Bloque 5: T cells Flashcards
Molecules present in all T cells
CD3 is a marker of T cells. CD3 complex is constituted by three non-covalently bound heterodimers:
- Heterodimer formed by gamma and epsilon chains
- Heterodimer formed by delta and epsilon chain
Tau chain is present as an homodimer containing short extracellular region of 9 amino acids. It is linked to processes occurring inside the cell.
CD3 and Tau structures are not responsible for antigen recognition but for signal transduction thanks to their conserved intracellular domain known as ITAM (immunoreceptor tyrosine-based activation motifs)
TCR: heterodimer formed by alpha and beta chains covalently linked by disulphide bridge between the extracellular cysteine. Each of these chains consist of one Immunoglobulin-like N-terminal variable (V) domain, a hypdrophobic transmembrane region, one Ig-like constant (C) domain and a short cytoplasmic region.
Alpha and beta chains of TCR recognise the antigen present in MHC molecules.
There’s another type of TCR made from gamma and delta heterodimers. It contain variable, constant, diversity and joining extracellular regions and transmembrane short cytoplasmic regions associated to CD3 and Tau. It appear in less than 5% of T cells (CD4- and CD8-). As they are in CD4 and CD8 negative T cells, they won’t recognise classical MHC-Antigen complexes, they recognise lipids and other molecules. They seem to be related to some kind of first line defense in the epithelium as they are abundant there.
TCR positive charged intermembrane region interact with negative charges of the aspartate amino acid present in CD3 and Tau molecule.
Signal transduction in T cells
One of the most important mechanisms to regulate cellular changes is phosphorylation. Not all amino acids can be phosporylated, only those ones with an hydroxyl group (serine, threonine and tyrosine).
Both CD3 and tau chains have a cytoplasmic conserved region known as immunoreceptor tyrosine-based activation motifs or ITAM, characterised by the following structure: Tyr-X-X-Leu. This sequence is fundamental in signal transduction.
Natural Killer T cells
Natural Killer T cells (NKT) are lymphocytes expressing markers of T cells and NK cells. They have a semi-invariant TCR with limited repertoire.
NKT recognise lipids and glycolipids presented by CD1, which is a non-classical MHC molecule.
Natural Killer T cells
Natural Killer T cells (NKT) are lymphocytes expressing markers of T cells and NK cells. They have a semi-invariant TCR with limited repertoire.
NKT recognise lipids and glycolipids presented by CD1, which is a non-classical MHC molecule.
Genetically modified T cells
Genetical modification has become an essential tool in oncology.
Genetically modified T cells can be used in children with leukaemia. Different T cells are obtained from the patients and modified with tumor-targeting TCR. After that, they are cloned and inserted into viral vector encoding tumour-specific TCR. After the vector is inserted to the patient, those cells infected by the virus will express TCR against tumour cells.
T accessory molecules functions
BINDING of T cells to OTHER CELLS like antigen presenting cells or to STRUCTURES that enable T cells to MOVE such as endothelial cells or extracellular matrix.
Transduction into lymphocytes to regulate their response.
Used as markers to distinguish different cells or different developmental state.
Accessory molecules of T cells
Co-receptors: CD4 (monomer) and CD8 (dimer). They are expressed once the cell reaches the mature status. They are able to bind classical MHC, so they are involved in the adhesion to APCs and antigen recognition. Regarding APC adhesion, they have a secondary role, as their affinity for MHC molecules is low. T cell adhesion to APCs is mainly produced by other adhesion molecules.
CD4 and CD8, together with TCR, help lymphocyte activation by transducing signals. Signal transduction is produced by Lck tyrosine kinase, which is non-covalently associated to cytoplasmic tails of CD4 and CD8. Lck is also needed for T cell maturation.
Co-stimulators: Accessory molecules have been implicated in the two-signal model of T cell activation, in which signal 1 would be the MHC-antigen complex and signal two would be those co-stimulatory molecules.
Accessory molecules regulating T cell activation:
- CD28: Membrane protein responsible for signal transduction in order to activate naïve T cells. It binds to CD80 and CD86 of APCs
- CTLA4: Membrane protein whose function is to inhibit T cella activation induced by TCR and CD28. So it is involved in T cell response finalisation. On its cytoplasmic structure, it has ITIMs or immunoreceptor tyrosine-based inhibitory motif that recruits a kinase that dephosphorylate the protein. This molecule also binds to CD80 and CD86 in APCs and can be used as a target in the treatment of some tumours such as melanoma to avoid the immune response scape.
Other signalling molecules:
- CD45: Membrane glycoprotein that is present in different isoforms of T cells, depending on the maturation status.
- CD2: Glycoprotein expressed by most mature T cells and by NK cells.
- LFA-3: Adhesion molecule and signalling molecule.
Adhesion molecules
Integrins:
Heterodimeric surface adhesion molecules present on T cell surface that mediate the binding of T cells to antigen presenting cells, endothelial cells or to extracellular matrix. These molecules also have cytoplasmic domains bound to the cell’s cytoskeleton.
They combine 2 chains: alpha and beta. As there are different types of alpha and beta chains, their combination leads to a wide variety of integrins. Cells will express one type of integrin or another one depending on their activation status or the cell population in which they are.
Integrin avidity for their ligands increases after TCR or chemokine receptor has been activated.
Integrins present on T cell membrane are upregulated by T-cell activation, while the ones present in APCs or endothelial cells are upregulated by inflammation.
Selectins:
Adhesion proteins present on leukocytes and platelets and able to bind sugars (C-type lectins).
There are different types of selectins:
- L-selectins or CD62L are selectins expressed on naïve lymphocytes and in leukocytes. They bind to sugars present on endothelial cells in order to allow naïve T-cells migration to lymph nodes, antigen encounter and T cell activation.
- E-selectins or CD62E are selectins expressed on cytokine activated endothelial cells
- P-selectins or CD62P are stored in granules in platelets and endothelial cells and translocated upon activation.
Features of T cell maturation
T cells precursors are originated in bone marrow. After that, T cell maturation takes place in the thymus.
T cell maturation is MHC-restricted.
The affinity that a TCR has for an antigen is always the same. There is no hypermaturation, the cell doesn’t change during its divisions.
There are different pathways of maturation from which different cells are generated: alpha-beta TCR T cells, gamma-delta TCR T cells, T helper cells, T regulatory cells and cytotoxic T cells.
T Lymphocyte maturation stages
Early maturation:
Bone marrow pluripotent progenitor cells originate different hematopoyetic cells populations, one of which is Lymphocytes. T cells then move to the thymus to continue their maturation process (the cell moves from pro-T lymphocyte to pre-T lymphocyte, double positive lymphocyte and single positive lymphocyte).
Early maturation of T lymphocytes is characterised by a high IL-7 induced proliferation.
Gene recombination and expression of antigen receptor:
Expression of antigen receptor is the main feature of lymphocyte maturation. Their expression and their signals are necessary for repertoire diversity, selection process and T lymphocyte survival. Those lymphocytes that don’t express TCR die by neglect.
Selection of lymphocyte repertoire:
Once immature lymphocytes express their antigen receptor, useful lymphocytes are maintained (positive selection) and the ones that are potential auto-reactive cells are eliminated (negative selection)
Acquisition of functional capacities:
During the late maturation stages, lymphocytes become functional to respond to antigens and display effector mechanisms.
Early maturation of T lymphocytes
Bone marrow pluripotent progenitor cells originate different hematopoyetic cells populations, one of which is Lymphocytes. T cells then move to the thymus to continue their maturation process (the cell moves from pro-T lymphocyte to pre-T lymphocyte, double positive lymphocyte and single positive lymphocyte).
Early maturation of T lymphocytes is characterised by a high IL-7 induced proliferation.
Gene recombination and expression of antigen receptor:
Expression of antigen receptor is the main feature of lymphocyte maturation. Their expression and their signals are necessary for repertoire diversity, selection process and T lymphocyte survival. Those lymphocytes that don’t express TCR die by neglect.
Selection of lymphocyte repertoire:
Once immature lymphocytes express their antigen receptor, useful lymphocytes are maintained (positive selection) and the ones that are potential auto-reactive cells are eliminated (negative selection)
T cell ontogeny process
T cell ontogeny is the process through which a progenitor stem cell becomes a mature T lymphocyte. This process takes place in three different parts of the body: bone marrow, thymus and periphery.
In bone marrow, progenitor cells give rise to different cellular populations, one of which are lymphocytes. Once the cell has sufficiently developed, it leaves the bone marrow and goes to the thymus, where the maturation process continues.
In the thymus, different developmental stages will be seen. First of all, the cell is called pro-T lymphocyte and is placed in the capsule and subcapsular regions of the thymus. Pro-T lymphocytes are characterised by having germinal DNA and different surface markers: c-Kit, CD44 and CD25.
Pro-T cell will undergo beta chain rearrangements and will become a pre-T lymphocyte. Pre-T lymphocytes express the same surface markers than pro-T lymphocytes, but they also have a pre-TCR associated to CD3 and tau chains.
The next stage of T cell development happens once the cells reach the thymic cortex and it becomes a double-positive lymphocyte. This name comes from the presence on the surface of both cell markers (CD4+ and CD8+). CD3 is also present but its presence will be increased in the following stages. In these cells, the recombination of both beta and alpha chains’ genes can be seen. That is the beginning of the expression of TCR.
The last cell population that appears in the thymus (in the medulla) is the single-positive lymphocyte which can be either CD4+/CD8- or CD4-/CD8+ depending on the antigen-MHC complex that they recognise (phenotypic development). Only those TCRs able to recognise MHC-Antigen complex (5%) will continue their maturation process thanks to the so-called positive selection. T cells also undergo negative selection, meaning that they die if they recognise self-antigens. The thymus can express tissue-specific antigens thanks to AIRE transcription factors.
The activation and proliferation of mature T lymphocyte will take place in the periphery.
