B Cell Activation PDF

WELCOME! BMS 545 IMMUNOLOGY OCTOBER 30, 2024 ANNOUNCEMENTS  Halloween costume contest Friday, November 1st, 12 pm in LH2  OBJECTIVES  What is required for B cell activation?  Describe the steps of B cell activation with antigen (hint: the brief intracellular stuff)  Define the components...

WELCOME! BMS 545 IMMUNOLOGY OCTOBER 30, 2024 ANNOUNCEMENTS  Halloween costume contest Friday, November 1st, 12 pm in LH2  OBJECTIVES  What is required for B cell activation?  Describe the steps of B cell activation with antigen (hint: the brief intracellular stuff)  Define the components of the B cell co-receptor & how it functions  Compare & contrast B cell activation from surface bound antigen & soluble antigen  List the steps of naïve B cell activation by a TFH  Compare & contrast T-dependent and T-independent activation of B cells  Describe how TFH activate B cells  Elaborate on the primary and secondary focus of B cell expansion. What are they, what are their “focuses,” & why? What immune location do they occur in?  What is the consequence of being unable to form germinal centers?  List the cytokines that drive B cell activation, differentiation, and isotype switching  Identify where B cells undergo somatic hypermutation and affinity maturation after B cell activation.  Describe the location, histology, composition, and function of the lymph node and germinal centers  What is the consequence of somatic hypermutation and affinity maturation and how does it occur?  Antibody production by B lymphocytes 9-1 B-cell activation requires cross-linking of the B-cell receptor Figure 9.1 Cross-linking of B-cell receptors by antigens initiates a cascade of intracellular signals (Part 1)  The B-cell receptor (BCR) complex on a mature naive B cell is composed of monomeric IgM that binds antigen (Ag) & associated Igα (blue) & Igβ (orange) chains for intracellular signaling  Remember naïve? Means it has not encountered its antigen yet Example: 1. Multivalent antigen with regularly arrayed epitopes on bacterial surface to which IgM binds 2. Once IgM binds, cross-linking & clustering of antigen receptors occurs 3. Receptor-associated tyrosine kinases Blk, Fyn, & Lyn phosphorylate tyrosine residues in ITAMs of the cytoplasmic tails of Igα & Igβ Figure 9.1 Cross-linking of B-cell receptors by antigens initiates a cascade of intracellular signals (Part 2) Example continued: 4. Syk binds to phosphorylated ITAMs of Igβ chains  Because Igβ chains are close to each other within the cluster, they activate each other by transphosphorylation, leading to further signaling 5. Intracellular signals are relayed to nucleus of B cell, where they induce changes in gene expression that initiate B-cell activation  Antibody production by B lymphocytes 9-2 B-cell activation requires signals from the B-cell co-receptor Figure 9.2 Structure and function of the B-cell co-receptor 1 2 3 1. B-cell co-receptor is composed of CR2, CD19, & CD81 protein subunits  CR2- Complement Receptor 2 for complement fragments iC3b & C3d fixed on pathogen surfaces; Extracellular portion is a long flexible tail composed of a series of 16 CCP structural modules  CD19- cooperates with antigen receptor to produce activating signals when antigen is bound  CD81- brings CD19 to surface & organizes the B-cell co-receptor within membrane Figure 9.2 Structure and function of the B-cell co-receptor 1 2 3 2. B cells carry both CR1 & CR2  On binding to C3b fragments on pathogen’s surface, CR1 facilitates their cleavage by factor I, which forms the iC3b intermediate & then forms C3d  In this sequence of reactions, CR1 acts as a cofactor for CR2 because it facilitates the production of C3d, the ligand for CR2 3. The CR2 component of B-cell co-receptor binds to C3d on the pathogen surface Figure 9.3 Signals generated from the B-cell receptor and B-cell co-receptor combine to activate B cells in response to surface and soluble antigens  (L) Interaction of BCR with surface antigens: 1. Binding of BCR to regularly arrayed epitopes on pathogen clusters BCRs & brings them close to B- cell co-receptors, where CR2 binds to C3d on pathogen 2. Cytoplasmic tail of CD19 is then phosphorylated by tyrosine kinases associated with BCR 3. Phosphorylated CD19 generates intracellular signals that synergize with those from BCR  (R) Interaction of BCR with soluble antigens:  If soluble antigen is tagged with C3d it cross-links BCR & co-receptor, increasing strength & efficiency of intracellular signaling  Because CR2 is more elongated & flexible than IgM, it facilitates formation of cross-links with a variety of antigens of different shapes & sizes  Antibody production by B lymphocytes 9-3 Effective B cell–mediated immunity depends on help from CD4 TFH cells In primary immune responses, the activation of most naïve B cells requires conjugation with CD4 TFH cell that recognizes pathogen- derived peptides resented by MHC class II on the B cell Figure 8.28 Activation of a naive B cell by a TFH cell 1. Naive B cells bind specific antigen with its BCR 2. The antigen is internalized by receptor-mediated endocytosis & phagocytosis & processed 3. Peptides from antigen are presented on B cell’s MHC class II molecules 4. TFH cell specific for the pMHC complex presented by the B cell forms a cognate pair with the B cell 5. Cognate interaction leads to expression of CD40 ligand (CD40L) on T cell, which interacts with CD40 on the B cell 6. Triggers secretion of the cytokines IL- 4, IL-5, & IL-6 by TFH cell, activating B cell WHAT IS A COGNATE PAIR?  A cognate pair is a CD4+ effector T cell bound via its antigen receptor to its target cell (in this case, a B cell)  Also used to describe pairs of interacting cells, one of which is a lymphocyte  AKA “conjugate pair” Figure 9.14 Normal lymph node compared with that from a person with CD40 ligand deficiency  A person without CD40L is unable to form germinal centers  Known as CD40 ligand deficiency  Light micrograph of a section through a normal lymph node  Note the presence of prominent germinal centers (GC, germinal center)  A section through a lymph node from a person with CD40 ligand deficiency  Germinal centers are absent  Antibody production by B lymphocytes 9-3 Effective B cell–mediated immunity depends on help from CD4 TFH cells In primary immune responses, the activation of most naïve B cells requires conjugation with CD4 TFH cell that recognizes pathogen-derived peptides resented by MHC class II on the B cell HOWEVER….  Figure 9.4 The signals generated by B-cell receptors and co-receptors are sufficient to activate a minority B-cell population (without T cell activation) (TI) *2nd panel more detailed version of the first panel T-Cell Independent Activation (Predominantly in B-1 cells):  The epitopes usually occur in dense & regular arrays on pathogen surfaces which produces dense clustering of BCRs & co-receptors at B-cell surface which generates sufficient signaling to stimulate B-cell proliferation & differentiation without T cells  Because response to these antigens does not require help from T cells, they are known as Thymus- independent antigens (TI antigens)  Antibody production by B lymphocytes 9-4 Follicular dendritic cells in the B-cell area store intact antigens and display them to B cells Figure 9.5 The dendrites of follicular dendritic cells take up intact pathogens and antigens and preserve them for long periods 1. You can’t tell me science isn’t cool… I mean LOOK AT THIS. 2. Scanning electron micrograph of a Follicular Dendritic Cell with a well-defined cell body & numerous dendritic processes with thread-like or filiform appearance When pathogens and antigens are bound to complement receptors on the FDC surface they become clustered, forming prominent beads at intervals along the dendrites Figure 9.6 Naive B cells recognize antigens captured by subcapsular sinus macrophages & follicular dendritic cells  Antigens tagged with C3d are brought to lymph node from the infected tissue via the afferent lymph  By binding to C3d, CR2 on subcapsular sinus macrophages & follicular dendritic cells (FDCs) tethers antigen to the cell surface, where it can be screened by naive B cells arriving either from blood via a HEV or from the afferent lymph  Antibody production by B lymphocytes 9-5 Antigen-activated B cells move close to the T-cell area to find a TFH cell Figure 9.7 B cells activated by antigen in the B-cell area move to the area boundary, where they are helped by antigen-specific effector TFH cells coming from the T-cell area Figure 9.8 TFH cells help antigen-activated B cells through cell-surface interactions between CD40L & CD40 & by the targeted delivery of secreted cytokines to the B-cell surface  Top: antigen-specific B & TFH cells forming a cognate pair & a synapse that is stabilized by strong adhesive interactions between TFH-cell LFA-1 & B-cell ICAM-1  TFH cell then synthesizes CD40 ligand (CD40L)  MTOC- microtubule-organizing center  Center: interaction of B-cell CD40 receptor with TFH-cell CD40 ligand is accompanied by reorientation of the cytoskeleton, via talin protein (red) & MTOC  Bottom: soluble cytokines (green) are synthesized in cytoplasm & translocated to endoplasmic reticulum of TFH cell  Cytokines are carried in exocytic vesicles through Golgi apparatus & secreted, by vesicle fusion with plasma membrane, onto a localized area of B-cell surface  Antibody production by B lymphocytes 9-6 The primary focus of clonal expansion in the medullary cords produces plasma cells secreting IgM Figure 9.9 B cells activated by antigen and T-cell cytokines differentiate into plasma cells in two waves, occurring at different sites in the lymph node 1. Cognate pairs of mutually activated antigen-specific B cells & TFH cells move from boundary region to medullary cords, proliferate & form large colonies  Some members of each clone of cognate pairs differentiate into plasma cells secreting antigen-specific IgM, while the other cognate pairs return to the primary follicle in the cortex  This is primary focus of B-cell expansion 2. Returning cognate pairs proliferate in the follicle forming a germinal center  A germinal center is where B cells undergo affinity maturation & isotype switching of their BCRs  This is secondary focus of expansion  Antibody production by B lymphocytes 9-7 Somatic hypermutation and isotype switching occur in the specialized microenvironment of the primary follicle, known as the germinal center Figure 9.11 Anatomy of the germinal center: the place where activated B cells undergo affinity maturation and isotype switching (Part 1)  A schematic diagram showing distribution of cells within a germinal center & location of germinal centers within a lymph node (inset)  Centroblast- Rapidly dividing B cells forming dark zone of center  Centrocytes- mature non-dividing B cells that interact with FDC’s forming light zones *Follicular dendritic cell = FDC Figure 9.11 Anatomy of the germinal center: the place where activated B cells undergo affinity maturation and isotype switching (Part 2)  A section of a germinal center that has been stained with fluorescent antibodies & roughly corresponds to schematic diagram on previous slide  Rapidly dividing B cells (centroblasts) form “dark zone” of germinal center  As they mature, centroblasts stop dividing & become small centrocytes that interact with antigen-bearing FDCs to form the “light zone”  Centroblasts = green stain  FDCs = red stain  T cells = blue stain  Smaller number of T cells in light zone are TFH (which activate B cells) Figure 9.11 Anatomy of the germinal center: the place where activated B cells undergo affinity maturation and isotype switching (Part 3)  Antibody production by B lymphocytes 9-8 Antigen-mediated selection of centrocytes drives affinity maturation of the B-cell response in the germinal center Figure 9.12 After undergoing somatic hypermutation, centrocytes with high-affinity antigen receptors are rescued from apoptosis (Part 1)  In germinal center, TFH cells induce dividing centroblasts to undergo somatic hypermutation (they then become centrocytes)  Centrocytes expressing mutant IgM then test their BCRs against intact antigens displayed by FDCs  L: Centrocytes whose BCRs have acquired mutations that REDUCE affinity for antigen are induced to die by apoptosis  R: Centrocytes whose BCRs have INCREASED affinity for antigen are induced to express Bcl-xL (become plasma cells)  BCL-xL- an intracellular protein that prevents apoptosis & ensures cell’s survival Figure 9.12 After undergoing somatic hypermutation, centrocytes with high-affinity antigen receptors are rescued from apoptosis (Part 2)  L: Centrocytes whose BCRs have acquired mutations that REDUCE affinity for antigen are induced to die by apoptosis  R: Centrocytes whose BCRs have INCREASED affinity for antigen are induced to express Bcl-xL (become plasma cells)  BCL-xL- an intracellular protein that prevents apoptosis & ensures cell’s survival  Antibody production by B lymphocytes 9-9 Cytokines made by TFH cells guide B-cell switching of immunoglobulin isotype Figure 9.13 Different cytokines induce B cells to switch to different immunoglobulin isotypes  IL-21- the cytokine considered to be the most potent inducer of terminal B-cell differentiation in humans  *IL-10 & IL-21 also help drive to plasma or memory (see slide 35)  Yes, please know these… But notice how they are similar & often overlap… I will be generous with this question  Antibody production by B lymphocytes 9-10 TFH cells also determine the differentiation of antigen-activated B cells into plasma cells or memory cells Figure 9.15 Cytokines made by TFH cells enable centrocytes to differentiate into either plasma cells or memory B cells  IL-10 & IL-21 drive centrocytes (B) to differentiate into antibody-producing plasma cells or into memory B cells at different stages of the immune response  ALSO A SCIENTIST  Ellen Vitetta, M.D.  Professor of Microbiology & Immunology, Director of the Cancer Immunobiology Center, & the Sheryl Simmons Patigian Distinguished Chair in Cancer Immunobiology at the University of Texas Southwestern Medical Center  Published over 500 papers, coinventor on 24 issued patents, one of the top 100 most cited biomedical scientists in the world  Co-discoverer of IL-4, her group demonstrated that IL-4 was a “switch” factor for Ig on B cells  Has developed antibody-based “biological missiles” to destroy cancer cells and cells infected with HIV  Developing vaccines to protect against agents of biological warfare  1st female biomedical scientist from Texas ever elected to the National Academy of Ellen Vitetta, M.D., Jonathan Uhr, M.D. Sciences and laboratory assistant