T cell differentiation

Introduction

Within the blood and lymphoid organs the majority of CD4+ T cells are antigen-naive T cells. Only a small proportion are memory T cells.

Naive T cells have yet to encounter antigen and can only be activated by antigen that is presented by dendritic cells. After initial antigenic activation, naïve T-cells develop into an intermediate stage cell called the TH₀ cell which can then be activated by any antigen-presenting cell, e.g. Dendritic cells, macrophages or B-cells.

The TH₀ cells have the capacity to differentiate into TH₁ and TH₂ cells. The type of cell that develops depends on the antigen presenting cell type. Macrophages cause the TH₀ cell to develop into a TH₁ cell induced by IL-12 production following macrophage-antigen interaction. B cells cause the TH₀ cell to develop into a TH₂ cell induced by IL-10 production following B cell-antigen interaction. On antigenic stimulation the TH₁ or TH₂ cells become activated, undergo clonal expansion and secrete a range of different cytokines.

For any one cell the cytokine-secreting activation state is short-lived, lasting between 4 - 40 hours. After this time these cells either die or mature into the long-lived memory cells. The proliferation of T cells continues until the presentation of antigen ceases.

Dendritic Cells

Diagram showing the 2 Lineages of dendritic cells - Copyright Prof Dirk Werling DrMedVet PhD MRCVS

• There are two different lineages of dendritic cells:
  • From myeloid precursor cells
  • From plasmacytoid precursor cells
• Stimulate a primary T cell response
• Migrate through tissues, track to T cells dependent areas of lymph node and cluster with T cells
• Dendritic cells have unique capabilities to take up antigen by pathways involving phagocytosis, macropinocytosis and clathrin-coated pits
• Cell-surface antigen phenotype distinguishing it from Monocytes/macrophages and B cells
• Cell-surface expression pattern according to their differentiation/maturation/activation
• Main function is priming of T helper cells
• Produces chemokines

Maturation signals

• Exogenous
  • Bacteria or their products (LPS, LTA, lipoproteins)
  • Viruses or their products (dsRNA, G-RSV)
  • Protozoa or their products
  • Helminths (SEA, ES 62)
• Endogenous
  • Inflammatory mediators (IL-1/TNF-a, hsp, FcR)
  • Immune cells (CD40L, CD47, FasL)

Maturation of Dendritic Cells - Copyright Prof Dirk Werling DrMedVet PhD MRCVS

Antigen Presentation by Dendritic Cells

• Dendritic cells present antigen to T cells

• Circulating Monocytes differentiate to form immature dendritic cells called Langerhans Cells

• Langerhans cells sample the tissue fluid by endocytosis
  • Foreign organisms are internalised
  • Within the dendritic cells, antigen is digested to peptides
  • Some of the peptides formed bind to the cell’s MHC molecules

• The Langerhans cells leave the epithelium and travel via the afferent lymph. They are now called Veiled Cells

• Veiled cells enter the paracortical region of the lymph node where they present antigen to T cells. They are now called Interdigitating Dendritic Cells

TH₁ Cells

TH-1 cells boost macrophage response - B. Catchpole, RVC 2008

• TH1 cells help macrophages digest bacteria (organisms in vesicles)
• TH₁ cells secrete a range of cytokines.
• Cytokines secreted include:
  • IL-2.
    • Gives proliferation of both CD4+ and CD8+ T-cells.
    • This stimulation of proliferation of T cells is the main function of the TH₁ cell.
  • Interferon gamma (IFNγ).
    • Activates tissue macrophages
    • Is the principal effector mechanism in the defence against intracellular bacteria and parasites.
      • E.g. Mycobacteria, Brucella, Rickettsia (bacteria) and Leishmania, Coccidia, Babesia (parasites).
      • Activates macrophages and stimulates them to produce enzymes triggering the intracellular killing mechanisms, e.g.
        • Superoxide dismutase and myeloperoxidase.
          • Produce H₂O₂ and trigger the "superoxide burst".
        • Nitric oxide synthase.
          • Produces nitric oxide.
      • This is another example of the immune system working through the innate immune response.
    • Can act to suppress antibody synthesis.

TH₂ Cells

TH2 cells incite increased antibody production - B. Catchpole, RVC 2008

• TH2 cells help B cell produce antibody (organism in tissue fluid)
• The TH2 population influences B cell activation, proliferation and immunoglobulin production.
• The TH2 T cell population secrete a range of cytokines.
  • IL-4
    • Stimulates B cell growth.
    • Gives heavy chain switch from IgM to IgG , IgA and IgE.
    • Proliferation of basophils/ mast cells.
    • Can inhibit some T cell responses.
  • IL-5
    • Activates B cells.
    • Stimulates high rate B cells proliferation.
    • Promotes immunoglobulin synthesis.
    • Proliferation and differentiation of eosinophils.
  • IL-6
    • Activates B cells.
    • Stimulates high rate B cell proliferation.
    • Promotes immunoglobulin synthesis.

Common Functions of Th₁ and TH₂ Cells

• Both TH1 and TH2 cells produce IL-3 and granulocyte-macrophage colony stimulating factor (GM-CSF).
  • These act to activate and induce proliferation of neutrophils and also macrophages.
    • Neutrophils are the major phagocytic cells in the blood and the principal cells in acute inflammatory lesions.
      • Function chiefly in the defence against extracellular bacteria.
  • Therefore, one of the major biological functions of the activation of either TH subset is cytokine-controlled reactive haematopoiesis.

Cytotoxic T-Cells

• Cytotoxic T cells kill virus infected cells (organisms in cytoplasm)
• Viruses are intracellular pathogens that use the host cell machinery for pathogen protein synthesis.
  • Viral peptides associate with MHC class I and are expressed on the cell surface.
    • CD8+ cytotoxic T-lymphocytes (CTL) recognise the antigen-MHC complex.
• Cytotoxic T-cells secrete a pattern of cytokines similar to that of TH₁ cells.
  • I.e. IFNγ but not IL-2.
    • The IFNγ shifts the balance of the immune response in favour of TH₁ cells.
      • There is therefore an increased level of T-cell proliferation.
        • The initiation of the immune response via CTL leads to the selective proliferation of CTL - enhances the main mechanism of killing of virally-infected cells.

Killing Mechanism

• The CTl killing mechanism is initiated by direct CTL-target cell contact.
• The cells involved bind by antigen/MHC class I-TcR interaction.
  • This results in the CTL's intracellular granules becoming localised to the area of contact.
    • These granules contain most of the molecules responsible for cytotoxicity.
• Direct cell contact stimulates the release of the granule contents into the area of contact between the two cells.
• The granules contain two groups of cytotoxic molecules.
  • Perforin.
    • Structurally related to the complement component, C9.
    • Forms pores in the cell membrane.
  • Granzymes.
    • Proteolytic enzymes.
    • Target cell nucleases.
    • Cause programmed cell death.

T-Cell Activation

• T cells function only after recent activation by antigen.
• CD4 binds MHC class II.
  • CD4+ T-cells therefore recognise antigen only in association with MHC class II
• CD8 binds MHC class I.
  • CD8+ T-cells recognise antigen only in association with MHC class I.
• Activation of T cells requires two distinct signals.
  • Signal 1
    • The interaction of the TcR with the antigenic peptide/MHC complex on the antigen presenting cell.
  • Signal 2
    • The interaction of CD28 on the T cells with its ligand, CD80, on the antigen-presenting cell.
      • APC expression of CD80 only occurs after:
        • The engagement of pattern recognition or Fc receptors.
        • Activation with cytokines.
          • Interferons, IL-1β or TNFα.
      • Therefore signal 2 only occurs after the recognition of danger.

Scenarios

• No signal 1
  • T cells is not activated as there is no antigen.
• Both signal 1 and signal 2
  • T cells is activated into clonal expansion.
    • Produces cytokines or becomes cytotoxic.
  • This is the response to a dangerous antigen.
• Signal 1 but no signal 2
  • T-cell is triggered into apoptosis and dies.
  • This is the basis of "clonal deletion".
    • A major mechanism of tolerance.
    • Ensures that T-cells do not react with self (non-dangerous) antigens.

TCR Complex

TCR Complex - Copyright Prof Dirk Werling DrMedVet PhD MRCVS

• The TCR is always associated with CD3
  • The TCR and CD3 molecule is referred to as the TCR complex
• TCR is expressed on the surface of T cells in noncovalent association with a complex of transmembrane polypeptides
• CD3 contains 3 distinct polypeptide chains:
  • γ, ε, and δ
  • Members of the Ig superfamily
  • The ε chain associates with both γ and δ
  • chaperone role in transporting newly synthesized TCR molecules to the cell surface
  • Expressed exclusively on T cells
• CD3 also contains 2 identical chains:
  • ζ
  • 16 kDa
  • Found on T cells, macrophages and NK cells
  • Mice also can have an ε (eta) form

Response to Activation

• The response of the T cells to obtaining Signals 1 and 2 is:
  • To express the receptor for the cytokine interleukin-2 (IL-2).
  • In CD4+ T-cells only, the secretion of IL-2.
• The final trigger for clonal expansion is the engagement of IL-2R with IL-2.
  • The IL-2 can come from any activated CD4+ T-cell.
  • IL-2 produced by a CD4+ cell may also stimulate clonal expansion of that cell.

T-Helper Cell Function

TH1 and 2 selection is influenced by infection - B. Catchpole, RVC 2008

• The function of T helper cells is to regulate the immune response.
  • The cytokines they secrete exert their influence on other cell populations.
    • Most of the different effector cells of the immune system are affected by one or more of the cytokines secreted by TH cells.
• TH cells secrete cytokines for only a short period after they have been activated.
• The range of cytokines that TH cells secrete after activation chiefly determines their function.
  • Different T-helper cell subpopulations secrete different sets of cytokines.
    • Th₁ and TH₂ cells.