Transcript 6 B cell
DEFINITION AND PROPERTIES OF ANTIGEN • Antigen (Ag) - any substance, which is recognized by the mature immune system of a given organism – antigenicity - specific reactivity with cells or molecules of the immune system – immunogenicity - capability to elicit an immune response – tolerogenicity - capability to induce immunological tolerance ANTIGENIC DETERMINANT (=EPITOPE) part of the antigen which is recognized by a defined immunoglobulin (BCR / antibody) or by T cell receptor B cell epitope T cell epitope recognized by B cells recognized by T cells proteins polysaccharides lipids DNA steroids etc. (many artificial molecules) proteins mainly (8-23 amino acids) requires processing by APC cell or matrix associated or soluble !! !! IMMUNOGLOBULIN STRUCTURE • 2x Heavy chain (light blue) • 2x light chain (dark blue) • Variable regions antigen binding disulfide bond • Constant regions carbohydrate CL VL CH1 VH CH2 hinge region CH3 ANTIBODY DOMAINS AND THEIR FUNCTIONS !! Antigen recognition antigénkötés s s s s s s VH s s CH1 s s s s Variable domain va riábilis d om ének s s VL s ss ss s konsta ns dom ének CH2 s effektor funkc iók Effector functions CH3 ss s s s s s s s Constant domain s s CL BCR (B cell receptor) Associated chains for signaling Antibody !! Transmembrane domain Cytoplasmic domain MEMBRANE BOUND! Antigen recognition and B cell activation SOLUBLE (freely circulating) Antigen recognition and effector functions. Produced by plasma cells ANTIGEN BINDING Antigen Binding Fragment (Fab) Complement binding site Constant fragment (Fc) Binding to Fc receptors on phagocytic cells Placental transfer DIFFERENT VARIABLE REGIONS DIFFERENT ANTIGEN-BINDING SITES DIFFERENT SPECIFICITIES !! EFFECTOR FUNCTIONS OF ANTIBODIES Antibody-mediated immune responses • NEUTRALIZATION • OPSONIZATION ADCC (NK cell-mediated killing) MAST CELL DEGRANULATION • COMPLEMENT FIXATION VARIABILITY IN DIFFERENT REGIONS OF THE Ig DETERMINES Ig SPECIFICITY OR CLASS Isotype Sequence variability of H/Lchain constant regions Sequence variability of H/Lchain constant regions OPSONIZATION !! Opsonization facilitate and accelerate the recognition of the pathogen by phaogocytes, opsonins form a bridge between pathogen and a phagocyte connecting them. Main opsonins: antibodies Complement fragments Acute-phase proteins OPSONIZATION Flagging a pathogen Antigen binding portion (Fab) binds the pathogen, the Fc region binds phagocytic cells Fc-receptors speeding up the process of phagocytosis FcR Affinity for Immunoglobulin Cell Distribution Function FcγRI (CD64) High (Kd < 10-9 M); binds IgG1 and IgG3, can bind monomeric IgG Macrophages, neutrophils; also eosinophils Phagocytosis; activation of phagocytes FcγRIIA (CD32) Low (Kd > 10-7 M) Macrophages, neutrophils; eosinophils, platelets Phagocytosis; cell activation (inefficient) FcγRIIB (CD32) Low (Kd > 10-7 M) B lymphocytes Feedback inhibition of B cells FcγRIIC (CD32) Low (Kd > 10-7 M) Macrophages, neutrophils, NK cells Phagocytosis, cell activation FcγRIIIA (CD16) Low (Kd > 10-6 M) NK cells Antibody-dependent cell-mediated cytotoxicity FcγRIIIB (CD16) Low (Kd > 10-6 M); GPI-linked protein Neutrophils Phagocytosis (inefficient) FcΕRI High (Kd > 10-10 M); binds monomeric IgE Mast cells, basophils, eosinophils Cell activation (degranulation) FcΕRII (CD23) Low (Kd > 10-7 M) B lymphocytes, eosinophils, Langerhans cells Unknown !! EFFECTOR FUNCTIONS OF ANTIBODIES Antibody-mediated immune responses • NEUTRALIZATION • OPSONIZATION ADCC MAST CELL DEGRANULATION • COMPLEMENT FIXATION NEUTRALIZATION Covering of the pathogen’s surface prevents replication and growth ADAPTIVE IMMUNE SYSTEM Diversity of receptor strucure How can the antigen receptors of lymphocytes recognize extremly diverse antigens Kb. 10 – 1000 million (107 - 9) different antigen receptors, unique specificity of B cells Kb. 10 – 1000 million (107 - 9) different antigen receptors, unique specificity of T cells Random hands, millions of variations Random selection of gene segments ensures millions of different receptor (variable domains) SOMATIC REARRANGMENT OF THE HEAVY CHAIN GENE SEGMENTS 65 VH VH1 VH2 27 D VH3 D D D 6 JH D JH JH JH JH During B-cell development VH1 VH2 VH3 VH1 D D JH JH VH2 D D JH JH IMMUNOGLOBULIN CHAINS ARE ENCODED BY MULTIPLE GENE SEGMENTS ORGANIZATION OF IMMUNOGLOBULIN GENE SEGMENTS Chromosome 2 kappa light chain gene segments Chromosome 22 lambda light chain gene segments Chromosome 14 heavy chain gene segments HOW MANY IMMUNOGLOBULIN GENE SEGMENTS Gene segments Light chain kappa Heavy chain lambda Variable (V) 40 30 65 Diversity (D) 0 0 27 Joining (J) 5 4 6 VARIABILITY OF B-CELL ANTIGEN RECEPTORS AND ANTIBODIES B cells of one individual 2 3 1 4 V-Domains C-Domains VH D JH VL VH-D-JH JL VL-JL Estimates of combinatorial diversity Taking account of functional V D and J genes: 65 VH x 27 DH x 6JH = 10,530 combinations 40 Vk x 5 Jk = 200combinations 30 Vl x 4 Jl = 120 combinations = 320 different light chains If H and L chains pair randomly as H2L2 i.e. 10,530x 320 = 3,369,600 possibilities Due only to COMBINATORIAL diversity In practice, some H + L combinations do not occur as they are unstable Certain V and J genes are also used more frequently than others. There are other mechanisms that add diversity at the junctions between genes - JUNCTIONAL diversity GENERATES A POTENTIAL B-CELL REPERTOIRE Clonal expansion of B cells Forms of antibodies -memrane bound (BCR) -soluble (Plasma cells do not express antibody, but secrete it!!) Membrane bound and soulble antibodies recognize the same antigen Differenciation Plazma cell Secreted antibodies ! ! ! Several antibodies are expressed on B cells, (arround 100.000) but all of them with the same specificity Antigen recognition by specific BCR induces clonal expansion of the sepcific B cells. ! B cell Antigen receptor, BCR A n ti g e n Ag Activation Clonal expansion A n ti g e n A n ti g e n A n ti g e n Clonal antigen receptors are expressed exclusively on Tand B lymphfocyties. ! Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells. Specific B cells Non-specific B cells Activation Clonal expansion Differencaition Memory B cells Circulation Restricted lifespan (few days) Apoptosis Plasma cells Antibody production ! Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells. Plasma cells, antibody production A n ti g e n 2.Differen tiation Activation of specific B cells 1. Clonal expansion A n ti g e n MEMORY B CELLS A n ti g e n ! A n ti g e n A n ti g e n ! During the B cell development in the bone marrow, self reactive B cells are deleted Several epitops of one microbes can be recognized by different B cells POLICLONAL Response B cell repertoar Ag Specific, activated B cells Plasma cells Antigen specific antibodies Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells. A n ti g e n Activation 1. Clonal expansion Plasma cells 2.Differen tiation A n ti g e n MEMORY B cells A n ti g e n A n ti g e n A n ti g e n ! B – CELL ACTIVATION Where and how do all these things take place? B-cell recycling in the absence of antigen (lymph node) T cell area B cells in blood B cell area Efferent lymph Recirculating B cells are trapped by foreign antigens in lymphoid organs Antigen enters node in afferent lymphatic YY Y B cells proliferate rapidly B cells leave blood & enter lymph node via high endothelial venules Y YY Y Y Y GERMINAL CENTRE Transient structure of Intense proliferation YY Y Germinal centre releases B cells that differentiate into plasma cells Immune complex (1)antigen-(2) recognising antibody (3) complement components Complement proteins The structure of the germinal centre LZ FDC DZ LZ: light zone DZ: dark zone FDC: follicular dendritic cell Antigen is bound on the surface of follicular dendritic cells (FDC) FDC FDC-s bind immune complexes (Ag-Ab ) Ag detectable for 12 months following immunization A single cell binds various antigens B cells recognize Ag on the surface of FDC 1. B cells compete for the antigen High affinity B cells can grab the antigen ! B cell antigen presentation ! Antigen prezentation 1 B-sejt MHCII + peptid T-sejt CD4 TCR 2 citokinek +++ polysaccharides are not presented 2. B cells compete for T cell help’ High affinit B cell can present the antigen to T cells ! INNATE IMMUNITY II Effector functions, elimination of pathogens 1. Phagocytosis 2. Killing with soluble mediators 3. Complement system 4. NK cell activation !! Monocita/ makrofág DC Hízó Sejt Granu locita NK sejt B-sejt T-sejt recogni tion comm unicati on Effector function neutralizáció Komp lement EFFECTOR FUNCTIONS OF ANTIBODIES NEUTRALIZATION OPSONIZATION COMPLEMENT ACTIVATION Binding of antibody increases phagocytosis PLASMA CELL FcR Opsonization by C3b Complement C3b FcR FcR CR1 PHAGOCYTES ENGULFMENT, DEGRADATION Immunoglobulin Structure-Function Relationship • cell surface antigen receptor on B cells allows B cells to sense their antigenic environment connects extracellular space with intracellular signalling machinery • secreted antibody neutralization opsonization complement fixation NK cell –mediated killing