Unit 5: Cytokines; The complement system - Subjective Questions

Cytokines are low molecular weight (usually < 30 kDa) soluble proteins or glycoproteins secreted by various cells of the immune system and other cell types. They act as chemical messengers that regulate the intensity and duration of immune responses.

General Properties of Cytokines:

• Pleiotropy: A single cytokine can act on different cell types and produce different biological effects (e.g., IL-4 acts on B cells, T cells, and mast cells).
• Redundancy: Multiple cytokines can mediate the same or similar functions (e.g., IL-2, IL-4, and IL-5 all promote B-cell proliferation).
• Synergy: The combined effect of two cytokines is greater than the sum of their individual effects.
• Antagonism: One cytokine may inhibit the effect of another (e.g., IFN- antagonizes IL-4).
• Cascade induction: One cytokine can induce the production of others, creating a cascade.

Modes of Action:

• Autocrine: Acts on the same cell that secretes it.
• Paracrine: Acts on nearby cells.
• Endocrine: Acts on distant cells via the bloodstream.

Cytokines exert their effects by binding to specific high-affinity receptors on target cells, triggering intracellular signaling and altered gene expression.

Cytokines regulate nearly every aspect of the immune response. Their major functions include:

• Regulation of innate immunity: Cytokines like TNF-, IL-1, and IL-6 mediate inflammation and the acute-phase response. Type I interferons (IFN-/) provide antiviral defense.
• Regulation of adaptive immunity: IL-2 promotes T-cell proliferation; IL-4 drives B-cell class switching to IgE; IL-5 stimulates eosinophils.
• Hematopoiesis: Colony-stimulating factors such as GM-CSF, G-CSF, and IL-3 stimulate the growth and differentiation of blood cells from bone marrow progenitors.
• Chemotaxis: Chemokines (e.g., IL-8/CXCL8) direct the migration of leukocytes to sites of infection.
• Th cell differentiation:
  • IL-12 + IFN- Th1 cells
  • IL-4 Th2 cells
  • TGF- + IL-6 Th17 cells
  • TGF- Treg cells
• Anti-inflammatory regulation: IL-10 and TGF- suppress excessive immune activation.

Thus cytokines act as the communication network coordinating immune cell activity.

The JAK-STAT pathway is a major intracellular signaling mechanism used by many cytokine receptors (Type I and Type II cytokine receptors) that lack intrinsic kinase activity.

Components:

• JAK (Janus Kinases): A family of receptor-associated tyrosine kinases (JAK1, JAK2, JAK3, TYK2).
• STAT (Signal Transducers and Activators of Transcription): Latent cytoplasmic transcription factors (STAT1–6).

Steps of the Pathway:

1. Cytokine binding to its receptor causes receptor dimerization/oligomerization.
2. Receptor-associated JAKs are brought into proximity and transphosphorylate each other, becoming activated.
3. Activated JAKs phosphorylate tyrosine residues on the cytoplasmic tail of the receptor.
4. These phosphotyrosines serve as docking sites for STAT proteins via their SH2 domains.
5. STATs are then phosphorylated by JAKs, causing them to dimerize.
6. STAT dimers translocate to the nucleus and bind specific DNA sequences to regulate gene transcription.

Importance:

• Provides a direct and rapid route from the cell surface to the nucleus.
• Mediates responses to interferons, interleukins, and growth factors.
• Regulated by SOCS proteins (negative feedback) and phosphatases.
• Defects cause immunodeficiencies (e.g., JAK3 mutations cause SCID).

Cytokines can act at different distances from the cell that produced them:

Key Points:

• Most cytokines act locally (autocrine/paracrine) because they are secreted in small amounts and act at high affinity.
• Endocrine action occurs when cytokines are produced in large amounts, such as during severe infection or septic shock.
• The mode of action depends on the concentration, half-life, and receptor distribution of the cytokine.

The complement system is a group of over 30 soluble and membrane-bound proteins (mainly synthesized by the liver) that form a major part of the innate immune defense. These proteins circulate as inactive precursors (zymogens) and become activated in a sequential enzymatic cascade.

Key Features:

• Proteins are designated with the letter C followed by a number (e.g., C1–C9), plus factors B, D, P, etc.
• Activation involves proteolytic cleavage, producing an active fragment (larger fragment usually labeled b, smaller a; exception is C2).
• Functions as a cascade with amplification at each step.

Major Biological Roles:

• Cell lysis: Formation of the Membrane Attack Complex (MAC) to destroy pathogens.
• Opsonization: Coating of pathogens with C3b to enhance phagocytosis.
• Inflammation: C3a, C4a, and C5a act as anaphylatoxins, promoting inflammation and chemotaxis.
• Clearance of immune complexes: Solubilization and removal of antigen–antibody complexes.
• Bridge between innate and adaptive immunity: Enhances antibody responses.

The classical pathway is typically initiated by antigen–antibody complexes and is a key link between adaptive and innate immunity.

Initiation:

• Triggered when C1q binds to the Fc region of IgM or IgG antibodies bound to an antigen. (IgM is most efficient due to its pentameric structure.)

Sequence of Events:

1. C1 complex (C1q, C1r, C1s) binds antibody; C1r activates C1s.
2. Activated C1s cleaves C4 into C4a and C4b. C4b binds the pathogen surface.
3. C1s then cleaves C2 into C2a and C2b. C2a binds C4b.
4. This forms the C4b2a complex = C3 convertase of the classical pathway.
5. C3 convertase cleaves C3 into C3a (anaphylatoxin) and C3b.
6. C3b binds to C4b2a to form C4b2a3b = C5 convertase.
7. C5 convertase cleaves C5 into C5a and C5b, initiating the terminal pathway (MAC formation).

Summary of Convertases:

• C3 convertase:
• C5 convertase:

The pathway thus amplifies the response and leads to opsonization, inflammation, and lysis.

The alternative pathway is part of innate immunity and does not require antibodies. It is activated directly by microbial surfaces such as bacterial cell walls, lipopolysaccharide (LPS), and fungal cell surfaces.

Initiation – Spontaneous C3 Tickover:

1. C3 undergoes spontaneous, low-level hydrolysis in plasma to form C3(HO).
2. C3(HO) binds Factor B.
3. Factor D cleaves the bound Factor B into Ba and Bb, forming a fluid-phase C3 convertase C3(HO)Bb.
4. This cleaves more C3 into C3a and C3b.

Surface Amplification:

5. C3b binds covalently to microbial surfaces.
6. Surface-bound C3b binds Factor B, which is cleaved by Factor D to form C3bBb = alternative pathway C3 convertase.
7. This convertase is stabilized by Properdin (Factor P).
8. Addition of another C3b forms C3bBb3b = C5 convertase.

Key Points:

• Provides a rapid, antibody-independent first line of defense.
• Acts as an amplification loop for C3b generated by any pathway.
• Host cells are protected by regulatory proteins (e.g., Factor H, DAF).

The lectin pathway is an antibody-independent activation route triggered by carbohydrate patterns on microbial surfaces.

Initiation:

• Begins when Mannose-Binding Lectin (MBL) or ficolins recognize and bind mannose or N-acetylglucosamine residues commonly found on the surface of bacteria, viruses, and fungi.
• MBL is structurally similar to C1q.

Sequence of Events:

1. MBL associates with MASPs (MBL-Associated Serine Proteases), particularly MASP-1 and MASP-2.
2. Activated MASP-2 functions like C1s, cleaving C4 and C2.
3. This forms the C4b2a = C3 convertase, identical to the classical pathway.
4. The pathway then proceeds like the classical pathway (C3 and C5 cleavage, MAC formation).

Comparison with the Classical Pathway:

Both pathways converge at the formation of the same C3 convertase.

The Membrane Attack Complex (MAC) is the terminal product of all three complement pathways and is responsible for direct lysis of target cells.

Formation Steps:

1. C5 convertase cleaves C5 into C5a (anaphylatoxin) and C5b.
2. C5b binds to C6, forming C5b6.
3. C7 binds, forming C5b67, which inserts into the lipid bilayer of the target membrane.
4. C8 binds to form C5b678, which begins to disrupt the membrane.
5. Multiple C9 molecules (–) polymerize and bind, forming a transmembrane pore.

Final Complex:

Function:

• The MAC forms a cylindrical pore (approx. 70–100 Å diameter) through the membrane.
• This causes loss of osmotic balance, allowing free flow of ions and water.
• The target cell swells and undergoes osmotic lysis (cell death).

Significance:

• Especially effective against Gram-negative bacteria such as Neisseria.
• Regulated by CD59 (protectin) on host cells to prevent self-damage.

Because complement activation is rapid, amplifying, and potentially destructive, tight regulation is essential to prevent damage to host cells. Regulation occurs at multiple stages using soluble and membrane-bound regulatory proteins.

Why Regulation is Necessary:

• To prevent self-tissue damage.
• To limit excessive inflammation.
• To control the spontaneous activation (tickover) of the alternative pathway.

Key Regulatory Proteins:

• C1 Inhibitor (C1-INH): Dissociates C1r and C1s from C1q; deficiency causes hereditary angioedema.
• Factor H: Binds C3b and promotes its inactivation; competes with Factor B (alternative pathway).
• Factor I: A serine protease that cleaves C3b and C4b (with cofactors like Factor H, MCP).
• C4b-binding protein (C4BP): Regulates the classical pathway C3 convertase.
• DAF (CD55, Decay-Accelerating Factor): Accelerates decay of C3 convertases on host cells.
• MCP (CD46, Membrane Cofactor Protein): Cofactor for Factor I-mediated cleavage of C3b/C4b.
• CD59 (Protectin): Blocks C9 polymerization, preventing MAC formation on host cells.

These ensure complement targets pathogens while sparing host cells.

Complement activation produces several important biological effects that contribute to host defense:

• 1. Cell Lysis (Cytolysis):

  • The MAC (C5b-9) forms pores in target membranes, causing osmotic lysis of bacteria, enveloped viruses, and other cells.

• 2. Opsonization:

  • C3b and C4b deposited on pathogen surfaces are recognized by complement receptors (CR1) on phagocytes, enhancing phagocytosis.

• 3. Inflammation (Anaphylatoxins):

  • C3a, C4a, and C5a trigger:
  • Mast cell and basophil degranulation (histamine release)
  • Increased vascular permeability
  • Smooth muscle contraction
  • C5a is also a potent chemoattractant for neutrophils and monocytes.

• 4. Clearance of Immune Complexes:

  • C3b binds immune complexes and transports them via CR1 on erythrocytes to the liver and spleen for removal.

• 5. Link to Adaptive Immunity:

  • C3d bound to antigen enhances B-cell activation via CR2 (CD21), lowering the threshold for antibody production.

Thus complement provides defense, inflammation, clearance, and immune enhancement.

Anaphylatoxins are small, biologically active complement fragments produced during complement activation that mediate inflammation by promoting the release of vasoactive substances.

The Anaphylatoxins are:

• C3a
• C4a
• C5a

Functions:

• Degranulation of mast cells and basophils, releasing histamine.
• Increased vascular permeability, leading to edema.
• Contraction of smooth muscle.
• Chemotaxis (mainly C5a) — attracts neutrophils, monocytes, and eosinophils to the site of infection.
• Activation of neutrophils and upregulation of adhesion molecules.

Relative Potency:

• C5a is the most potent and the only strong chemoattractant.
• C4a is the weakest.

Regulation:

• Their activity is controlled by carboxypeptidase N, which removes a C-terminal arginine to form less active des-Arg forms.

Excessive anaphylatoxin production can contribute to anaphylactic shock and tissue injury.

All three complement pathways converge on the cleavage of C3 and lead to formation of the MAC, but differ in their initiation.

Common Terminal Pathway:

• All converge at C5 cleavage and proceed through C5b–C9 to form the Membrane Attack Complex (MAC).

Note: The alternative pathway also acts as an amplification loop for C3b generated by the other two pathways.

Cytokines can be classified in several ways based on structure and function.

A. Structural Classification (based on receptor families):

• Type I cytokines (hematopoietin family): e.g., IL-2, IL-4, IL-6, GM-CSF.
• Type II cytokines (interferon family): e.g., IFN-, IFN-, IFN-, IL-10.
• TNF family: e.g., TNF-, TNF- (lymphotoxin).
• IL-1 family: e.g., IL-1, IL-18.
• Chemokine family: e.g., IL-8 (CXCL8), classified as CXC, CC, C, and CX3C.

B. Functional Classification:

• Interleukins (ILs): Mediate communication between leukocytes (IL-1 to IL-38).
• Interferons (IFNs): Antiviral and immunomodulatory (IFN-, , ).
• Tumor Necrosis Factors (TNFs): Mediate inflammation and apoptosis.
• Colony-Stimulating Factors (CSFs): Stimulate hematopoiesis (G-CSF, GM-CSF).
• Chemokines: Mediate chemotaxis (CXCL8/IL-8, CCL2/MCP-1).
• Growth Factors: e.g., TGF-.

C. Based on Immune Response:

• Pro-inflammatory: IL-1, IL-6, TNF-.
• Anti-inflammatory: IL-10, TGF-.

Naïve CD4 T cells differentiate into distinct effector subsets depending on the cytokine environment during antigen presentation. Each subset produces its own signature cytokines.

1. Th1 Cells:

• Inducing cytokines: IL-12 and IFN-
• Master transcription factor: T-bet
• Secretes: IFN-, IL-2, TNF-
• Function: Cell-mediated immunity; activates macrophages; defense against intracellular pathogens.

2. Th2 Cells:

• Inducing cytokine: IL-4
• Master transcription factor: GATA-3
• Secretes: IL-4, IL-5, IL-13
• Function: Humoral immunity; helps B cells; defense against helminths; involved in allergy.

3. Th17 Cells:

• Inducing cytokines: TGF- + IL-6 (with IL-23 for maintenance)
• Master transcription factor: RORt
• Secretes: IL-17, IL-22
• Function: Defense against extracellular bacteria and fungi; recruits neutrophils; linked to autoimmunity.

4. Treg (Regulatory T) Cells:

• Inducing cytokine: TGF- (without IL-6)
• Master transcription factor: FoxP3
• Secretes: IL-10, TGF-
• Function: Immune suppression and maintenance of self-tolerance.

Cytokines thus act as a decision-making switch that shapes the type of immune response.

Cytokine receptors are transmembrane proteins that bind cytokines with high affinity and transmit signals into the cell. They are grouped into families based on structure.

Major Receptor Families:

• 1. Type I Cytokine Receptors (Hematopoietin family):

  • Contain conserved cysteine residues and a WSXWS motif in the extracellular region.
  • Bind IL-2, IL-3, IL-4, IL-6, GM-CSF.
  • Often share common subunits (e.g., common chain, common chain).

• 2. Type II Cytokine Receptors (Interferon family):

  • Bind interferons (IFN-, , ) and IL-10.
  • Contain conserved cysteines but lack the WSXWS motif.

• 3. Immunoglobulin (Ig) Superfamily Receptors:

  • Contain Ig-like domains.
  • e.g., IL-1 receptor, M-CSF receptor.

• 4. TNF Receptor Family:

  • Contain cysteine-rich extracellular domains.
  • Some contain death domains that trigger apoptosis (e.g., TNFR1, Fas).

• 5. Chemokine Receptors (G-protein coupled receptors):

  • Seven-transmembrane structures.
  • Signal via G-proteins; e.g., CXCR4, CCR5.

Signaling:

• Many Type I and Type II receptors use the JAK-STAT pathway since they lack intrinsic kinase activity.

Opsonization is the process by which pathogens are coated with molecules (opsonins) that enhance their recognition and phagocytosis by immune cells such as macrophages and neutrophils.

Role of Complement in Opsonization:

• The most important complement opsonin is C3b (and its cleavage product iC3b).
• During complement activation, C3 is cleaved into C3a and C3b.
• C3b binds covalently to the pathogen surface via its reactive thioester group.

Mechanism of Enhanced Phagocytosis:

1. C3b-coated pathogen is recognized by Complement Receptor 1 (CR1/CD35) on phagocytes.
2. iC3b is recognized by CR3 (CD11b/CD18) and CR4.
3. Binding triggers engulfment and destruction of the pathogen.

Significance:

• Greatly increases the efficiency of phagocytosis.
• Especially important for clearing encapsulated bacteria (e.g., Streptococcus pneumoniae).
• Works synergistically with antibody opsonins (IgG via Fc receptors).

Thus opsonization is a key effector function linking complement with cellular immunity.

Hereditary Angioedema (HAE) is a genetic disorder characterized by recurrent episodes of severe swelling (edema) of the skin, gastrointestinal tract, and airways. Airway involvement can be life-threatening.

Cause – Complement Regulation Defect:

• HAE results from a deficiency or dysfunction of C1 Inhibitor (C1-INH).
• C1-INH normally regulates the classical complement pathway by inactivating C1r and C1s, and also regulates the kinin (contact) system by inhibiting kallikrein and factor XIIa.

Pathophysiology:

• Without functional C1-INH:
  • Uncontrolled activation of C1 leads to excessive consumption of C4 and C2.
  • Overproduction of bradykinin (from the kinin system) increases vascular permeability.
• The result is fluid leakage into tissues, causing swelling.

Laboratory Findings:

• Low C4 levels (a diagnostic marker).
• Reduced or dysfunctional C1-INH.

Key Point:

• HAE demonstrates the importance of complement regulatory proteins in preventing harmful, uncontrolled activation.

Interferons (IFNs) are a group of cytokines named for their ability to interfere with viral replication. They are crucial in antiviral defense and immune regulation.

Types of Interferons:

• Type I IFNs: IFN- (produced by leukocytes) and IFN- (produced by fibroblasts).
• Type II IFN: IFN- (produced by T cells and NK cells).
• Type III IFNs: IFN-.

Properties:

• Low molecular weight glycoproteins.
• Produced rapidly in response to viral infection or immune stimulation.
• Act via Type II cytokine receptors using the JAK-STAT pathway.

Functions:

• Antiviral action (Type I):
  • Induce an antiviral state in neighboring cells by activating enzymes (e.g., 2'-5' oligoadenylate synthetase, protein kinase R) that inhibit viral protein synthesis.
• Activation of NK cells and macrophages.
• Upregulation of MHC class I (Type I) and MHC class II (IFN-) molecules, enhancing antigen presentation.
• IFN-: Major macrophage activator; promotes Th1 responses.
• Anti-tumor and immunomodulatory effects.

Interferons are used therapeutically in viral infections (hepatitis), cancers, and multiple sclerosis (IFN-).

Although complement is traditionally considered part of innate immunity, it plays a vital role in enhancing and regulating adaptive immune responses, thus serving as a bridge between the two systems.

1. Enhancement of B-cell Responses:

• The complement fragment C3d binds to antigens.
• The antigen-C3d complex binds simultaneously to the B-cell receptor (BCR) and to CR2 (CD21) on B cells.
• This co-stimulation dramatically lowers the threshold for B-cell activation (by up to 1000–10,000 fold), enhancing antibody production.

2. Antigen Presentation and Trapping:

• Follicular dendritic cells in lymph nodes use complement receptors (CR1, CR2) to trap and display antigen-complement complexes, promoting memory B-cell development.

3. Clearance of Immune Complexes:

• C3b binds immune complexes and transports them via CR1 on erythrocytes for safe clearance, preventing tissue damage and autoimmunity.

4. Modulation of T-cell Responses:

• Complement components (e.g., C3a, C5a) influence dendritic cell function and T-cell activation.

5. Link via Innate Recognition:

• The lectin and alternative pathways provide immediate innate defense, while the classical pathway is activated by antibodies from adaptive immunity.

Thus complement integrates rapid innate defense with specific adaptive responses.