B Cell Effector Function PDF

WELCOME! BMS 545 IMMUNOLOGY NOVEMBER 1, 2024 You GOT this!  OBJECTIVES  Compare & contrast naïve B cell and plasma cell  Elaborate on where different immunoglobulin isotypes (& their many forms) are found throughout life & development  Describe the movement of antibodies across & through...

WELCOME! BMS 545 IMMUNOLOGY NOVEMBER 1, 2024 You GOT this!  OBJECTIVES  Compare & contrast naïve B cell and plasma cell  Elaborate on where different immunoglobulin isotypes (& their many forms) are found throughout life & development  Describe the movement of antibodies across & through the tissues  Understand antibody-antigen interactions- describe the effector function of the various antibodies  Differentiate agglutination, neutralization, opsonization, complement & provide example  Provide examples of how the different immunoglobulins provide effector functions (e.g.. how do each protect the body? Crossing epithelial barriers, activating complement pathways, etc.)  Describe the role Fc receptors play in opsonization  What is the role of IgE in regard to parasitic infections & allergies?  Describe what happens to excess immunoglobulin molecules CHAPTER OPENER Figure 9.10 Plasma cells have a distinctive appearance that distinguishes them from all other forms of B cell  The nucleus of a plasma cell has a characteristic ‘clockface’ pattern  Also has extensive rough endoplasmic reticulum (rough ER), a typical feature of cells that synthesize & secrete large quantities of proteins.  In this case- monoclonal antibody (mAb) Figure 9.16 Comparison of key properties of naive B cells and plasma cells  During an adaptive immune response, quiescent naive B cells evolve into highly active plasma cells  Shown here are key properties & capacities that distinguish naive B cells from plasma cells  Antibody effector functions 9-14 Before and after birth, mothers provide their children with protective antibodies Figure 9.21 Immunoglobulin isotypes are distributed selectively in the human body and some are passed by mothers to their young  In healthy adults, IgM, IgG, & monomeric IgA predominate in blood, while IgG & monomeric IgA are major isotypes in extracellular fluid  Dimeric IgA predominates in secretions at mucosal epithelia  IgE is associated mainly with mast cells in connective tissue beneath epithelial surfaces, (esp. skin, respiratory & GI tracts)  The brain is devoid of antibodies  During prenatal development the fetus cannot make its own immunoglobulin  IgG is selectively transported from maternal to fetal circulation in the placenta  Protects against infections mother has experienced  After birth & with breast-feeding, GI tract of infant is supplied with protective maternal dimeric IgA (major constituent of breastmilk) Figure 9.22 The first year of life is a period when infants have a limited supply of IgG and are particularly vulnerable to infections  Before birth, high levels of IgG are provided by mother; after birth, maternally derived IgG declines  Although infants produce IgM before birth, production of IgG antibodies does not begin for ~6 months  Concentration of IgG in blood reaches a minimum within 1st year & then gradually increases until adulthood  Antibody effector functions 9-11 IgM, IgG, & monomeric IgA protect the internal tissues of the body Figure 9.17 The receptor FcRn transports IgG from the bloodstream into the extracellular spaces of tissues FcRn- an Fc receptor that transports IgG across epithelia and has a structure resembling an MHC class I molecule  At apical (luminal) side of endothelial cell, IgG & other serum proteins are actively taken up by fluid-phase endocytosis in a basic pH environment  In endocytic vesicle pH becomes acidic & each IgG molecule associates with two molecules of FcRn  IgG is carried by FcRn to basolateral face of cell & away from degradative activity of lysosomes  At basal side of cell, more basic pH dissociates IgG & FcRn complex, & IgG is released into extracellular space  Antibody effector functions 9-12 Dimeric IgA and pentameric IgM protect mucosal surfaces of the body Figure 9.18 Transcytosis of dimeric IgA antibody across epithelia is mediated by the poly-Ig receptor Dimeric IgA is made by plasma cells from epithelial basement membranes of mucosal tissues, e.g., gut 1. IgA dimer diffuses across basement membrane & is bound by poly-Ig receptor (pIgR) on basolateral surface of an epithelial cell  Binding to pIgR is mediated by CH3 domains 2. Bound complex crosses cell (transcytosis) in a membrane vesicle & delivered to apical surface 3. Receptor undergoes cleavage, which releases a complex of dimeric IgA bound to a fragment of receptor called “secretory component” 4. Carbohydrate of secretory component tethers IgA to mucus that coats apical surface, preventing antibody from being washed away 5. Residual membrane-bound fragment of pIgR receptor is nonfunctional & degraded  HOW DO ANTIBODIES PROTECT THE BODY? EFFECTOR FUNCTIONS!  IMMUNOGLOBULINS  Antibodies recognize and bind to microbial epitopes: 1. to prevent microbial spread by immobilization (agglutination) 2. prevention of microbial attachment to host cells (neutralization) 3. promotion of microbial phagocytosis and clearance (opsonization) 4. targeting of microbial destruction by soluble molecules (complement) or by leukocytes Figure 4.30 Each human immunoglobulin isotype has specialized functions correlated with distinctive properties  Major effector functions of each isotype (+++) are shaded in dark red; lesser functions (++) are shown in dark pink, and minor functions (+) in pale pink  Opsonization refers to the ability of the antibody itself to facilitate phagocytosis  Antibodies that activate complement system indirectly cause opsonization via complement -Properties of IgA1& IgA2 are similar & combined under “IgA” *IgG2 acts as an opsonin in the presence of one genetic variant of its phagocyte Fc receptor, which is found in ~50% of people of European origin. 1. AGGLUTINATION AGGLUTINATION CAN HELP PREVENT PATHOGENS FROM SPREADING VIA IMMOBILIZATION. FREQUENTLY SEEN WITH IGM BECAUSE IT HAS MORE BINDING SITES. 2. NEUTRALIZATION  Antibody effector functions 9-15 High-affinity neutralizing antibodies prevent viruses and bacteria from infecting cells Figure 9.23 Viral infections are blocked by neutralizing antibodies Vaccine Twin Study  Effect of exposure to this year’s influenza virus between vaccinated & unvaccinated  Vaccinated made neutralizing anti-influenza IgA antibodies  Unvaccinated has no anti-influenza antibodies  To replicate itself, influenza uses its hemagglutinin protein to bind to sialic acid attached to human cell-surface proteins  Internalization of virus, with subsequent fusion of viral & endosomal membranes, releases viral RNA into cytoplasm, where replication occurs  Neutralizing antibodies stop infection by blocking viral hemagglutinin’s binding site for sialic acid *Because influenza infects epithelial cells of respiratory tract, effective antibodies are IgA dimers (shown here) Figure 9.24 Disease-causing bacterial infections at mucosal surfaces are prevented by neutralizing antibodies Pharynx of child who has previously had ‘strep throat’ & is making neutralizing IgA against Streptococcus pyogenes vs. Strep naïve child without neutralizing IgA  Antibodies coat the bacteria & impair capacity of protein F of bacteria to bind to cilia or attach to fibronectin in the extracellular matrix, preventing bacteria from remaining in the pharynx  Keeps bacteria to volume that doesn’t cause disease  Without antibodies, bacterial population is not controlled; it can expand, causing damage to mucosal surface & inducing inflammation  Triggers primary immune response that produces neutralizing antibodies against S. pyogenes  Antibody effector functions 9-16 High-affinity IgG and IgA antibodies neutralize microbial toxins and animal venoms Figure 9.25 Many common diseases are caused by bacterial toxins  Several examples of bacterial exotoxins are shown here Figure 9.26 Neutralization of toxins by IgG antibodies protects cells from toxin action  Protein toxins produced by many bacteria comprise two functional models 1. Binds to a component at surface of a human cell, which allows toxin to be internalized 2. A poison that interferes with a vital function of the cell  High-affinity neutralizing IgG antibodies cover up binding site in toxin’s first module, preventing its attachment to human cells 3. OPSONIZATION Figure 4.3 The Y-shaped immunoglobulin molecule can be dissected using a protease  Remember Fc fragment & Fc region??  Fc fragment- a proteolytic fragment of an antibody that consists of the carboxy-terminal halves of the two heavy chains disulfide bonded to each other by the residual hinge region  An Fc receptor is a cell surface for the Fc region of an immunoglobulin isotype.  There are different Fc receptors for the different isotypes & subtypes  E.g. FcγRI (that binds IgG) & Fcε (that binds IgE) HOW DO CELLS KNOW TO BIND ONTO ANTIBODIES THOUGH?  Fc receptors!  Fc Receptors are proteins found on surface of leukocytes that are made to recognize antibodies  They contribute to protective functions of immune system, by binding to antibodies that are attached to infected cells or invading pathogens Figure 9.37 Comparison of structure and cellular distribution of the Fc receptors for IgG, IgE, and IgA  There are FC receptors unique for every single immunoglobulin molecule & their specificity is identified by the Greek letter after “Fc,” e.g. Fcγ ←Some examples & where they’re found  Fc receptors for IgG & IgE are related in structure & encoded by closely linked genes  Fc receptor for monomeric IgA belongs to a different family than Fc receptors for IgG & IgE, despite having a similar structure  Antibody effector functions 9-21 Fcγ receptors enable effector cells to bind IgG and be activated by IgG bound to pathogens Figure 9.33 FcγRI on myeloid cells is an Fc receptor that binds with high affinity to IgG1 and IgG3  IgG-binding function of FcγRI is a property of 3 extracellular, immunoglobulin-like domains of receptor α chain  Signaling function of receptor is property of γ chain, which forms a homodimer  Enables IgG bound to FcγRI at surfaces of macrophages, dendritic cells, & neutrophils to trap pathogens & target them for uptake Of Fc receptors for IgG, only FcγRI binds to IgG in absence of & degradation antigen, as shown here for IgG3 Figure 9.34 Fc receptors on phagocytes trigger the uptake and breakdown of antibody-coated pathogens  Specific IgG molecules bind their antigens pathogen’s surface & expose their Fc regions  Fc receptors of phagocytic cells bind to Fc regions, tethering bacterium to phagocyte’s surface  Signals from Fc receptors enhance both phagocytosis & subsequent destruction of bacterium in lysosomes  Fc receptors provide a mechanism for eliminating pathogens that have been opsonized with a coating of specific antibody  These Fc receptor–mediated mechanisms work together with complement receptors (CR1 & CR2) that recognize C3 fragments that become attached to pathogens after classical pathway of complement activation PHAGOCYTOSIS  https://digital.wwnorton.com/immunesystem5  Antibody effector functions 9-23 An Fc receptor also acts as an antigen receptor for NK cells Figure 9.36 The Fc receptor expressed by NK cells recognizes IgG-coated tumor cells and signals them to die Illustrated here by use of anti-CD20 IgG mAbs as a common & effective therapy for certain types of B-cell tumor  When therapeutic antibodies bind to CD20 antigen on tumor cells, FcγRIII receptors on NK cell bind to Fc regions of cell-bound IgG  Multiple interactions between IgG & FcγRIII molecules establish stable binding between NK cell & its target  Signals from FcγRIII activate NK cell to form a conjugate pair & synapse with tumor cell  By secreting contents of its lytic granules on tumor cell surface, NK cell condemns tumor cell to die by apoptosis 4. ACTIVATING COMPLEMENT PATHWAY  Antibody effector functions 9-17 Binding of IgM to antigen on a pathogen’s surface activates complement by the classical pathway Figure 9.27 Classes and subclasses of antibodies differ in their capacity to activate and fix complement  IgM & IgG3 isotypes most effective at activating complement cascade  IgG1 is the runner-up Figure 9.28 Binding of IgM to antigen on a pathogen’s surface initiates the classical pathway of complement activation b  When soluble pentameric IgM in ‘planar’ conformation has established multipoint binding to antigens on a pathogen surface, it adopts ‘staple’ conformation & exposes its binding sites for C1q component of C1  Activated C1 then cleaves C2 & C4, & C2b and C4b fragments form classical C3 convertase on pathogen surface  Conversion of C3 to C3b leads to attachment of C3b to pathogen surface & recruitment of effector functions  Multiple copies of C3b bound to C2bC4b on pathogen surface act as C5 convertase of classical pathway, leading to formation of membrane-attack complex (MAC) & lysis of pathogen Figure 9.29 A bird’s-eye view of the fixation of C4b and C3b fragments on a pathogen surface around an antigen:antibody complex b 1. Antibody bound to antigen on a microbial surface binds C1 2. Leads to deposition of C4b (pink circles) around antigen:antibody complex 3. When C4b binds C2b to form classical C3 convertase, a limited # of C3b molecules (green rectangles) are produced 4. These can bind Bb to form alternative convertase, C3bBb (yellow rectangles), which leads to deposition of many more C3b fragments on microbial surface (green rectangles)  CLASSICAL PATHWAY OF COMPLEMENT ACTIVATION  Classical pathway of complement- activated by conformational changes that occur in antibodies when they bind to epitope  Antibodies (usually IgM & IgG) facilitate sequential binding of C1, C4, C2, & C3 components  Results in production of C3b, a “sticky” fragment of C3 that readily binds to surfaces (of cells, microbes, or particles) as a highly effective opsonin (aka induces opsonization)  Release of small pro-inflammatory fragments such as C5a, C4a, and C3a, & assembly of membrane attack complex  Antibody effector functions 9-19 Complement activation by IgG requires the participation of two or more IgG molecules Figure 9.31 At least two molecules of IgG bound to pathogens or soluble antigens are required to activate the complement cascade Activation of complement by IgG bound to antigens on a pathogen surface  C1q molecule needs to find pathogen- bound IgG molecules that are close enough to each other for C1q molecule to span them  Activation of complement by C1q binding to two IgG molecules in a soluble immune complex 5. LAGNIAPPE (AKA EXTRA)  Antibody effector functions 9-13 IgE provides a mechanism for rapid ejection of parasites and pathogens from the body Figure 9.19 Cross-linking IgE on mast-cell surfaces leads to the rapid release of mast-cell granules containing inflammatory mediators  Mast cells have numerous granules containing inflammatory mediators, e.g. histamine & serotonin  Mast cells have high-affinity Fc receptors (FcεRI) on their surface that are occupied by IgE molecules in the absence of antigen  When antigen cross-links the complexes of IgE & FcεRI, the mast cell is activated, leading to degranulation & release of inflammatory mediators into surrounding tissue Figure 9.20 Eosinophil attack on a schistosome parasite is facilitated by specific antibody binding to parasite surface antigens Schematic showing eosinophil binding to a schistosome larva via IgE antibodies bound to antigens on parasite surface  Fcε receptors on eosinophils bind to Fc portion of IgE  Fcε receptors transduce signals that activate eosinophils to secrete contents of their granules directly onto the parasite’s surface HOW DO OUR BODIES GET RID OF EXTRA ANTIBODIES?  Antibody effector functions 9-20 Erythrocytes facilitate removal of immune complexes from the circulation Figure 9.32 Erythrocyte CR1 helps to clear immune complexes from the circulation  Small soluble immune complexes bind to CR1 on erythrocytes, which transport them to liver & spleen  Here they are transferred to CR1 of macrophages & taken up for degradation