Effector Mechanisms of Humoral Immunity Prof.Dr.Pongsak Utaisincharoen Department of Microbiology, Faculty of Science Pongsak.uta@mahidol.ac.th

Properties of Antibodies that determine effector function Antibodies function throughout the body and in the lumens of mucosal organs Protective antibodies are produced during the first (primary) response and in large amounts during subsequent (secondary responses) Antibodies use Ag-binding (Fab) regions to bind to microbes and Fc regions to activate effector mechanisms that eliminate microbes Heavy-chain isotype (class) switching and affinity maturation enhance the protective function Heavy-chain isotype switching: results in the production of Ab with Distinct Fc regions capable of different effector function

Antibody isotype Effector functions IgG IgM IgA IgE Neutraliztion of microbes and toxins Opsonization of antigens for phagocytosis by macrophages and neutrophils Activation of the classical pathway of complement Antibody-dependent cellular cytotoxicity mediated by NK cells Neonatal immunity: transfer of maternal antibody across placenta and gut Feedback inhibition of B cell activation IgM IgA Mucosal immunity: secretion of IgA into lumens of gastrointestinal and respiratory tracts, neutralization of microbes and toxins IgE Defense against helminthes Mast cell degranulation (immediate hypersensitivity reactions) IgM

Affinity maturation is induced by prolonged or repeated stimulation with Ag results in the production of Ag with higher affinity to Ag

Where is antibody produced from? Cells: B lymphocyte and plasma cells Organ: Lymphoid organs and bone marrow

How antibody is produced ? Naïve B cell Differentiation Ab producing cells (plasma cell) Memory cells located in lymphoid organ Ag (e.g. microbe)

Heavy-chain class switching: optimize for combating the microbes Affinity maturation: increase binding affinity to Ag by repeated or prolong stimulation

Neonatal Fc receptor (FcRN) - expressed in placenta, endothelium, phagocytes and few other cells - protecting IgG antibodies from intracellular catabolism - found in endosomes of endothelial cells - once bond to FcRn, IgG is recycled back into the circulation (avoiding lysosomal degradation) * This unique mechanism for protecting IgG to have longer half live than other Ig

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Neutralization of microbes and microbial toxins Ab binds to microbes and prevent them to infect the cells Ab binds to toxins and bock the toxin to damage the cells

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Opsonization and Phagocytosis Ab coat microbes promote phagocytosis (FcgRI) major defense mechanism against encapsulated bacteria eg. Pneumococci

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FcγRIIB important not for shutting down antibody production and reduce inflammation (negative feedback inhibition of B cell) also inhibits activation of macrophages and dendritic cells

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Antibody-Dependent Cellular cytotoxicity (ADCC) - FcγRIII on NK cells 8.5

Immunoglobulin E and Eosinophil/ mast cells defense against helminthic parasites involve in allergic diseases use FceRI on eosinophil or mast cells 8.6

Complement system a collection of circulating and cell membrane proteins activation involves sequential proteolytic cleavage of these proteins leading to generation of effector molecules activated complement proteins covalently attached to the cell surfaces where the activation occurs

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Function of the complement system elimination of microbes during innate and adaptive immune response provide stimuli for the development of humoral immune response

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Regulation of Complement Activation purpose to preventing complement-mediated damage to host cells Decay accelerating factor (DAF) is a lipid-linked cell surface protein, disrupts the binding of Bb to C3b Membrane cofactor protein (MCP) serves as a cofactor for cleavage of C3b thus destroying C3b C1 inhibitor (C1 INH) stop complement activation at stage of C1

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*Inherited deficiencies of regulatory proteins cause uncontrolled and pathologic complement activation e.g. Deficiency of C1INH case disease called “hereditary angioneurotic edema” (excessive C1 activation)

Inherited deficiencies of complement often lead to an increase susceptibility to infection

Functions of Antibodies at special anatomic sites - Ab function at any site in the body to which Ab gain access - Ab serve protective function at two special anatomic sites 1) Mucosa organs 2) the fetus

Mucosal Immunity IgA is produced in mucosal lymphoid tissues, transported across epithelia and bind and neutralizes microbes in the lumens of espiratory or gastrointestinal tract IgA 60-70% produced daily The propensity of B cells in mucosal epithelia tissues to produce IgA is influenced by Transforming Growth Factor b (TGF-b) which induces class switching B-1 cells also can produce IgA (T cell independent) Poly-Ig receptor important for transport IgA across epithelial cell

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Neonatal Immunity Transported across the placenta to the fetus across the gut epithelium of neonates protecting the newborn from infection Naturally occurring passive immunity Neonates acquire maternal IgG by two routes both of which rely on neonatal Fc receptor (FcRN) which help IgG to transport into fetal circulation FcRN express in the placenta during pregnancy and facilitate IgG to transport into the fetal circulation After birth neonate ingest maternal Ab from milk and FcRN express in the neonate’s intestinal epithelial cell IgA from milk provide mucosal immune protection

Evasion of Humoral Immunity by Microbes

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Vaccination 8.13

Vaccine that stimulate Cell Mediated Immunity To elicite T cell mediated response, the Ag must be delivered to the interior of antigen-presenting cell (APC, DC). Incorporate microbial Ag into viral vectors which infect host cells and produce the Ag inside the cells Immunize individuals with DNA encoding a microbial Ag in a bacterial plasmid which will ingested by APC and Ag is produced inside the cells