate and adaptive immunity
– Stimulators of hematopoiesis
– Cytokines are soluble “messenger” proteins that play a key role in the host defense against pathogens and provide links between innate and adaptive immune responses