The Development of B and T Lymphocytes PDF

The Development of B and T Lymphocytes Literature: Chapter 8, Janeway‘s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024 ...

The Development of B and T Lymphocytes Literature: Chapter 8, Janeway‘s Immunobiology ETH Zurich Lecture on “Pharmaceutical Immunology I” Prof. Dr. Cornelia Halin Winter 535-0830-00L HS 2024  Revision from Chapter 1 5. Lymphoid organs in the body Central or primary lymphoid organs Generation / maturation of lymphocytes Bone marrow (B cells and initially T cells) Thymus (T cells) Peripheral or secondary lymphoid organs Maintenance of lymphocytes and induction of adaptive immune responses Lymph nodes Spleen Peyer’s patches Appendix Þ In this Chapter we will learn about the different stages of B cell and T cell development in the bone marrow and thymus and how these are syn- chronized with the rearrangement of its B and T cell receptor, respectively. 1 Content 1. Development of B lymphocytes 2. Development of T lymphocytes 3. Positive and negative selection 2  Lymphocytes derive from hematopoietic stem cells (HSC) in the bone marrow Specialized microenvironment of the bone marrow provides signals for the development of all hemato- poietic cells Stromal cells provide adhesion, growth and differentiation factors. First differentiation step: multipotent progenitor (MPP) NK cells, B and T cells derive from the same common lymphoid progenitors (CLPs) G. Sannia et al., 2015, Corpus ID: 54714197  1. Development of B lymphocytes The early stages of B cell development are dependent on bone marrow stromal cells immunoglobulin rearrangement Important factors produced by stromal cells: CXCL12: chemokine that retains precursors in the bone marrow niche Interleukin-7 and stem cell factor (SCF) : growth and survival factors for developing B cells Cell adhesion molecules: provide cellular support 4  B-cell development begins by rearrangement of the heavy- chain locus Rearrangement of gene segments requires activity of RAG-1/-2 enzymes (recombination activating genes) Starts in the early pro-B cell stage with heavy chain rearrangement Once the heavy chain is productively rearranged: pre-B cell stage 5 B-cell development begins by rearrangement of the heavy-chain locus A productively rearranged immunoglobulin gene is immediately expressed as a protein by the developing B cell. To allow for expression of the rearranged heavy chain, it is co-expressed on the cell surface together with a surrogate light chain 6  Critical stage of B cell development: Successful rearrangement of the heavy chain (pro-B cell => pre-B cell stage) Successfully rearranged heavy chain gets ex- pressed on the cell surface Heavy chain is crosslinked by secreted VpreB protein (part of the surrogate light chain) Crosslinking induces intracelular signaling via the ITAM motifs in Igab => survival signal signal In case of no productive heavy chain rearrange- ment: no survival signal and cell death by apoptosis 7  Pre-B cells rearrange the light-chain locus and express cell- surface immunoglobulin The pre-B cell stage is accompanied by several rounds of cell division: 30-60 fold expansion! After division, pre-B cells undergo light chain rearrangement: Cells with the same heavy chain c can get different light chains! Nonproductive light chain rearrangements can be rescued by further rearrangements: => increase chances of success !! BUT: if they fail: apoptosis 8 Immature B cells are tested for autoreactivity before they leave the bone marrow Process known as “Central B cell tolerance”: Binding to a self molecules in the bone marrow can lead to inactivation or to cell death or of the immature B cells 9 Anergy: State of immunologic unresponsiveness  Lymphocytes that encounter sufficient quantities of self antigens for the first time in the periphery are eliminated or inactivated Mechanism known as “Peripheral B cell tolerance”: Immature (transitional) B cells: Recent Recent emigrants emigrants from from the the bone bone marrow If If they they bind bind to to abundant abundant multivalent multivalent / soluble self antigens in the first days after exiting to the periphery, they they undergo undergo apoptosis apoptosis oror become become anergic anergic (immunologically (immunologically unresponsive) unresponsive) If they bind low-affinity, non- crosslinking crosslinking self-antigen self-antigen they they may may become become ignorant ignorant Mature B cells: Have past the transitional B cell stage: Can be activated by antigen. 10 Summary of the development of the human conventional B- lineage cells 11 1. Summary: B cell development B cells develop in the bone marrow, where rearrangement of the B cell receptor (surface immunoglobulin) occurs. They then migrate to peripheral lymphoid organs, where activation by antigen occurs. 12 2. Development of T lymphocytes T cell progenitors migrate from the bone marrow to the thymus, where the main stages of T cells development (incl. T cell receptor rearrangement) takes place Mature T cells then migrate to SLOs, where activation by antigen occurs 13 SLO: secondary lymphoid organs Cellular organization of the human thymus Cortex: immature thymocytes, cortical epithelial cells, macrophages Medulla: maturing thymocytes, medullary epithelial cells, dendritic cells, phagocytic cells (macrophages; Hassall's corpuscule) 14 The epithelial cells of the thymus form a network (“house”) surrounding the developing thymocytes Immunodeficiencies due to failing development of the thymus: DiGeorge’s syndrome (humans) thymic epithelium fails to develop normally => No development of mature T cells; fewer B cells and defective Ab production Nude mice: Mutations in a transcription factor (Foxn1) required for terminal epithelial differentiation => in absence of Foxn1: no hair and no thymus, impaired adaptive immunity 15 The stages of a:b T cell development in the mouse thymus correlate with the program of gene rearrangement, and the expression of cell-surface proteins, signaling proteins, and transcription factors 16 Thymus: a site of massive cell expansion and cell death TWO main goals of thymic education / T cell selection: T cells are able to bind to MHC molecules to screen for presence of foreign antigen (“MHC-restriction”) T cells are not autoreactive, i.e. do not get activated by binding to MHC or self-peptides presented on MHC 1) Immature thymocytes proliferate in the cortex prior to starting T cell receptor rearrangement Red: apoptotic cells Blue: macrophages Top: cortex overview 2) Approx. 98% of all thymocytes undergo apoptosis, because: Bottom: high magnification No productive gene rearrangement newly generated T cell receptor does not display low- affinity binding to MHC (“not MHC-restricted”) newly generated T cell receptor bind autoantigens present in the thymus  Thymocytes at different developmental stages are found in distinct parts of the thymus Rearrangement of the T cell receptor b chain already starts in the double- negative stage (DN2) Once the b chain is rearranged, thymocytes enter the double-positive stage: => i.e. upregulate CD4 and CD8 They undergo massive proliferation and start to rearrange the a chain Once they express the complete T cell receptor, they enter into positive and negative selection 18 Successive stages in the development of thymocytes are marked by changes in cell-surface molecules I. Progenitor cells arrive in thymus (medulla) II. Interactions with the stroma induces differentiation into “double-negative” thymocytes (CD4-CD8-) III. They subsequently migrate to the cortex, proliferate and differentiate into CD4+CD8+ “double-positive” cells (1) IV. Thymic selection determines whether they become a CD4+ (MHCII-restricted) or CD8+ (MHCI-restricted) “single-positive” thymocyte (2) 8% 7 87% FACS plot of thymocytes 2 CD4 1 2 2 3 3% 2% CD8 19 3. Positive and negative selection of T cells TWO main goals of thymic education / T cell selection: T cells are able to bind to MHC molecules to screen for presence of foreign antigen T cells are not autoreactive, i.e. do not recognize self-peptides presented on MHC Revision from Chapters 5&6: The TCR binds the peptide: MHC complex The less variable CDR1 and CDR2 loops of a T-cell receptor mainly contact the relatively less variable MHC component of the ligand The highly variable CDR3 regions mainly contact the unique peptide component Revision from Chapter 6 Characteristics of MHC molecules: Polygenic 3 genes encoding MHC class I and 3 encoding MHC class II Polymorphic many different alleles co-dominantly expressed Remember: Allele = variant of a gene 3. Positive and negative selection of T cells POSITIVE SELECTION Thymocytes proliferate in the subcapsular region. Immature double-positive cells have an average life-span of only 3-4 days Only 10-30% of thymocytes have receptors with the ability to interact with self peptide:self MHC complexes Such self peptide:self MHC complexes are present on thymic cortical epithelial cells Thymocytes that can interact with self peptide:self MHC via their T cell receptors receive survival signals: => POSITIVE SELECTION 22  The MHC molecules that induce positive selection determine differentiation into CD4+ or CD8+ T cells Remember: CD4 and CD8 bind to invariant sites of the MHC molecule the T cell receptor is interacting with. Positive selection determines: cell phenotype (CD8+ or CD4+ expression) differentiation into a cytotoxic or helper T cell Fig. 8.27 shows an important, historic experiment performed to T cell receptor (TCR) transgenic 23 mice, in which all thymocytes express a rearranged TCR which either recognizes MHCI or MHCII. Thymic cortical epithelial cells mediate positive selection of developing thymocytes Historic experiment demonstrating the role thymic cortical epithelial cells in positive selection: 24 Negative Selection: T cells that react strongly with ubiquitous self antigens are deleted in the thymus Negative selection of thymocytes can occur both in the cortex and in the medulla 25 Negative Selection: T cells that react strongly with ubiquitous self antigens are deleted in the thymus AIRE: autoimmune regulator Transcriptional regulator expressed in medullary stromal cells Facilitates transcription of tissue-specific proteins (e.g. insulin) in the thymus Mutations in AIRE give rise to a rare autoimmune disease: autoimmune polyendocrinophathy- candidiasis-ectodermal dystrophy (APECED) Green: AIRE. RED: MTS10,marker of thymic medullary epithelial cells 26 The specificity and/or the strength of signals for negative and positive selection must differ Affinity model of T-cell selection T cells with newly arranged TCRs that fail to sufficient binding strength to self peptide: self MHC complexes on thymic epithelium die by neglect. The rest is positively selected. However, cells that have excessively strong reactivity o self peptide:self MHC complexes are sent into apoptosis, i.e. negative selection. 27 Summary of the development of human a:b T cells 28  Take-home messages Chapter 8 B and T cells derive from uncommitted hematopoietic stem cells Somatic gene rearrangements account for the highly diverse repertoire of antigen receptors; immunoglobulins (B cells) and T cell receptors (T cells) Somatic recombination occurs between V(D)J segments and involves RAG (recombination activating gene) proteins In case of B cells, first the heavy and then the light chain rearranges (bone marrow) In the case of T cells, first the b- and then the a-chain rearranges (thymus) Cells in which receptor rearrangement is not successful die by aptotosis In the case of T cells, positive and negative selection in the thymus guarantees MHC restriction and elimination of autoreactive T cells In the case of B cells, autoreactive cells are eliminated in the bone marrow or shortly after their release into the circulation/lymphoid organs (transitional B cells) 29