B-Cell Mediated Immunity II PDF

11/26/2023 B Cells: An Overview generate unique B cells circulate B cells through bod...

11/26/2023 B Cells: An Overview generate unique B cells circulate B cells through body Adaptive Immunity: each with a unique BCR blood, lymph, 2° lymphoid tissues eliminate self-reactive B cells activate antigen-specific B cells B-Cell Mediated Immunity II negative selection proliferate into clone produce mature B cells differentiate activated B cells positive selection plasma cells & memory B cells MCB 11338 – Immunology LS Meadows Immature to Mature B Cell: A Recap LECTURE OUTLINE immature (self-tolerant) B cell [IgM+, IgD-] I. Overview VI. B Cell Activation leaves BM, enters blood, begins expressing both IgM and IgD BCRs II. B Cell Receptor Diversity A. Major Events enters 2° LT, interacts with FDC == becomes mature (naïve) B cell B. BCR Complex III. B Cell Development C. FDC Interactions mature B cell [IgMlow, IgDhigh] circulates through blood, lymph, 2° LT in search of its specific antigen IV. B Cell Maturation D. TFH Interactions V. B Cell Abnormalities E. Clonal Expansion VI. B Cell Activation F. Somatic Hypermutation G. Affinity Maturation VII. Antibody Structure & H. Isotype Switching Function I. Memory B Cells VIII. Antibody Receptors J. B-1 Cells When Specific Antigen Encountered... Newly Activated B Cells: Two “Fates” B cell forms cognate pair some proliferate in medullary with ag-specific effector TFH cords of LN or spleen Primary Clonal Expansion cell differentiate into IgM-secreting previously activated by DC plasma cells cognate TFH cell other cognate “B-T” pairs secretes cytokines to activate migrate to 1° follicle to form 2° B cell follicle containing a germinal center Secondary Clonal Expansion activated B cell proliferates & activated B cells differentiate into differentiates lymphoblasts 1  11/26/2023 Plasma Cells Lymphoblasts (Centroblasts) stop dividing; stop expressing large, proliferating B cells surface IgM & MHC II mature into slowly dividing unresponsive to antigen centrocytes do not interact with T cells centrocytes some migrate to bone marrow undergone isotype switching & major site of antibody production affinity maturation high affinity BCRs selected by antigen some migrate to MALT divide to form more lymphoblasts site of mucosal antibody migrate to BM or 2° LT == plasma production cells secreting high affinity antibodies PRIMARY IMMUNE (ANTIBODY) RESPONSE Memory B Cells isotype switched, high affinity BCRs formed in germinal centers as primary antibody response subsides permanent part of B cell repertoire persist throughout lifetime only require intermittent stimulation in lymphoid follicle easily activated to rapidly form plasma cells secrete more IgG than IgM SECONDARY IMMUNE (ANTIBODY) RESPONSE Figure 6.23 Summary of the main stages in B-cell development Activation of Mature (Naïve) B Cells cross-linking of B-cell receptors by specific antigen binding of B-cell co-receptor to complement fragment interaction with TFH cells 2  11/26/2023 B-Cell Receptor Complex: Two Parts Mature Naïve B Cells membrane-bound immunoglobulins IgM & IgD BCRs very short cytoplasmic segments cross-linked & aggregated by multimeric proteins or multiple identical can’t be transported directly to cell membrane epitopes on pathogen surface can’t interact with intracellular signaling proteins intracellular signals generated by Igα & Igβ using cytoplasmic tyrosine kinases (Blk, Fyn, Lyn, Syk) Igα and Igβ protein complex associates with Ig molecules destined for cell membrane required for transport of Ig to cell membrane “connect” Ig to intracellular signaling pathways Igα & Igβ B Cell Co-Receptor: Three Proteins contain ITAMS (Immunoreceptor Tyrosine-Based Activation Motifs) complement receptor 2 (CR2) phosphorylated by Blk, Fyn, or Lyn binds iC3b and C3d Syk binds P-ITAM on Igβ phosphorylated → intracellular signaling pathway initiated CD19 CD81 signaling chain CD19 CD81 brings CD19 to B-cell surface coordinates interaction of BCR & co-receptor deficiency or defective CD19 or CD81 = impaired antibody-mediated immunity Complement Receptors Generation of iC3b & C3d C3b on pathogen’s surface bound by CR1 on B cell cleaved by factor I → iC3b iC3b cleaved by factor I → C3d C3d bound by CR2 component of B-cell co-receptor 3  11/26/2023 Figure 9.2 Structure and function of the B-cell co-receptor Binding of C3d by B-cell Co-Receptor CR2 B-cell receptor & co-receptor associate CD19 phosphorylated ↑↑↑ intracellular signals Δ gene expression simultaneous ligation of BCR & co-receptor multivalent insoluble pathogen multiple molecules of soluble monovalent antigens Follicular Dendritic Cells (FDCs) Follicular Dendritic Cells (FDCs) organize B-cell areas (LN) into primary follicles use CR2 (& CR1) to extract lack phagocytic abilities antigens from lymph display bound antigens on cell long-term storage/display of intact antigens surface required for BCR recognition subcapsular sinus macs required for B cell maturation & survival poor phagocytes also capture & display intact dependent on TNF-α, LT-α, LT-β from lymphocytes antigens intact ag NOTE: Medullary sinus macrophages filter lymph (remove & destroy antigens). Naïve B Cells from Blood Naïve B Cells from Lymph attracted to T cell area by CCL21 & CCL19 survey antigens on subcapsular sinus via CCR7 macrophages first CXCL13 then attracted to primary follicle by CXCL13 then enter primary follicle to survey ag via CXCR5 on FDCs survey antigens on FDCs CCL19 if BCR binds specific antigen, cell enters CCL21 B-cell area of follicle if BCR binds specific antigen, cell enters B-cell area of follicle 4  11/26/2023 Antigen-Sensitized B Cells Antigen-Sensitized B Cells express CD69 increase expression of CCR7 prevents surface expression of S1P receptor binds CCL21 & CCL19 (sec. by LN stroma & DCs) B cells remain in LN B cell moves to boundary between B- and T-cell areas CXCL13 endocytose BCR:ag complexes interact with newly differentiated TFH cells produced by activation of naïve T cells by myeloid dendritic cells in T-cell area of LN present peptides with MHC-II CCL19 CCL21 NOTE: B cells that are not sensitized express S1P receptor & exit the LN via efferent lymphatics. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877811/ Figure 9.7 B cells sensitized 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 TFH Cells reduce expression of CCR7 express CXCR5 CXCL13 binds CXCL13 (sec. by FDC) move to boundary between B- and T-cell areas CCL19 CCL21 “screen” MHC-II:peptide complexes on B cells TFH Cells Clonal Expansion: Primary Focus form cognate pair with B cell if TCR recognizes B:TFH cognate pairs move into MHC-II: peptide complex medullary cords express CD40 ligand → binds CD40 on B cell → activates NFкB in B cell both cells divide & proliferate B cell increases expression of ICAM-1 binds to LFA-1 on TFH cell plasmablasts produced dividing B lymphoblasts immunological synapse forms secrete IgM → lymph → blood → MTOC, Golgi, talin “point” toward B cell infection site TFH cell delivers cytokines directly onto B cell surface 5  11/26/2023 Clonal Expansion: Primary Focus some plasmablasts (B lymphoblasts) remain in medullary cords differentiate into plasma cells stim. by IL-5 & IL-6 from TFH cells BLIMP1 transcription factor req. for plasma cell formation halts B lymphoblast division increases antibody production & secretion 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 Clonal Expansion: Secondary Focus other plasmablasts (B lymphoblasts) and their cognate TFH cells move from medullary cords into primary follicle both cells divide to form germinal center of secondary follicle Germinal Center Germinal Center FDCs centroblasts secrete IL-6, IL-15, 8D6, BAFF stop expressing surface Ig undergo somatic hypermutation, affinity maturation, & isotype B cells switching divide rapidly & differentiate into centroblasts large population of B cells produced with … TFH cells different C-region genes divide & use CD40L to bind to CD40 on centroblasts diverse V-region genes containing random mutations 6  11/26/2023 Germinal Center “Anatomy” Variable Domains dark zone = densely packed centroblasts hypervariable regions (HVs) divide rapidly to produce centrocytes AA loops most distal to C domains 3 per VL and VH light zone = centrocytes form antigen-binding site at end of each Fab “arm” divide more slowly determine antibody specificity express “mutated/isotype switched” surface Ig compete for access to CD40L+ TFH cells & framework regions (FRs) antigen on surface of FDCs β strands & other loops 4 per VL and VH mantle zone = naïve B cells entering LN to structurally stable search for specific antigen HVs = CDRs (complementarity-determining regions) Somatic Hypermutation Somatic Hypermutation introduction of point mutations mutations concentrated in heavy and throughout rearranged V domain light chain HV regions coding sequence affects all 3 HV regions one mutation per round of cell division 1,000,000-fold increase in typical mutation rate Somatic Hypermutation Affinity Maturation activation-induced cytidine deaminase (AID) production of antibodies of progressively higher affinity for the enzyme produced only in dividing B cells infecting pathogen converts cytosine to uracil in ssDNA during transcription DNA repair & modification enzymes occurs by selection of B cells convert uracil to another base during DNA replication expressing higher affinity BCRs produced during somatic hypermutation (SHM) 7  11/26/2023 Affinity Maturation High-Affinity BCR Centrocyte somatic hypermutation of “founder” BCRs cross-linked B cell’s receptor genes → population internalizes & presents antigenic peptides with MHC-II to of centrocytes with BCRs of low, CD40L+ TFH cell medium, and high affinity for antigen CD40L+ TFH cell recognizes MHC-II:peptide centrocytes with high-affinity binds CD40 on centrocyte BCRs “out compete” others stim. centrocyte to express Bcl-xL (anti-apoptosis protein) for antigen displayed on FDCs BCRs cross-linked by antigen → activation centrocyte → divides → plasma cells → high-affinity abs Low-Affinity BCR Centrocyte Isotype (Class) Switching BCRs not crosslinked by antigen on FDC DNA recombination events that connect the rearranged variable- region exon with different heavy-chain constant region exons centrocyte → apoptosis only occurs in proliferating, activated B cells does not internalize/present peptides with MHC-II during an active immune response cannot present to CD40L+ TFH cell Bcl-xL (anti-apoptosis protein) not expressed permits production of antibodies with different effector functions Isotype (Class) Switching Example: IgM to IgG1 requires switch sequences (S regions) During Ig gene transcription, AID highly repetitive DNA sequences flanking 5’ side of C H region genes (except Cδ) converts cytosines in Sμ & Sɣ1 regions requires 3 enzymes to uracils AID = activation-induced cytidine deaminase UNG removes the uracils UNG = uracil-DNA glycosylase APE1 removes the “base-less” nts, APE1 = apurinic/apyrimidinic endonuclease 1 creating nicks in the DNA in the Sμ & Sɣ1 regions nicks promote recombination between Sμ & Sɣ1 regions intervening DNA sequences excised 8  11/26/2023 Isotype (Class) Switching Isotype (Class) Switching can occur between Sμ and any other S region occurs in germinal center can occur sequentially (example: IgM to IgG1 to IgA1) requires CD40:CD40L interaction pattern of switching controlled by cytokines from effector TFH cells cytokine secretion determined by type of infection DC that activated the TFH cell IL-21 = most important inducer of terminal B-cell differentiation Figure 4.37 Changes in the immunoglobulin genes that occur during a B cell’s lifetime Hyper-IgM Syndrome one form caused by mutations in AID gene == immunodeficiency produce large quantities of low affinity IgM antibodies no somatic hypermutation no isotype switching inc. susceptibility to infections with pyogenic bacteria esp. sinuses, ears, & lungs prevented by injections of intravenous immunoglobulin Figure 9.16 Comparison of key properties of naive B cells and plasma cells 9  11/26/2023 Plasma Cell versus Memory B Cell IL-21 & IL-10 during infection → plasma cells at end of infection → memory cells Co-stimulatory molecules involved (?) ICOS (also req. for TFH cells differentiation) similar to CD28 ICOS = inducible T-cell co-stimulator ICOS ligand (also expressed by DCs) similar to B7 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3991608/ CD27 not on naïve B cells B-1 Cells (CD5+ B Cells) Activation of B-1 Cells 5% of adult human B cells recognize repetitive carbohydrate or protein produced early in embryonic development epitopes capable of self-renewal (stim. by IL-10) “thymus-independent” (TI) antigens cross-link many BCRs & co-receptors not dependent on T cell help no affinity maturation; no isotype switching low affinity, IgM antibodies mainly against common bacterial polysaccharides (multivalent antigens) Figure 9.4 The signals generated by B-cell receptors and co-receptors are sufficient to activate a minority B-cell Figure 6.14 Comparison of the properties of B-1 cells and B-2 cells population BCR = IgM > IgD BCR = IgD > IgM CD5+ CD5- 10