Lecture 12 - Immunology Studyguide PDF

Lymphocyte maturation involves: 1. Commitment of progenitor cells to either the B lymphoid or T lymphoid lineage 2. Proliferation of progenitor cells and immature committed cells 3. Sequential rearrangement (recombination) of antigen receptor chains 4. Selection of cells expressing functional antige...

Lymphocyte maturation involves: 1. Commitment of progenitor cells to either the B lymphoid or T lymphoid lineage 2. Proliferation of progenitor cells and immature committed cells 3. Sequential rearrangement (recombination) of antigen receptor chains 4. Selection of cells expressing functional antigen receptors & elimination of cells that strongly recognize self antigens 5. Differentiation of B and T cells into distinct subpopulations Certainly, let's delve into the details of lymphocyte maturation: 1.Commitment to B or T Lymphoid Lineage: 1. Lymphocyte maturation begins with the commitment of hematopoietic stem cells (HSCs) to either the B lymphoid or T lymphoid lineage. 2. Hematopoietic stem cells in the bone marrow give rise to lymphoid progenitor cells, which undergo lineage commitment based on signaling cues from the microenvironment. 3. B lymphoid progenitors remain in the bone marrow, while T lymphoid progenitors migrate to the thymus for further development. 2.Proliferation of Progenitor Cells: 1. Upon commitment to the B or T cell lineage, progenitor cells undergo rapid proliferation to generate a pool of immature lymphoid cells. 2. Proliferation is driven by growth factors and cytokines present in the bone marrow or thymic microenvironment, such as interleukins and stem cell factor. 3.Sequential Rearrangement of Antigen Receptor Chains: 1. In both B and T cell maturation, a key step is the sequential rearrangement (recombination) of antigen receptor genes to generate a diverse repertoire of receptors capable of recognizing a wide range of 1 antigens. 2. B cells undergo V(D)J recombination in the bone marrow to assemble immunoglobulin genes, encoding the variable regions of B cell receptors (BCRs). 3. T cells undergo TCR gene rearrangement in the thymus to assemble TCR genes, encoding the variable regions of T cell receptors (TCRs). 1.Selection of Cells with Functional Antigen Receptors: 1. During maturation, immature B and T cells undergo selection processes to ensure the expression of functional antigen receptors while maintaining self-tolerance. 2. Positive selection in the thymus and bone marrow selects cells expressing antigen receptors with moderate affinity for self-antigens, ensuring recognition of foreign antigens. 3. Negative selection eliminates cells that strongly recognize self-antigens, preventing autoimmunity. This process occurs primarily in the thymus for T cells and in the bone marrow for B cells. 2.Differentiation into Distinct Subpopulations: 1. Mature B and T cells differentiate into distinct subpopulations with specialized functions, depending on their antigen encounter and microenvironmental cues. 2. B cells differentiate into plasma cells for antibody production or memory B cells for long-term immune memory. 3. T cells differentiate into various effector subsets, such as helper T cells (Th), cytotoxic T cells (CTLs), regulatory T cells (Tregs), or memory T cells, based on their cytokine milieu and antigen encounter. In summary, lymphocyte maturation involves commitment to the B or T cell lineage, proliferation, sequential rearrangement of antigen receptor genes, selection of cells expressing functional receptors, and differentiation into distinct subpopulations with specialized functions. These processes ensure the generation of a diverse and functional immune system capable of responding to a wide range of pathogens while maintaining self-tolerance. 1 Stages of lymphocyte maturation B cell shown Similar for T cell hematopoietic Certainly, let's detail the stages of lymphocyte maturation: 1.Stem Cell: 1. The process of lymphocyte maturation begins with hematopoietic stem cells (HSCs), which reside in the bone marrow. 2. Growth factors and cytokines induce commitment of HSCs to the lymphoid lineage, setting the stage for lymphocyte development. 2.Pro-lymphocyte: 1. Committed lymphoid progenitor cells undergo rapid proliferation, generating a pool of precursor cells known as pro-lymphocytes. 2. These pro-lymphocytes are characterized by their potential to differentiate into B or T cells. 3.Initiation of Antigen Receptor Gene Rearrangement: 1. Pro-lymphocytes initiate the process of antigen receptor gene rearrangement, which is essential for generating the diverse repertoire of antigen receptors. 2. In B cells, this process involves V(D)J recombination of immunoglobulin genes, while in T cells, it involves rearrangement of T cell receptor (TCR) genes. 2 4. Pre-lymphocyte: 1. Pre-lymphocytes are immature cells that express pre-antigen receptors on their surface. 2. These pre-antigen receptors are non-functional and serve as a checkpoint for the selection of cells capable of generating functional antigen receptors. 5.Selection of Cells Expressing Pre-Antigen Receptors: 1. Pre-lymphocytes undergo a selection process to ensure the expression of pre-antigen receptors with moderate affinity for self-antigens. 2. This process helps eliminate cells with non-functional receptors or those with high affinity for self-antigens, which could lead to autoimmunity. 6.Immature Lymphocyte: 1. Following successful selection, pre-lymphocytes differentiate into immature lymphocytes, which express functional antigen receptors on their surface. 2. Immature lymphocytes undergo further maturation and selection processes to ensure a diverse and functional repertoire of antigen receptors. 7.Mature Lymphocyte: 1. Mature lymphocytes are fully developed and functional cells capable of recognizing specific antigens. 2. B cells migrate from the bone marrow to peripheral lymphoid organs or tissues, while T cells migrate from the thymus to peripheral lymphoid organs. 3. These mature lymphocytes are poised to mount immune responses upon encountering their specific antigens. In summary, lymphocyte maturation involves a series of stages, starting from hematopoietic stem cells in the bone marrow or thymus, progressing through commitment, proliferation, antigen receptor gene rearrangement, selection, and maturation into fully functional cells. This process ensures the generation of a diverse and competent immune system capable of responding to a wide range of pathogens while maintaining self-tolerance. 2 Antigen receptor gene rearrangement in lymphocytes The number of genes that code for antigen receptors is relatively small BUT the typical human has 107 to 109 different B cell and T cell clones, each with a unique antigen receptor gene and protein Depends on: 1. Random recombination of gene segments 2. Introduction of nucleotides at segment joints - Were can there is recomnbination there is addition and sometimes deletion These create very large number of possible Variable region-encoding exons => Each lymphocyte “acquires” uenique exon sequence Occurs in immature B cells (marrow) and immature T cells (thymus) => Diverse receptors generated and expressed before antigens are encountered ( during the recombinations) 3 Gene rearrangement via V(D)J recombination of germline DNA The variable region of each antigen receptor is generated by combining different: Variable (V) segments + Diversity (D) segments + Joining (J) segments Basics are same for Ig heavy chains & light chains and both TCR chains, but details vary For example: Ig light chains lack D segments Exactly, the process of antigen receptor gene rearrangement in lymphocytes is crucial for generating the immense diversity of B cell and T cell receptors necessary for recognizing a wide range of antigens. Here's how this process works: 1.Random Recombination of Gene Segments: 1. Both B cells and T cells utilize a process called V(D)J recombination to generate diversity in their antigen receptor genes. 2. During V(D)J recombination, segments of genes encoding variable (V), diversity (D, only in immunoglobulin heavy chain genes), and joining (J) regions are randomly rearranged to form a functional gene that encodes the variable region of the antigen receptor. 3. The random combination of V, D, and J gene segments leads to an enormous potential for diversity in the variable region of the receptor. 2.Introduction of Nucleotides at Segment Joints: 1. During the recombination process, nucleotides can be added or removed at the junctions between the rearranged gene segments. 2. This process, known as junctional diversity, further increases the diversity of the variable region by introducing additional variability at the nucleotide level. 3.Addition and Deletion of Nucleotides: 4 1. The addition and deletion of nucleotides during V(D)J recombination occur through the action of enzymes called terminal deoxynucleotidyl transferase (TdT) and exonucleases. 2. TdT adds random nucleotides to the junctions between gene segments, creating additional diversity in the antigen receptor sequence. 1.Formation of Unique Exon Sequences: 1. Through the process of V(D)J recombination and junctional diversity, each lymphocyte acquires a unique combination of gene segments and nucleotide additions. 2. This results in the generation of a diverse repertoire of antigen receptors, with each lymphocyte expressing a unique variable region sequence. 2.Occurs in Immature B and T Cells: 1. V(D)J recombination occurs during the early stages of B cell maturation in the bone marrow and T cell maturation in the thymus. 2. Immature B cells express the rearranged immunoglobulin genes on their surface as B cell receptors (BCRs), while immature T cells express the rearranged T cell receptor (TCR) genes. 3.Diverse Receptors Generated Before Antigen Encounter: 1. The diversity generated by V(D)J recombination allows lymphocytes to express a wide range of antigen receptors even before encountering specific antigens. 2. This diversity ensures that the immune system is capable of recognizing and responding to a vast array of pathogens and foreign substances. In summary, antigen receptor gene rearrangement in lymphocytes is a highly regulated process that generates diversity in B cell and T cell receptors through the random recombination of gene segments and the introduction of nucleotides at segment joints. This process ensures that each lymphocyte expresses a unique antigen receptor capable of recognizing specific antigens encountered in the body. 4 Germline organization of human Ig heavy chain V = Variable (45) D = Diversity (23) J = Joining (6) C = Constant (9) In the germline organization of the human immunoglobulin heavy chain locus, there are several key components and features that contribute to the diversity and functionality of the antigen receptors expressed by B cells. Let's break down the organization and function of these components: 1.V (Variable), D (Diversity), and J (Joining) Segments: 1. The heavy chain locus begins with the V segment region at the 5' end, where 45 potential V segments are located. 2. Each V segment contains a promoter region and a leader (L) sequence, which facilitates the co-translational import of the protein. 3. Following the V segments are the D segments, with 23 potential segments, and then the J segments, with 6 potential segments. 4. During B cell development, one V, one D, and one J segment are randomly selected and rearranged to form the variable region of the heavy chain. 2.Random Recombination: 1. The process of V(D)J recombination is random, meaning that the selection and rearrangement of gene segments occur without regard to their specific sequences. 2. This randomness contributes to the vast diversity of antigen receptors 5 generated by the immune system, allowing B cells to recognize a wide range of antigens. 3. C (Constant) Segments: 1. At the 3' end of the heavy chain locus are the C (constant) gene segments, of which there are 9 in humans. 2. Each C gene consists of multiple exons, including transmembrane (TM) and cytoplasmic (CYTO) domains, as well as intervening introns. 3. The TM and CYTO domains are important for anchoring the antibody molecule to the B cell membrane and for signaling and intracellular interactions, respectively. 4.Exons and Domain Structure: 1. The constant region of the heavy chain is encoded by the exons of the C gene segments. 2. The TM domain is encoded by one or more exons within the C gene, and it anchors the antibody molecule to the B cell membrane. 3. The CYTO domain, also encoded by exons within the C gene, plays a role in signaling through interactions with intracellular signaling molecules and cytoskeletal elements. 5.Function of TM and CYTO Domains: 1. The TM domain ensures that the antibody molecule remains associated with the B cell membrane, allowing it to function as a B cell receptor (BCR) for antigen recognition. 2. The CYTO domain transduces signals from the BCR upon antigen binding, initiating intracellular signaling cascades that lead to B cell activation, proliferation, and differentiation. In summary, the germline organization of the human immunoglobulin heavy chain locus includes V, D, and J segments for variable region diversity and C segments for constant region structure. The TM and CYTO domains encoded within the C gene segments are essential for anchoring the antibody molecule to the B cell membrane and for intracellular signaling, respectively, thereby contributing to the functionality of the B cell receptor. 5 Domain structure of Ig proteins Certainly! The domain structure of immunoglobulin (Ig) proteins, which include antibodies and T cell receptors (TCRs), plays a crucial role in their function and diversity. Let's break down the domain structure and the significance of the complementarity-determining region 3 (CDR3): 1.General Structure of Ig Proteins: 1. Ig proteins are composed of two identical heavy chains and two identical light chains, linked together by disulfide bonds. 2. Each heavy and light chain consists of multiple domains, including variable (V) domains and constant (C) domains. 3. The variable domains are responsible for antigen recognition and contain the hypervariable regions, also known as complementarity-determining regions (CDRs). 4. The constant domains determine the effector functions and structural characteristics of the Ig protein. 2.Complementarity-Determining Regions (CDRs): 1. CDRs are loops within the variable domains of Ig proteins that directly interact with antigen molecules. 2. There are three CDRs in each variable domain: CDR1, CDR2, and CDR3. 3. CDR3 is the most diverse and variable region among antibodies, TCRs, 6 and B cell receptors (BCRs). 4. CDR3 is formed by the joining of V, D (in the case of heavy chains), and J gene segments during the rearrangement process. 5. The diversity of CDR3 arises from the random recombination and addition or deletion of nucleotides at the junctions between these gene segments. 1.Significance of CDR3 Diversity: 1. The high diversity of CDR3 sequences allows Ig proteins to recognize a vast array of antigens with specificity. 2. The unique sequence of CDR3 contributes significantly to the antigenbinding affinity and specificity of each Ig protein. 3. In antibodies, the diversity of CDR3 is critical for the recognition and binding of diverse epitopes on antigens. 4. In TCRs, the diversity of CDR3 is essential for recognizing peptide antigens presented by major histocompatibility complex (MHC) molecules. 2.Non-Germline Sequences in CDR3: 1. During V(D)J recombination, non-germline sequences are introduced at the junctions between V, D, and J gene segments, particularly in CDR3. 2. These non-germline sequences result from the addition or deletion of nucleotides by the enzyme terminal deoxynucleotidyl transferase (TdT) during the recombination process. 3. Non-germline sequences contribute to the diversity and specificity of CDR3, allowing for the recognition of a wide range of antigens. In summary, the domain structure of Ig proteins, particularly the diversity of CDR3, is crucial for antigen recognition and immune function. The variability of CDR3 sequences, including the presence of non-germline sequences, allows Ig proteins to recognize and bind diverse antigens with high specificity, contributing to the effectiveness of the adaptive immune response. 6 Germline organization of human TCR a and b chains Similar organization principles as Ig heavy chain Certainly! The germline organization of human T cell receptor (TCR) α and β chains shares similar principles with the immunoglobulin (Ig) heavy chain in terms of their genetic organization and diversity generation. Here's a detailed explanation: 1.V (Variable), D (Diversity), and J (Joining) Segments: 1. Similar to Ig heavy chains, the TCR α and β chains undergo V(D)J recombination to generate diversity in their antigen receptor genes. 2. The germline organization of TCR genes includes multiple V, D (only in TCR β chains), and J gene segments. 3. These gene segments are arranged in a specific order along the chromosome, with the V segments located upstream, followed by the D segments (in the case of TCR β chains), and finally the J segments. 2.Random Recombination: 1. During T cell development, precursor cells undergo random rearrangement of V, D, and J gene segments to form functional TCR genes. 2. The process of V(D)J recombination is similar to that observed in Ig heavy chains and results in the generation of diverse TCR variable regions capable of recognizing a wide range of antigens. 3.Junctional Diversity: 7 1. As with Ig heavy chains, the junctions between rearranged V, D, and J gene segments in TCRs can undergo the addition or deletion of nucleotides. 2. This process, mediated by the enzyme terminal deoxynucleotidyl transferase (TdT), introduces additional diversity at the junctions and contributes to the variability of the TCR variable regions. 1.Generation of CDR3: 1. The complementarity-determining region 3 (CDR3) in TCRs, analogous to Ig variable domains, is the most diverse region and plays a crucial role in antigen recognition. 2. CDR3 is formed by the joining of rearranged V, D, and J gene segments, with additional diversity introduced by nucleotide addition or deletion. 3. The diversity of CDR3 allows TCRs to recognize and bind specific peptide antigens presented by major histocompatibility complex (MHC) molecules. 2.Constant Domains: 1. Similar to Ig proteins, TCRs also contain constant domains that determine their effector functions and structural characteristics. 2. The constant regions of TCR chains are responsible for mediating intracellular signaling upon antigen recognition and binding. In summary, the germline organization of human TCR α and β chains follows similar principles as the Ig heavy chain, including V(D)J recombination, junctional diversity, and the generation of diverse variable regions. These processes ensure the generation of a diverse repertoire of TCRs capable of recognizing and responding to a wide range of peptide antigens presented by MHC molecules. 7 Domain structure of TCR proteins The domain structure of T cell receptor (TCR) proteins consists of distinct regions within the alpha (α) and beta (β) chains, as well as associated signaling molecules such as CD3. Let's break down the domain structure of TCR proteins: 1.Alpha (α) Chain: 1. The alpha chain of the TCR consists of multiple domains, including variable (V), constant (C), transmembrane (TM), and cytoplasmic (CYTO) domains. 2. The variable domain of the alpha chain, formed by the V region, is responsible for antigen recognition and binding. 3. The constant domain, located at the C terminus of the alpha chain, is involved in signal transduction upon antigen binding. 4. The TM domain anchors the alpha chain to the cell membrane, ensuring proper localization of the TCR. 5. The CYTO domain, located intracellularly, interacts with signaling molecules to initiate downstream signaling cascades upon TCR activation. 2.Beta (β) Chain: 1. The beta chain of the TCR also consists of similar domains as the alpha chain, including V, C, TM, and CYTO domains. 2. The variable domain of the beta chain is responsible for antigen 8 recognition and binding, similar to the alpha chain. 3. The constant domain of the beta chain participates in signal transduction upon antigen binding. 4. The TM domain anchors the beta chain to the cell membrane, ensuring proper localization of the TCR. 5. The CYTO domain interacts with intracellular signaling molecules to initiate downstream signaling events upon TCR activation. 1.CD3 Complex: 1. The CD3 complex is associated with the TCR and is essential for TCR signaling and activation. 2. It consists of multiple subunits, including CD3γ, CD3δ, CD3ε, and ζ (zeta) chains. 3. The CD3 complex contains immunoreceptor tyrosine-based activation motifs (ITAMs) within its cytoplasmic tails. 4. Upon TCR engagement with antigen-MHC complexes, the CD3 ITAMs are phosphorylated, initiating downstream signaling pathways that lead to T cell activation. 2.Junctional Diversity and Non-Germline Sequences: 1. Junctional diversity refers to the variability introduced at the junctions between V, D, and J gene segments during TCR gene rearrangement. 2. Non-germline sequences, resulting from the addition or deletion of nucleotides at the junctions, contribute to the diversity of the TCR variable regions, particularly in the CDR3 region. 3. These non-germline sequences enhance the ability of the TCR to recognize a wide range of antigenic peptides presented by major histocompatibility complex (MHC) molecules. In summary, the domain structure of TCR proteins includes variable, constant, transmembrane, and cytoplasmic domains in both the alpha and beta chains, as well as associated signaling molecules such as the CD3 complex. Junctional diversity and non-germline sequences within the TCR variable regions contribute to the diversity and specificity of antigen recognition by T cells. 8 Diverse antigen receptor genes from the same germline DNA random V + random D + random J = V(D)J exon = Variable region of Ig heavy chain or TCR b chain random V + random J = VJ exon = Variable region of Ig light chain or TCR a chain N/P nucleotides are non-germline sequences added to segment junctions during gene rearrangement N = non-template P = palindromic Certainly! The generation of diverse antigen receptor genes from the same germline DNA is a fundamental process in the adaptive immune system, allowing B cells and T cells to recognize a vast array of antigens. Let's explore this process in detail: 1.Random V(D)J Recombination: 1. The diversity of antigen receptor genes, both immunoglobulin heavy chains (IgH) and T cell receptor beta chains (TCRβ), arises from the random rearrangement of gene segments known as V (Variable), D (Diversity), and J (Joining). 2. During B cell development in the bone marrow or T cell development in the thymus, precursor cells undergo V(D)J recombination, where one V, one D (in the case of IgH), and one J segment are randomly selected and rearranged to form a functional exon, known as the V(D)J exon. 3. This process results in the generation of a diverse repertoire of V(D)J exons with different combinations of V, D, and J gene segments, contributing to the diversity of antigen receptor genes. 2.Variable Region of Antigen Receptors: 1. The V(D)J exon forms the variable region of the antigen receptor, which is responsible for antigen recognition and binding. 2. In the case of IgH and TCRβ chains, the variable region of the receptor is 9 encoded by the V(D)J exon. 3. The diversity of the variable region allows B cells and T cells to recognize a wide range of antigens with high specificity. 1.VJ Recombination: 1. In addition to V(D)J recombination, Ig light chains (both kappa and lambda) and TCR alpha chains undergo a simplified recombination process known as VJ recombination. 2. During VJ recombination, one V and one J gene segment are randomly selected and rearranged to form a functional VJ exon, which encodes the variable region of the Ig light chain or TCR alpha chain. 3. This process contributes to the diversity of antigen receptors by generating a diverse repertoire of VJ exons with different combinations of V and J gene segments. 2.Non-Germline Sequences: 1. During V(D)J recombination, and to a lesser extent VJ recombination, nucleotides known as N or P nucleotides are added or deleted at the junctions between the rearranged gene segments. 2. These non-germline nucleotides are added by the enzyme terminal deoxynucleotidyl transferase (TdT) and contribute to the diversity of the variable region by introducing additional sequence variability. 3. The presence of N/P nucleotides at the junctions enhances the antigenbinding specificity of the receptor by creating unique amino acid sequences in the complementarity-determining regions (CDRs), which are responsible for antigen recognition. In summary, the diverse repertoire of antigen receptor genes in B cells and T cells is generated through the random rearrangement of V, D, and J gene segments, as well as the addition of non-germline nucleotides at the junctions between these segments. This process ensures that each B cell and T cell expresses a unique antigen receptor capable of recognizing and responding to specific antigens encountered in the body. 9 B cell and T cell antigen receptor diversity: a summary Combinatorial diversity Different combinations of gene segments that are united by V(d) J recombination Enhance by the association of two different randomly =-generated protein chains Junctional diversity largest contribution to antigen receptor diversity Addition or deletions of nucleotides occurs at junctions between segments These create new, non-germline sequences May lead to inactive receptor but critical for increasing diversity Occurs in CDR3 (most diversity and mistakes come into play) Certainly! The diversity of B cell and T cell antigen receptors is essential for their ability to recognize and respond to a wide range of antigens. This diversity is achieved through two main mechanisms: combinatorial diversity and junctional diversity. 1.Combinatorial Diversity: 1. Combinatorial diversity refers to the diversity generated by the different combinations of gene segments brought together through V(D)J recombination. 2. During B cell development in the bone marrow or T cell development in the thymus, precursor cells undergo V(D)J recombination, where one Variable (V), one Diversity (D) (in the case of immunoglobulin heavy chains), and one Joining (J) gene segment are randomly selected and rearranged to form a functional exon. 3. This process results in the generation of a diverse repertoire of V(D)J exons with different combinations of V, D, and J gene segments. 4. Combinatorial diversity is further enhanced by the association of two different randomly generated protein chains, such as the pairing of immunoglobulin heavy and light chains or T cell receptor alpha and beta chains. 2.Junctional Diversity: 10 1. Junctional diversity is the largest contributor to antigen receptor diversity and arises from the addition or deletion of nucleotides at the junctions between the rearranged gene segments. 2. During V(D)J recombination, the enzyme terminal deoxynucleotidyl transferase (TdT) adds random nucleotides at the junctions between gene segments, resulting in the introduction of non-germline sequences. 3. These non-germline sequences create new amino acid sequences in the complementarity-determining regions (CDRs) of the antigen receptor, which are responsible for antigen recognition. 4. Junctional diversity significantly increases the diversity of antigen receptors by generating unique CDR sequences, allowing B cells and T cells to recognize a wide variety of antigens. 5. While junctional diversity may occasionally lead to the generation of inactive receptors due to frame shifts or stop codons, it is critical for increasing the overall diversity of the antigen receptor repertoire. In summary, the diversity of B cell and T cell antigen receptors is generated through combinatorial diversity, which arises from different combinations of gene segments through V(D)J recombination, and junctional diversity, which results from the addition or deletion of nucleotides at the junctions between gene segments. These mechanisms ensure that each B cell and T cell expresses a unique antigen receptor capable of recognizing and responding to specific antigens encountered in the body. 10