(9/8) Woodbury Lecture: Complement System Complement System: Produced by the liver, the complement system is a cascade of plasma proteins for innate immunity

- Function: Serves to complement the action of antibodies in the destruction of bacteria and the eradication of foreign substances from the body. It has potential for great harm, thus regulation is key

o Opsonization (C3b), Cell Lysis, Enhance B Cell response, Remove immune complexes, remove necrotic cells and subcellular membranes, respond to viruses

o Anaphylatoxin production (C5a, C3a, C4a) à anaphylaxis, blood pressure drop (vasodil), swell, edema - Activation: Variety of Stimuli: Ig, Lipid A, Polynucleotides, CRP, viruses, poly- and lipopolysaccharides - Action: Act at biomembranes to induce cytolysis, mediate inflammatory response, assist in opsonization - Mechanism: 3 pathways: Classical, Lectin, and Alternative

Classical Pathway- Begins with Ab attached to Ag-determinant of target cell membrane - C1 (large multi-subunit protein {broccoli-like}) requires Ca2+ ions and subunit Q to

maintain C1’s structural integrity. Subunit Q and S have enzymatic activity - à C1Q binds to Fc domain of Ab-Ag-determinant complex (2 adjacent IgG or IgM) - This binding activates enzymatic activity through a conformational change,

whereupon C1R enzymatically cleaves C1S à activating C1S - C1S cleaves incoming C4 into C4A & C4B. C4B has an active thiol group that

covalently binds to Ag surface - C1S cleaves incoming C2 into C2A & C2B. C2A engages the Ag-C4B. This forms

the C3 Convertase (C4B-Ag-C2A). C2B dissociates away. - C3 Convertase cleaves C3, forming C3A & C3B, withholding C3B. This forms the

C5 Convertase (C4B-Ag-C2A-C3B). C3A dissociates away. - At a very high rate, C5 Convertase acts on incoming C5 to form C5A & C5B. C5B adheres solo to the Ag. Many

C5Bs adhere to the Ag. C5A dissociates away. - Terminal Phase: Sequentially, C6, C7, C8, and C9 line up and associate with C5B to form the Membrane Attack

Complex (MAC). Additional C9s arrive, forming a multimer that acts as a polyC-9 pore extending across the membrane. Water and ions can pass through this pore, causing the pathogen cell to swell and lyse due to osmotic pressure

Lectin Pathway- A lectin is a cell-surface protein that recognizes and binds sugar - MBL is a mannose-binding lectin that recognizes terminal mannose residues

(“mannan”), found on bacteria. MBL in circulation is generally complexed with mannose-associated serine proteases (MASPs) (similar to C1R/S)

- à MBL binds mannan residue of an oligosaccharide on the surface of a bacteria, activating MASP - MASP cleaves incoming C4 and C2 to create C4B2A (C3 Convertase). Thereafter, à classical pathway - Ficolin: A serum protein similar in structure to MBL, capable of binding N-acetylglucosamine/galactosamine - FYI: In lecture, it was pointed out that MBL has structural similarity to IgM. No genetic relation. Collagen stem.

Alternative Pathway- Pathway dependent solely on Complement protein C3’s ability to autocleave - àSpontaneously, at low rates, C3 can autocleave, forming C3A and C3B. C3B recognizes and binds to Ag - Factor B comes and binds in association with C3B to the Ag, forming a small

complex - Factor D comes and interacts with C3B-FactorB-Ag, cleaving off part of Factor

B, leaving Bb. This complex, C3B-Bb-Ag is the active C3 Convertase. Thereafter à Classical Pathway

Complement Regulation- Host must conserve complement components as a reserve - Why? Complement system can generate potent cleavage products capable of strongly activating inflammation and

damaging the host. Dysregulation of complement plays a role in several disease of the immune system, such as Rheumatoid Arthritis, Age-related macular degenederation, and many more. Plus, must conserve for other threats

- Classical Pathway Regulation o C1 Esterase Inhibitor (C1-INH) binds to C1R/S preventing complement activation o Proteins may induce dissociation of the C3 Convertase to stop the cascade (C4BP, DAF, CR1/35, MCP) o Factor I can cleave C4B after C2A dissociates (whether induced or spontaneous), to halt the cascade

- Alternative Pathway Regulation o Factor H, a serum protein, can bind to C3B-Bb complex to displace Bb. Factor I may then cleave C3B

o Self-protection mechanism: Mammalian cells have Sialic acid on their surface, which promotes the binding of Factor H to prevent the complement cascade from destroying “self” cells. Bacterial cells typically lack Sialic acid, thus factor B in the alternative pathway may bind readily to C3B

- Terminal Phase Regulation o Self-protection mechanism: Mammalian cells have CD59 membrane protein capable of binding to the

C5b-C8 partial MAC, which will prevent C9 polymerization, thereby preventing “self” MAC attack o In general, there are other proteins capable of binding to the various C5 fluid-phases, preventing its

interaction with the membrane and MAC attack Complement Deficiencies

- C3 Deficiency: Severe infections and inflammation - MBL Deficiency: Bacterial Infections - Lack of C1 Inhibitor: Angioedema

(9/11) S. Patel Lecture: MHC: Major Histocompatibility Complex Immunity Review Adaptive Immunity

- Humoral: Antibody-mediated immunity. Major player = B-lymphocytes. CAN DIRECTLY INTERACT W/ Ag o à Responds to extracellular microbes o à Block infections and eliminate extracellular microbes

- Cell-Mediated: Cellular Immunity. Major player = T-Lymphocyte. REQUIRE APC TO BE ACTIVATED o à Responds to intracellular microbes o à Activate macrophages to kill phagocytosed micrboes, Kill infected cells and eliminate infectious

reservoirs Antigen Presenting Cells (APC)- 3 types each with their own particular function. Generally, present Ag to T Cells

- Dendritic Cells (DC): Most potent for Naïve T Cell activation, clonal expansion, and differentiation into effectorT - Macrophages: More effective on previously activated (effector). Thus, activate effectorT, [Cell-mediated] - B Lymphocytes: Effector T & B Cell activation + antibody production [Humoral Immunity] - General APC Function:

o (1) Naïve T Cell Activation à Differentiation into effector T Cells o (2) Effector T Cell Activation à Activation of macrophages and B Cells

- On the surface of APC are MHC, the Major Histocompatibility Complex which holds part of the Ag on display o Used for autorecognition and tolerance

MHC: The Major Histocompatibility Complex- A complex/set of cell surface proteins discovered in 1940s by Snell - Roles: Display Ag to T Cells, Allorecognition + Tolerance, Governs T&B Cell interactions, Controls +/- selection - Composition: Human Leukocyte Antigens - Genetics: 2 Polymorphic MHC genes (I, II), and a third locus that codes for

inflammation/complement factors. Haplotype: Set of alleles on a chromosome o MHC I Genes: HLA-A, HLA-B, HLA-C o MHC II Genes: HLA-DP, HLA-DQ, HLA-DR o Humans express 2 haplotypes, 1 from each parent. HLA is incredibly polymorphic, Ex. HLA-A (47) o HLA Expression is Codominant: Proteins encoded by maternal and paternal expressed equally.