Unit 5: Cytokines; The complement system

Immunology depends on two soluble effector networks that translate cellular recognition into coordinated action: cytokines, which carry information between cells, and complement, which delivers direct effector force. Both operate through cascades, receptors and tight regulation; both bridge innate and adaptive immunity.

I. Orientation: Soluble Effector Networks

Immune responses are governed by regulated signalling and controlled proteolysis. Cytokines are low-molecular-weight secreted proteins that instruct target cells, while complement is a set of plasma zymogens activated in sequence.

• Cytokine defined: low-molecular-weight (typically 8–30 kDa) glycoproteins secreted by leukocytes and other cells that bind specific receptors to alter target-cell behaviour.
• Complement defined: ~30 heat-labile serum and membrane proteins, mostly zymogens (e.g. C1–C9, factors B, D), activated by limited proteolysis into a self-amplifying cascade.
• Shared logic: both use signal amplification — one enzyme cleaves many substrates — so responses are fast and disproportionate to the trigger.
• Shared constraint: both are potentially damaging to host tissue and are therefore held in check by short half-lives, receptor antagonists and dedicated regulatory proteins.
• Nomenclature: cytokines are grouped as interleukins (IL-), interferons (IFN), tumour necrosis factors (TNF), colony-stimulating factors (CSF) and chemokines; complement components are numbered by discovery, so activation order (C1, C4, C2, C3) is not strictly sequential.

II. Cytokines — Chemical Messengers of Immunity

Cytokines coordinate the intensity, duration and character of immune responses by binding high-affinity receptors and reprogramming gene expression in target cells.

A. Overview of cytokines

Cytokines form a communication language linking innate and adaptive immunity.

• Cellular sources: macrophages and monocytes (IL-1, IL-6, TNF-α), T-helper cells (IL-2, IL-4, IFN-γ), and non-immune cells such as fibroblasts and endothelium.
• Functional families:
  • Interleukins: leukocyte-to-leukocyte signals, e.g. IL-2 drives T-cell proliferation.
  • Interferons: IFN-α/β are antiviral; IFN-γ activates macrophages.
  • Chemokines: small (8–10 kDa) chemotactic cytokines, e.g. IL-8/CXCL8 recruits neutrophils along concentration gradients.
  • CSFs: GM-CSF and G-CSF drive haematopoiesis of myeloid lineages.
• Th subset signatures: Th1 secrete IFN-γ (cell-mediated immunity); Th2 secrete IL-4, IL-5 (humoral, allergic responses) — the two are mutually cross-inhibitory.

B. Properties and functions of cytokines

Cytokines share defining behavioural attributes that explain both their potency and their tight control.

• Pleiotropy: one cytokine acts on many cell types — IL-4 promotes B-cell class switching and Th2 differentiation.
• Redundancy: several cytokines produce the same effect — IL-2, IL-4 and IL-5 all support B-cell proliferation.
• Synergy and antagonism: IFN-γ + TNF-α together boost MHC class I expression beyond either alone (synergy); IFN-γ blocks IL-4-driven IgE switching (antagonism).
• Cascade induction: one cytokine induces production of another, e.g. IFN-γ prompts macrophage TNF release.
• Action modes:
  • Autocrine: acts on the secreting cell — IL-2 sustains the activated T cell that produced it.
  • Paracrine: acts on nearby cells — cytokines in an inflamed site.
  • Endocrine: enters circulation to act distantly — IL-1 and IL-6 reach the liver and hypothalamus.
• Core functions: cell growth and differentiation, inflammation, chemotaxis, and acute-phase responses. IL-1, IL-6 and TNF-α act as endogenous pyrogens inducing fever and hepatic acute-phase proteins.

C. JAK-STAT pathway

The JAK-STAT pathway is the principal route by which class I/II cytokine receptors convert extracellular binding into rapid transcriptional change without second messengers.

• Components:
  • JAK (Janus kinase): cytoplasmic tyrosine kinases (JAK1, JAK2, JAK3, TYK2) constitutively associated with receptor cytoplasmic tails.
  • STAT (Signal Transducer and Activator of Transcription): latent cytoplasmic transcription factors (STAT1–6) with an SH2 domain.
• Step sequence:

1. Cytokine binds → receptor chains dimerise/oligomerise
2. Apposed JAKs transphosphorylate each other (activation)
3. JAKs phosphorylate tyrosines on receptor tail (docking sites)
4. STAT SH2 domains bind phosphotyrosines → STATs phosphorylated by JAK
5. Phospho-STATs dimerise (SH2–phosphotyrosine reciprocal binding)
6. STAT dimer translocates to nucleus → binds DNA → gene transcription

• Specificity: particular receptor–JAK–STAT combinations give distinct outcomes — IFN-γ signals through JAK1/JAK2 → STAT1; IL-4 through JAK1/JAK3 → STAT6.
• Negative regulation: SOCS proteins (Suppressors Of Cytokine Signalling) are STAT-induced feedback inhibitors that bind JAKs; phosphatases dephosphorylate the components.
• Clinical anchor: JAK3 mutations cause X-linked SCID because γc-chain cytokine signalling (IL-2, IL-4, IL-7, IL-15) fails, halting lymphocyte development.

III. The Complement System — Cascade of Serum Effectors

Complement complements antibody and phagocyte action by tagging pathogens for destruction, recruiting inflammatory cells and lysing target membranes.

A. Overview of the complement system

Complement is a proteolytic cascade whose central event is cleavage of C3 into effector fragments.

• Nature of components: inactive zymogens circulating in plasma; activation cleaves each into a large fragment (b) that continues the cascade and a small fragment (a) that diffuses as an inflammatory mediator (exception: C2, where C2a is the larger fragment).
• Convertases: multi-subunit enzyme complexes are the engines — a C3 convertase cleaves C3, and a C5 convertase cleaves C5.
• Convergence point: all activation routes generate a C3 convertase, so C3 is the pivotal molecule of the system.
• Effector outputs: opsonisation (C3b), inflammation (C3a, C5a), and membrane lysis (the C5b–C9 membrane attack complex).

B. Complement system pathways

Three initiation pathways converge on C3, differing only in how the first C3 convertase is assembled.

1. Classical pathway (antibody-dependent):
   • Trigger: C1q binds the Fc region of antigen-bound IgM or IgG.
   • Activation: C1r/C1s cleave C4 and C2 → C4b2a (the classical C3 convertase).
2. Alternative pathway (antibody-independent):
   • Trigger: spontaneous C3 hydrolysis ("tick-over"); C3b deposits on microbial surfaces lacking regulators.
   • Activation: factor B binds C3b, factor D cleaves it, stabilised by properdin → C3bBb (the alternative C3 convertase), giving rapid amplification.

• Lectin pathway: mannose-binding lectin (MBL) recognises mannose on microbial surfaces; associated MASP proteases cleave C4 and C2 exactly as in the classical route, forming C4b2a — antibody-independent but mechanistically classical.
• Terminal (common) pathway: each C3 convertase adds C3b to become a C5 convertase (C4b2a3b or C3bBb3b); this cleaves C5, and C5b nucleates C6, C7, C8 and multiple C9 to form the membrane attack complex (MAC), a pore that lyses the target.

C. Regulation of the complement system

Because activation is amplifying and indiscriminate, host cells are protected by regulators acting at each cascade step.

• Initiation control: C1 inhibitor (C1-INH) dissociates C1r/C1s from C1q; its deficiency causes hereditary angioedema.
• Convertase control:
  • Decay-accelerating factor (DAF/CD55): dislodges Bb or C2a from convertases on host cells.
  • Factor H and factor I: factor H binds host C3b; factor I then cleaves C3b to inactive iC3b.
  • MCP (CD46) and C4b-binding protein: cofactors for factor I-mediated cleavage.
• MAC control: CD59 (protectin) blocks C9 recruitment into the forming MAC on host membranes.
• Self versus non-self basis: host surfaces carry these membrane regulators and sialic acid that recruit factor H; microbial surfaces lack them, so amplification proceeds only on pathogens.
• Disease anchor: in paroxysmal nocturnal haemoglobinuria, loss of GPI-anchored DAF and CD59 leaves red cells unprotected against complement lysis.

D. Biological consequences of the complement system

Complement fragments produce four physiologically distinct effects that together clear pathogens and shape inflammation.

• Cell lysis: the MAC (C5b–9) forms transmembrane pores causing osmotic lysis of Gram-negative bacteria and enveloped cells.
• Opsonisation: C3b and iC3b coat microbes; phagocytes bearing complement receptor CR1 (CD35) bind and engulf them, greatly enhancing clearance.
• Inflammation (anaphylatoxins): C3a, C4a and C5a trigger mast-cell degranulation, increase vascular permeability and smooth-muscle contraction; C5a is also a potent neutrophil chemoattractant.
• Immune-complex clearance: C3b-tagged complexes bind CR1 on erythrocytes, which ferry them to the liver and spleen for removal, preventing tissue deposition.
• B-cell activation link: C3d bound to antigen engages CR2 (CD21) on B cells, lowering the threshold for antibody responses and connecting innate complement to adaptive immunity.