B Cell Activation and Function PDF

B Cell Activation and Function Reading assignment and LOs Overview of B Cell Activation Before We Get Into the Details! Review of Previous Content B Lymphocytes and Plasma Cells B Cell Plasma cell The job of the B cell is to make and secrete antibodies. It...

B Cell Activation and Function Reading assignment and LOs Overview of B Cell Activation Before We Get Into the Details! Review of Previous Content B Lymphocytes and Plasma Cells B Cell Plasma cell The job of the B cell is to make and secrete antibodies. It does this by differentiating into an antibody-secreting plasma cell following activation by antigen Review of B Cell Maturation Review of B Cell Antigen Receptor Generation- Heavy Chain Rearrangement as Example  Review of Lymphocyte Circulation Lymphocytes develop from bone marrow stem cells, mature in the generative lymphoid organs (bone marrow and thymus for B and T cells, respectively), and then circulate through the blood to secondary lymphoid organs (lymph nodes, spleen, mucosal lymphoid tissues). Fully mature T cells leave the thymus, but immature B cells leave the bone marrow and complete their maturation in secondary lymphoid organs. Naive lymphocytes may respond to foreign antigens in these secondary lymphoid tissues or return by lymphatic drainage to the blood and recirculate through other secondary lymphoid organs.  Antibodies Proteins secreted by plasma cells Bind to and aid in disposal or inactivation of antigens Fab 4 polypeptide chains composed of 2 heavy and 2 light chains Parts of the antibody molecule: Heavy chains Determines class of antibody molecule IgA, IgG, IgD, IgM, IgE chains designated as a, g, d, m, e Light chains kappa and lamda (k) (l) Fc Antigen-binding site Exquisite specificity! Hypervariable regions Fc Fab What’s an Antigen? Molecule recognized by specific receptors on T or B cells ; may be microbial or otherwise foreign to the body, Host cells neoplastic or normal host cell in origin Pathogen proteins, Animal dander carbohydrates and lipids Pollen Viral proteins  More Definitions T-dependent antigens Require T cell help for B cells to respond Multiple distinct epitopes Proteins T-independent antigens No T cell help is required for B cells to respond Repeating units of identical epitopes Polysaccharides, lipids  T-dependent (Td) vs T-independent (Ti) B cell activation A bacterium displaying both Td and Ti antigens Flagella antigens Capsule antigens What are epitopes? = multiple sites on a single protein where different antibodies can bind (example HA antigen of Influenza Virus) What’s an Opsonin? A substance that coats an antigen to increase effectiveness of phagocytosis Which would I rather eat- the opsonized or the non-opsonized? =  Opsonization Facilitates Phagocytosis Opsonins include C3b- a component of the complement system IgG antibody Phagocytes express receptors for opsonins Review Structure of the Lymph Node Steps in B Cell Activation by T—dependent Antigens (example- in lymph nodes) Find the slides in this ppt that correspond to these steps 1. Mature, naive B and T cells recirculate through secondary lymphoid organs in search of the antigen to which they are specific 2. T and B cells enter the lymph nodes through the venules, and cross into the node itself across high endothelial venules and migrate to their respective regions- T cells to paracortes and B cells to the follicles in the cortex 3. Dendritic cells capture antigens in the tissues, migrate to paracortex of draining lymph nodes, process and present antigen to T cells expressing receptors specific for the antigen(s) presented 4. Opsonized antigens are also brought via the lymphatics. As the lymph filters through the channels and parenchyma of the node, the opsonized antigens to bind to follicular dendritic cells (FDCs) within follicles of the cortex 5. B cells specific for the captured antigen interact with antigen, “pluck it off” the FDC, endocytose and process the antigen to present to T cells  Steps in B Cell Activation by T-dependent Antigens (example- in lymph nodes) Find the slides in this ppt that correspond to these steps 6. B cells specific for the captured antigen interact with antigen, “pluck it off” the FDC, endocytose and process the antigen to present to T cells; they also undergo clonal expansion 7. B cells within the follicles migrate to the margin of the follicles, while T cells that interacted with the antigen in the paracortex migrate toward the follicles 8. B cells then present the antigen to CD4 T cells. T cells assist in B cell activation via co-stimulatory molecules (ie CD40/CD40L) interactions and cytokines (many) 9. This interaction stimulates the B cell to move toward the center of the follicle, undergo further clonal expansion and to differentiate into plasma cells and memory cells (although some stay in the marginal zone, and become short-lived, IgM-secreting plasma cells 10. The interaction also stimulates B cells to undergo class switching from IgM secretion to the secretion of other classes of antibody with different function than IgM has. 11. The class of antibody that is switched to depends on the cytokines to which the B cell is exposed during activation and clonal expansion 12. Affinity maturation occurs at this time, as somatic mutations in the variable genes of the B cell receptor lead to stronger binding to the antigen still held on the membrane of FDCs. 13. Plasma cells produced from affinity matured B cells migrate to the bone marrow where they continue to produce antibodies for months to years, even after clearance of the antigen, thus providing immediate protection should the pathogen be encountered again B Cell Activation- How and Where Does It Happen? Example- lymph nodes  Antigen Presentation to T and B Cells in the Lymph Node Antigens are brought to lymph nodes via lymphatics T and B cells enter lymph nodes via arterial blood supply, cross from blood into node across high endothelial venules B cells home to cortex due to chemokines secreted by follicular dendritic cells; T cells home to paracortex due to chemokines secreted by fibroblastic reticular cells Antigen is presented to T cells by dendritic cells in the paracortex; B cells bind to antigen bound to membranes of follicular dendritic cells  What is a follicular dendritic cell? Dendritic cell found in follicles of lymph node and other secondary lymphoid tissue Express abundant FcR and Complement receptors (for C3b) on their membranes Capture opsonized antigens filtering through lymph nodes and hold them for prolonged period for interaction with antigen-specific B cells Antigen delivery to follicles for interaction with B cells  B Cell Interaction With Antigen Antigen B cell Ag receptor B Cell Lets back up a moment and see what happens immediately following B cell interaction with antigen on FDC: BCR-Mediated Signal Transduction Role of Complement in B Cell Activation Consequence of BCR-Mediated Activation Antigen presentation to CD4 T cells by B cells Molecules involved in T/B cell interaction for B cell Activation T Cell Help for B Cell Activation The Germinal Center Reaction  Class Switching in activated B cells Isotype (=Class) switching takes place on the edge of the follicles and within the germinal center. It is stimulated via CD40/CD40L interactions between B and T cells This interaction “tells” the B cells they “need” to switch to another heavy chain class Which class they switch to is directed by cytokines Affinity Maturation in Antibody Responses As clonal expansion occurs in germinal center, somatic mutations occur in variable genes of B cell recptor B cells within germinal center continue to “test” their antigen receptors for the strongest fit for the antigen Those with strongest fit survive and multiply, those with weaker fit undergo apoptosis Long-lived plasma cells Plasma cells produced from affinity matured B cells migrate to the bone marrow where they continue to produce antibodies for months to years, even after clearance of the antigen, thus providing immediate protection should the pathogen be encountered again Characteristics and Functions of Antibody Molecules  B Lymphocytes and Plasma Cells B Cell Plasma cell The job of the B cell is to make and secrete antibodies. It does this by differentiating into an antibody-secreting plasma cell following activation by antigen Plasma cells secrete 100’s of antibody molecules per second, at least initially What do these antibodies do? Structure of Antibody Classes Primary and Secondary Antibody Responses Antibody Levels in Fetus, Newborn and Adult During fetal and early neonatal life, circulating antibody is acquired from the mother though transfer across the placenta and from breast milk. The newborn starts producing antibody at about 6 months of age. This explains the increased incidence of infection in newborns and infants. Total IgG refers to IgG transferred from the mother and that produced in the newborn. Normal adult levels of immunoglobulin classes are IgA 70–400 mg/dl, IgG 700–1600 mg/dl and IgM 40–230 mg/dl Overview of Effector Functions of Antibodies Neutralization of microbes and toxins Antibody-mediated opsonization Antibody-mediated cellular cytotoxicity Complement in the Humoral Immune Response Activation Pathways Effector Functions of Complement  Role of Secretory IgA in Defense IgA is secreted by plasma cells which home to mucosal sites IgA is a dimer (2 chains connected by a J (joining) chain. It binds to a receptor on basal membrane of mucosal epithelial cell and is endocytosed by the cell It is transported across cell and deposited in lumen carrying with it a piece of the receptor, now referred to as secretory piece. Secretory piece helps protect IgA from degradation in secretions IgA provides immune exclusion- Immune exclusion is a process that uses secretory IgA (SIgA) to prevent microorganisms and antigens from accessing mucosal membranes. Mast Cells and Basophils by IgE Mast cell or basophil sensitized with IgE IgE is secreted by plasma cells following Ag (allergen) activation of B cell IgE attaches to mast cell or basophils’s FcRε Mast Cells and Basophils by IgE Cross-linking of IgE following re-exposure to allergen leads to…… Mast Cells and Basophils by IgE ….degranulation with release of histamine and other mediators Histamine causes increased vascular permeability, smooth muscle contraction Role of IgE in Host Defense Eosinophil FcR IgE Parasitic worm (Trichinella spiralis) Role of Eosinophils in Host Defense Role of Eosinophils in Host Defense Function of IgE in Atopy Summary of Functions of Antibody Complement Activation IgM, IgG Opsonization IgG Neutralization of toxins or viruses Antibody-dependent IgG, IgM, IgA cytotoxicity Sensitization of mast cells IgG, IgE IgE Protection of fetus IgG Protection of newborn intestine IgA  Summary of Characteristics and Functions of Antibody Classes IgM Pentamer- 5 monomers held together by J (joining chain) Complement activation Neutralization of viruses and toxins First antibody produced by infants First Ab produced in primary immune response IgG Opsonization Neutralization of viruses and toxins Complement activation Cross placenta to protect fetus ADCC Longest half-life IgA Dimer with secretory piece Protection at mucosal surfaces by binding to pathogens to prevent entrance into body proper Protection of newborn via presence on breast milk Short half-life Slow to develop in children (~ 2yrs to reach adult levels) IgE Protection against helminth parasites through ADCC and participation of eosinophils IgD B cell antigen receptos