unit 4, lesson 3, part 2 Complement System and regulation

• The complement system is part of humoral and innate immunity, and it works alongside cellular components such as neutrophils, monocytes, macrophages, natural killer cells, and mast cells.

Complement System Definition and Role

• The complement system is a defense mechanism and part of the innate immune response found in the blood.
• Exposure to the external environment occurs through tissue injury or when pathogens enter the bloodstream.
• It offers a non-specific, rapid, first-line response that activates instantly upon pathogen detection via pattern recognition receptors.
• The complement system amplifies quickly, recruits immune cells, supports phagocytosis, and directly kills pathogens by generating a lytic pore protein complex.
• It is a killing mechanism by attaching complexes to membranes.

Complement System: Key Aspects

• Is an innate immune response as it is inherited, originating over 550 million years ago and co-evolving with the adaptive immune system.
• Is a non-cellular humoral immune component composed of proteins circulating in blood plasma along with membrane-bound proteins expressed on tissues.
• Immune cells both express and respond to complement proteins, such as C3 and C5aR.
• Functions as a proteolytic cascade involving serine proteases, where zymogens require proteolytic cleavage to become active.
• "Tickover" allows for low-level sustained activation and rapid, powerful response to pathogens.
• The cascade amplifies the response since each activated enzyme generates multiple activated molecules.
• Soluble and cell membrane-associated proteins tightly regulate the cascade.

Complement Activation Pathways

• Integral to the innate immune response and is made of approximately 40 plasma proteins produced mainly in the liver, but also immune cells.
• Continuously activated and regulated and amplified rapidly by small triggers such as invading pathogens or wound/trauma.
• The complement system has three activation pathways that involve enzymatic/protease cascades.
• (1) Immunoglobulins activate the Classical Pathway
• (2) Foreign surfaces activate the Alternative Pathway
• (3) Bacterial carbohydrates activate the MBL Pathway

Classical Pathway

• Antigen-antibody (immunoglobulin) complexes trigger the classical pathway.
• The antigen-antibody complex binds to the C1 complex.
• The C1 complex is a molecular complex formed of 1x C1q, 2x C1r, and 2x C1s.
• This pathway is linked to the adaptive immune response through antibodies and needs prior activation of B cells to generate antibodies.
• The pathway is only activated is there was some prior generation of antibodies against an antigen.
• Note that antibodies or antigens alone cannot be detected by the C1 complex, and instead the antibody-antigen complex must be recognized by the complement C1 complex.

MBL or Lectin Pathway

• MBL and ficolins recognize many pathogens, including viruses, bacteria (LPS) and fungi through an antibody-independent pathway.
• Mannose-binding lectin (MBL) then binds to carbohydrates on the surface of pathogens (e.g. mannan).
• Bacterial carbohydrates bind to the MBL-MASP1-MASP1 complex.

Alternative Pathway

• Triggers foreign surfaces and the surface of microbes/damaged cells.
• The "tickover" mechanism involves constant low-level activation of complement, idling and ready to go through C3 converting into C3(H2O).
• Conformational changes in C3 that occur upon thioester hydrolysis produce product C3(H2O).
• A C3 convertase with Factors B and D is formed, cleaving C3 into C3b and C3a and permitting the continuous production of C3b from C3.
• The presence of an activating surface (e.g., microbe, damaged cell) causes C3b to covalently bind.

Physiological Roles of Complement

• Direct killing of invading bacteria and infected cells via the MAC
• Enhancement of phagocytosis through opsonization
• Initiation and enhancement of inflammation at sites of injury or infection through anaphylatoxin generation.
• Clearance of debris, dead and dying cells, which is linked to apoptosis.
• Cell activation, such as the triggering of oxidase response.
• Immune complex handling.
• B cell activation, linking to adaptive immunity.
• Coagulation factor expression, linking to coagulation.

Biological Functions

• Opsonization of micro-organisms by binding complement proteins to the surface, labeling them with C3b opsonin for example.
• Activation of leukocytes where activation fragments generated through proteolytic cascade such as C3a and C5a activate neutrophils/macrophages and stimulate respiratory burst.
• Lysis of target cells by the membrane attack complex (MAC) inserts into the microbial membrane of the pathogen and destroys the cell.

Generation of C3 Convertase & Terminology

• C3 (195 kDa) constitutes the most abundant complement protein and has a central role in the cascade, with high plasma levels of 1.2 g/L. -C number refers to the order of discovery, not appearance. –Factors include factor B or D.
• Cleavage products are distinguished from precursors using suffixes 'a' or 'b'.
• Suffix 'b' denotes the larger fragment that remains with the membrane.
• Suffix 'a' denotes the smaller diffusing fragment.

Complement Regulation

• CD55 (DAF) decays/dissociates the C3-cleaving enzyme C3bBb into its subunits C3b and Bb.
• MCP acts as a cofactor for enzyme factor I, cleaving remaining C3b into inactive iC3b.
• Factor H can both decay the C3 convertase and act as a cofactor for factor I.

Terminal Pathway

• Occurs when the membrane attack complex (MAC) is formed
• CD59 is a regulator of the terminal pathway that inhibits C9-mediated pore formation.