T cells originate from bone marrow stem cells but emigrate to mature in the thymus. These lymphocytes were called thymus-dependent lymphocytes (T cells=T lymphocytes).
Two lineages of T cells develop in the thymus : α:β and γ:δ T cells. These lineages develop in parallel from a common presursor. Whilst in the thymus, developing thymocytes also start to express other cell surface proteins related to their eventual effector functions. Among these are the CD4 and CD8, which are essential for the T cell response to cells presenting antigens.
The thymus is designated primary lymphoid organ because it is concerned with the production of useful lymphocytes, not with their application to the problems of infection. Unlike the secondary lymphoid organs, which perform the latter function, the thymus is not involved in lymphocytes recirculation; neither does it receive lymph from other tissues. The blood is the only route by which progenitor cells enter the thymus and by which mature T cells leave.
In the embryonic development of the thymus, the epithelial cells of the cortex arise from ectodermal cells, whereas those of the medulla derive from endodermal cells. Together, these two types of epithelial cell form a rudimentary thymus (thymic anlage), which subsequently becomes colonized by cells from the bone marrow. The progenitor cells give rise to thymocytes and also to dendritic cells, which populate the medulla of the thymus. The thymus is also colonized by bone-marrow-derived macrophages, which, although concentrated in the medulla, are also found scattered throughout the cortex. As the thymocytes mature, they tend to move progressively from the outer subcapsular region of the cortex radially towards the inner cortex and the medulla. One of the functions of the macrophages in both cortex and medulla is to remove the many thymocytes that fail to mature properly. A characteristic feature of the medulla is Hassall’s corpuscles, which are believed to be sites of cell destruction.
The thymus is most active in the young and it atrophies with age. The reduced production of new T cells by the thymus with age not impair T cell immunity, neither does thymectomy (removal of the thymus) in adults. Once established, the repertoire of mature peripheral T cells seems to be long lived and/or self-renewing. In this it seems to differ from the mature B cell repertoire, which is composed of short-lived cells that are continually being replenished from the bone marrow.
Changes in the cell-surface proteins expressed at different developmental stages are used to distinguish between different populations of developing thymocytes.
As the double negative thymocytes mature, they first express the adhesion molecule CD44 and the CD25, a component of the receptor for the cytokine IL-2. In time the expression of CD44 decrease and T cell receptor gene rearrangements commence. T cells have two lineages, which are distinguished by the expression of an α:β or a γ:δ T cell receptor. Commitment to one or other lineage does not occurs as a consequence of a race between the different loci to obtain a productive rearrangement. Thymocytes start to rearrange their β-, γ-, and δ-chain genes at about the same time. This is the first major difference from B-cell development, in which each type of immunoglobulin gene is rearranged in turn and in set order. If both a productive γ- and δ- chain gene rearrangement are made before a productive β-chain gene rearrangement, then the appearance of the γ:δ receptor on the cell surface signals the cell to stop β-chain rearrangement and to develop as a γ:δ T cell. The more frequent outcome is for the β-chain gene to rearrange productively before a functional γ:δ receptor can be assembled. In this situation, the β chain assembles with a surrogate α chain called pTα and expression of this pre-T-cell receptor signals the cell to halt rearrangement of the β-, γ- and δ-chain genes and to enter a phase of proliferation. Once that is over, the recombination machinery is reactivated and becomes targeted to the α-chain locus as as to the γ and δ loci. In a minority of these cells, successful completion of γ- and δ-chain gene rearrangements before the α-chain gene has rearranged leads to their commitment to the γ:δ lineage. In the majority of these cells, however, productive rearrangement of α-chain gene occurs first, and leads to expression of an α:β receptor and commitment to differentiation as an α:β T cells. Lineage commitment depends on whether a functional α:β and γ:δ T cell receptor is made first. Cells committed to one lineage can contain productive gene rearrangements for the T cell receptor genes of the other lineage. The exception is that α-chain gene rearrangements are not found in γ:δ cells. Legend: Blue- immature T lymnphocytes Orange – γ:δ T cells Green – α:β T cells
The T cell receptor β -chain genes rearrange first in CD4 - CD8 - (double negative) thymocytes expressing the cell surface protein CD25 and low levels of CD44. As with immunoglobulin heavy-chain genes, D to J rearrangement occurs first and a V gene segment then rearranges to DJ. The red arrows in the second panel represent small amounts of transcription from the gene segments to be rearranged, which open up the chromatin. The black arrow represents transcriptions to produce a functional mRNA. The kink in the arrow represents sequences that are spliced out of the primary transcript. The β chain is expressed within the cell and then appears at low levels on the cell surface in a complex with a surrogate α chain-pT α - and the CD3 chains. This complex is called the pre-T-cell receptor or pT α : β . Expression of the pre-T-cell receptor signals the cell to halt β -chain rearrangement and to undergo cycles of cell division. At the end of proliferation, CD4 and CD8 are expressed at the cell surface and the α chain now rearranges. When a functional α chain is produced, it pairs with the β chain to form the α : β T cell receptor. This appears on the surface with the CD3 complex. This double-positive thymocytes express CD4, CD8 and the α : β receptor in association with CD3. They are ready to undergo selection. These cells are found predominantly in the inner cortex of thymus, where they interact with the branching network of epithelial cells.
Just as an immunoglobulin light chain locus can undergo several successive gene rearrangements, so can the TCR α-chain locus. It, therefore, has a greater chance of achieving a successful rearrangement than the β-chain locus. This difference results from the presence of many Vα and over 50 Jα gene segments, which allows many successive VJα rearrangements to be tried. For the T cell receptor α-chain genes, the multiplicity of V and J gene segments allows successive rearrangement events to jump over unproductively rearranged VJ segments, deleting the intervening gene segments. This process continues until either a productive rearrangement occurs or the supply of V and J gene segments is exhausted, where upon the cell dies. Successful rearrangement of one copy of the α-chain gene and cell-surface expression of a functional α:β receptor do not prevent rearrangement at other copy of the α-chain gene. Therefore, many T cells express two α chains and have two different TCR at the surface at this stage in their development. As the T cell enters the next phase of development, the forces of positive and negative selection can act on either of the two receptors. Because the proportion of TCR that succeed in positive selection is so small, however, it will be a vanishingly rare cell that has two TCR that can both be activated by peptides presented by self-MHC. Thus, in the vast majority of mature T cells that express two receptors, one receptor will be nonfunctional.
The primary TCR repertoire has a bias towards interaction with MHC molecules. Thus, gene rearrangement provides an extensive repertoire of TCR that could be used with the hundred of MHCI and MHCII present in the human population. However, the TCR genes possessed by a given individual are not tailored specifically towards making receptors that interact with the particular forms of MHC molecule expressed by the same individual. Only a small subpopulation of the double-positive thymocytes, at most 2% of the total, have receptors that can interact with one of the MHCI o MCHII isoforms expressed by the individual, and will, therefore, be able to respond to antigens presented by these MHC molecules.
Positive selection takes place in the cortex of the thymus. It is mediated by the complexes of self-peptides and self-MHC molecules present on the surface of the cortical epithelial cells. In the absence of infection MHC molecules assemble with self-peptides derived from the normal breakdown of the body’s own proteins. The cortical epithelial cells form a web of cell processes that envelop and make contact with the double-positive CD4 CD8 thymocytes. Thymic cortical epithelium expresses both MHCI and MHCII. At regions of contact, potential interactions of the α : β receptor of a thymocytes with the self-peptide:self-MHC complexes on the epithelial cell are tested. If a peptide:MHC complex is bound within 3-4 days of the thymocytes expressing a functional receptor, then a positive signal is delivered to the thymocytes, which continues its maturation. Cells that do not receive such a signal within this period die by apoptosis and are removed by macrophages.
Positive selection not only selects a repertoire of cells that can interact with an individual’s own MHC, but it is also instrumental in determining whether a double-positive T cell will become a CD4 T cell or a CD8 T cell.
CD4 interact with MHC II. CD8 interact with MHCI. During positive selection, the double positive CD4CD8 T cell interacts through its α : β receptor with a particular peptide:MHC complex. When the interacting MHCI, CD8 are recruited into the interaction, whereas CD4 are excluded. Coversely, when the selecting MHCII, CD4 is recruited and CD8 excluded. The mechanism by which this interaction stops expression of the nonbinding co-receptor and turns double-positive thymocytes into single-positive thymocytes is still unknown.
Rearrangement at the α -chain locus continue throughout the 3-4 days period of positive selection. Through the use of different gene segments and the production of different α chains, a T cell can change the specificity of the antigen receptor that it expresses. In this way, double-positive thymocytes can explore the usefulness of different receptors successively, thereby improving the chance of their positive selection. Once a T cell has been positively selected, α -chain gene rearrangement stops.
Such T cells are potentially autoreactive, and, if allowed to enter the peripheral circulation, could cause tissue damage and autoimmune disease.
As thymocyte mature they move from the subcapsular region deeper into the thymus. Double positive cells are found in the cortex where they undergo positive selection on cortical epithelial cells. The positively selected cells encounter dendritic cells and macrophages at the cortico-medullary junction; this is where most negative selection occurs (apoptosis). The surviving mature single-positive T cell leave the thymus and enter the blood circulation at venules in the medulla.
Only a small fraction of T cells survive the obstacle course of positive and negative selection and leave the thymus. Mature T cells are longer lived than mature B cells and, in the absence of their specific antigen, continue to circulate through the body for many years.
Unlike B cells, which have just one terminally differentiated state-the antibody-secreting plasma cell-there are several different types of effector T cells. Which type of effector CD4 T cell predominates depend on the nature of the pathogen and the type of immune response required to clear it.
Bone marrow transplantation is a common treatment fro leukemia, lymphoma and inherited immunodeficiency diseases (SCID). The patient’s diseased hematopoietic system is destroyed by chemotherapy and irradiation. An infusion of bone marrow obtained from a healthy HLA-matched donor is then given. Over a period of months the hematopoietic stem cell in the graft reconstitute the patient with a healthy hematopoietic system. As part of this process, T cells developing from stem cells in the bone marrow graft migrate to the thymus, where they mature into T cells under the influence of the thymic epithelial cells of the recipient and of the HLA molecules that these cells express. The developing T cells are, therefore, positively selected in the recipient’s HLA allotypes. To reconstitute T cells function, the new T cells must be able to respond to antigens presented by the professional antigen-presenting cells (dendritic cells, B cells, and macrophages), which are derived from bone marrow and will now all be of donor origin and donor HLA type. To satisfy this requirement, the donor and recipient must have at least one HLA class I and one HLA class II allotype in common (Next slide).
The donor and recipient in a bone marrow transplant must share HLA-I and HLA-II molecules in order to reconstitute T-cell function. After bone marrow transplantation, donor-derived thymocytes are positively selected on the recipient’s thymic epithelium. The top panels show the hypothetical situation where none on the recipient’s HLA (red) is the same as the donor’s HLA (blue). The lower panels show the situation when the recipient and donor share the HLA indicated by blue. In clinical practice, bone marrow transplant donors and recipients are chosen to share as many HLA I and II as possible. APC – antigen presenting cell.
The development of t lymphocytes
The development of T lymphocytes Dr. Glorivee Rosario Pérez BIOL 4056 Parham P. (2009). The Immune System. Third Edition. Garland Publishing, New York.
T lymphocytes cont.T cells lineages (develop in the thymus): α:β T cells (majority) CD4 γ:δ T cells (minority) CD8
Thymus: Isa lymphoid organ in the upper anterior thorax just above the heart. Itcontains immature T cells (thymocytes), which are embedded in a network of epithelial cells (thymic stroma). These elements form: Cortex Medulla
Thymus development: Birth – the human thymus is fully developed. Puberty – increase in size. Adult (30 years) – degeneration of the thymus.
B vs T cells The bone marrow is continually turning over the B cell repertoire during the whole of person’s lifetime. Thethymus works principally during youth, when it serves to accumulate a repertoire of T cells that can then be used throughout life.
Two lineage of T cells:α:β and γ:δ Maturation of thymocytes into mature T cell occurs in distinct stages. These are marked by changes in the status of the: T-cell receptor gene. expression of the T-cell receptor protein. production of other T-cell surface glycoprotein: CD4, CD8 and CD3 complex.
Two lineage of T cells:α:β and γ:δ cont. Progenitor Cells Subcapsular region of the outer cortex(no cell-surface glycoproteins) Progenitor Cells Thymic stromal cells (Proliferate) (epithelial cells) A week T-cell express specific adhesion molecule : CD2 No expression of : TCR complex, CD4, CD8, CD3 (double negative thymocytes: CD4-CD8-)
Positive and negativeselection of the T-cell repertoire
Positive selection : MHC Is a process in the thymus that selects immature T cells with receptors that recognize peptide antigens presented by self-MHC molecules. Only cells that are positively selected are allowed to continue their maturation.
Positive selection : MHC cont. Positive selectiontakes place in the cortex ofthe thymus.
Positive selection :CD4 and CD8 co-receptor Single-positive thymocytes – double positive thymocytes mature into cells that express just one or other of the two co-receptors. CD4+CD8- CD4+CD8+ CD4-CD8+
Positive selection :CD4 and CD8 co-receptor cont.
Thymus →secondary lymphoid tissue cont. Naïve T cells are activated by their specific antigens in T cell rich areas of secondary lymphoid tissues ↓ Final phases of T cell development and differentiation ↓The mature T cell divide and differentiate into effector T cells CD8 T cells CD4 T cells (TH1/TH2) ↓ Some stay in the lymphoid tissues while others migrate to sites of infection