8.1 Complement activation pathways

Complement Activation Pathways

The complement system is a network of over 30 plasma proteins that work together to destroy pathogens, promote inflammation, and bridge innate and adaptive immunity. Understanding the three activation pathways and how they converge is central to immunobiology, since complement dysfunction underlies diseases ranging from recurrent infections to autoimmune tissue damage.

The Three Activation Pathways

Each pathway has a different trigger, but all three converge on the same critical step: formation of C3 convertase.

Classical pathway — antibody-dependent. This pathway links adaptive immunity to complement. When IgG or IgM antibodies bind antigens on a pathogen surface, the C1 complex recognizes the Fc regions of those antibodies and kicks off the cascade. Because it requires antibody, the classical pathway typically activates after an adaptive immune response is already underway (or during a secondary response when antibody is already circulating).

Alternative pathway — antibody-independent and always "on." C3 in plasma undergoes slow, spontaneous hydrolysis (called C3 tickover), generating small amounts of $C3(H_2O)$. On host cells, regulatory proteins quickly shut this down. On microbial surfaces (e.g., bacterial cell walls, which lack those regulators), the process is amplified. This makes the alternative pathway a true innate sentinel: it doesn't need prior recognition of the pathogen.

Lectin pathway — pattern-recognition driven. Mannose-binding lectin (MBL) and ficolins circulate in plasma and bind specific carbohydrate patterns (particularly terminal mannose residues) on microbial surfaces. These sugar arrangements are common on bacteria, fungi, and some viruses but rare on healthy human cells, giving the lectin pathway selectivity without requiring antibody.

Steps in Complement Activation

Classical pathway:

1. The C1 complex (C1q + two copies each of C1r and C1s) binds to the Fc regions of IgG or IgM in antigen-antibody complexes. C1q must engage multiple Fc regions for stable binding, which is why pentameric IgM is especially efficient at activating this pathway.
2. C1s cleaves C4 into C4a and C4b; C4b deposits on the pathogen surface.
3. C1s then cleaves C2 into C2a and C2b. C2a binds C4b.
4. The resulting C4b2a complex is the classical pathway C3 convertase.

Alternative pathway:

1. Spontaneous hydrolysis of C3 produces $C3(H_2O)$, which adopts a C3b-like conformation.
2. Factor B binds $C3(H_2O)$ and is cleaved by Factor D, releasing Ba and leaving Bb attached.
3. The initial fluid-phase convertase $C3(H_2O)Bb$ cleaves more C3, generating true C3b that deposits on nearby surfaces.
4. Surface-bound C3b recruits more Factor B (again cleaved by Factor D), forming the alternative pathway C3 convertase: C3bBb.
5. Properdin (Factor P) stabilizes C3bBb on microbial surfaces, extending its half-life and boosting amplification.

Lectin pathway:

1. MBL (or ficolins) binds mannose or other carbohydrate patterns on the pathogen surface.
2. MBL-associated serine proteases (MASP-1 and MASP-2) activate. MASP-2 is the key enzyme.
3. MASP-2 cleaves C4 and C2 in the same manner as C1s in the classical pathway.
4. The result is the same C3 convertase: C4b2a.

Role of C3 Convertase and the Amplification Loop

C3 convertase is where all three pathways funnel together, and it's the main amplification point of the entire system.

• C3 convertase cleaves C3 into C3a (a small anaphylatoxin) and C3b (a large fragment that deposits on the target surface).
• Each molecule of C3 convertase can cleave many C3 molecules, and each new C3b can seed more C3 convertase (especially via the alternative pathway). This positive feedback loop means a single triggering event can coat a pathogen with thousands of C3b molecules within minutes.

C3b has three major downstream effects:

1. Opsonization — C3b (and its cleavage products iC3b and C3dg) tags pathogens for recognition by complement receptors on phagocytes, dramatically enhancing phagocytosis.
2. C5 convertase formation — When C3b binds to an existing C3 convertase, it forms C5 convertase (C4b2a3b for the classical/lectin pathways; C3bBb3b for the alternative pathway).
3. Amplification — C3b feeds back into the alternative pathway regardless of which pathway initiated activation, making the alternative pathway the dominant amplification loop for all three pathways.

From C5 convertase to the membrane attack complex (MAC):

1. C5 convertase cleaves C5 into C5a (a potent anaphylatoxin and chemoattractant) and C5b.
2. C5b sequentially recruits C6, C7, C8, and multiple copies of C9.
3. C9 polymerizes to form a pore (the MAC, also called C5b-9) that inserts into the target membrane, disrupting osmotic balance and lysing the cell.

Regulation of Complement Activation

Without tight regulation, complement would damage host tissues. The system uses multiple layers of control to confine activation to pathogen surfaces.

Membrane-bound regulators (on host cell surfaces):

• DAF (CD55) — accelerates the decay (dissociation) of both C3 convertases, preventing amplification on host cells.
• MCP (CD46) — serves as a cofactor for Factor I, enabling it to cleave and inactivate C3b and C4b deposited on host surfaces.
• CR1 (CD35) — has both decay-accelerating and cofactor activity; also facilitates clearance of immune complexes.
• CD59 (protectin) — blocks C9 polymerization, preventing MAC assembly on host cells.

Fluid-phase regulators (in plasma):

• Factor H — binds C3b in the fluid phase and on host surfaces (recognizing host sialic acid residues), acting as a cofactor for Factor I and accelerating alternative pathway C3 convertase decay.
• C4-binding protein (C4BP) — the equivalent regulator for the classical/lectin pathway convertase; accelerates decay of C4b2a and serves as a cofactor for Factor I-mediated cleavage of C4b.
• C1 inhibitor (C1-INH) — inactivates C1r, C1s, and MASPs, limiting initiation of the classical and lectin pathways.

Enzymatic inactivation:

• Factor I is the central protease that cleaves C3b into iC3b (and eventually C3dg) and cleaves C4b into C4c and C4d. It always requires a cofactor (MCP, Factor H, CR1, or C4BP) to function.

Intrinsic instability:

• C3 convertases have short half-lives and decay spontaneously. This built-in instability means activation must be continuously reinforced on a surface to persist, which only happens where regulators are absent (i.e., on pathogens, not host cells).