Cytokines: Key Points and Functions

Overview

• Cytokines are small proteins secreted by immune and non-immune cells for communication.
• They bind to receptors, triggering responses in target cells.
• Functions include activation, proliferation, differentiation of immune cells, and causing fever.

Signaling Mechanisms

• Autocrine Signaling: Cytokine acts on the cell that secreted it.
  • Example: Interleukin-2 (IL-2) from CD4+ T cells promotes T cell proliferation.
• Paracrine Signaling: Cytokine affects nearby cells.
  • Example: IL-2 assists nearby CD8+ T cells.
• Endocrine Signaling: Cytokine affects distant cells.
  • Example: IL-1β, IL-6, TNF-α produced by macrophages travel to liver/brain, causing fever and acute phase reactants.

Classes of Cytokines

1. Interleukins (ILs)

• Numbered by discovery order, not function.
• Act on leukocytes and non-leukocytes.

2. Tumor Necrosis Factors (TNFs)

• Named for tumor-killing ability by inducing inflammation.
• TNF-α, TNF-β (LT-α), and LT-β activate endothelial cells and increase vascular permeability.

3. Interferons (IFNs)

• Interfere with viral replication.
• Type 1: IFN-α and IFN-β, limit viral replication.
• Type 2: IFN-γ, activates macrophages and CD4+ T cells.

4. Colony Stimulating Factors (CSFs)

• Stimulate hematopoietic stem cell proliferation/differentiation.
• Example: GM-CSF promotes granulocytes and macrophages development.

5. Transforming Growth Factors (TGFs)

• TGF-β suppresses cell proliferation and aids in regulatory T cell development.

Cytokine Responses

Acute Inflammatory Response

• IL-1β, TNF-α, IL-6 target immune/endothelial cells, liver, and brain.

Pro-inflammatory Response

• Includes IFN-α, IFN-β, IL-1β, TNF-α, IL-6.

Adaptive Immune Response

• Th1 Cells: IL-2, IL-12 help differentiation, produce IFN-γ.
• Th17 Cells: IL-6, IL-23, TGF-β aid differentiation, produce IL-17.

Anti-Parasitic and Allergy Response

• IL-4, IL-5, IL-13 induce Th2 helper T cells.

Regulatory Immune Response

• IL-10, TGF-β inhibit inflammation, promote regulatory T cells.

Growth and Differentiation

• CSFs and IL-7 promote progenitor cell differentiation.

Chemotactic Agents

• Chemokines such as IL-8 guide cells to inflammation sites.
• IL-17 produced by Th17 cells induces chemokine production.

Recap

• Cytokines communicate through autocrine, paracrine, and endocrine signaling.
• Five main classes: Interleukins, TNFs, Interferons, TGFs, CSFs.
• Different cytokine combinations lead to varied immune responses.

These notes provide an overview of cytokine functions and classifications, focusing on their role in immune responses and signaling mechanisms.