Immunology Block 1.2: Lymphocyte Development

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Lymphocyte Development - CRN No: 15569(Male), 15581 (Female) - Block 1.2 - Subject/Discipline: Immunology

• Expert: Dr.Sayed A.Quadri
• Block Coordinator: Dr.Sayed A.Quadri
• Topic: Lymphocyte Development

Learning Objectives

• Explain how diversity develops in the specificity of immunoglobulins and T-cell receptors
• Describe how mature B lymphocytes are produced in the bone marrow from common lymphoid progenitor cells
• Discuss the role of rearrangement of the membrane immunoglobulin genes in this process
• Explain the term "allelic exclusion" and its relevance

Learning Objectives - Maturation and Autoreactive Cells

• Explain how the maturation of autoreactive cells is prevented during development of B lymphocytes in bone marrow (central tolerance)
• Describe T-lymphocyte production in the thymus, including rearrangement of T-cell receptor genes
• Explain how the interaction between TCR and MHC determines the fate of double-positive thymocytes
• Detail the difference between positive and negative selection, and how potential threats by autoreactive cells are prevented

Cells of the Immune System

• Shows a hierarchical diagram of immune cells branching from stem cells, lymphoid and myeloid progenitors
• Illustrates different types of lymphocytes and granulocytes, including B cells, T cells, natural killer cells, neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages, and their relationships.

Development of Immune Repertoires

• Early lymphocyte development
• Production of diverse antigen receptors
• Maturation and selection: B lymphocyte, T lymphocyte

Development of Immune Repertoires - Complex Process

• 1000 million lymphocyte clones
• 325 million base pairs of genes
• Not enough genes in the human genome to encode every possible receptor
• The immune system develops diverse antigen receptors from limited genes, linked to B and T lymphocyte maturation process

Lymphocyte Maturation

• B lymphocytes mature in bone marrow.
• T lymphocytes mature in the thymus.

Lymphocyte Maturation - Steps

• Illustrates the progression from pro-lymphocyte to pre-lymphocyte to immature lymphocyte to mature lymphocyte.

Lymphocyte Maturation - Detailed Steps

• Common lymphoid progenitors in bone marrow differentiate into B or T cell progenitors.
• Transcription factors are activated for Ig and TCR gene accessibility.
• Immature lymphocytes proliferate stimulated by cytokines, predominantly IL-7.
• Antigen receptors (ARs) are produced through gene recombination.
• Lymphocytes selected for maturation occur at different stages of maturation.

Lymphocyte Maturation - Selection

• Lymphocytes are selected at multiple steps during maturation to preserve specificities.
• Selection is based on intact antigen receptor expression and recognition.
• Cells with functional antigen receptors survive and proliferate.
• Pre and immature lymphocytes failing to express receptors die by apoptosis.

Lymphocyte Maturation - Immature T cells

• Immature T cells selected in the thymus to recognize self MHC molecules (positive selection).
• Ensures complete maturation of cells to recognize antigens displayed by the same MHC molecules on APCs.
• Strongly self-reactive B or T cells eliminated to prevent autoimmune responses (negative selection).

Lymphocyte Maturation - Diagram

• Illustrations depicts detailed stages of B and T cell development, including checkpoints and selection processes.

Production of Diverse Antigen Receptors

• Somatic recombination of AR variable region leads to unique antigen specificities.
• Hematopoietic stem cells and early lymphoid progenitors contained multiple V region gene segments, one or few, C region, and D and J genes between V and C genes.
• Random selection and recombination of V, D, J gene segments lead to functional genes with unique variable regions (Ig and TCR).
• All antigen receptor gene loci = V, J, C genes.
• Ig heavy chain and TCR beta chain loci = V, J, C + D gene segments.

VDJ Recombination

• Lymphocyte progenitor → B lymphocyte = recombination of first one D gene segment with one J segment, followed by rearrangement of a V segment to the fused D-J element
• Somatic recombination occurs with V, D, or J gene segments in immature B and T lymphocytes via VDJ recombinase for efficient processing and expression.
• VDJ recombinase expressed only in immature B and T lymphocytes composed of the recombinase-activating gene 1 and 2 (RAG-1 and RAG-2).
• Recognizes DNA sequences in antigen receptor V, D, and J gene segments for efficient recombining.

VDJ Recombination - Details

• Recombinase brings two Ig or TCR gene segments close together and cleaves DNA.
• DNA breaks are repaired by ligases, producing a full-length recombined V-J or V-D-J without intervening DNA segments.
• Intact Ig heavy-chain and light-chain genes are rearranged and expressed exclusively in B cells.
• TCR alpha and beta genes are rearranged and expressed exclusively in T cells.

VDJ Recombination - Diversity

• Combinatorial diversity: different combinations of V, D, and J gene segments produce diverse antigen receptors.
• Junctional diversity: changes in nucleotide sequences at the junctions of recombining V, D, and J gene segments increase receptor diversity.

VDJ Recombination - Unlimited Diversity

• Junctional diversity is unlimited by three sequence changes removing nucleotides, addition of nucleotides (N-region or P-nucleotides)

Maturation & Selection of B Lymphocytes

• Progenitors proliferate pro-B cells.
• Cells that generate productive VDJ rearrangements develop into pre-B cells in the cytoplasm.
• μ chain and surrogate light chains associate with Igα and Igβ signaling molecules to form the Pre-B cell receptor (pre-BCR) complex.

Maturation & Selection of B Lymphocytes - Pre-BCR

• pre-BCR delivers signals to promote survival and proliferation of B lineage cells.
• Pre-B cells unable to make productive rearrangements or μ protein fail to express pre-BCR signals and die by apoptosis.

Maturation & Selection of B Lymphocytes (Ig protein stages)

• Illustrates stages of B cell development, starting from stem cell to final mature state, showing the evolution of Ig DNA, RNA and Ig expression

Allelic Exclusion

• Expressing only one of two Ig alleles
• Mechanism: When a functional heavy chain is made the pre-BCR is formed signaling redirecting Rag1 & Rag2 to the Ig light loci, and Rag1 and Rag2 expression turns off once a functional light chain is made.
• Interaction with self-antigen: Receptor editing—B cells can continue to rearrange Ig L genes.

Allelic Exclusion - Prevents Unwanted Responses

• Only one allele is expressed while the other is silenced in B lymphocytes
• Successful heavy-chain gene rearrangement from one chromosome, results in the shutdown of rearrangement of genetic material.

Anergy (Non-responsiveness)

• A lack of reaction in the body defense mechanisms to foreign substances
• An individual in a state of anergy indicates the immune system is unable to mount a normal immune response against a specific antigen (usually a self-antigen)
• A process that induces tolerance to prevent self-destruction (in addition to clonal deletion and immunoregulation)

Clonal Deletion

• Elimination of B and T cells expressing receptors for self-antigens before they become immunocompetent.
• Responsible for immune tolerance.
• Autoreactive T or B cells produced, reacting to body's own proteins (autoantigens) need neutralization prior to circulation to prevent potential attack on healthy tissues.

Maturation & Selection of T Lymphocytes

• T cell progenitors migrate from the bone marrow to the thymus.
• Immature progenitors (pro-T cells) formed, not expressing CD4 or CD8,
• Cells expand in influence of IL-7 production in thymus
• Successful VDJ recombination for TCR beta chain protein synthesis
• TCR β chain protein is expressed on the surface of protein Pre-Ta to form pre-T cells.

Positive and Negative selection in T cells

• Negative selection: Strong interaction resulting in the recognition of self peptide / MHC complexes
• Positive selection: Results from a weak interaction resulting in recognition of self-peptide / MHC complexes

Summary - Antigen Receptor Production

• Genes for antigen receptors are present in the germline, brought together during lymphocyte maturation.
• In B cells, Ig gene segments undergo recombination during maturation in bone marrow.
• In T cells, TCR gene segments undergo recombination during maturation in the thymus.

Summary - Specificity Generation & Elimination of Self-Reactive Cells

• Receptors of different specificities are generated by different combinations of V, D, and J gene segments.
• MHC restriction eliminates immature lymphocytes that strongly recognize self-antigens.

Thank You

Explore the intricacies of lymphocyte development in this quiz from Block 1.2 of the Immunology course. Focus on key topics such as immunoglobulin diversity, B lymphocyte production, and allelic exclusion. Perfect for students aiming to enhance their understanding of immunological concepts.