Immunology Chapter 7


Published on

Published in: Technology, Health & Medicine
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Immunology Chapter 7

  1. 1. Chapter 7 Development Of T Lymphocytes
  2. 2. Chapter 7 <ul><li>The development of T cells in the thymus </li></ul><ul><ul><li>Stages of gene rearrangement that produce the 1  repertoire of T-cell receptors </li></ul></ul><ul><li>Positive and negative selection of T-cell repertoire </li></ul><ul><ul><li>Processes of + & - selection that act on the primary repertoire of T-cell receptors in the thymus to produce the circulating population of mature; naïve T cells </li></ul></ul>
  3. 3. The Development Of T Cells Compared To The Development Of B Cells <ul><li>Similarities in the development of T and B lymphocytes: </li></ul><ul><ul><li>Derive from bone marrow stem cells </li></ul></ul><ul><ul><li>Undergo gene rearrangement  antigen receptors </li></ul></ul><ul><ul><ul><li>B cells rearrange in the bone marrow </li></ul></ul></ul><ul><ul><ul><li>Precursors of T cells leave the bone marrow ->thymus </li></ul></ul></ul><ul><li>The formation of 2 distinct T cell lineages </li></ul><ul><ul><ul><ul><li> :  Receptors (1-2% of the primary repertoire) </li></ul></ul></ul></ul><ul><ul><ul><ul><li> :  Receptors (not as stringent selection) </li></ul></ul></ul></ul>
  4. 4. Primary Lymphoid Tissue For T Cell Development Is The Thymus <ul><li>Major function of the thymus function is to ensure that mature T cells that leave thymus are restricted to the particular MHC class expressed by an individual person ( self -MHC ) </li></ul><ul><li>Two selection processes: </li></ul><ul><ul><ul><ul><li>Positive selection leads to the death of immature T cells having receptors that do not interact with any self-MHC class I and II </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Negative selection induces the death of those immature T cells that are autoreactive (receptors bind too strongly to a self-MHC molecule) </li></ul></ul></ul></ul><ul><li>Mature T cell leaving the thymus to circulate in the secondary lymphoid organs is: </li></ul><ul><ul><ul><li>Rendered tolerant of self-antigens </li></ul></ul></ul><ul><ul><ul><li>Responsive to foreign antigens </li></ul></ul></ul><ul><ul><ul><li>Ready to fight infection </li></ul></ul></ul>
  5. 5. Only 1-2% of  T cells exit the thymus and go into circulation.
  6. 6. The Development Of T Cells In The Thymus
  7. 7. The Development Of T Cells In The Thymus <ul><li>T cells originate from bone marrow stem cells </li></ul><ul><ul><li>Emigrate to mature in the thymus </li></ul></ul><ul><ul><li>Named thymus-dependent lymphocytes  T cells </li></ul></ul><ul><ul><ul><li>Majority are  :  cells </li></ul></ul></ul><ul><ul><ul><li>Minority are  :  cells </li></ul></ul></ul><ul><ul><ul><ul><li>2 lineages develop in parallel from common precursors </li></ul></ul></ul></ul><ul><li>While developing in the thymus </li></ul><ul><ul><li>T cells also express other cell-surface proteins related to their eventual functions </li></ul></ul><ul><ul><ul><li>Examples are CD4 and CD8 glycoproteins </li></ul></ul></ul>
  8. 8. T Cells Derive From Bone Marrow Stem Cells <ul><li>From the bone marrow , T cell precursors migrate through the blood to the thymus </li></ul><ul><li>Thymus is where the development of T cells occurs </li></ul><ul><li>Mature T cells then leave the thymus in the blood and enter the secondary lymphoid organs, such as the spleen or lymph nodes. </li></ul><ul><li>In the absence of activation mature T cells recirculate between the blood , the secondary lymphoid tissues , the lymph and the GALT . </li></ul>
  9. 9. T Cells Develop In The Thymus <ul><li>Thymus is found in the upper anterior thorax above the heart </li></ul><ul><li>Immature T cells - called thymocytes - are embedded in epithelial cell network called the thymic stroma </li></ul><ul><li>Thymus is primary lymphoid organ: </li></ul><ul><ul><li>Involved in the development of T cells </li></ul></ul><ul><ul><li>Not involved in lymphocyte recirculation via lymph </li></ul></ul><ul><ul><li>Blood is the only route through which T cells enter and exit </li></ul></ul>
  10. 10. T Cells Develop In The Thymus <ul><li>In the embryonic development of thymus </li></ul><ul><ul><ul><li>Epithelial cells of cortex  outer </li></ul></ul></ul><ul><ul><ul><li>Epithelial cells of medulla  inner </li></ul></ul></ul><ul><ul><ul><li>Rudimentary thymus called thymic anlage </li></ul></ul></ul><ul><ul><ul><ul><li>Is colonized by cells from bone marrow </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Progenitor cells  thymocytes & dendritic cells  populate medulla </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Bone marrow derived macrophages  also populate medulla (also macrophages scattered throughout the cortex of thymus </li></ul></ul></ul></ul></ul><ul><li>Thymocyte mature  progressively move from outer subcapsular region to the inner cortex and the medullar </li></ul>
  11. 11. The Epithelial Cells Of The Thymus Form A Network Surrounding Developing Thymocytes <ul><li>Scanning electron micrograph of thymus </li></ul><ul><li>Developing thymocytes (spherical cells) occupy the interstices of an extensive network of epithelial cells </li></ul>
  12. 12. Immature thymocytes Immature thymocytes Cortical epithelial cells Macrophages Mature thymocytes Medullary epithelial cells Dendritic cells Macrophages Mature thymocytes
  13. 13. The Cellular Organization Of The Thymus <ul><li>Macrophages in both cortex and medulla remove the many thymocytes that fail to mature properly </li></ul><ul><li>Hassall’s corpuscles </li></ul><ul><ul><li>Characteristic feature of </li></ul></ul><ul><ul><li>The medulla </li></ul></ul><ul><ul><li>(? Sites of cell destruction?) </li></ul></ul><ul><li>Thymus= multi-lobal (stained with hematoxylin and eosin, viewed by light microscope) </li></ul><ul><li>The darker staining of cortex vs with the lighter stained medulla </li></ul>
  14. 14. DiGeorge’s Syndrome <ul><li>Example of importance for the development a functional T-cell repertoire </li></ul><ul><li>A deletion in chromosome 22 in which the thymus fails to develop and T cells are absent </li></ul><ul><li>Susceptibility to wide range of opportunistic infections  resembles SCID (severe combined immunodeficiency disease </li></ul>
  15. 15. Thymus And Aging <ul><li>Thymus fully developed before birth; is most active in the young ; atrophies with age </li></ul><ul><ul><li>Progressively shrinks, fat gradually claiming areas once packed with thymocytes = involution of the thymus </li></ul></ul><ul><ul><li>Reduced production of new T cells not noticeably impairing T cell immunity </li></ul></ul><ul><ul><ul><li>Nor does thymectomy = removal of the thymus affect T cell immunity of adults </li></ul></ul></ul><ul><li>Once established, the repertoire of mature peripheral T cells is long lived and/or self-renewing </li></ul><ul><ul><li>Differs from the mature B-cell = shorted lived cells that are continually being replenished from the bone marrow </li></ul></ul>
  16. 16. The Two Lineages Of T Cells Arise From A Common Thymocyte Progenitor <ul><li>Maturation of thymocytes into mature T cells occurs in distinct steps </li></ul><ul><ul><li>Marked by changes in the status of the TCR genes </li></ul></ul><ul><ul><li>Expression of the TCR protein </li></ul></ul><ul><ul><li>Production of other T-cell surface glycoproteins </li></ul></ul><ul><ul><ul><li>CD4, CD8, CD3 complex </li></ul></ul></ul><ul><li>Changes in cell surface proteins expressed at each developmental stage is a way to distinguish between subpopulations of developing thymocytes </li></ul>
  17. 17. The Two Lineages Of T Cells Arise From A Common Thymocyte Progenitor <ul><li>Progenitor T cells that enter the thymus lack the cell surface glycoproteins (CD4, CD8, CD3) of mature T cells but they do have CD34 (a cell surface glycoprotein of stem cells). </li></ul><ul><li>The TCR genes are in germline configuration </li></ul><ul><li>Upon interaction with thymic stromal cells, the progenitor T cells will proliferate and differentiate </li></ul><ul><li>Approximately one week later, progenitor T cells will express the T-cell specific adhesion molecule CD2 and other surface markers such as CD5 but no TCR complex </li></ul><ul><ul><li>No CD4 or CD8  called “double negative” thymocytes </li></ul></ul><ul><li>IL-7 receptor on T-cells is essential for binding IL-7 secreted by thymic stromal cells – helps tell the T-cell what to do next in its maturation. </li></ul><ul><li>Notch 1 – at all stages of maturation in the thymus signals are sent through this receptor to drive the T-cell in their differentiation. </li></ul>
  18. 18.
  19. 19. T-cell Development in the Thymus is Driven by the Receptor Notch 1
  20. 20. T Cells Have Two Lineages Distinguished By The Expression Of An  Or A  TCR <ul><li>Commitment does not occur before TCR rearrangement but is a race to obtain a productive rearrangement </li></ul><ul><li>Thymocytes will rearrange their  ,  and  -chain genes about the same time </li></ul><ul><ul><li>Different from B-cell development (recall: each type of Ig gene is rearranged in turn and in a set order) </li></ul></ul><ul><li>Productive  and  -chain gene rearrangement made prior to a productive  -chain rearrangement leads to  receptor which signals cell to stop rearrangement of  chain </li></ul><ul><li>More frequently the  chain productively rearranges before the  and  -chains </li></ul><ul><li> -Chain assembles with a surrogate  chain = pt  (pre-T-cell receptor) which signals the cell to halt rearrangement of  ,  and  -chain genes and begin to proliferate </li></ul>
  21. 21.
  22. 22. <ul><li>After expression of the pre-TCR, the recombination machinery is reactivated & targeted towards the  chain loci (and the  and  loci) </li></ul><ul><li>In a minority of these cells successful completion of  and  chain gene rearrangements occurs before the  chain gene has rearranged  the  lineage </li></ul><ul><li>In a majority of these cells, productive rearrangement of the  -chain gene occurs first  an  T cell </li></ul><ul><li>Recall the  chain locus is located within the  -chain locus…a rearrangement at an  -chain locus results in the deletion of the complete  chain locus from the chromosome </li></ul>T cells have two lineages distinguished by the expression of an  or a  TCR
  23. 23. T Cells Have Two Lineages Distinguished By The Expression Of An  Or A  TCR <ul><li>Cells committed to one lineage can contain productive rearrangements for the TCR genes of the other lineage (except for  -chain). </li></ul>
  24. 24. Immature T Cells That Undergo Apoptosis Are Ingested By Macrophages In The Thymic Cortex <ul><li>Failure to make a productive rearrangement results in death by apoptosis (fate of about 98% of thymocytes) </li></ul><ul><ul><li>Macrophages in thymus continually remove dead cells </li></ul></ul><ul><ul><li>Cells have been stained for apoptosis with a red dye </li></ul></ul><ul><ul><li>Apoptotic cells are scattered throughout the cortex but are rare in the medulla </li></ul></ul><ul><ul><li>Higher magnification red for apoptotic cells and blue for macrophages </li></ul></ul><ul><ul><li>Apoptotic cells are visible within macrophages </li></ul></ul>cortex medulla
  25. 25. Production of T-cell receptor β chain stops β -chain rearrangement and leads to expression of CD4 and CD8 <ul><li> Receptor most abundant type found on T cells. </li></ul><ul><li>TCR  -chain locus = variable (V), diversity (D) and joining (J) gene segments and is rearranged first (similar to heavy chain in Ig’s) </li></ul><ul><li>TCR  -chain locus has no D segments and is rearranged after the  -chain (similar to light chain in Ig’s) </li></ul>
  26. 26. Production of T-cell receptor β chain stops β -chain rearrangement and leads to expression of CD4 and CD8 <ul><li>Production of a functional  -chain gene   -chain translation and assembly with a surrogate  -chain (preT  ), CD3 proteins and  chain to form a pre-T cell receptor  transported to the cell surface </li></ul><ul><li>Role of pre –T-cell receptor is analogous to the pre-B-cell receptor in B-cell development </li></ul><ul><ul><li>Triggers the thymocyte to proliferate and halt  -chain gene rearrangement </li></ul></ul><ul><ul><li>Ensures only one type of T-cell receptor  -chain is expressed by the T cell </li></ul></ul>
  27. 27. Unproductive Rearrangement At One  -Chain Locus Can Lead To Rearrangement Of The  -Chain On The Homologous Chromosome, Rearrangement At the Same Locus Can Also Occur <ul><li>This is not the case for Ig H-chain genes </li></ul><ul><ul><li>Exists with T-cell  -chain because 2 sets of D  J  and C  gene segments are tandemly associated with the V  gene segments. </li></ul></ul><ul><ul><ul><li>Potential to “try out” up to 4 gene rearrangements = 80% of T cells make successful rearrangement of the  -chain gene. </li></ul></ul></ul><ul><ul><ul><li>Only 55% success rate for productive H-chain gene rearrangement by B cells. </li></ul></ul></ul>
  28. 28. Rescue Of Unproductive  -Chain Gene Rearrangements
  29. 29. Unproductive Rearrangement At One  -Chain Locus Can Lead To Rearrangement Of The  -Chain On The Homologous Chromosome, Rearrangement At the Same Locus Can Also Occur <ul><li>Successful rearrangement of a  -chain gene induces expression of the two co-receptors (CD4 & CD8) </li></ul><ul><ul><li>Called “double-positive” thymocytes </li></ul></ul><ul><ul><li>Found predominantly in the inner cortex of the thymus </li></ul></ul><ul><ul><li>Interact intimately with the network of epithelial cells </li></ul></ul><ul><li>During cell proliferation initiated by signaling through the pre-TCR, expression of the RAG-1 and RAG-2 genes is repressed (allelic exclusion of the beta chain) </li></ul><ul><li>No rearrangement of the  -chain genes occurs until the double-positive cells stop dividing </li></ul><ul><ul><li>Ensures each cell with a productive  -chain gene rearrangement produces many daughter cells that have the potential to express a different  -chain gene </li></ul></ul>
  30. 30. T-cell Receptor  -Chain Genes Can Undergo Several Successive Rearrangements <ul><li>The TCR  -chain can undergo several successive gene rearrangements </li></ul><ul><ul><ul><li>Presence of many V  and over 50 J  gene segments allows many successive rearrangements (like Ig  ) </li></ul></ul></ul><ul><ul><ul><li>Almost every developing T cell will make a productive  -chain rearrangement </li></ul></ul></ul><ul><ul><ul><li>The  chain loci is much larger than the  -chain loci which adds to is flexibility </li></ul></ul></ul>
  31. 31. Successive Gene Rearrangements Allow The Replacement Of One T-cell Receptor  Chain By Another
  32. 32.
  33. 33. Checkpoints in T-cell Development <ul><li>Check-point 1 - Once a beta chain is produced it is sent the ER to make sure it can bind to the surrogate alpha chain (pT  ) – Pre-TCR </li></ul><ul><li>Check-point 2 – Once an alpha chain is made it is sent to the ER to make sure it can bind to the beta chain - TCR </li></ul>
  34. 34. Stages of  T cell development correlate with TCR gene rearrangement and expression of cell surface proteins.
  35. 35. Positive And Negative Selection Of The T-cell Repertoire
  36. 36. Positive And Negative Selection Of The T-cell Repertoire <ul><li>Second phase of T cell development involves selection of T cells bearing TCRs that can recognize an individual’s own MHC presenting peptides. </li></ul><ul><li>This selection process involves only  T cells and not  T cells. </li></ul>
  37. 37. T-cells That Can Recognize self-MHC Are Positively Selected In The Cortex of the Thymus <ul><li>Gene rearrangement produces a repertoire of T cells bearing TCRs that can interact with the hundreds of MHC class I and II molecules present in a population. </li></ul><ul><li>Therefore, TCRs expressed on the T cells of one individual are not made to specifically interact with only that individual's MHC molecules. </li></ul><ul><li>Only a small population (2%) of double positive thymocytes will be able to bind to a specific MHC the rest die by apoptosis in the cortex. </li></ul><ul><li>Positive selection is the process by which this small population of T cells that reacts with the individual’s own MHC molecules is selected. </li></ul>
  38. 38. T-cells That Can Recognize self-MHC Are Positively Selected In The Thymus <ul><li>Positive selection takes place in the cortex of the thymus and is mediated by cortical epithelial cells bearing complexes of class I and class II self-MHC and self-peptides . </li></ul><ul><li>Cortical epithelial cells form web of cell processes that envelope CD4,CD8 double-positive thymocytes. </li></ul><ul><li>At the point of contact – interactions between the  TCR of thymocytes with self-MHC and self-peptide are tested. </li></ul><ul><ul><li>If a peptide:MHC complex is bound by a thymocyte within 3-4 days of expressing a functional TCR, then a positive signal is delivered to the thymocyte. </li></ul></ul><ul><ul><li>A thymocyte that does not receive a signal dies by apoptosis and is removed by macrophages. </li></ul></ul>
  39. 39. T-cells that can recognize self-MHC are positively selected in the thymus
  40. 40. T-cells that can recognize self-MHC are positively selected in the thymus <ul><li>Self-peptides presented in the MHC molecules of cortical epithelial cells are derived from self-proteins present in the thymus. </li></ul><ul><li>The number of different peptides that can be presented by one individual’s MHC molecule is estimated to be about 10,000 . </li></ul><ul><li>For someone who is heterozygous for the six major HLA genes about 120,000 self-peptides could be presented by 12 different MHC class I and II molecules. </li></ul><ul><li>The mature T-cell repertoire is about tens of millions or more, so most of the self-peptides:MHC complex will bind a T cell receptor during positive selection. </li></ul>
  41. 41.
  42. 42. Positive Selection Controls Expression Of The CD4 Or CD8 Co-receptor <ul><li>Positive selection also determines whether a double positive thymocyte matures into a CD8 or CD4 T cell, known as “single-positive” thymocytes. </li></ul><ul><li>CD4 T cells only interact with MHC class II molecules and CD8 T cells interact with MHC class I molecules. </li></ul><ul><li>During positive selection… </li></ul><ul><ul><li>When a CD4 CD8 double-positive thymocyte interacts through its TCR with a class I MHC molecule, CD8 is recruited and CD4 is excluded. </li></ul></ul><ul><ul><li>When a CD4 CD8 double-positive thymocyte interacts through its TCR with a class II MHC molecule, CD4 is recruited and CD8 is excluded. </li></ul></ul>
  43. 43.
  44. 44. Rearrangement Of  -Chain Stops Genes Once A Cell Has Been Positively Selected <ul><li>Rearrangement of the  -chain locus continues throughout the 3-4 days of positive selection – hence a T cell can change the specificity of the TCR it expresses. </li></ul><ul><li>Once a T cell is positively selected, rearrangement of the  -chain stops . </li></ul><ul><li>Some double positive thymocytes can express two  -chains (one from maternal allele and one from the paternal allele) and thus two types of TCR and undergo positive selection by engagement of one of these receptors. </li></ul><ul><li>The number of positively selected cells is very small, therefore it is rare that one cell will have two selected TCRs (one receptor is usually nonfunctional). </li></ul>
  45. 45. T-cells Specific For Self-antigens Are Removed In The Thymus By Negative Selection <ul><li>Negative selection serves to delete T cells whose antigen receptors bind too strongly to the complexes of self-peptides and self-MHC molecules presented by thymic cells. </li></ul><ul><li>Negative selection is mediated by several cell types, most important of which are the bone marrow-derived dendritic cells and macrophages . </li></ul><ul><li>Engagement of the MHC molecule of one of these specialized thymic antigen-presenting cells by the TCR of a thymocyte causes that cell to undergo apoptosis and phagocytosis by macrophages. </li></ul>
  46. 46.
  47. 47. T-cells Specific For Self-antigens Are Removed In The Thymus By Negative Selection
  48. 48. T-cells Specific For Self-antigens Are Removed In The Thymus By Negative Selection <ul><li>Negative selection cannot eliminate T cells bearing TCRs that can bind to self-peptides not present in the thymus – such cells enter the periphery but are rendered anergic or inactivated. </li></ul><ul><li>How the positive and negative selection processes used in the thymus lead to death or growth is not well understood but likely involves differences in the affinity of the ligand-receptor interactions . </li></ul><ul><li>Because of the diversity of HLA types in a population an individual’s T cell component becomes highly personalized. </li></ul>
  49. 49. Regulatory T cells <ul><li>CD4 T-cells </li></ul><ul><li>Express </li></ul><ul><ul><li>CD25 </li></ul></ul><ul><ul><li>FoxP3 a transcriptional repressor is used by the regulatory T-cells (unique to regulatory T-cells) </li></ul></ul><ul><li>Distinct from naïve T-cells </li></ul><ul><li>Contact with MHC II – self-antigen can suppress proliferation of naïve T-cells responding to self-antigens </li></ul><ul><li>Suppressive effects require contact between the two T-cells and secretion of non-inflammatory cytokines </li></ul>
  50. 50.
  51. 51. T-cells Undergo Further Differentiation In Secondary Lymphoid Tissues After Encounter With Antigen <ul><li>The T cells that survive the selection processes in the thymus become mature, naïve T cells that recirculate through blood into the secondary lymphoid organs. </li></ul><ul><li>Mature T cells are longer-lived than B cells and in the absence of specific antigen stimulation will continue to circulate in the body for many years. </li></ul><ul><li>The T cell zones of lymphoid organs are sites where naïve T cells are activated by antigen – which provokes the final phase of T-cell development and differentiation. </li></ul><ul><li>Mature T cells become effector cells that can stay in lymphoid tissues or migrate to sites of infection. </li></ul>
  52. 52. T-cells undergo further differentiation in secondary lymphoid tissues after encounter with antigen <ul><li>There are several different types of effector T cells. </li></ul><ul><li>CD8 T cells become activated cytotoxic T cells . </li></ul><ul><li>CD4 T cells differentiate under the influence of cytokines into TH1 or TH2 helper T cells . </li></ul><ul><li>Which type of CD4 T cell predominates depends on the nature of the pathogen and immune response needed. </li></ul><ul><li>In an healthy individual there are about twice the number of CD4 T cells to CD8 T cells . </li></ul><ul><li>In patients with AIDS this proportion changes becomes the AIDS virus infects and kills CD4 T cells. </li></ul>
  53. 53. Stages of T-cell Development in the Thymus
  54. 54. Stages of T-cell Development in the Thymus