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Biểu hiện đầu tiên của sự biệt hóa thành dòng tế bào B là sự sắp xếp lại các gian nhỏ giữa các nhóm gien V, D, J để hình thành gien của chuỗi nặng rồi chuỗi nặng H (H) được tổng hợp và có mặt trong bào tương. Ở giai đoạn này tế bào được gọi dưới tên tế bào pré B. Sau đó là đến lượt sắp xếp lại các gian nhỏ V, J của chuỗi nhẹ L và tổng hợp chuỗi nhẹ. Khi có cả chuỗi nặng u và chuỗi nhẹ, phân tử IgM được hình thành và biểu lộ lên bề mặt tế bào tức là các IgM bề mặt (Surface IgM = S.IgM hay còn viết m.IgM; m=membrane). Các S.IgM này đóng vai trò thụ thể KHÁNG NGUYÊN của tế bào B. Lúc bề mặt chỉ mới biểu lộ riêng S.IgM thì tế bào B vẫn còn chưa trưởng thành (immature B cells). Nếu tiếp xúc với KHÁNG NGUYÊN ở giai đoạn chưa trưởng thành thì tế bào có thể bị bất hoạt hay l
Overview of Lymphocyte Development The maturation of B and T lymphocytes involves a series of events that occur in the primary (also called generative or central) lymphoid organs (Fig 8.1) These events include the following: • Commitment of progenitor cells to the B lymphoid or T lymphoid lineage • Proliferation of progenitors and immature commi ed cells at specific early stages of development, providing a large pool of cells that can generate useful lymphocytes • The sequential and ordered rearrangement of antigen receptor genes and the expression of antigen receptor proteins (The terms rearrangement and recombination are used interchangeably.) • Selection events that preserve cells that have produced functional antigen receptor proteins and eliminate potentially dangerous cells that strongly recognize self antigens These selection processes during development ensure that lymphocytes that express functional receptors with useful specificities will mature and enter the peripheral immune system • Differentiation of B and T cells into functionally and phenotypically distinct subpopulations B cells develop into follicular, marginal zone, and B-1 cells, and T cells develop into CD4+ and CD8+ αβ T lymphocytes, γδ T cells, natural killer T (NKT) cells, and mucosa-associated invariant T (MAIT) cells The properties and functions of these different lymphocyte populations are discussed in later chapters FIGURE 8.1 Stages of lymphocyte maturation.Development of both B and T lymphocytes involves the sequence of maturational stages shown B cell maturation is illustrated, but the basic stages of T cell maturation are similar Commitment to the B and T Cell Lineages and Proliferation of Progenitors Multipotent stem cells in the fetal liver and bone marrow, known as hematopoietic stem cells (HSCs), give rise to all lineages of blood cells, including lymphocytes (see Chapter 2) HSCs mature into common lymphoid progenitors that can give rise to B cells, T cells, NK cells, and innate lymphoid cells (Fig 8.2) The maturation of B cells from progenitors commi ed to this lineage occurs before birth in the fetal liver and after birth in the bone marrow, with the final steps being completed in the spleen Fetal liver–derived stem cells give rise mainly to a type of B cell called a B-1 cell, whereas bone marrow–derived HSCs give rise to the majority of circulating B cells (follicular B cells) and a subset of B cells called marginal zone B cells Precursors of T lymphocytes emerge from the fetal liver before birth and from the bone marrow later in life and circulate to the thymus, where they complete their maturation T cells that express γδ T cell receptors (TCRs) arise from fetal liver HSCs, and the majority of T cells, which express αβ TCRs, develop from bone marrow–derived HSCs In general, the B and T cells that are generated early in fetal life have less diverse antigen receptors Despite their different anatomic locations, the early maturation events of both B and T lymphocytes are fundamentally similar Commitment of common lymphoid progenitors to the B or T cell lineage depends on transcriptional regulators that drive development toward either B cells or T cells Key events in the commitment of precursor cells to the B cell or T cell lineage are expression of the proteins involved in antigen receptor gene rearrangements, described later in the chapter, and the generation of accessibility, at the level of chromatin, of particular antigen receptor gene loci to these proteins In the case of developing B cells, the immunoglobulin (Ig) heavy chain locus, initially in a closed chromatin configuration, is opened so that it becomes accessible to the proteins that will mediate Ig gene rearrangement and expression In developing αβ T cells, the TCR β gene locus is made accessible first These changes in the chromatin accessibility of antigen receptor loci during development are initiated by sets of lineage-specific transcription factors Numerous transcription factors are involved in the maturation of T and B cells (see Fig 8.2) Notch1 and GATA3 commit developing lymphocytes to the T cell lineage The Notch family of proteins are cell surface molecules that are proteolytically cleaved when they interact with specific ligands on neighboring cells (see Fig 7.2) The cleaved intracellular portions of Notch proteins migrate to the nucleus and modulate the expression of specific target genes Notch1 is activated in lymphoid progenitor cells, and together with GATA3 it induces expression of a number of genes that are required for the further development of αβ T cells Some of these genes encode components of the pre-T cell receptor (pre-TCR) and the RAG1 and RAG2 proteins, which are required for V(D)J recombination, described later The EBF, E2A, and PAX5 transcription factors induce the expression of genes required for B p p g q cell development These include genes encoding the RAG1 and RAG2 proteins, components of the pre-B cell receptor (pre-BCR), and proteins that contribute to signaling through the pre-BCR and the BCR The role of these proteins in T and B cell development will be considered later in this chapter FIGURE 8.2 Multipotent stem cells give rise to distinct B and T lineages.Hematopoietic stem cells (HSCs) give rise to distinct progenitors for various types of blood cells One of these progenitor populations (shown here) is called a common lymphoid progenitor (CLP) CLPs give rise to B and T cells and also contribute to natural killer (NK) cells, innate lymphoid cells (ILCs), and some dendritic cells (not depicted here) Pro-B cells can eventually differentiate into follicular (FO) B cells, marginal zone (MZ) B cells, and B-1 cells Pro-T cells may commit to either the αβ or γδ T cell lineages Commitment to different lineages is driven by various transcription factors, indicated in italics During B and T cell development, commi ed progenitor cells proliferate first in response to cytokines and later in response to signals generated by a pre-antigen receptor that select cells that have successfully rearranged the first set of antigen receptor genes Proliferation ensures that a large enough pool of progenitor cells will be generated to eventually produce a highly diverse repertoire of mature, antigen-specific lymphocytes In mice, widely used for basic research of lymphocyte development, the cytokine interleukin-7 (IL-7) drives proliferation of early T and B cell progenitors; in humans, IL-7 is required for the proliferation of T cell progenitors but not of progenitors in the B lineage The factors that drive the proliferation of human progenitor B cells remain to be identified IL-7 is produced by stromal cells in the bone marrow and by epithelial and other cells in the thymus Mice with targeted mutations in the gene encoding either IL-7 or the IL-7 receptor show defective maturation of lymphocyte precursors beyond the earliest stages and, as a result, profound deficiencies in mature T and B cells Mutations in the human gene encoding the common γ chain, a protein that is shared by the receptors for several cytokines, including IL-2, IL-7, and IL-15, give rise to an immunodeficiency disorder called X-linked severe combined immunodeficiency disease (X-SCID) (see Chapter 21) This disease is characterized by a block in T cell and NK cell development, but normal B cell development, because IL-7 is required for T cell development in humans and IL-15 for NK cells The greatest proliferative expansion of lymphocyte precursors occurs after successful rearrangement of the genes encoding one of the two chains of the T or B cell antigen receptor, producing a pre- antigen receptor (described later) Signals generated by pre-antigen receptors are responsible for far greater proliferation of developing lymphocytes (which have successfully rearranged the Ig heavy chain gene or the TCR β chain gene, as the case may be) than the earlier proliferation driven by cytokines such as IL-7 Role of Epigenetic Changes and MicroRNAs in Lymphocyte Development Many nuclear events in lymphocyte development are regulated by epigenetic mechanisms Epigenetics refers to the control of gene expression and phenotypes by mechanisms other than changes in the coding sequences themselves In developing lymphocytes, epigenetic mechanisms also control antigen receptor gene rearrangement events DNA exists in chromosomes tightly bound to histones and nonhistone proteins, forming what is known as chromatin DNA in chromatin is wound around a protein core of histone octamers, forming structures called nucleosomes, which may be either well separated from other nucleosomes or densely packed Chromatin may therefore exist as relatively loosely packed structures, called euchromatin, wherein genes can be accessed by transcription factors and are transcribed, or as tightly packed heterochromatin in which genes are maintained in a silenced state The structural organization of portions of chromosomes varies in different cells, making certain genes available for transcription factors to bind to, while these very same genes may be unavailable to transcription factors in other cells Epigenetic mechanisms regulate the accessibility and activity of genes by inducing changes in promoter and enhancer regions of genes These changes include: the methylation of DNA on certain cytosine residues that generally silences genes; post-translational modifications of the histone tails of nucleosomes (e.g., acetylation, methylation, and ubiquitination) that may render genes either active or inactive depending on the histone modified and the nature of the modification; active remodeling of chromatin by protein machines called remodeling complexes that can also either enhance or suppress gene expression; and the silencing of gene expression by noncoding RNAs Some critical components of lymphocyte development are regulated by epigenetic mechanisms • Histone modifications in antigen receptor gene loci are required for recruitment of proteins that mediate gene recombination to form functional antigen receptor genes This process is discussed later in the chapter • Commitment of developing T cells to the CD4 or CD8 lineage depends on epigenetic mechanisms that silence the expression of the CD4 gene in CD8+ T cells Silencing involves chromatin modifications that place the CD4 gene into an inaccessible heterochromatin state • In Chapter 7, we discussed microRNAs (miRNAs) in the context of T cell activation They contribute in significant ways to modulating gene and protein expression during development as well As mentioned in Chapter 7, Dicer is a key enzyme in miRNA generation Deletion of Dicer in the T lineage results in a preferential loss of regulatory T cells and the consequent development of an autoimmune phenotype similar to that seen in the absence of FOXP3 (discussed in Chapters 15 and 21) The loss of Dicer in the B lineage results in a block at the pro-B to pre-B cell transition (discussed in more detail later), primarily due to enhanced apoptosis of pre-B cells Gene ablation studies have revealed that many specific miRNAs are involved in lymphocyte development Antigen Receptor Gene Rearrangement and Expression The rearrangement of antigen receptor genes is an essential event in lymphocyte development, and this process is responsible for the generation of a diverse adaptive immune repertoire As we discussed in previous chapters, each clone of B or T lymphocytes produces an antigen receptor with a unique antigen-binding structure In any individual, there may be 107 to 109 different B and T lymphocyte clones, each with a unique receptor The ability of each individual to generate these large and diverse lymphocyte repertoires has evolved in such a way that a fairly small number of genes can give rise to a vast number of distinct Ig and TCR molecules, each capable of binding to a different antigen Functional antigen receptor genes are produced in immature B cells in the bone marrow and in immature T cells in the thymus by a process of gene rearrangement In this process, segments of antigen receptor genes are randomly recombined and nucleotide sequence variations are introduced at the joints, resulting in the production of a large number of variable region–encoding exons The DNA rearrangement events that lead to the production of antigen receptors are not dependent on or influenced by the presence of antigens In other words, as the clonal selection hypothesis had proposed, diverse antigen receptors are generated and expressed before encounter with antigens (see Fig 1.7) We will discuss the molecular details of antigen receptor gene rearrangement later in this chapter Selection Processes That Shape the B and T Lymphocyte Repertoires The process of lymphocyte development contains numerous steps, called checkpoints, at which the developing cells are tested and continue to mature only if a preceding step in the process has been successfully completed One of these developmental checkpoints is based on the successful production of one of the polypeptide chains of the two-chain antigen receptor protein, and a second checkpoint requires the second chain and thus assembly of a complete receptor The requirement for traversing these developmental checkpoints is a quality control mechanism that ensures that only lymphocytes that produce complete antigen receptors and are therefore likely to be functional are selected to mature Additional selection processes operate after antigen receptors are expressed and serve to eliminate potentially harmful, self-reactive lymphocytes and to commit developing cells to particular lineages (Note that the term checkpoints is also used to describe very different phenomena in the context of peripheral immune activation and cancer immunotherapy [see Chapter 18]) We will next summarize the general principles of these events Pre-antigen receptors and antigen receptors deliver signals to developing lymphocytes that are required for the survival of these cells and for their proliferation and continued maturation (Fig 8.3) Pre-antigen receptors, called pre-BCRs in B cells and pre-TCRs in T cells, are signaling structures expressed during B and T cell development that contain only one of the two polypeptide chains present in a mature antigen receptor Cells of the B lymphocyte lineage that successfully rearrange their Ig heavy chain genes express the µ heavy chain protein and assemble a pre-BCR In an analogous fashion, developing T cells that make a successful TCR β chain gene rearrangement synthesize the TCR β chain protein and assemble a pre-TCR The assembled pre-BCR and pre-TCR form complexes with proteins that generate signals for survival, proliferation, and the phenomenon of allelic exclusion (discussed later), and for the further development of B and T cells Because of the random addition of nucleotides at junctions between segments of antigen receptor genes that are joined together during lymphocyte development and the triplet base pair code for determining amino acids, only about one in three antigen receptor gene rearrangements is in frame and, therefore, capable of generating a proper full-length protein Such a successful rearrangement is sometimes called a productive rearrangement If cells make out-of-frame or nonproductive gene rearrangements at the Ig µ or TCR β chain loci, the pre-antigen receptors are not expressed, the cells not receive necessary survival signals, and they undergo programmed cell death Thus, expression of the pre-