Week 2 Fusion Session TBL Acute Inflammatory Response & Complement PDF

1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Clinical markers of inflammation encompass local signs such as redness and swelling, systemic protective effects like fever and increased immune response,...

1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Clinical markers of inflammation encompass local signs such as redness and swelling, systemic protective effects like fever and increased immune response, as well as systemic pathologic effects including tissue damage and organ dysfunction. For review (and because we have referred to these often in this text): Transcript Top Pattern recognition receptors (PRRs) recognize and bind to pathogen-associated molecular patterns (PAMPs), with PAMPs being found in extracellular, cytosolic, and endosomal https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 31/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 compartments, triggering immune responses to combat infections. Transcript Plasma membrane-bound pattern recognition receptors (PRRs) are specialized in recognizing extracellular pathogen-associated molecular patterns (PAMPs) and endosomal PRRs are specialized in recognizing intracellular pathogen-associated molecular patterns (PAMPs), which trigger immune responses against intracellular infections.  Top Cytokines (comm only referred to in this and later terms) https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 32/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 You will need to know the bolded ones. The other ones (not bolded) you will see later. For a more exhaustive list of functions, secreting cells, and target cells, refer to Appendix II of your required book (Molecular and Cellular Immunology). Also, some of the non-bolded cytokines and chemokines play important roles in diseases. Anyone remember SARS-CoV-2? Some of these are extremely important in some diseases, to name only Acute Respiratory Distress Syndrome. IL-1ß: initiation of inflammatory response, induction of other proinflammatory cytokine synthesis and secretion (IL-1ß, TNF-α, IL-6, & CXCL8) as well as other inflammatory mediators (LTs, prostaglandins) and adhesion molecules (endothelial cell activation, i.e., expression of selectins and integrins); fever, synthesis of acute-phase reactants by the liver; CD4 T lymphocyte differentiation into pathologic TH17 cell subset (autoimmune diseases); signals mainly through IL-1R TNF-α: vasodilation, chemotaxis, endothelial activation, transendothelial migration, and activation of neutrophils and macrophages; fever, increased muscle and fat catabolism (cachexia); signals mainly through TNFR LTα (Lymphotoxin α; TNF family) LTß (Lymphotoxin ß; TNF family) IL-6: fever, synthesis of acute-phase reactants by the liver; leukocyte proliferation; CD4 T lymphocyte differentiation into TH17 cell subset (for responses against extracellular microbes); signals mainly through IL-6R IL-12: activation of NK cells for the synthesis and release of IFN-γ; CD4 T lymphocyte differentiation into TH1 cell subset (for responses against intracellular microbes); activation of Cytotoxic T Lymphocytes (CTLs; cytotoxic CD8 subset); signals mainly through type I cytokine receptors IFN-γ (Type II IFN): activation of macrophage microbicidal functions (M1 macrophages); CD4 T lymphocyte differentiation into TH1 cell subset (for responses against intracellular microbes); isotype-switching to IgG; involved in Type IV hypersensitivity (Delayed-Type Hypersensitivity or DTH); signals mainly through type II cytokine receptors (specifically the IFNGR) IL-33: nuclear alarmin cytokine released during tissue injury and involved in the control of the immune system; also involved in different pathological processes such as asthma, allergy etc. IL-2: proliferation of T and B lymphocytes, as well as NK cells; signals mostly through common γ-chain receptors, but also through type I cytokine receptors IL-15: development and proliferation of NK cells, and proliferation of T lymphocytes; signals mainly through common γ-chain receptors, but also type I cytokine receptors Top IL-4: CD4 T lymphocyte differentiation into TH2 cell subset (for responses against worm infections); isotype-switching to IgE; alternative activation of macrophages (M2 macrophages); involved in Type I hypersensitivity reactions (immediate or allergic & asthma); signals mainly through type I cytokine receptors, but also through common γ-chain receptors https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 33/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 IL-5: proliferation and activation of eosinophils; involved in Type I hypersensitivity reactions (immediate or allergic & asthma); signals mainly through type I cytokine receptors, but also through GM-CSF receptors IL-13: increased mucus production by goblet cells; isotype-switching to IgE; alternative activation of macrophages (M2 macrophages); signals mainly through type I cytokine receptors IL-17: increased chemokine production (mainly CXCL8) by macrophages, epithelial cells, fibroblasts, and keratinocytes at site of infection to attract neutrophils (mainly) and monocytes; increased leukocyte production (neutrophils mainly & monocytes) by the bone marrow (GM-CSF, G-CSF, & M-CSF); signals mainly through the IL-17R IL-22: increased barrier protection and defensin (small cationic proteins capable of destabilizing microbial membranes) production by epithelial cells and neutrophils (& other cells) at site of infection; signals mainly through type II cytokine receptors IL-23: CD4 T lymphocyte differentiation into pathologic TH17 cell subset (autoimmune diseases); upregulation of TH17 cells; signals mainly through type I cytokine receptors IL-21: B lymphocyte activation and proliferation; inhibition of NK cells and CTLs; signals mainly through type I cytokine receptors, but also through common γ-chain receptors IL-18: induction of antiviral mediators; activation of non-cytotoxic CD8 T lymphocytes; signals mainly through IL-6R IFN-α (Type I IFN): antiviral state (intracellular response); signals mainly through type II cytokine receptors IFN-ß (Type I IFN): antiviral state (intracellular response); signals mainly through type II cytokine receptors IFN-γ (Type III IFN; IL-28, IL-29): antiviral state (intracellular response); signals mainly through type II cytokine receptors TGF-ß: anti-inflammatory; CD4 T lymphocyte differentiation into TH17 cell subset (for responses against extracellular microbes); mainly signals through the TGF-ßR IL-10: anti-inflammatory; inhibition of IL-12 expression; signals mainly through type II cytokine receptors IL-3 and stem cell factor: hematopoiesis (immature cells); signals mainly through type I cytokine receptors IL-7: proliferation of NK and T lymphocyte progenitors; maturation of B cells following activation (IL-7 does, however, stimulate some subsets of B cell progenitors); signals mainly through type I cytokine receptors and common γ-chain receptors GM-CSF (granulocyte/macrophage colony stimulating factor): maturation of granulocytes and monocytes; macrophage activation; signals through type I cytokine and GM-CSF receptors Top G-CSF (granulocyte colony stimulating factor): maturation of granulocytes; signals mainly through type I cytokine receptors M-CSF (macrophage colony stimulating factor): maturation of monocytes; signals mainly through type I cytokine receptors https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 34/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Platelet-activating factor (PAF): involved in inflammation and thrombotic cascades; signals mainly through seven transmembrane G-protein-coupled receptors CXCL8 (IL-8; chemokine): chemotaxis, neutrophils especially; signals mainly through seven transmembrane G-protein-coupled receptors CCL3 (MIP-1α or macrophage inflammatory protein 1-α; chemokine): recruitment and activation of leukocytes through binding to the receptors CCR1, CCR4 and CCR5; fever, but mechanism independent from prostaglandins and cyclooxygenase; induce the production of IL-1ß, IL-6, and TNF-α CCL4 (MIP-1ß or macrophage inflammatory protein 1-ß; chemokine): recruitment and activation of leukocytes through binding to the receptors CCR1, CCR4 and CCR5; fever, but mechanism independent from prostaglandins and cyclooxygenase; induce the production of IL-1ß, IL-6, and TNF-α CXCL2 (MIP-2 or macrophage inflammatory protein 2; chemokine): chemotaxis of leukocytes to site of injury and activation of neutrophils; signals through CXCR1 and CXCR2 CCL20 (MIP-3α: or macrophage inflammatory protein 3-α; chemokine): chemotaxis of activated B cells, memory T cells, and immature dendritic cells in mucosa and skin; migration of dendritic cells and lymphocytes in secondary lymphoid tissue; signals through CCR6 CCL19 (MIP-3ß: or macrophage inflammatory protein 3-ß; chemokine): chemotaxis of mature dendritic cells to lymph nodes; chemotaxis of naïve T lymphocytes and B lymphocytes to the lymph node; signals through CCR7 CCL2 (MCP-1 or monocyte chemoattractant protein 1; chemokine): chemotaxis and infiltration of monocytes, memory T lymphocytes, and NK cells; signals through CCR2 CXCL10 (IP-10 or interferon-γ-inducible protein 10; chemokine): chemotaxis of monocytes and macrophages, T lymphocytes, NK cells, and dendritic cells; T cell adhesion to endothelial cells; angiogenesis; signals through CXCR3 CCL5 (RANTES): T lymphocyte and NK cell chemotaxis; signals mainly through seven transmembrane G-protein-coupled receptors CCL11, CCL24 and CCL26 (eotaxin-1, eotaxin-2, & eotaxin-3 respectively): eosinophil chemotaxis; signal mainly through seven transmembrane G-protein-coupled receptors VEGF-A (vascular endothelial growth factor A): involved in angiogenesis, endothelial cell growth, vascular permeability (inflammation), and chemotaxis; signals through VEGFRs, which are tyrosine kinase receptor Top  C h e c k Yo u r K n o w l e d g e https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 35/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 What are the cardinal signs of inflammation?  Loss of function  Erythema  Pain  Warmth  Edema  Check  Transcript Question 1 What are the cardinal signs of inflammation? Edema (correct) Erythema (correct) Loss of function (correct) Pain (correct) Warmth (correct) Question 2 Which receptors are involved in the recognition of injury? DAMP IL-1β NLR (correct) PAMP TLR (correct) Question 3 Top What molecule(s) is(are) secreted to initiate an acute inflammatory response? Caspase 1 IL-1β (correct) https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 36/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 IL-18 (correct) NF-κB NLR Question 4 What is the role of NF-κB? Activation of the inflammasome Cleavage of pro-IL-1β Degradation of IκB Recognition of PAMPs Recruitment of RNA pol II (correct) Question 5 What is the role of NF-κB? Activation of the inflammasome Cleavage of pro-IL-1β Degradation of IκB Recognition of PAMPs Recruitment of RNA pol II (correct) Question 6 What enzyme is responsible for cleaving inactive pro-IL-1β into its biologically-active form IL1β? Caspase 1 (correct) IκB NF-κB Proteasome RNA pol II Question 7 The IL-1R signals through which pathway(s)? Akt/Tor Ca2+/Calmodulin JAK/STAT MAPK (correct) NF-κB (correct) Top Question 8 During the initial stages of acute inflammation, what inflammatory mediators are secreted in response to macrophage IL-1β stimulation? https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 37/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 IFN-γ IL-1β (correct) IL-6 (correct) Prostaglandins (correct) TNF-α (correct) Question 9 What cytokine(s) stimulate(s) the expression of endothelial cell and circulating leukocyte adhesion molecules? IFN-γ IL-1β (correct) IL-12 TGF-β TNF-α (correct) Question 10 What cytokine(s) stimulate(s) the expression of endothelial cell and circulating leukocyte highaffinity integrins? CXCL8 (correct) IFN-γ IL-1β IL-6 TNF-α (correct) Question 11 What inflammatory mediator(s) promote(s) vascular leakage? Bradykinin (correct) Histamine (correct) Integrins (correct) Prostaglandins (correct) TNF-α (correct) Question 12 What cytokine(s) stimulate(s) the expression of tissue factor? IFN-γ IL-1β (correct) IL-12 TGF-β TNF-α (correct) Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 38/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Question 13 What inflammatory mediator(s) stimulate(s) the activation of Factor VII? Histamine Leukotrienes Prostaglandins Tissue factor (TF) (correct) TNF-α Question 14 What cell(s) is(are) the main source(s) of IFN-γ during the acute stage of an inflammatory response? B lymphocyte T lymphocyte Macrophage NK cell (correct) Type 1 innate lymphoid cell (ILC1) (correct) Question 15 What signaling pathway acts downstream of the interferon-gamma receptor (IFNGR)? Akt/Tor Ca2+/Calmodulin JAK/STAT (correct) MAPK NF-κB Question 16 In which tissue are acute-phase reactants primarily synthetized? Bone marrow Liver (correct) Lymph node Spleen Thymus Question 17 Which cytokine(s) is(are) pyrogenic? Top IL-1β (correct) IL-6 (correct) IL-12 TGF-β https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 39/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 TNF-α (correct)  The Complement System: General Concepts What is Complement? The complement system is composed of a set of soluble serum and cell surface proteins that interact with one another as well as with other molecules of the body to activate important innate and adaptive immune responses such as the destruction of microbes by lysis, inflammation, the activation of antibody-producing cells (B lymphocytes), and the clearance of immune complexes. Features of Complement Complement (C) is activated, or fixed (which is synonymous when discussing complement), by microbial surfaces or molecules that activate complement when bound to microbes. Complement activation, or fixation, results from the sequential proteolysis of complement proteins, which in turn generate enzyme complexes that also possess proteolytic activity (similar to the coagulation & kinin systems); thus, complement cascades are sequential steps of complement activation with each component being activated by the previous components. Also, some components of the complement system can mediate several proteolytic reactions, thus ensuring a rapid amplification of the response. Some biologically active cleavage products resulting from complement activation covalently bind to microbe surfaces, antibodies bound to microbes and other antigens, as well as apoptotic bodies. Byproducts of complement activation stimulate inflammation. Complement cascades can result in: Microbe lysis; Microbe and apoptotic body opsonization (‘tagging’) resulting in microbe or apoptotic body phagocytosis by neutrophils and macrophages; Soluble byproducts that trigger and further stimulate inflammation. To prevent deleterious effects of complement to the host, complement is inhibited by complement Top inhibitors associated with host cells. H o w D o e s C o m p l e m e n t Wo r k ? https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 40/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 As previously described, complement reactions meant to kill microorganisms and/or induce inflammation usually act in a stepwise cascade of events, with some steps involved in amplifying complement action; furthermore, each step requires the activation of the previous step through enzymatic proteolysis. Complement Proteins There are about 30 known complement proteins, and these makeup about 10% of serum proteins. Most complement proteins are zymogens which is an inactive precursor enzyme that is altered to become an active protease that cleaves and thereby induces the proteolytic activity of the next complement protein in the cascade. Finally, complement components are heat-labile, which is relevant to certain clinical diagnostic tests. Important Concepts: Opsonin and Opsonization Opsonization is the process of a molecule, such as IgG or complement fragments, binding to a cellular surface for the purpose of triggering phagocytosis. An opsonin is a molecule that binds a cellular surface to trigger phagocytosis. A molecule that becomes attached to the surface of a microbe and can be recognized by surface receptors of neutrophils and macrophages and that increases the efficiency of phagocytosis of the microbe. Opsonins include IgG antibodies, which are recognized by the Fcγ receptor on phagocytes, and fragments of complement proteins, which are recognized by CR1 (CD35) and by the leukocyte integrin Mac-1 (CD11b/CD18). Where do Complements Proteins Come From? Most serum complement proteins are primarily synthesized by the liver (hepatocytes), especially when the liver is stimulated by proinflammatory cytokines (especially IL-6), but some complement proteins are synthesized by monocytes, macrophages, epithelial cells, as well as fibroblasts. How Many Complement Pathways Are There? There are three recognized complement cascades: Alternative Lectin Classical Top Alternative (alternate) Pathway (innate) https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 41/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 This pathway is theoretically the first one to be activated, and it is triggered by spontaneous fixation of C3b (complement component 3b) on microbial surfaces. Lectin Pathway (also innate) The lectin pathway is theoretically activated later after the onset of inflammation since its action draws on acute-phase reactants (acute-phase proteins synthesized and released by the liver following stimulation by proinflammatory cytokines, especially IL-6. This pathway is triggered by plasma lectins that bind microbe carbohydrates; Classical Pathway (acquired, syn. adaptive) This is the last pathway to be activated since it requires antibody binding to microbes.  Functions of Complement The 5 main functions of complement are: 1. opsonization of microbes and apoptotic bodies for phagocytosis 2. initiation and maintenance of inflammation 3. microbial lysis 4. B lymphocyte activation, and 5. the clearance of immune complexes. Opsonization of microbes and apoptotic bodies for phagocytosis: Opsonins fixed on membranes can be bound by their receptors present on the surface of phagocytic cells to trigger phagocytosis of opsonized material. See the figure below: Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 42/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Opsonization enhances phagocytosis by coating target cells or particles with proteins, facilitating their recognition and ingestion by phagocytic cells. 2. Initiation of and maintenance of inflammation: Complement fixation results in the release of soluble complement fragments (C3a & C5a), which then perform other roles. Both C3a and C5a released in an inflamed site act as chemokines to attract leukocytes. Furthermore, mast cells possess receptors for C3a and C5a; engagement of these receptors results in the release of inflammatory mediators such as histamine and other vasoactive amines, prostaglandins, and leukotrienes. See the figure below: Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 43/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Stimulation of inflammatory reactions, including the action of complement proteins C3a and C5a, induces the release of proinflammatory mediators and chemotaxis, contributing to the immune response. 3. Microbial lysis: Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 44/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Complement-mediated cytolysis, involving the complement protein C3b, leads to the formation of membrane attack complexes that puncture and destroy target cells by creating pores in their membranes. 4. Lymphocyte activation: Complement fragment 3d (C3d) bound to microbe engages complement receptor type 2 (CR2) on the surface of B lymphocytes. CR2 (CD21) is part of the B cell receptor (BCR) complex necessary for B cell activation. As you will learn later in the B Lymphocyte Biology lecture, the BCR is a complex of several transmembrane molecules (the BCR per se, which is coupled with Igα/Igß, as well as CD19, CD21, & CD81). The BCR per se is a membrane-bound immunoglobulin whose only function is antigen (Ag) recognition; the BCR has no signaling capacity (no phosphate binding motifs), which is why the BCR is a complex of molecules. The signaling that leads to the B cell’s response is conferred by signaling molecules Igα/Igß, CD19, and CD21 (remember this dichotomy in receptor function, which will be developed in the lecture on B Lymphocyte Biology; it is very similar to that of the T Cell Receptor (TCR) seen in a later lecture called Ag Presentation and T Lymphocyte Biology); the function of CD81 is still under investigation. The figure below shows some of the signaling details pertaining to CR2. Keep this figure in mind when you tackle the B Lymphocyte Biology lecture, as well as the Ag Presentation and T Lymphocyte Biology lecture; there are a lot of parallels to be drawn between the TCR and the BCR, and many of the events surrounding these receptors form the basis of many pharmacological interventions. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 45/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript B lymphocyte activation involves the binding of antigens to the B cell receptor (BCR), leading to the differentiation of B cells into antibody-producing plasma cells and memory B cells. 5. Clearance of immune complexes: Complement receptor type 1 (CR1) present on the surface of erythrocytes, coupled with complement fragments bound to IgGs, are paramount for clearing immune complexes. Inefficient immune complex clearance (i.e., deficiency in some complement components) can lead to serious, sometimes fatal, disease manifestations… Immune complexes activate complement. These immune complexes can be bound to erythrocytes through C3b/CR1 interaction. In the spleen, splenic macrophages uncouple erythrocytes from immune complexes; immune complexes are phagocytosed and destroyed by splenic macrophages, and erythrocytes are recycled. These events are depicted in the figure below: Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 46/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Small antigen-antibody complexes develop within the bloodstream. The immune complex becomes coated with many molecules of C3b due to complement activation. Erythrocytes' complement receptor CR1 binds to immune complexes through C3b that is attached to them. Phagocytic cells in the spleen and liver clear immune complexes from the surface of erythrocytes. Some complement functions, such as complement-mediated phagocytosis, clearance of immune complexes, and B lymphocyte activation, also require the use of other proteins of the complement system; these proteins are Complement Receptors (CRs). There are four complement receptors: CR1, CR2, CR3, and CR4. Complement receptors 1, 3, and 4 (CR1, CR3, & CR4) are primarily involved in phagocytosis; CR2 is part of the B cell receptor and, therefore, is involved in B lymphocyte activation (as you will later see in the B Lymphocyte Biology lecture). The names, main cell distribution, and functions of complement receptors are summarized in the table below: C3 Fragment Receptors Receptor Type I complement receptor (CR1, CD35) Ligands C3b> C4b> Top iC3b> Cell Distribution Mononuclear phagocytes, neutrophils, B and T cells, erythrocytes, eosinophils, FDC's Function Phagocytosis Clearance of immune complexes Promotes dissociation of C3 https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 47/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 convertases by acting as cofactor for cleavage of C3b, C4b Coreceptor for B cells activation B Lymphocytes, Type 2 complement C3d, C3dg> FDCs, receptor (CR2, CD21) iC3b nasopharyngeal epithelium Trapping of antigens in germinal centers Receptor for EBV Type 3 complement receptor (CR3, Mac1, CD11bCD18) Type 4 complement receptor (CR4, iC3b, ICAM-1; also Mononuclear phagocytes, binds microbes neutrophils, NK cells Phagocytosis Leukocyte adhsesion to endothelium (via ICAM-1) Mononuclear iC3b p150,95,CD11cCD18) phagocytes, Phagocytosis, cell neutrophils, NK adhesion? cells CCPRs, Complement control protein repeats: EBV, Epstein-Barr virus; FDCs, follicular dendritic cells; ICAM-1, intercellular adhesion molecule 1: NK, natural killer Top  Complement Cascades All 3 complement cascades converge on C3 cleavage and a common pathway for the membrane attack complex (MAC), i.e., the complex responsible for cell lysis. Although the three complement https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 48/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 cascades differ in their early stage, there are obvious similarities between the lectin and classical pathways; furthermore, all three pathways lead to C3 and C5 convertase complex formation, with the terminal phase (membrane attack complex or MAC) being identical for all three pathways. Transcript The classical pathway is initiated by antibodies, while the alternative and lectin pathways are not; however, the central event inTop all three pathways is the cleavage of C3 by C3 convertase (specifically, in the classical and lectin pathways, and for the alternative pathway). Light blue indicates molecules with protease activity. https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 49/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Note on C2a/C2b Nomenclature Congruity By convention, the smaller fragment that is released and solubilized is given the suffix 'a', whereas the larger catalytic fragment is given the suffix 'b.' However, in the initial report on the activity of C2, the larger, catalytically active fragment remaining attached to C4b was named C2a, and the smaller, released fragment was named C2b. Consequently, the C4bC2a (syn. C4b2a) nomenclature for designating the C3 convertase was established. But many authors wishing to maintain a coherent and logical nomenclature system based on released fragments being attributed to the ‘a’ suffix and the bound catalytically-active fragments, the suffix ‘b,’ developed the alternative nomenclature by which the larger catalytic fragment was renamed C2b and the smaller released fragment C2a hence the C4bC2b (syn. C4b2b) C3 convertase; as a result, a second nomenclature was firmly established as well. Given that both nomenclatures are deeply rooted, both are commonly in use, but they are considered one and the same. I use C4bC2a (or C4b2a) because your book uses that nomenclature, and I wanted to avoid this confusion and be coherent with the resources you are provided with. This being said, however, always remember that both C4b2a and C4b2b are one and the same complex and that the difference between the two is rooted in semantics exclusively… Watch this video: Complement System - An Introduction (https://www.youtube.com/watch?app=desktop&v=DPNnZE4OtCM) The Alternative Pathway of Complement Fixation (Activation): 1% to 2% of total plasma C3 is spontaneously cleaved into fragments C3a and C3b per hour; this is called C3 tickover. Fluid-phase C3b is rapidly inactivated by hydrolysis. However, spontaneous cleavage of C3 in proximity to a complement-activating surface, such as a cell membrane or a cell wall, leads to binding of C3b to the activating surface (& the release of the soluble fragment C3a, which then promotes inflammation & leukocyte chemotaxis) and a conformational change that exposes Top a binding site for Factor B; these events lead to C3b/Factor B being covalently bound to the surface. Factor D, a plasma serine protease, then cleaves Factor B to yield a C3bBb complex and a small soluble fragment, Ba. The C3bBb complex, which is called the C3 convertase, is then https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 50/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 stabilized by another molecule of the complement system, properdin. The Bb moiety of the C3 convertase is another serine protease that cleaves C3 into C3b and C3a. It is important to understand that the C3 convertase (C3bBb) is an enzymatic complex that catalyzes the cleavage of several C3 molecules; therefore, with the formation of the C3 convertase (C3bBb), the alternative complement pathway is amplified. C3b fragments generated by the C3 convertase in turn, bind the surface and result in 2 things: 1. surface opsonization with C3b and 2. the production of more C3 convertase on the surface as a result of plasma Factor B and Factor D interaction (also stabilized by properdin). This way, 1. further C3b opsonization of the surface occurs, which can trigger phagocytosis by phagocytes (e. g. macrophages & neutrophils), and 2. C3b fragments can bind the C3 convertase complex to yield the C5 convertase (C3bBbC3b) whose role is to cleave C5 into C5b and C5a. In a manner similar to the C3 convertase, the C5 convertase catalyzes the cleavage of many C5 molecules and is another stage at which the alternative pathway can be amplified. These alternative pathway amplification stages, C3 convertase, and C5 convertase, also yield great amounts of soluble C3a and C5a, respectively, thereby stimulating further inflammation and chemotaxis of leukocytes. As you remember from the Inflammation and Cytokines lecture, C3a and C5a are chemotactic for leukocytes, and mast cells possess C3a and C5a receptors, which, upon engagement, trigger mast cell degranulation and release of vasoactive and other inflammatory molecules. C5b initiates the terminal phase of complement activation, resulting in the membrane attack complex (MAC) and the creation of pores in the surface that trigger cellular lysis (seen later below). Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 51/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript The alternative pathway of complement activation is an innate immune response that can be initiated independently of antibodies and relies on the spontaneous hydrolysis of C3 to C3(H2O), which triggers a cascade of reactions leading to opsonization and inflammation. 1. Spontaneous hydrolysis and cleavage of C3 2. Cleavage of Factor B by Factor D. Stabilization by Properdin Top 3. Cleavage of additional C3 molecules by cell-associated C3 convertase 4. C3b binds covalently to microbe surface, binds to C3bBb to form C5 convertase 5. Cleavage of C5, initiation of late steps of complement activation. https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 52/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 The Lectin Pathway of Complement Activation (Fixation): The lectin pathway of complement activation is, theoretically, the second complement pathway to be activated. This is because, in theory, the lectins that trigger this pathway are part of the socalled acute-phase reactants (or acute-phase proteins) which are generated as a result of inflammation (e.g., IL-6 and other cytokines action on the liver). Top Transcript https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 53/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 The lectin pathway of complement activation is an immune response triggered by the binding of mannose-binding lectin (MBL) or ficolins to specific sugar molecules on the surface of pathogens, leading to the activation of the complement system and promoting opsonization and inflammation. 1. Mannose-binding lectin (MBL) binds to sugar molecules on pathogen surface 2. Cleavage of C4 and C2 3. C4b and C2a bind together into C3 convertase and bind to the cell surface 4. Cleavage of additional C3 molecules by cell-associated C3 convertase 5. Cleavage of C5, initiation of late steps of complement activation Plasma lectins or ficolins, such as the mannose-binding lectin and different types of ficolins respectively, bind polysaccharides on the surface of microorganisms. Of note, these lectins and ficolins are collagen-like proteins whose structure is reminiscent of C1q, the first complement component of the classical pathway of complement activation – remember this when it comes to comparing the lectin and classical pathways of complement activation (the two pathways are identical, except for the complex that cleaves C4 and C2, which are still very similar nonetheless!). Lectins or ficolins associate with mannose-associated serine proteases 1 to 3 (MASP1, MASP2, & MASP3). MASPs cleave plasma C4 into fragments C4b and C4a, resulting in the deposition of C4b on the cell surface and release of soluble C4a in fluid phase. Then plasma C2 is cleaved by MASPs to yield C2a cell surface deposition and release of C2b in fluid phase. The surface-bound C4bC2a is the C3 convertase which catalyzes the cleavage of C3 into C3b and C3a to yield (1) microorganism opsonization and release of inflammatory soluble C3a (in analogy with the alternative pathway, this is an amplification step of the lectin pathway), and (2) C3b binding to C4bC2a to yield the C5 convertase (C4bC2aC3b). In turn, the C5 convertase catalyzes the cleavage of numerous C5 molecules into C5b and C5a. The actions of the lectin pathway C3 convertase and C5 convertase result in the same effects as those described for the alternative pathway, namely stimulation of inflammation, opsonization for phagocytosis, and pore formation as a result of the MAC. As for the relationship of lectins and ficolins with C1q (these molecules are quite similar structurally), note also that MASPs are homologous to the serine proteases C1r and C1s of the classical pathway of complement activation – again, remember this when it comes to comparing the lectin and classical pathways of complement activation. These two complement pathways are identical except for the opsonins that trigger their activation! – This should reduce the time required for the memorization of these pathways. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 54/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Lectins and ficolins are pattern recognition molecules, similar to C1q in the classical pathway, that initiate complement activation by binding to specific pathogen-associated molecular patterns, facilitating the recognition and elimination of pathogens. Top The Classical Pathway of Complement Fixation (Activation): https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 55/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 The classical pathway of complement activation is the last complement pathway to be activated; this is because antibody (immunoglobulin) production usually takes at least a couple of days (and on average 1-2 weeks). C1 (C1qr2s2) is a large multimeric protein complex composed of C1q, C1r, and C1s subunits; C1q is a hexamer that binds the Fc portion of immunoglobulin molecules (Fc stands for Fragment crystallizable, & is the part of the antibody molecule that determines its function, namely complement fixation in this case), C1r (dimer) is a serine protease which cleaves C1s to activate it, and C1s (dimer) is the actual serine protease that cleaves C4 and C2. C1q cross-linking of Fc portions of antibodies bound to multivalent Ag (either between the Fc portion of two or more adjacent IgG molecules or two or more Fc portions from the same IgM molecule – IgMs possess 5 Fc portions per molecule as opposed to IgG molecules which possess only one, and this explains why IgMs are better at activating complement as compared to any other immunoglobulin) – and triggers the cleavage of C1s by C1r. Activated C1s then cleaves C4 into C4b and C4a; C4b then covalently binds the pathogen surface, whereas C4a is released in the fluid phase where it mediates inflammatory processes (such as mast cell activation & chemotaxis of leukocytes). Then plasma C2 is cleaved by C1s to yield C2a cell surface deposition and association with C4b and release of C2b in fluid phase; the resulting C4bC2a complex is termed, as seen with the lectin pathway, the C3 convertase. The C3 convertase then catalises the cleavage of C3 into C3b and C3a to yield (1) microorganism opsonization and release of inflammatory soluble C3a (in analogy with the alternative and lectin pathways, this is an amplification step of the classical pathway), and (2) C3b binding to C4bC2a to yield the C5 convertase (C4bC2aC3b). In turn, the C5 convertase catalises the cleavage of numerous C5 molecules into C5b and C5a. The actions of the classical pathway C3 convertase and C5 convertase result in the same effects as those described for the alternative and lectin pathways, namely stimulation of inflammation, opsonization for phagocytosis, and pore formation as a result of the MAC. As for the relationship of C1q with lectins and ficolins (C1q is a hexamer of 6 collagen-like arms that each possess a globular head, the head being the part that binds the Fc portion of antibodies), note also that C1r and C1s are serine proteases homologous to the MASPs of the lectin pathway of complement activation – again, remember this when it comes to comparing the lectin and classical pathways of complement activation. These two complement pathways are identical except for the opsonins that trigger their activation! Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 56/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript The classical pathway of complement activation is initiated by the binding of antibodies to antigens, leading to the formation of C1 complexes and subsequent activation of the complement cascade, which enhances immune responses by opsonization, inflammation, and target cell lysis. Top Terminal Phase – Membrane Attack Complex (MAC): https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 57/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 The terminal phase of complement activation, which is common to all 3 activation pathways, is the elaboration of the membrane attack complex, or MAC, that is responsible for cell lysis. The action of the C5 convertase (C3bBb3b of the alternative pathway or C4b2a3b of the lectin and classical pathways) results in the cleavage of serum C5 into C5b and C5a: C5a is released in fluid-phase where it mediates further inflammatory reactions, whereas C5b binds serum C6 to induce a conformational change in C6 that allows the C5b/C6 complex to bind the membrane surface. Serum C7 then binds C5b to form a C5bC6C7 complex; membrane-bound C7 is amphiphilic and penetrates the membrane. Serum C8 then binds C5b of the C5bC6C7 complex and stably penetrates the membrane to form an unstable pore (0.4-3.0 nm in diameter); large numbers of C5bC6C7C8 unstable pores can result in cellular lysis, but the recruitment and polymerization of serum C9 creates larger stable pores (1.0-11.0 nm in internal diameter) that act as channels through which water and ions move freely – the resulting osmotic swelling culminates in cellular rupture or lysis. Serum C9 molecules bound to the surface C5bC6C7C8 complex therefore polymerize to create a large stable pore in the plasma membrane to yield an active membrane attack complex (MAC). Note that C9 is structurally homologous to the perforin secreted by NK cells and cytotoxic T lymphocytes, and that the pores formed by C9 are similar to the pores that result from the polymerization of perforin… Top Transcript https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 58/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 The terminal phase, also known as the membrane attack complex (MAC), involves the assembly of C5b, C6, C7, C8, and multiple C9 molecules to form a pore-like structure on the target cell membrane, leading to cell lysis and destruction.  Regulation of Complement Left unchecked, complement activation can damage normal host cells and unnecessarily prolong inflammation hence the complement system is normally tightly regulated. Below are some of the most important complement inhibitors. Complement Component 1 Inhibitor (C1 INH): C1 INH is a serine protease inhibitor (serpin) that acts by binding to and inactivating the active site of serine proteases. Its targets include C1r2s2 and MASPs (the mannose-associated serine proteases of the lectin pathway of complement activation) hence C1 INH is an inhibitor of both the lectin and classical pathways of complement activation. The binding and inactivation resulting from C1 INH action also results in the dissociation of the C1r2s2 tetramer from the C1q hexamer, or the dissociation of MASPs from MBL and other lectins or ficolins. The stoichiometry of the reaction is 1:1, meaning that one C1 INH is used up for the inactivation of each serine protease. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 59/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript C1r2s2 activated by binding of C1q to antigen-complexed antibodies. C1r2s2 is prevented from becoming proteolytically active by C1 INH. Complement Component 1 Inhibitor (C1 INH) is a regulatory protein that controls the classical complement pathway by inhibiting the activation of C1 complexes and preventing excessive complement activation.] Factor I Factor I (FI, also called C3b/C4b inactivator) is a trypsin-like serine protease that accelerates the decay of C3b and C4b; it acts by cleaving C3b to yield iC3b and C3f (membrane-bound and fluid phase), as well as C4b to yield C4c and C4d. Because of its very low catalytic activity, Factor I requires cofactors to cleave C3b and C4b efficiently; cofactors involved depend on the site of Factor I action: C4b-binding protein (C4BP) and Factor H (FH; see next figure) for fluidphase inactivation, and membrane cofactor protein (MCP) and complement receptor 1 (CR1) for Top inactivation of membrane-bound C3b and C4b. https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 60/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript C3b (or C4b) binds covalently to cells. iC3b is produced when MCP and CR1 act as cofactors for Factor I-mediated proteolytic cleavage of C3b. Factor I (FI), also known as C3b/C4b inactivator, is a serine protease that plays a key role in regulating the complement system by cleaving and inactivating complement components C3b and C4b, preventing uncontrolled complement activation. Factor H Inactivation of the alternative complement pathway. A. C3b binds FB, which is then converted to Bb, by the serine protease factor D (not depicted). The resulting C3bBb is the alternative pathway C3 convertase that cleaves additional C3 to C3b, thus amplifying the process. Properdin stabilizes this convertase (not depicted). B. This process is controlled by regulatory proteins such as FH. It decays Bb from C3b and is a cofactor for degradation of C3b to iC3b by the serine protease FI. FH attaches to surfaces through its heparin/anionic-binding sites in its carboxyl-terminus (CCPs 16– Top 20), while the decay-accelerating activity and cofactor activity sites are at the amino terminus (CCPs 1–4). https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 61/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Factor H is a regulatory protein in the complement system that helps prevent excessive complement activation by promoting the decay of C3 convertases and enhancing the clearance of C3b from host cell surfaces. C4 Binding Protein (C4BP) C4BP prevents the formation and accelerates the decay of the C3 convertase of the lectin and classical pathways of complement activation; it does so by cleaving C4b into C4c and C4d. Also, remember that C4BP is also a cofactor for Factor I that leads to C3b and C4b inactivation. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 62/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript 1. Cleavage of C3 molecules by cell-associated C3 convertase 2. DAF, C4BP, and CR1 displace C2a from the C4b2a complex. C4b bound by C4BP, MCP, or CR1 is cleaved by a soluble protease I to inactive forms C4c and C4d. C4 binding protein (C4BP) is a regulatory protein that controls the classical complement pathway by binding to C4b and preventing the formation of excessive C3 convertases, thereby modulating complement activation. Decay-accelerating Factor (DAF) Decay accelerating factor (DAF or CD55) prevents the assembly of the C3 convertase of all 3 complement fixation pathways by dissociating C4b from C2a as well as C3b with Bb. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 63/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript Decay-accelerating factor (DAF) is a cell surface protein that regulates the complement system by accelerating the decay of C3 and C5 convertases, protecting host cells from excessive complement activation CD59 (Protectin) Protectin (CD59) prevents the incorporation of C9 into the membrane attack complex (MAC) thereby preventing stable pore formation of the late stage of complement fixation. Furthermore, deposition and fixing of terminal components of complement (C5bC6C7C8, which can also create unstable pores in lipid membranes…) on normal host cells can lead to the destruction of these cells; this phenomenon is referred to as the innocent bystander effect. This innocent bystander effect is limited by the action of CD59, and another host complement regulator, the S protein which we will not be discussing. Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 64/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Transcript CD59, also known as Protectin, is a cell surface protein that inhibits the formation of the membrane attack complex (MAC) in the complement system, protecting host cells from complement-mediated lysis.  Examples of Complement Deficiencies Complement deficiencies can contribute to a number of pathologies. It is important to realize that complement deficiencies could relate to either complement components or their regulators (inhibitors). Their effects mainly manifest as (1) a greater susceptibility to infections in which the membrane-attack complex is an important defense mechanism (e.g., Gram-negative bacteria, which tend to possess a thin cell wall – the model organism, as you will later see, is Neisseria sp., especially Neisseria meningitidis), (2) immune complex-mediated disease in which C3 or CR1 deficiencies (or C3 depletion when C3 use exceeds its synthesis) lead to deficient immune Top complex clearance by splenic macrophages and deposition in the microvasculature or the kidney glomeruli with resulting microinflammation and loss, or alteration of function (e.g., vasculitis & glomerulonehritis), and (3) a bystander effect (e.g., paroxysmal nocturnal hemoglobinuria). https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 65/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 One last important deficiency is a deficiency in complement receptors 3 or 4 (CR3 or CR4) which leads to what is referred to as Leukocyte Adhesion Deficiency (LAD). To comprehend this deficiency, you need to understand that the β-chain (CD18) of CR3 and CR4 is shared with the CD11/CD18 family of integrins (e.g., Lymphocyte Function-associated Antigen 1, or LFA-1), a mutation in this β-chain results in a loss of function of these integrins which are important for leukocyte adhesion prior to transendothelial migration, and this effect is particularly obvious for neutrophils. Individuals with LAD present with recurring bacterial infections.  G e n e r a l Te s t s f o r C o m p l e m e n t D e f i c i e n c y Total complement activity test (CH50): tests for C1-C9 function, i.e., the classical pathway of complement activation; an abnormal result indicates a deficiency in the classical pathway; Alternative complement activity test (AH50): tests for the alternative pathway of complement activation; in conjunction with a normal CH50, an abnormal AH50 result indicates a deficiency in the alternative pathway. Further determination of complement deficiency is based on the following algorithm: Read this article to learn more about leukocyte recruitment and hemodynamic changes. Shih, A.R. and Murali, M.R. (2015), Laboratory tests for disorders of complement and complement regulatory proteins (https://onlinelibrary.wiley.com/doi/10.1002/ajh.24209) . Am. J. Hematol., 90: 1180- 1186. https://doi.org/10.1002/ajh.24209  Evasion of Complement by Microbes Top As you will later learn in Microbiology, some microorganisms possess, or have evolved, mechanisms that protect them from complement. For example, some bacterial possess thick cell walls preventing the function of the MAC. Some microorganisms are capable of recruiting host complement regulatory factors to avoid complement activation (e.g., factor H, DAF, & CD59). https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 66/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Others produce factors that mimic host complement regulatory factors (e. g. C1q-binding protein (E. coli), staphylococcal complement inhibitor, gpC-1 (herpesviruses), gp160 (T. cruzi), & others). Finally, some microbes can produce inflammation inhibitors, such as the chemokine inhibitory protein of staphylococci (CHIPS), in an effort to dampen inflammation (C5a inhibitors which diminish leukocyte chemotaxis & inflammation).  C h e c k Yo u r K n o w l e d g e What is an opsonin?  A molecule that binds the surface of an antigen to promote apoptosis  A molecule that binds the surface of an antigen to promote lysis  A molecule that binds the surface of an antigen to promote chemotaxis  A molecule that binds the surface of an antigen to promote necrosis  A molecule that binds the surface of an antigen to promote phagocytosis  Check  Transcript Question 1 What is an opsonin? A molecule that binds the surface of an antigen to promote apoptosis A molecule that binds the surface of an antigen to promote chemotaxis A molecule that binds the surface of an antigen to promote lysis Top of an antigen to promote necrosis A molecule that binds the surface A molecule that binds the surface of an antigen to promote phagocytosis (correct) Question 2 https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 67/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 Associate the complement receptor with its function(s): B lymphocyte activation Clearance of immune complexes Inflammation Phagocytosis CR2 CR1 CR3 CR1 CR4 CR3 Question 3 Associate the C5 convertase with the correct complement activation pathway(s): Alternative Lectin Classical C3bBb3b C4b2a3b C4b2a3b Question 4 Associate the serine proteases responsible for C4 and C2 proteolysis with the correct complement activation pathway(s): Alternative Lectin Classical MASP-1/MASP-2 C1r2s2 Question 5 Associate the convertase with its substrate: C3bBb C3bBb3b C4b2a C4b2a3b C3 C5 C3 C5 Question 6 What is the composition of the membrane-attack complex? Top C3bBb C3bBb3b C4b2a C4b2a3b https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 68/71 1/25/24, 12:55 AM Week 2: Fusion Session | TBL: Acute Inflammatory Response & Complement: Hemtlgy Onclgy Infectn Imm - January 2024 C56789n (correct) Question 7 Associate the inhibitor with the correct complement activation pathway(s): Alternative Lectin Classical C1 INH C1 INH Factor I Factor I C4BP C4BP DAF DAF DAF CD59 CD59 CD59 Factor H Factor I Question 8 A patient with a suspected complement deficiency has his complement activity tested. Both the CH50 and AH50 return results below normal values. A deficiency in which complement component could explain these results? C1 C2 C3 (correct) C4 Factor B Question 9 A patient with a suspected complement deficiency has his complement activity tested. The CH50 returns results consistent with normal function but the AH50 returns results below normal values. A deficiency in which complement component could explain these results? C1 C3 C4 C9 Factor B (correct) Top https://rossmed.instructure.com/courses/3318/pages/week-2-fusion-session-%7C-tbl-acute-inflammatory-response-and-complement 69/71