T-Cell Maturation and Function

T-cell Maturation and Development

T-cell Maturation Location

  • Thymus: A primary lymphoid organ located above the heart, responsible for T-cell maturation.

  • Thymocytes: Immature T cells originating from hematopoietic stem cells in the bone marrow that migrate to the thymus for selection and maturation into functional, self-tolerant T lymphocytes.

Similarities and Differences between B Cell and T Cell Development

Feature Comparison

  • Origin:

    • B Cells: Develop and mature in the bone marrow.

    • T Cells: Originate in bone marrow, mature in thymus.

    • Shared: Both derived from hematopoietic stem cells.

  • Maturation Site:

    • B Cells: Bone marrow.

    • T Cells: Thymus.

    • Shared: Both undergo gene rearrangement (VDJ recombination) for unique antigen receptors.

  • Receptor:

    • B-cell Receptor (BCR): Membrane-bound antibody.

    • T-cell Receptor (TCR).

    • Shared: Both undergo positive and negative selection to ensure self-tolerance.

  • Lineages:

    • B Cells: Plasma cells (secrete antibodies), memory B cells.

    • T Cells: Helper T cells (CD4⁺), cytotoxic T cells (CD8⁺), regulatory T cells.

    • Shared: Both form effector and memory cells.

  • Antigen Recognition:

    • B Cells: Recognize free-floating (soluble) antigens.

    • T Cells: Recognize antigens presented on MHC by other cells.

    • Shared: Both are part of adaptive immunity.

Initial Stages of T-cell Maturation and Migration in the Thymus

  1. Entry: Thymocytes enter through blood vessels at the corticomedullary junction.

  2. Migration Pattern:

    • Move outward to the cortex to proliferate and rearrange TCR genes.

    • Migrate toward the subcapsular region for further development.

    • Return inward through the cortex to the medulla as they undergo selection.

  3. Cellular Components:

    • Macrophages phagocytose apoptotic thymocytes that fail selection.

    • Dendritic cells present self-antigens during negative selection to eliminate self-reactive T cells.

Macrophages in the Thymus

  • Role:

    • Remove dead or nonfunctional thymocytes (cells that fail selection).

    • Help maintain thymic structure and self-tolerance.

  • Hassall’s Corpuscles:

    • Found in the medulla of the thymus.

    • Composed of epithelial cells arranged concentrically.

    • Secrete cytokines (such as thymic stromal lymphopoietin) that aid in the maturation of regulatory T cells.

DiGeorge’s Syndrome

  • Cause:

    • A chromosomal deletion at 22q11.2 disrupts normal development of the third and fourth pharyngeal pouches during embryogenesis.

  • Consequences:

    • Lack of a functional thymus leads to immature T cells and severe immunodeficiency, increasing infection susceptibility.

    • Associated with hypocalcemia (absent parathyroids) and heart defects.

Thymus Aging and T-cell Immunity

  • Thymic Involution: The thymus decreases in size with age, with tissue replacement by fat.

  • Impact on T-cell Function:

    • T-cell immunity does not immediately become defective due to long-lived, self-renewing mature T cells in peripheral tissues.

    • New naïve T-cell production declines, reducing response capacity to new antigens over time.

  • Thymectomy Consequences:

    • Minimal effect on immunity in adults due to pre-existing mature T cells.

    • Severe immune impairment in infants/children due to incomplete T-cell repertoire.

Life of a T Cell vs. B Cell

Feature Comparison

  • Lifespan:

    • T Cells: Long-lived, especially memory T cells.

    • B Cells: Short-lived unless differentiated into memory or plasma cells.

  • Self-renewing:

    • T Cells: Yes, memory T cells can divide to maintain numbers.

    • B Cells: Mostly no, mature B cells do not self-renew.

  • Reason for Lifespan Differences:

    • T Cells: Continuous circulation and immune surveillance with self-renewal for long-term protection.

    • B Cells: Depend on bone marrow for continual generation.

Thymocyte Stages: Double-Negative (DN) vs. Double-Positive (DP)

Double-Negative (DN) Thymocytes

  • Definition:

    • Lacking both CD4 and CD8 co-receptors (CD4⁻CD8⁻), representing the earliest stage of T-cell development.

  • Surface Proteins:

    • Express CD2, CD5, CD44, and CD25 (in certain DN sub-stages).

    • Do NOT express CD4, CD8, or a TCR yet.

  • Importance of Notch1:

    • Essential for T-cell lineage commitment, directing progenitor cells to become T cells instead of B cells.

    • Activates transcription factors promoting TCR gene rearrangement.

Double-Positive (DP) Thymocytes

  • Definition:

    • After successful β-chain rearrangement and pre-TCR signaling, express both CD4 and CD8, thus becoming double-positive (CD4⁺CD8⁺).

  • Surface Proteins:

    • Express CD4⁺, CD8⁺, TCR (αβ upon α-chain rearrangement).

    • Also express CD3 and other accessory signaling molecules.

T-cell Lineages: αβ and γδ

General Overview

  • Majority vs. Minority:

    • α:β T cells: Majority (~95%).

    • γ:δ T cells: Minority (~5%).

  • Common Precursor:

    • Both types derive from a common DN thymocyte precursor.

  • Loci Rearrangement:

    • β, γ, and δ loci begin rearranging simultaneously in DN thymocytes.

Competition for Lineage Determination

  • Whichever rearrangement occurs first determines the T-cell lineage:

    • If γ and δ chains are made first: Cell becomes γ:δ T cell.

    • If a β-chain is made first: It pairs with pTα → α:β T cell lineage is chosen.

  • Result: β-chain rearrangement is more frequent, leading to predominantly α:β T cells.

α and β Chain Rearrangement Concepts

Gene Segments Involved

  • β-chain: V, D, J segments (similar to Ig heavy chain).

  • α-chain: V, J segments (similar to Ig light chain).

Rearrangement Attempts

  • β-chain: Two attempts possible (one per chromosome).

  • α-chain: Many more attempts allowed due to multiple J segments and absence of allelic exclusion until a productive α-chain is formed.

RAG Genes and 12/23 Rule

  • RAG-1 and RAG-2: Needed for both α and β rearrangements.

  • 12/23 rule: Ensures correct joining of gene segments (a segment with a 12-bp spacer can only join to one with a 23-bp spacer).

Gene Expression and Thymocyte Stage

Stage Characteristics

  • DN Thymocytes:

    • Phenotype: CD4⁻CD8⁻.

    • Active Genes/Proteins: RAG-1/2 (for rearrangement), Notch1, CD25.

    • Inactive: No CD4/CD8 expression.

  • DP Thymocytes:

    • Phenotype: CD4⁺CD8⁺.

    • Active Genes/Proteins: TCRα, TCRβ, CD3 complex.

    • Inactive: RAG downregulated after successful TCR formation.

  • Single-Positive (SP) Thymocytes:

    • Phenotype: CD4⁺ or CD8⁺.

    • Active: Lineage-specific genes (helper or cytotoxic T cell markers).

    • Inactive: Opposite co-receptor turned off.

The Pre-T-Cell Receptor (Pre-TCR)

Components

  • Surrogate α chain:

    • pTα: Acts as a placeholder pairing with the rearranged β chain until a real α chain is produced.

  • Other Components:

    • Includes CD3 complex proteins for signal transduction.

    • Structure: Pre-TCR = β chain + pTα + CD3 + ζ chains.

Roles of the Pre-TCR

  • Triggers:

    • Cell proliferation and expansion of functional β chain cells.

    • Expression of CD4 and CD8, transitioning to DP thymocyte.

    • Initiation of α-chain rearrangement.

  • Inhibits:

    • Further β-chain rearrangement (allelic exclusion).

α-Chain Gene and Relation to Immunoglobulin Light Chains

  • Comparison:

    • TCR α-chain gene organized similarly to immunoglobulin κ and λ light-chain genes (uses V and J segments only).

  • Delta Locus Location:

    • The δ locus is located within the α locus; α rearrangement deletes δ gene segments, committing the cell to α:β lineage.

Co-expression of CD4 and CD8

  • Timing:

    • Occurs after β-chain rearrangement due to pre-TCR signaling, which induces expression of both co-receptors, moving the cell to the DP stage.

T-cell Development Checkpoints

Checkpoint Overview

  • Checkpoint 1:

    • Stage: After β-chain rearrangement.

    • Testing: Functionality of β chain via pre-TCR.

    • Successful Outcome: Cell proliferates, expresses CD4/CD8, and starts α rearrangement.

  • Checkpoint 2:

    • Stage: After α-chain rearrangement.

    • Testing: Formation of functional TCR (αβ).

    • Successful Outcome: Proceed to positive/negative selection.

  • Summary:

    • Pre-TCR = Checkpoint 1.

    • TCR (αβ) = Checkpoint 2.

Positive vs. Negative Selection

Feature Comparison

  • Mediating Cells:

    • Positive Selection: Cortical epithelial cells.

    • Negative Selection: Dendritic cells and macrophages (mostly medullary).

  • What’s Checked:

    • Positive Selection: Can TCR recognize self-MHC (with self-peptide)?

    • Negative Selection: Does TCR bind too strongly to self-antigen?

  • Outcomes:

    • Positive Selection: Moderate binding survives → ensures MHC restriction; strong binding → dies → prevents autoimmunity.

    • Negative Selection: Majority of thymocytes (~95%) die either by strong binding or by neglect if they fail positive selection.

Summary Flow of Selection Steps

  1. Double-Negative (DN): CD4⁻CD8⁻ stage → rearrangement of β, γ, δ chains → pre-TCR checkpoint.

  2. Double-Positive (DP): CD4⁺CD8⁺ stage → rearrangement of α → full TCR checkpoint.

  3. Single-Positive (SP): After positive/negative selection → exit thymus as mature T cell.

Continuing Rearrangement of the α-Chain Gene and Positive Selection

  • Flexibility:

    • The α-chain gene undergoes rearrangement without allelic exclusion, facilitating multiple attempts to produce α-chain variants.

  • Importance for Selection:

    • Each new TCR variant can recognize a slightly different specificity.

    • This increases the chances of a TCR successfully binding to self-MHC, enhancing positive selection success.

Importance of Self-Peptides and Self-MHC in Negative Selection

  • Function:

    • Negative selection is vital for ensuring self-tolerance by eliminating T cells that bind too strongly to self-peptides.

    • Allows surviving T cells to respond only to foreign antigens presented by self-MHC.

  • Cells Involved:

    • Medullary epithelial cells, dendritic cells, and macrophages present a variety of self-peptides for testing.

    • Central Tolerance: Mechanism to eliminate or inactivate self-reactive T cells in the thymus through negative selection and Treg development.

AIRE (Autoimmune Regulator)

  • Definition: A transcription factor vital for promoting presentation of tissue-specific self-antigens in the thymus, specifically in medullary thymic epithelial cells (mTECs).

  • Function:

    • Allows exposure of developing T cells to a diverse range of body antigens, ensuring elimination of T cells reacting strongly against these self-antigens.

  • Clinical Relevance:

    • AIRE failure results in autoimmune polyendocrine syndrome type 1 (APS-1).

Regulatory T Cells (Treg)

  • Origin: Derived from CD4⁺ T cells.

  • Distinguishing Surface Molecule: CD25 (IL-2 receptor α-chain), which is highly expressed on Treg cells.

  • Key Transcription Factor: FoxP3

    • Required for Treg differentiation and function; mutations in FoxP3 lead to IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-linked).

  • Function:

    • Suppress activation of self-reactive or overactive immune cells.

    • Maintain peripheral tolerance to prevent autoimmunity.

Activation of Naïve T Cells

  • Site of Activation:

    • The thymus is solely for T-cell maturation; activation occurs in secondary lymphoid organs (e.g., lymph nodes, spleen).

  • Activation Process:

    • Naïve T cells encounter specific antigens presented by professional antigen-presenting cells (APCs) such as:

    • Dendritic cells (most important).

    • Macrophages.

    • B cells.

  • MHC Interaction:

    • MHC I: Activates CD8⁺ cytotoxic T cells.

    • MHC II: Activates CD4⁺ helper T cells.

Central vs. Peripheral Tolerance

Comparison Table

  • Type of Tolerance:

    • Central Tolerance:

    • Location: Thymus (for T cells); Bone marrow (for B cells).

    • Mechanism: Negative selection leading to deletion of self-reactive cells.

    • Purpose: Prevent autoreactive cells from entering circulation.

    • Peripheral Tolerance:

    • Location: Peripheral tissues / secondary lymphoid organs.

    • Mechanism: Anergy, suppression by Treg cells, or activation-induced cell death.

    • Purpose: Prevent self-reactive cells that escaped central tolerance from causing autoimmunity.

AIDS and Effector T Cells

Key Information

  • Compromised Effector T Cells: CD4⁺ helper T cells.

  • Impact of Loss:

    • Weakens both cell-mediated and humoral immunity due to dependency on helper T cells for activating cytotoxic T cells and B cells.

  • Diagnostic Threshold:

    • AIDS is diagnosed when CD4⁺ T cell count falls below 200 cells/μL of blood.

  • Consequences:

    • Increased susceptibility to opportunistic infections and certain cancers.

Summary of T-cell Development and Function (Flow 17-23)

  1. α-chain Rearrangement: Increases odds of positive selection.

  2. Negative Selection: Ensures self-tolerance using self-peptides + self-MHC.

  3. AIRE Function: Allows broad antigen presentation in mTECs → enhances negative selection.

  4. Treg Role: Suppress overactive immune responses → maintain peripheral tolerance.

  5. Activation Location: Occurs in secondary lymphoid tissues, not in the thymus.

  6. Central vs. Peripheral Tolerance: Central tolerance in thymus; peripheral tolerance in tissues.

  7. AIDS Consequences: Loss of CD4⁺ helper T cells (<200/μL) leads to immune collapse.