中科院细胞与分子免疫学课程Chapter9

Chapter 9 B Cell Activation and Antibody Production,General Features of Humoral Immune Responses,1. Sequential phases are needed in the process of activation of B cells to produce antibody.,2. Whether antibody responses to antigen require CD4+ helper T cells depends on the antigen character.3. Activated B cells differentiate into secreting B cells and into long-lived memory cells.,4. Heavy chain isotype switching and affinity maturation are typically seen in helper T cell-dependent humoral immune responses to protein antigens.5. There are differences between primary and secondary antibody responses to protein antigens,Q: How antigens initiate the process of B cell activation? A: Signal transductionSignal Transduction by the B lymphocyte antigen receptor complex 1) B cell antigen receptor complex,Membrane IgM and IgD, have short cytoplasmic tails consisting of only three amino acids. Signals are transduced by two other molecules, called Iga and Igb.Just like TCR complex, the cytoplasmic domains of Iga and Igb contain ITAMs.Cross linking of membrane Ig brings several ITAMs into proximity, and this triggers subsequent singnaling events.,2) Signaling events initiated by the BCR complex,Ig induces phosphorylation of the tyrosines in the ITAMs of Iga and Igb by tyrosine kinases such as Lyn, Blk and Fyn.Syk just like ZAP-70 in T lymphocytes, Syk is phosphorylated and activated.The adapter protein SLP-65(BLNK), is important in the formation of a scaffold of other adapter proteins such as Grb2 and SOS.Three signals have been activated in the proceeded procedure.,2) Signaling events initiated by the BCR complex,Ras-MAP kinase pathway is activated in atigen-stimulated B cells.PLCg2 is activated and activates downstream signaling parthways.IP3 mediated DAG mediated.iii. The resulted signal pathway ultimately activated the transcription factors such as AP-1, NFkB and NFAT.,2. Second Signals for B Cells Provided by Complement Receptors,1) The component of the second signals are: C3d, CR2-CD19-C81.2) C3d: a component of complement degraded fragment, which can bind to the microbial surface.3) CR2(CD21), the receptor of C3d on B cell .4) CD19 and CD81 expressed with CD2 as complex which is called B cell coreceptor complex.5) Antigen was bound by the Ig and C3d at the same time, thus, CD19 was brought to proximity to BCR-associated kinases and CD19 became phosphorylated.6) The phosphorylated CD19 activated several signaling pathway.,3. Functional Responses of B cells to Antigen Recognition,Stimulate the entry of previously resting cells into the G1 stage.Activated B cells show increased expression of MHCII molecules and costimulators.The cytokine receptors for T cell produced are also increased.The B cells change their expression of chemokine receptors, which enables them to migrate to and interact with helper T cells.5) Different antigen induced different activity.,Helper T Cell-Dependent Antibody Responses to Protein Antigen,Antibody responses to protein antigens require 1 recognition of the antigen by helper T cells 2 cooperation between the antigen-specific B and T lymphocytes.,3. Helper T lymphocytes stimulate B cell into four phases: 1) Clonal expansion 2) Isotype switching 3) Affinity maturation 4) Differentiation into memory B cells4. Different phases of T- dependent B cell activation occur in different anatomic regions within peripheral lymphoid organs.,Early Events in T cell-Dependent Antibody Responses:,Antigen-induced activation of the two cell types T and B cells;Physical contact between T- B cell interaction;Antigen presentation by B cells to differentiated helper T cells;Activation of the helper T cells;Expression of membrane and secreted molecules by the helper T cells .The molecules expressed by T cells bind to and activate the B cells.,Antigen-induced activation of the two cell types T and B cells; ( we have discussed this part in chapter 8 and earlier of this chapter.)Physical contact between T- B cell interaction i Antigen-induced migration of B cells and helper T cells,The T or B cell with particular antigen specificity is very low: 1 in 105 to 106 ; Elicited one problem: how will the necessary cells and antigen all be in the right place at the same time? The problem is solved: the lymphocyte can move in lymphoid organ. Within 1 or 2 days after antigen administration: Nave CD4+ T cells recognize antigens presented by APCs, then the expression of CCR7 is reduced and begin to leave T cell zones. B cells recognize the antigens in the follicles and are activated, the expression of CCR7 is increased and begin to enter T cell zones.,i Antigen-induced migration of B cells and helper T cells,Migration and interactions of B cells and helper T cells,3) Antigen presentation by B cells to differentiated helper T cells,B cells make function as APCs to T cell.For example: hapten-carrier conjugates Dinitrophenol can bind to B cell membrane Ig, but can not immunogenic by themselves.B cells are specific to hapten and bind the antigen through the hapten determinant, after being endocytosed, present peptides derived from the carrier protein. T cells are specific to carrier and T cells are activated. iii. Antigen binding to Ig enhances the expression of costimulators.,4) Helper T Cell activates B Cell through interaction between CD40 and CD40L,Activated helper T cells express a surface molecule: CD40 ligand. B cells constitutively express CD40.CD40-CD40L interaction stimulates B cell proliferation and differentiation.,Role of CD40 in contact-dependent T cell help of B cells,CD40L binding to CD40 can result:In B cells:Activation of transcription factors:NF-k B and AP-1. Isotype switching.Enhanced expression of B7.,Helper T Cell activates B Cell through secreting cytokinesIncrease B cell proliferation and differentiation: IL-2, IL-4, and IL-5.Promote switching to different heavy chain isotypes.,5),B Cell Differentiation into Antibody Secreting Cells,Molecular required in the antibody synthesis: IL-2, IL-4, IL-5 and IL-6, CD40L.Antibody secreting cells located in:Within lymphoid organs: the red pulp of the spleen and the medulla of the lymph nodes.,Plasma cells with abundant secreting antibody migrated to bone marrow, at 2-3 weeks after immunization , the marrow may be a major site of antibody production.,The antibody can change its form in two ways: 3.1: Through RNA processing, which is regulated by RNA cleavage and the choice of polyadenylation sites, determines whether the Ig is membrane of secreted form.,3.2 Through DNA recombination, heavy chain isotype switching.,localization:In peripheral lymphoid tissueAt the edges of the folliclesIn germinal center2) molecular:CD40 and cytokinesIL-4 : produced by CD4+ T cell is the principal switch factor for IgE.IFN-g : secreted by T cells and by NK cells for IgG2a.,3) Isotype switching in response to different types of microbes is regulated by the types of helper T cells.IgM: the major protective humoral immune response to bacteria with polysaccharide-rich capsules.IgG: many viruses and many bacteria activate helper T cells of the TH1 subset, which produce the cytokine IFN-g.IgE: many helminthic parasites, because helminths activate the TH2 subset of helper T cells, which produces IL-4.,4) In different anatomic sites switch to different isotypes.B cells in mucosal tissues switch to IgA, which is most efficiently transported through mucosal secretion and is stimulated by TGF-b and IL-5 derived from T cells.,5) In all, T cells function as controllers of immune responses.,3、B细胞激活所需第二信号：,C3d, CR2-CD19-C81,T依赖抗原是怎样使B细胞产生各种各样抗体的：,T、B细胞被抗原激活激活的T、B细胞相互迁移，发生接触。B细胞作为抗原递承细胞呈递抗原给T细胞。T细胞表面表达的CD40L，B细胞表面表达CD40。CD40L与CD40相互作用。T细胞分泌的细胞因子刺激B细胞,B细胞分化为可产生抗体的细胞,Molecular mechanisms of heavy chain isotype switching,1. Nucleotide sequences involved in the isotype switching:Switch regions: GAGCT GGGGGTI exon: initiator of transcription.2. Procedure:Upon signal triggering, a germline transcript is produced.The germline transcripts can mediate the switch recombination.The switch recombination occurs between the S region.,Molecular mechanisms of heavy chain isotype switching,1. The same character with VDJ recombination:DNA recombination2. The different character with the VDJ recombination:All isotype switch recombination is productiveIt uses different recombination signal sequences and does not require the RAG enzymesIt happens after antigen stimulation and not during B-cell development in the bone marrow.Not random but by external signals.,Late Events in T Cell-Dependent Antibody Responses:Germinal Center Reactions:Affinity maturationThe generation of memory B cellsIn the germinal centers of lymphoid follicles,1 Germinal center formation:,Within 4-7 days after antigen exposure, the activated B cells migrate deep into the follic and begin to proliferate, forming the germinal center.In germinal center, the B cells are smaller so called centrocytes.Another important cells in the germinal center are FCs.,Cells localization in germinal center,In identifiable light zone of the germinal center, accumulate proliferating B cells but few FDCs.2. In the basal dark zone, there exits the small non-dividing B cells and abundant FDCs.,2 Affinity Maturation: Somatic Mutations in Ig Genes and Selection of High- Affinity B Cells,The V regions of the Ig showed hypermutation.The nucleotide sequences of IgG antibodies derived from one clone of B cells can diverge as much as 5% from the original germline sequence.Helper T cells and CD40:C40L interactio