Lymphocyte Trafficking

• 1. cells and B-cells


  Produced in the primary lymphoid organs, the thymus and the bone marrow
• Naïve lymphocytes


  Must circulate through secondary or peripheral lymphoid tissue, called the lymph nodes, in order to encounter foreign proteins for the first time and become activated
• Lymphocytes


  Have no functional activity until they encounter antigen, when they proliferate and gain specialised functions
• Activated T-cells and B-cells

  1. Initiate the adaptive immune response

  2. Continuous recirculation through the secondary lymphoid tissues ensures that rare antigen-specific lymphocytes encounter their specific antigen
• Lymphocytes


  Circulate between the blood and the lymph and back to blood
• Pathogens


  Enter the body and set up infection anywhere, but lymphocytes eventually meet antigens in the lymph nodes
• Drainage of lymph fluid from sites of infection to the lymph nodes


  Is important in carrying the pathogen either directly or enclosed in phagocytic cells to the lymph nodes
• If infection is established

  Large amounts of antigen is taken up by the phagocytes, but free antigens in the lymph fluid must be trapped by antigen-presenting cells in the lymph node
• 1. cells


  Must be activated first because the B-cell antibody response is dependent on the presence of activated T-cells
• Within the lymph nodes


  Antigen-presenting cells present the antigens to specific naïve T-cells which become activated and proliferate
• Trafficking of lymphocytes


  Is very important because it allows antigen-specific naive T-cells to come into contact with specific antigens, and allows naïve B-cells to come into contact with activated T-cells
• Proliferation of lymphocytes in lymph nodes during an infection results in 'swollen glands'
• Antigen Presenting Cells (APC)

  • Present antigen to T-Cells and stimulate T-Cells response
  • APC are specialised for capturing antigens and these cells accumulate in peripheral lymphoid tissue
  • APC in the lymph nodes capture antigens from lymph fluid, and APC in the spleen capture antigens from blood
• Macrophage


  • Specialised to internalise and present particulate antigens to T-cells
  • Resting macrophages have few MHC molecules on the cell surface, but expression is increased following ingestion of microbes
  • Bacteria are engulfed and degraded in the lysosomes, generating peptides that can be presented by MHC molecules on the surface of the macrophage to the TCR on naïve T-cells
• Dendritic Cell


  • Specialised to trap antigens in tissue (lung and skin) and migrate to the lymph nodes to present antigens to T-cells, sole function
  • Dendritic cells have high levels of MHC molecules on the cell surface and are specialised to cope with viral infection
• 1. Lymphocyte


  Cell-surface immunoglobulins allow B-cells to internalize large amounts of specific antigen and present it to T-cell
• 1. Cells


  • Recognise foreign antigens bound to MHC molecules on antigen presenting cells
  • T-cells are specialised to recognise foreign antigens as peptide fragments bound to proteins of the major histocompatability complex (MHC)
• Antigens


  Are presented to T-cells as a complex with major histocompatability complex (MHC) proteins
• MHC genes


  Produce two classes of MHC molecule, MHC class I and MHC class II, which differ in subtle ways but share major structural features
• MHC class I molecules

  • Collect fragments of viral proteins, which are made in the cytoplasm, and display them on the cell surface
  • MHC class I is made by all cells of the body (except red blood cells)
  • MHC class I protein is coded by three polymorphic genes called HLA-A, HLA-B and HLA-C. One gene is inherited from each parent and equally expressed so cells can express 6 different Class I molecules
• MHC class II molecules

  • Bind peptides derived from proteins in intracellular vesicles, such as those internalised by phagocytosis by macrophages or dendritic cells, or by cell surface immunoglobulins on B-cells
  • MHC class II is made by macrophages, dendritic cells and B-cells
  • There are also 3 sets of polymorphic MHC class II genes, HLA-DR, HLA-DQ and HLA-DP
  • MHC Class II is made up of two chains, and since the alpha chain of one allele may associate with the beta chain of the other allele, a number of MHC class II variants can arise
• The two classes of MHC molecule

  • Are recognised by different groups of T-cells
  • MHC class I is recognised by CD8+ cytotoxic T-cells and MHC class II is recognised by CD4+ helper T-cells
• Dendritic cells


  Are infected by a wide range of viruses and present viral peptides on either MHC class I too naïve CD8 T-cells or on MHC class II to naïve CD4 T-cells
• MHC Class II


  Traps peptides derived from proteins degraded in intracellular vesicles
• Macrophage


  1. Bacterium infects macrophage, degradation in lysosome produces peptide fragments

  2. Bacterial fragments are bound by MHC Class II in vesicles

  3. Bound peptides are transported to the cell surface for presentation to T-Cells
• Antigen Processing & Presentation

  1. Vesicles within macrophages contain peptides derived from bacteria

  2. Extracellular bacteria and their toxic products are taken up by macrophages by the process of phagocytosis and degraded within vesicles, generating these peptides

  3. There are also some bacteria, such as the mycobacteria that cause TB and leprosy, that live in vesicles that are resistant to fusion with lysosomes, but also generate peptides

  4. The process of modifying a foreign protein and generating antigenic peptides is called antigen processing

  5. MHC molecules are formed in the endoplasmic reticulum (ER)

  6. Fragments of ER containing MHC molecules bud off and fuse with vesicles containing the antigenic peptides

  7. MHC molecules bind peptides within the vesicle and carry them to the cell surface

  8. The MHC molecule displays the peptide at the cell surface, this is antigen presentation

  9. The complex of peptide with MHC molecules is very stable and irreversible, allowing long term display and effective antigen presentation
• The MHC (major histocompatability complex) molecules


  • Involved in antigen presentation are encoded in a cluster of closely linked genes
  • This was first recognised as the site of genes that cause T-cells to reject tissues transplanted from unrelated donors to recipients
  • Each MHC molecule is coded by a separate stable gene (unlike the Igs and TCRs)
  • However there are many genetic variants giving rise to a degree of MHC diversity or polymorphism which is the basis of tissue-type and the chief cause of rejection of transplanted organs
  • When donor and recipient are of different MHC types, the immune system of the recipient normally makes a vigorous immune response against the donor's MHC molecules, which it perceives as foreign
• Antigen Presentation and Co-Stimulation Activates T-Cells


  1. The first encounter of a naïve T-cell with antigen on an antigen presenting cell (APC) in the lymph node stimulates the primary immune response, and also generates immunological memory which provides faster and more efficient protection against a pathogen the next time

  2. The MHC molecules promote the interaction between APC and naïve T-cells

  3. MHC class II molecules present bacterial peptides to specific T-cell receptors (TCR) on naïve CD4-positive T-cells. CD4 is a co-receptor for the TCR and interacts only with self-MHC class II

  4. MHC class I molecules present viral peptides to TCR on naïve CD8-positive T-cells. CD8 is a co-receptor for TCR on cytotoxic T-cells and interacts only with self-MHC class I

  5. APC accumulate in the lymph nodes

  6. T-cells leave the blood stream in the lymph nodes by crossing the walls of specialised blood vessels and come into contact with the APC

  7. Continual passage of T-cells past the APC allows for the recognition of antigen by the rare T-cells expressing the antigen-specific TCR

  8. When a T-cell recognises an antigen, it stops trafficking and becomes activated

  9. Binding of antigen to the specific TCR primes the cell for activation, but the most distinctive feature of all APC is co-stimulatory activity

  10. Molecules, which are only present on APC, bind to receptors on T-cells and stimulate the synthesis of IL-2 and the IL-2 receptor on the T-cell surface

  11. When IL-2 binds to its receptor, T-cells are stimulated to divide and undergo clonal expansion, one cell producing thousands of daughters each with the same TCR

  12. T-cells also differentiate, over 4-5 days, into armed effector T-cells. These are T-cells with the capacity to synthesise all of the proteins required for their specialised function as helper T-cells (Th1 or Th2) or cytotoxic T-cells

  13. Effector cells then act immediately, leaving the lymph node to find target cells at the site of infection
• Co-Stimulation


  • Neither chain of the TCR has a large cytoplasmic domain that can signal into the T-cell that the TCR has bound its antigen/MHC ligand complex
  • This signalling function is carried out by invariant proteins of the CD3 complex (CD3γ, CD3δ, CD3ε) and the ζ protein that are stably associated with the TCR on the cell surface
  • The proteins of the CD3 complex are required for the cell surface expression of TCR
  • All T-cells are CD3-positive. The cytoplasmic domains of all these proteins associate with protein tyrosine kinases in the cytoplasm, to initiate a cascade of intracellular signalling pathways following binding of antigen/MHC complex
  • T-cells fall into 2 major classes CD4 and CD8, that differ in the class of MHC molecule they recognise and in their effector functions
  • CD4 binds to parts of the MHC class II molecule and CD8 binds to parts of the MHC Class I molecule and play an important role in the differential recognition of MHC molecules
  • During antigen recognition, CD4 and CD8 associate with the TCR on the T-cell surface and are known as co-receptors
  • The cytoplasmic tails of CD4 and CD8 associate with tyrosine kinases and contribute to cell signalling, lowering the dose of antigen required for cell activation
  • The binding of antigen/MHC complex to the antigen specific TCR and either CD4 or CD8 co-receptor triggers the cell, but does not on its own stimulate the naïve T-cells to undergo proliferation and differentiation into armed effector T-cells
  • The clonal expansion of naïve T-cells requires a co-stimulatory signal, delivered by the same antigen presenting cell
  • The best characterised co-stimulatory molecules are the B7 glycoproteins that bind and activate CD28 on T-cells to provide the second activation signal leading to IL-2 and IL-2 receptor synthesis, and T-cell proliferation
  • Dendritic cells are professional antigen presenting cells that express co-stimulatory molecules constitutively. Macrophages, however, do not normally express MHC Class II or B7 molecules, but are stimulated by ingestion of bacteria to express both of these molecules
  • Without the co-stimulation presentation of antigen to T-cells makes them anergic, and unresponsive
• 1. Cells


  • Once activated, T-cells express CTLA4. CTLA-4 binds B7 strongly, competes with CD28, and delivers inhibitory signals to activated T-cells
  • Resting antigen presenting cells (APCs) express few or no B7 co-stimulatory molecules and fail to activate naïve T-cells
  • TCR-dependent recognition of antigen alone is not enough to stimulate T-cell proliferation
  • However, microbes and cytokines produced in response to microbes activate APCs and stimulate the expression of B7 co-stimulator molecules
  • B7-CD28 interactions stimulate the expansion and differentiation of naïve T-cells through increasing IL-2 and IL-2 receptor expression on T-cells
  • CTLA-4 is a high affinity receptor for B7 and expression of CTLA-4 is upregulated on activated T-cells
  • CTLA-4 has a higher affinity for B7 than CD28. T-cell responses are terminated by T-cell CTLA-4 interaction with B7 on APCs, ie feedback inhibition
• Function of Macrophages


  • Phagocytosis of opsonised bacteria at sites of infection
  • Regulation of neutrophil recruitment to sites of infection by cytokine synthesis
  • Presentation of foreign antigens to naïve T-cells in the lymph nodes
• Function of Activated T-Cells

  • Phagocytosis of opsonised bacteria at sites of infection
  • Regulation of neutrophil recruitment to sites of infection by cytokine synthesis
  • Presentation of foreign antigens to naïve T-cells in the lymph nodes