Innate Immunity II Flashcards
- The classical pathway is initiated by binding of the complement C1qrs protein complex to antibodies (that have already bound their antigen on the surface of a microbe). This interaction results in a conformational change in C1qrs, activating the C1s serine protease subunit in the complex.
- The lectin pathway is initiated by mannose-binding lectin (MBL) or ficolin (both complement family proteins) binding directly to the pathogen surface. These complement proteins recognize carbohydrate motifs on pathogens that aren’t present on host cells. MBL is similarly shaped to the C1qrs complex, and is associated with Mannose-associated serine proteases (MASPs). Upon binding of MBL to the pathogen surface, a conformational change leads to activation of the MASPs. This pathway is antibody independent. Active C1qrs (classical pathway) or MBL/MASP (lectin pathway) now begin the proteolytic cleavage cascade on the surface of the pathogen (Fig. 3-1): • cleave soluble C4 to deposit C4b on the pathogen surface (C4a is released). • cleave soluble C2 to deposit C2b (another serine protease) on the pathogen surface (C2a is released).
- The alternative pathway: In contrast to the classical and lectin pathways which require specific activation by either antibody/C1qrs or MBL/MASP interactions, the alternative pathway is continuously active at low levels and spontaneously activates on cells. Host cells encode inhibitory proteins to prevent this activation but microbes do not. C3 spontaneously hydrolyzes in the plasma to generate C3b and C3a, a term called “tickover”. C3b deposits on nearby cells and then binds to soluble Factor B, allowing it to be cleaved into Bb and Ba. The combination of C3bBb subunits together forms the alternative “C3 convertase”, which can now cleave many more molecules of C3 into C3b and C3a. This can all happen in the plasma, or on a cell surface. C3b and the C3bBb convertase are rapidly inactivated in the plasma. Normal host cells contain numerous inhibitory proteins to prevent this activation or rapidly inactivate it; pathogens do not. In addition to marking pathogens itself, this pathway is very important for amplifying the signals of the classical and lectin pathways.
Late STeps of Complement Activation
- C5 is cleaved by the C3 convertase to deposit C5b on the surface and release C5a.
- C5b promotes the assembly of C6, C7, C8 and C9 to form the membrane attack complex (aka terminal attack complex), which creates a hole in the pathogen surface
Functions of the Complement System
- Opsonization (“making pathogens tastier”). C3b coating on the pathogen surface facilitates binding to receptors on professional phagocytic cells (macrophages and neutrophils), leading to pathogen uptake and destruction.
- Direct pathogen destruction. The final stages of complement activation result in the formation of the membrane attack complex (MAC), inserting complement proteins directly into the microbe. This creates a pore, leading to death either by osmotic lysis or apoptosis, primarily in Gramnegative bacteria.
- Leukocyte recruitment. The C3a and C5a fragments are anaphylatoxins: strong chemoattractants for monocytes and neutrophils that recruit these professional phagocytes to the site of infection to phagocytose and destroy pathogens. These fragments also increase the permeability of blood vessels, and participate in anaphylaxis (hence the name).
- Mediates clearnace of immune complexes. Coating of soluble antigen-antibody complexes with complement allows their binding of complement receptor 1 (CR1) on red blood cells. These RBCs then circulate to the liver or spleen, where the immune complex is stripped from the RBC and phagocytosed by macrophages in these blood-filtering organs. This is possible because RBCs express lower levels of CR1 than the macrophages within the spleen and liver.
CR1
- CR1 is widely expressed on nucleated cells, and binds both C3b and C4b, the initial breakdown products of C3 and C4 that are covalently bound to targets.
- On erythrocytes, CR1 functions to bind immune complexes for transport to the spleen or liver.
- On neutrophils and macrophages, it promotes phagocytosis.
CR2
CR2 plays a critical role in B cell responses to generate antibodies. It primarily recognizes the breakdown products of C3b (C3dg, C3d, iC3b).
- On follicular dendritic cells within the germinal centers of secondary lymphoid organs, CR2 captures and holds antigens for long periods of time, promoting B cell memory development.
- It is also critical for B cell activation, serving as a coreceptor to enhance B cell recognition of C3b-bound antigens.
- It is the primary receptor for Epstein-Barr virus entry
CR3 and CR4
•CR3 and CR4 are primarily expressed on phagocytic cells that can bind to C3b breakdown products to promote phagocytosis.
C5a and C3a
•The C5a and C3a receptors recognize the anaphylatoxic subunits released from the pathogen surface during C5 or C3 cleavage. Both are expressed on endothelial cells, mast cells within tissues, and tissue-localized macrophages. Upon ligand binding, these receptors stimulate proinflammatory responses, including increased vascular permeability and upregulation of adhesion molecules. Ultimately, these events result in recruitment of leukocytes into the tissue to help combat the pathogen that triggered the activation of the complement cascade.
- Numerous regulators control the inappropriate activation of complement on host cells, as well as limit the continued amplification of the enzymatic cascade once it has been triggered.
- In general, regulators can be classified as fluid phase or soluble (distributed in the plasma and body fluids), or membrane bound (present in host cell membranes).
- Numerous regulators control the inappropriate activation of complement on host cells, as well as limit the continued amplification of the enzymatic cascade once it has been triggered.
- In general, regulators can be classified as fluid phase or soluble (distributed in the plasma and body fluids), or membrane bound (present in host cell membranes).
There are 3 broad mechanisms used to regulate complement:
- Inhibit activating proteases such as C1qrs (Classical Pathway) and MASPs (mannose-associated serine proteases; Lectin Pathway). Example: C1INH
- Competitively inhibit or dissociate complement enzymes from their substrates. Example: DAF
- Serve as a cofactor to increase the breakdown of convertase enzyme complexes. Example: Factor I, C4BP
Because these regulatory molecules are present in nearly all locations in the body, the relative activation state of the complement system is determined by the local balance between activating and regulatory components.
Complement and Coagulation
- The coagulation system is built in a similar manner as the complement system: multiple pathways, intrinsic and extrinsic activation mechanisms, and a high level of regulation. Both systems are designed to act locally, in tightly controlled cleavage cascades. By physically walling off pathogens from the circulatory system, coagulation functions to avoid the spread of the infection throughout the body. Thus, both systems serve as innate defenses against microbial invasion.
- Uncontrolled inflammation can promote disseminated intravascular coagulopathy (DIC), characterized by massive thrombin production, platelet activation, and impaired fibrinolysis. This can be a significant contributor to the pathophysiological problems during sepsis.
Complement Deficiencies in C1, C2, C4 and MBL
- Deficiencies in C1, C2, C4 and MBL lead to recurrent infections with pyogenic bacteria (Strep, Staph, Gram+), particularly in infants with MBL deficiencies.
- Deficiencies in any of the components leading to MAC formation result in recurrent meningococcal and gonococcal infections with Neisseria species.
Complement Deficiencies in cell surface host regulatory proteins…
•Defects in cell surface host regulatory proteins causes paroxysmal nocturnal hemoglobinuria, a disorder in which inappropriate complement activation on RBCs results in their lysis.
Complement Deficiencies in early components (C1, C2, C4)…
•Defects in early components (C1, C2, C4) are associated with “immune complex disorders” including SLE, vasculitis, and glomerulonephritis. This highlights the critical role for C3b deposition in aiding immune complex clearance.
