BMS 150 Phys 3.07 - Immunology PDF

Phys 3.07 - Immunology B cell development & activation Dr. Rhea Hurnik BMS 150 Week 3 Overview B-cell development B-cell activation Location: lymph node Histology Antigen recognition Activation process Differentiation into plasma cells or memory cel...

Phys 3.07 - Immunology B cell development & activation Dr. Rhea Hurnik BMS 150 Week 3 Overview B-cell development B-cell activation Location: lymph node Histology Antigen recognition Activation process Differentiation into plasma cells or memory cells Class switching & somatic hypermutation Antibodies Functions Classes Learning Objectives Describe the steps of B-cell development in the bone marrow and spleen Describe the basic histology of the lymph node and identify the cortex, paracortex, and medulla, including: their basic function and major cell types found in each. Describe the journey of a partially mature naïve B-cell to the lymph node, through to its full maturation and activation and the secretion of antibodies Describe the B cell receptor structure and how antigen-binding leads to B-cell activation Outline the process that leads to optimal B-cell activation via Th input Define somatic hypermutation and describe: its location, timing (when during the immune response it occurs), and list the basic steps. Describe the location, timing, and basic steps of antibody class switching. List the cytokines needed to inducing class switching to IgG subtypes, IgA, and IgE Define how plasma cells differ from mature B-cells in terms of structure and function Outline the general role of memory B cells and how they allow for a faster and more forceful secondary response to a repeat antigen Learning Objectives Antibodies Briefly outline the following antimicrobial mechanisms used by antibodies: ADCC, Opsonization, Agglutination, Neutralization, Complement activation/cell lysis, degranulation Outline the main functions of the following antibody classes: IgA, IgD, IgE, IgG, IgM B-cell Introduction The function of a B cell is to give rise to plasma cells that secrete antibodies capable of binding to an organism, microbe or molecule The secreted antibodies have antigen-binding sites identical to those of the receptor molecules on the B-cell surface ▪ Antibodies belong to the class of proteins known as immunoglobulins (many proteins in this superfamily) ▪ Once secreted they can protect the host against a wide variety of pathogens B cell development: review Hematopoietic Stem Cells Myeloid Cells Granulocytes Lymphoid Cells Neutrophils T cells Basophils B Cells Eosinophils NK Cells RBCs Dendritic Cells Platelets Mast Cells Monocytes Macrophages Dendritic Cells B-cell development: review Names of progenitor cells FYI https://upload.wikimedia.org/wikipedia/commons/1/1f/Hematopoiesis_%28human%29_diagram_en.svg  B-cell development: details IL-7 B cells develop first from hematopoietic IL-7 stem cells, and then into common lymphoid progenitors under the influence of IL-7 B cells are released into circulation as immature, naïve B cells. Names of progenitor cells FYI https://upload.wikimedia.org/wikipedia/commons/1/1f/Hematopoiesis_%28human%29_diagram_en.svg  B-cell development: Bone Marrow Let’s take a closer look at the Bone Marrow: ▪ B cells develop from common lymphoid precursors into Pre-pro B cells ▪ B cells then become irreversibly committed to the B-cell lineage once they reach the pro-B cell stage. Preview – it is during this stage that immunoglobulin gene recombination begins, more to come in future lectures.  B-cell development: As B cells mature, their expression of surface B-cells markers change. ▪ These markers allow for communication with stromal cells in the bone marrow Specific surface B-cells markers in boxes FYI https://upload.wikimedia.org/wikipedia/commons/5/56/Early_B_cell_development.jpg B-cell development: The B-Cell Receptor The B-cell receptor (BCR): ▪ Membrane bound antibody that associates with a disulfide-linked heterodimer (Ig- ⍺ & Ig-β) Ig- ⍺ has a long cytoplasmic tail that associated with intracellular signaling molecules Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e 5/Figure_42_02_06.jpg B-cell development Let’s take a closer look at the Bone Marrow: ▪ As the pro-B cells matures to a pre-B cell it beings to express an immature BCR (B cell Receptor) If this BCR is productive the B cell passes the pre-B cell checkpoint and becomes a pre-B cell If this BCR is unproductive the B cells will undergo apoptosis B-cell development Let’s take a closer look at the Bone Marrow: ▪ Prior to maturation into an immature B cells, the pre-B cell goes through a second checkpoint At this point the BCR is checked for self-reactivity ▪ If (+) --> Preview: Pre-B cells tries to rearrange light chain genes of the BCR and will be checked again for self-reactivity If still (+) → Pre-B cell will undergo apoptosis ▪ If (-) → Becomes a immature (aka transitional) B cell and leaves the bone marrow B-cell development: Spleen From the bone marrow immature (aka transitional) B cells travel to the spleen. ▪ Once in the spleen the immature B cell begins expressing: CD21 – the complement co-receptor IgD in addition to IgM as the isomer of the portion of the BCR ▪ From here the B-cell is considered mature but naive and enters general circulation B-cell Activation: Lymph nodes From general circulation, the B cell will migrate to a lymphoid follicle within a lymph node. ▪ Lymph nodes are bean-shape structures encapsulated with a reticular network and packed full of lymphocytes, macrophages, and dendritic cells. 3 main regions ▪ Cortex ▪ Paracortex ▪ Medulla Kuby Immunology (6th ed) Figure 2-18, page 48 B-cell Activation: Lymph nodes Cortex: ▪ Contains lymphocytes (predominantly B-cells), macrophages and follicular dendritic cells arranged into primary follicles. After antigen exposure primary follicles will enlarge into a secondary follicles with germinal centers ▪ Relevance to B-cell activation coming soon. Kuby Immunology (6th ed) Figure 2-18, page 48 B-cell Activation: Lymph nodes Paracortex: ▪ Contains mostly T-lymphocytes and dendritic cells The dendritic cells express high levels of HLA-2 molecules & present them to T-helper cells for T-helper activation and polarization Medulla ▪ More sparsely populated with lymphoid-linage cells Often plasma cells actively secreting antibody molecules. Kuby Immunology (6th ed) Figure 2-18, page 48 B-cell Activation Within a lymph node, a naïve B-cell encounters an antigen in one of two ways: ▪ 1. The antigen flows into a lymph node through afferent lymphatics and binds to the BCR ▪ 2. Macrophages or dendritic cells encounter an antigen in the periphery and bring it to a lymph node. Here they will “present” the antigen to a B cell and the antigen binds to the BCR B-cell Activation: Ag binding to the BCR The antigen binds to the BCR with the help of: ▪ B-cell co-receptor: CD21: co-receptor that binds an antigen bound to the complement component C3d CD19: signal transduction protein ▪ Initiates downstream signaling cascade promoting B cell survival & proliferation FYI – for visualization only B-cell Activation - Review Once the antigen binds the BCR on a B-cell, what happens to the antigen? ▪ Hint: how does it get into the B-cell? What does the B cell do with the antigen next? B-cell activation Once the B-cell has presented it’s antigen it will also increase its expression of co-stimulatory molecules: ▪ CD80/86 Binds to ____ on TH cells for T-cell activation ▪ iCOSL Bind to ____ on TH cells to induce cytokine productions ▪ CD40 Bind to ____ on TH cells to increased antibody production Then the B-cell will present it’s antigen to which type of T-helper cell within germinal centers of a lymphoid follicle? B-cell activation Now the B-cell and Tfh cell are locked in an immunologic synapse ▪ Co-stimulatory interaction iCOSL and iCOS will stimulate production of cytokines by the Tfh cells ▪ Under the influence of IL-21, IL-4 (by Tfh cell), and CD40- CD40L interaction, the B cell will proliferate and begin to produce antibodies Adapted from: https://upload.wikimedia.org/wikipedia/co mmons/e/e8/T- Dependent_B_Cell_Activation.png Antibody production After it’s interaction with a Tfh cell: ▪ Some B-cells will differentiate into plasma cells for antibody secretion Early primary response Initially the antibodies will be medium-affinity ▪ Some B-cells will move into the germinal centers of lymphoid follicles: Late primary response The influence of particular cytokines secreted by TH1 or TH2 cells will induce antibody class switching Somatic hypermutation will result in production of high- affinity antibodies Antibody production: class switching During antibody class switching, the original antibody (IgM) may be switched to a different class ▪ Ie. IgA, IgE, IgG etc. Antibody classes coming soon ▪ During class-switching, the specific portion of IgM will be cut off and then re-attached with a different antibody. Antibody production: class switching Different cytokines secreted by the T-helper cells will induce class switching ▪ TH1 cells secrete INF-y which stimulates class switching to IgG subtypes ▪ TGF-beta stimulates class switching to IgA ▪ TH2 cells secrete IL-4 & IL- IL-21 5, which stimulates class IL-4 switching to IgE Also induce secretion of large amounts to IgM Kuby Immunology (6th ed) Figure 11-19, page 271 Somatic Hypermutation During the late primary response some B-cell move into the germinal center of lymph nodes to undergo somatic hypermutation ▪ During somatic hypermutation the variable region of an antibody will mutate at a very high rate. Then it will be tested for antigen-binding ▪ If it binds with higher affinity it is kept ▪ If it binds with lower affinity or self the B-cell will undergo apoptosis Why? Somatic hypermutation - steps 1. The B cell starts in the dark zone of the germinal center where it will loses it’s HLA expression and mutate it’s BCR genes ▪ FYI - Enzymes: AID and DNA-polymerase-eta Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e8/T-Dependent_B_Cell_Activation.png Somatic hypermutation - steps 2. The the BCR moves into light zone of the germinal center and samples follicular dendritic cells for the antigen ▪ Self-reactivity or binding with low-affinity to the antigen results in apoptosis Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e8/T-Dependent_B_Cell_Activation.png Somatic hypermutation 2. The the BCR moves into light zone of the germinal center and samples follicular dendritic cells for the antigen ▪ If the B-cell binds with high-affinity to the antigen, it will re-present it on an HLA-2 and be re-stimulated by a Tfh cell Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e8/T-Dependent_B_Cell_Activation.png Somatic hypermutation Once re-stimulated by a T-fh cell, the B-cell will either differentiate into a plasma cell or memory B cells Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e8/T-Dependent_B_Cell_Activation.png Plasma and Memory B cell functions Following B-cell selection in germinal center for high- affinity antibodies, the B cell differentiates into either: ▪ Plasma cells No longer have BCR and secrete a large number of antibody molecules ▪ Memory B cells Circulate in the periphery: ▪ Once they re-encounter their specific antigen, they will proliferate and differentiate into plasma cells Continue to express the BCR Much longer-lived than naive B-cells Differentiation into Memory B cells is unclear Antibody structure Antibodies are protein molecules ▪ Two heavy chains ▪ Two light chains The heavy chain can be divided into a “constant” region (Fc) and “variable” (Fab)region ▪ It is the variable regions that undergoes genetic shuffling The specificity of an antibody is determined by the Fab region and the antigen- binding portion of the light chain Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e 5/Figure_42_02_06.jpg General Antibody functions There are 6 categories of antibody functions: ▪ 1. Neutralization: Antibody binds to pathogen or toxin, inactivating in and preventing binding to cells General Antibody functions There are 6 categories of antibody functions: ▪ 2. Agglutination Prevents the pathogen from binding to cells and promotes clearance of pathogen General Antibody functions There are 6 categories of antibody functions: ▪ 3. Opsonization: Antibody binds to Fc receptor and promotes phagocytosis General Antibody functions There are 6 categories of antibody functions: ▪ 4. Complement activation: a) Initiates classical pathway of complement pathway. b) Promotes phagocytosis General Antibody functions There are 6 categories of antibody functions: ▪ 5. Anti-body dependent cell mediated cytotoxicity: Antibody activates Fc receptor on Natural killer cells & inducing apoptosis of infected cell General Antibody functions There are 6 categories of antibody functions: ▪ 6. Degranulation Antibody activates Fc granulocytes triggering degranulation Antibody classes There are 5 main classes of antibodies, each with different functions. ▪ IgM, IgG, IgD, IgE, IgA Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Antibody classes: IgM Monomer found as part of the BCR First class of antibody secreted during the primary immune response ▪ Secreted as a pentamer ▪ Tends to have a lower-affinity for it’s antigen Functions: ▪ Very good at activating complement Review – what was the purpose of complement activation? ▪ Good at agglutinating pathogens Immobilizes the pathogens allowing for phagocytosis Image adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Antibody classes: IgG Most common antibody found in serum ▪ Variety of different classes – IgG1, IgG2, IgG3, IgG4 Secreted as a monomer Produced later in the humoral response Functions: ▪ Opsonization Bind the Fc receptor on phagocytes & enhances phagocytosis ▪ IgG1 and IgG3 are good at activating complement ▪ IgG2alpha is good at triggering antibody-mediated cytotoxicity by Natural Killer cells Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Antibody classes: IgA Predominantly found as a dimer secreted into tears, saliva, breastmilk, and mucus Functions: ▪ Neutralizing pathogens ▪ Agglutinating pathogens Can also be found in blood as a monomer, with similar functions to IgG. ▪ Also good at inducing degranulation of granulocytes Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Antibody classes: IgE Secreted as a monomer in small quantities Functions: ▪ Binds to cells with and Fc receptor for IgE triggering degranulation of granulocytes Eosinophils, basophils, mast cells TOP: Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg BOTTOM: Kuby Immunology (6th ed) Figure 4-16, page 94 Antibody classes: IgD Secreted as a monomer in very small quantities Most prevalent in secretions of the upper respiratory tract ▪ Binds to basophils and mast cells Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Antibody classes: summary Signals inducing class switching Secreted early or late Antibody functions Adapted from: https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_of_Antibodies_new.jpg Study Guiding Questions List the stages of B-cell development in the bone marrow. ▪ Outline the two checkpoints in the Bone marrow ▪ What is the B-cell called when it leave the Bone marrow Where is the next stage of B-cell development? ▪ What is the B-cell called when it leave? Describe the 3 signals needed for B-cell activation ▪ Decribe the two ways B-cells encounter an antigen ▪ What do they do next with the antigen? ▪ Which lymphocyte is needed for their activation? How does this cell contribute to activation Study Guiding Questions Following activation: ▪ What happens during the early primary response? What is the main class of antibody secreted Describe the affinity of the antibodies secreted? ▪ What happens during late primary response? Describe the signals needed for class switching Describe the steps of somatic hypermutation ▪ What happens withing the light vs dark zones of the lymphoid follicle What are the 6 main functions of antibodies? What are the 5 classes of antibodies? ▪ Is the class secreted early or late ▪ What is main function(s) of this antibody class References Punt et al. Kuby Immunology. WH Freeman and Company Parham P. The Immune System. Garland Science. Images Adapted from: ▪ https://upload.wikimedia.org/wikipedia/commons/1/14/2221_Five_Classes_ of_Antibodies_new.jpg ▪ Adapted from: https://upload.wikimedia.org/wikipedia/commons/e/e8/T- Dependent_B_Cell_Activation.png ▪ https://upload.wikimedia.org/wikipedia/commons/5/56/Early_B_cell_develo pment.jpg ▪ https://upload.wikimedia.org/wikipedia/commons/1/1f/Hematopoiesis_%28 human%29_diagram_en.svg