Complement

Overview

Complement refers to a family of plasma proteins which continuously circulate through the blood and play an important role in immunity. Complement proteins bind microbes and assist in their killing through both direct toxicity and indirect recruitment of immune effector cells. Because activation of Complement is induced by general molecular patterns on microbes, many consider Complement to be a component of the Innate Immune Response; however, we treat it here separately for the sake of organizational simplicity.

Basic Architecture

The complement pathway is frequently referred to as a "Cascade" because of the sequential architecture by which these proteins activate one another. Because activated complement proteins can damage host tissues and are highly inflammatory, they are normally held inactive in the plasma. Most complement proteins are enzymes which are regulated as zymogens and consequently their activation occurs by enzymatic removal of an inhibitory domain within the protein. In most cases, the enzyme which removes the inhibitory domain is a different complement protein higher up in the complement "Cascade". Although three basic stimuli can activate Complement, the cascade ultimately converges on a single final common pathway which generates a highly toxic protein complex that can lyse a large variety of bacteria species. Importantly, the removed inhibitory domains of several complement proteins act as potent chemoattractants for other immune effector cells, allowing complement activation to be linked to cellular aspects of immunity.

Complement Initiation

As explained above, the complement cascade can be initiated by three basic stimuli. These initial steps ultimately converge into a final common complement pathway whose initiation requires the generation of an enzyme that can activate the complement protein C3. Consequently, the end-point of all three pathways is the generation of a functional "C3 Convertase".

The Classical Pathway is initiated by the complement protein C1q which exists as part of a complex with two other complement proteins C1s and C1r. C1q can directly bind to a variety of molecular patterns on bacteria or can bind to the Fc region of IgG and IgM when they are cross-linked by antigen. Consequently, the Classical Pathway can connect antibody to complement activation
Once C1q binds, the attached C1r protein is activated and cleaves C1s, thus activating C1s. Activated C1s cleaves and activates circulating complement proteins C2 and C4 into their active forms C2b and C4b. Together C2b and C4b act as a "C3 Convertase" which can initiate the final common complement pathway.

The MB-lectin pathway is initiated by the protein "Mannose-binding Lectin (MBL)" which exists as part of a complex with two other proteins MASP-1 and MASP-2. MBL can directly bind to a variety of sugars found on bacteria such as mannose. Once MBL binds, the attached MASP-2 is activated and cleaves C2 and C4 into their active forms C2b and C4b. As in the Classical Pathway, C2b and C4b act as a "C3 Convertase" which can initiate the final common complement pathway.

The Alternative Pathway is so named because it does not require recognition of microbial surfaces but rather depends on the absence of host regulatory factors on bacterial membranes. The alternative pathway is initiated by C3 itself, which spontaneously hydrolyzes at a very low rate in such a way as to generate a "C3 Convertase" with the aid of circulating Factor B and Factor D. Formally, the "C3 Convertase" generated is composed of C3bBb subunits and deposits randomly on membranes, whether host or microbial. Host cellular membranes possess a variety of inhibitory proteins, including "Decay Activating Factor", which rapidly inactivate the C3bBb convertase and thus prevent further cleavage of C3. However, microbial membranes do not possess these inhibitory proteins and thus the deposited C3bBb convertase can initiate the final common complement pathway.

Final Common Pathway

Once the "C3 Convertase" has been generated, this enzyme rapidly cleaves circulating C3 to C3a and C3b. While C3a is released, C3b binds to the microbial surface which quickly becomes coated, or "Opsonized", with C3b.

Bound C3b then complexes with its original "C3 Convertase" to form a "C5 Convertase". Therefore, when activated by the Classic or MB-lectin pathway, the C5 convertase is composed of C3bC2bC4b whereas when activated by the Alternative Pathway, the C5 convertase is composed of (C3b)₂Bb. The "C5 Convertase" cleaves circulating C5 into C5a and C5b. C5b then initiates the generation of the "Membrane Attack Complex".

The "Membrane Attack Complex" is a large multi-component protein complex which forms a large pore in bacterial membranes. The pore destroys the capacity of the microbe to regulate its membrane permeability and in consequence leads to microbial death. Assembly of the Membrane Attack Complex is initiated by C5b which sequentially coordinates recruitment and placement of circulating complement proteins C6, C7, C8, and C9 which form the components of the pore.

Recruitment of Cellular Immune Effectors

Although generation of the Membrane Attack Complex may appear to be the most efficacious anti-microbial mechanism of Complement, deficiency of proteins C5-C9 only results in a slight increased suseptibility to Neisseria species. Instead, the immunologically most important function of complement proteins is their capacity to recruit other immune effectors to the site of microbial pathogens and induce microbial phagocytosis.

As described above, activation of many complement proteins proceeds by enzymatic removal of inhibitory fragments which are then released into the local environment. The removed fragments of several complement proteins act as potent inflammatory mediators and induce changes in the local vasculature to promote entry of immune cells such as macrophages and neutrophils. These fragments are known as "Anaphylotoxins" with the most potent being C5a, although C3a and C4a do exhibit some activity as well. Consequently, activation of complement cannot only prove directly toxic to microbes but can inform the rest of the immune system about the presence of a localized infection.

Once activated, several complement proteins physically bind and coat microbial surfaces, a process known as "Opsonization". The immunologically most important opsonin is C3b which can densely cover the membrane of bacteria. Several immune cells, particularly macrophages, neutrophils, and dendritic cells possess membrane receptors, termed "Complement Receptors", which recognize bound C3b and then coordinate phagocytosis of the opsonized bacteria. Indeed, the capacity of bound complement to induce phagocytosis is likely the most important role that complement plays in host defense.

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