Chapter 7- Development and Activation of T Lymphocytes Flashcards
Where do T cells travel as they mature?
T cells originate in the bone marrow and then travel to the thymus, where VDJ recombination occurs. Mature T cells will leave the thymus and travel to secondary lymphoid tissues.
Self MHC
A person’s own set of MHC class 1 and class 2 molecules. A major function of the thymus is to ensure that a person’s mature T cells have T cell receptors that recognize the self MHC to prevent autoreactivity
2 lineages of T cells
- αβ T cells
- γδ T cells
Both develop in the thymus from a common T-cell precursor
CD4 and CD8
Cell surface glycoproteins found on T cells, which act as co-receptors. These glycoproteins begin to be expressed in the thymus. They differentiate the two lineages of αβ T cells- CD4 recognizes MHC class 2, and CD8 recognizes MHC class 1
Where is the thymus located?
In the upper anterior thorax, right above the heart
T cell developmental precursors
Common lymphoid precursor, then NK/T cell precursor, then the cell can differentiate into a T cell. It can also further differentiate into an effector T cell
Thymus organization
The thymus is a primary lymphoid organ that contains immature T cells (thymocytes) that are embedded in a network of epithelial cells (the thymic stroma). These 2 components form a closely packed cortex (outer) and more loosely packed medulla (inner). The thymus does not receive lymph from other tissues, the blood is the only route for progenitor cells to enter the thymus and for mature T cells to leave
DiGeorge syndrome
Prevalence is 1-2 in 4,000 people. It is a genetic disease where the thymus doesn’t develop and T cells are absent. Caused by a chromosome 22q11 deletion- T-box transcription factor TBX1. B cells are still made. These patients have almost no adaptive immunity and are frequently infected with a wide range of opportunistic infection.
Thymic involution
The maximal size of the thymus is reached at puberty, and then atrophy begins and steadily continues throughout life. The T cell producing tissue of the thymus is gradually replaced with fatty tissue. With atrophy, fewer naive T cells are produced, but this does not grossly impair T cell immunity. The repertoire of mature peripheral T cells seems to be long lived, self renewing, or both, in contrast to the short-lived B cell repertoire.
Immunosenescence
A gradual reduction of immune function with age
DiGeorge syndrome symptoms (6)
- Cardiac abnormalities
- Learning problems
- Specific facial features
- Thymic hypoplasia
- Cleft palate
- Opportunistic infections
Thymocyte commitment to the T cell lineage
T cell progenitors are not committed to becoming a T cell when they enter the thymus. At this point, they express markers like CD34 that identify them as stem cells. When they interact with the thymic stroma, the cells are signaled to divide, proliferate, and differentiate. After a week, the cells have lost all stem-cell markers and become committed thymocytes. They express CD2, CD5, and other T cell markers.
Double-negative thymocytes
Immature T cells at a very early stage of development in the thymus, which don’t have CD4 or CD8. T-cell receptor gene rearrangement up to the pre T-cell receptor stage occurs in double-negative thymocytes. DN thymocytes are the progenitor for both T cell lineages
Interleukin-7 (IL-7)
An important cytokine in T cell development. It is secreted by thymic stromal cells and binds to IL-7 receptors on CD34-bearing progenitor cells.
Notch1
An important regulator in T cell development- it is a thymocyte cell-surface receptor. It interacts with transmembrane protein ligands on thymic epithelial cells. Signals from Notch1 are necessary to drive cells along the path to T cell development in all stages. Notch1 keeps cells away from the pathway of B cell differentiation
Notch1 structure
The extracellular domain binds to its ligand on the thymic epithelium. This induces proteolytic cleavage which releases the Notch1 intracellular domain from the thymocyte plasma membrane. The intracellular domain then goes to the thymocyte nucleus. It becomes part of the transcription factor complex, which initiates the transcription of genes for T cell development
T-cell progenitors
Both αβ and γδ cell lineages come from a double negative thymocyte precursor. Rearrangement of the antigen receptor genes are then initiated to commit the cells to a certain lineage.
Which T cell lineage predominates?
αβ TCRs predominate, γδ T cells make up around 2% of total T cells
TCR gene rearrangement
Proliferation of T cell progenitors is followed by rearrangement of the β, γ, and δ chain genes. Cells that productively rearrange both a γ and δ chain gene, but not a β chain gene commit to the γδ lineage. Once the γδ TCR appears on the cell surface, the cells exit the thymus and travel in the blood to other tissues. In other cells, rearrangement of the β chain shuts down the recombination machinery. When this occurs, the β chain is made and is quality tested. If it passes, the cell proliferates and makes a clone of β chain positive cells. If these cells, the recombination machinery is reactivated at the α, γ, and δ chains occurs. When a productive α chain is made, an αβ receptor is assembled and the cell commits to the αβ lineage
Competition in TCR rearrangement
Rearrangement of the β, γ, and δ chain genes occurs at the same time. The γ and δ chain genes compete with the β locus to make productive gene rearrangements and functional T cell receptor chains. If a thymocyte makes a functional γδ receptor before a function β chain, it commits to the γδ lineage. A functional β chain predisposes the cell to the αβ lineage, but does not commit it. Gene rearrangement stops at this point, and the cell expresses CD4 and CD8 receptors
Double-positive thymocytes
A T cell at an intermediate stage of development in the thymus. It expresses both CD4 and CD8. The final steps of TCR receptor gene rearrangement to produce an αβ TCR gene occur in double positive thymocytes.
What happens to cells that don’t make a productive TCR gene rearrangement?
They die by apoptosis and are phagocytosed by macrophages in the thymic cortex. Apoptosis occurs with all except around 2% of thymocytes
β chain and δ chain rearrangements
β chain and δ chain loci contain V, D, and J gene segments. The first rearrangement joins D to J and the second joins V to VDJ
α and γ chain rearrangements
These loci only contain V and J segments. A single rearrangement joins V to J.
δγ cell development
If a thymocyte first makes productive δ and γ rearrangements, the δ and γ chains are made and form δγ heterodimers in the endoplasmic reticulum. Each heterodimer assembles with a CD3 signaling complex and moves to the cell surface. It also generates signals that stop β chain gene rearrangement. The cell is then committed to becoming a δγ cell. These cells do not undergo positive and negative selection. They leave the thymus through the blood and enter the circulation.
αβ cell development
A productive β chain gene rearrangement occurs more commonly. The β chain is made, translocated to the endoplasmic reticulum. There, it is tested for its capacity to bind to PTα. If heterodimers form a superdimer, PTα makes extensive contacts with the C and V regions of the β chain in order to test the β chain conformation and make sure it can interact with α chains and make a functional TCR. Then, a functional pre-T cell receptor is formed and checks the quality of the β chain
PTα
An invariant polypeptide that acts as a surrogate α chain. If the β chain binds to PTα, two of these heterodimers will form a superdimer.
Pre-T cell receptor
A receptor that is present on the surface of some immature thymocytes. It’s made of a TCRβ chain associated with PTα. To form it, each heterodimer superdimer assembles with the CD3 complex and ζ chain
Lck
An intracellular protein tyrosine kinase associated with the CD4 and CD8 co-receptors of T cells. It is involved in transmitting signals from these receptors
Pre-T cell receptor signaling
This receptor generates intracellular ligands rather than using an exogenous ligand. The signals are mediated by the CD3 complex and Lck. Assembly of the pre-T cell receptor signals the thymocyte to stop the rearrangement of the β, γ, and δ chains. Once a thymocyte has passed the pre-T cell receptor developmental checkpoint, it becomes a pre-T cell and moves on to the next stage of development.
Which factors bias thymocytes in favor of the αβ lineage? (2)
- Commitment to the αβ lineage occurs after the first productive β chain gene rearrangement, while commitment to the γδ lineage requires productive rearrangement of both chains
- The β chain locus has two different CB genes and their associated Db and Jb segments. Therefore, a nonproductive rearrangement of the β chain can be followed by a second, productive rearrangement at the same locus. Each person has 2 copies of the locus, so can make 4 rearrangements in total
Allelic exclusion in T cell development
Once the pre-T cell receptor is made, it signals the pre-T cell to stop gene rearrangements. This causes the degradation of the proteins of the RAG complex, and prevents the expression of RAG genes. This ensures that only one β chain gene has a productive rearrangement and is expressed, leading to allelic exclusion at the β chain locus. The pre-T cell is signaled to proliferate and makes clones of the cells with the same β chain
Rearrangement of the α-chain gene
This only occurs in pre-T cells- CD4 and CD8 are both expressed, and RAG1 and RAG2 are re-expressed after proliferation ends. Vα gene to Jα gene rearrangement
occurs on both alleles, similar to light chain rearrangement in B cells. Several α chains can be expressed until one is produced that can pass positive selection
RAG proteins
Essential for gene rearrangement, selectively expressed at the two stages in which β and α rearrangements are made.
ZAP70
A cytoplasmic tyrosine kinase in T cells that forms part of the signal transduction pathway from T cell receptors. Involved in signals from the pre-T cell receptor
Steps of the initial development of αβ T cells (7)
- Uncommitted progenitor cell enters the thymus through HEVs
- Proliferation
- DN T cells commit to the T lineage
- β genes are rearranged, the cell goes through the checkpoint for pre-TCR
- Proliferation of DN pre-T cells, immature double positive cells develop
- α genes are rearranged, the cell goes through the checkpoint for TCR
- Mature double positive cells
Positive selection
The process in the cortex of the thymus that selects for developing 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. Successful positive selection results in a single positive CD4 or CD8 cell
When does positive selection occur?
When the αβ receptor of a double positive thymocyte recognizes the complex of a self-peptide and a self-MHC molecule on a cortical epithelial cell. MHC class 1 and class 2 are both expressed by the epithelium, and thymocytes express CD4 and CD8, so the epithelial cells can select double positive thymocytes whose TCRs interact with a self-MHC class 1 or MHC class 2
What happens to a thymocyte with a TCR that doesn’t bind to self-MHC?
If the cell has run out α chain recombination options and there is no recognition of self-MHC, positive selection is unsuccessful. The cell is signaled to die by apoptosis. This occurs in 95% of thymocytes
How long does positive selection take?
A few hours up to 3-4 days if rearrangements are needed
MHC restriction
The property that a given TCR recognizes its peptide antigen only when the peptide is bound to a specific form of MHC molecule. This restriction is a consequence of positive selection in the thymus. The MHC class that mediates positive selection of a T cell is the one to which it becomes MHC restricted
Thymoproteasome
A version of the proteasome that is present in cortical epithelial cells of the thymus. It produces self peptides that are particularly effective at positive selection when bound to MHC class 1.
Thymoproteasome catalytic subunit
β5t. It is made as an inactive propeptide that is processed by cleavage to give the active protease. If this subunit is inactive, it interferes with positive selection and causes a reduction in the number of T cells in the blood
Bare lymphocyte syndrome
A rare recessive human immunodeficiency disorder. Mutations lead to the absence of MHC molecules on lymphocytes and thymic epithelial cells. No positive selection results in a lack of single positive T cells, which causes difficulty fighting infection. Type 1 of the syndrome is a lack of MHC class 1 due to TAP mutations, while type 2 is a lack of MHC class 2 due to CIITA mutations- this causes hypogammaglobulinemia
Single positive thymocyte
A T cell at a late stage of development in the thymus, characterized by the cell surface expression of a T cell receptor and either the CD4 or the CD8 co-receptor
Negative selection
A process in the thymus where developing T cells that recognize self antigens are induced to die by apoptosis. The cells signaled to die include those that have antigen receptors that bind too strongly to the self peptides and self MHC molecules in the thymus. These cells could be autoreactive and would be autoreactive and cause autoimmune disease if allowed to leave the thymus
Which cells mediate negative selection?
Dendritic cells and macrophages derived from the bone marrow are the most important cells for negative selection. When the receptors of an autoreactive thymocyte engage MHC molecules on one of the specialized dendritic cells, the thymocyte dies and is phagocytosed by macrophages
Autoimmune regulator (AIRE)
A transcription factor that causes several hundred tissue-specific genes to be transcribed by a subpopulation of epithelial cells in the medulla of the thymus. This allows the developing T cell population to become tolerant of antigens that normally occur only outside the thymus- different from the ubiquitously expressed self antigens. The presence of small amounts of tissue-specific proteins in the thymus means that peptides derived from the turnover and degradation of these proteins can be bound by MHC class 1 molecules to form complexes that participate in the negative selection of the T cell repertoire. Lack of a functional AIRE gene causes autoimmune disease
Central tolerance
Takes place in the thymus, ensures the immune system focuses on pathogens and avoids healthy tissues. Negative selection in the thymus produces central tolerance in the T cell repertoire
What happens if autoreactive T cells “escape” negative selection?
They are eliminated by peripheral tolerance
Regulatory T cells
These cells have T cell receptors that recognize self antigens. They are distinguished from other T cells by their expression of CD25 on the cell surface and the unique use of the transcriptional repressor protein called FoxP3. When regulatory T cells encounter self antigens presented by MHC class 2, the cells suppress the activation and proliferation of naive T cells responding to self antigens. This protects the organs and indicates that not all T cells specific for self antigens are detrimental to human health
Autoimmune polyendocrinopathy-
candidiasis-ectodermal dystrophy (APECED)
An autoimmune disease caused by the lack of the AIRE protein. Has a 1 in 100,000 incidence rate. These individuals are susceptible to Candida albicans infection (infection of the skin, nails, and mucosa). Patients can also experience autoimmune damage to the adrenal glands and parathyroid glands (Hypoparathyroidism, adrenal gland insufficiency)
Hypoparathyroidism symptoms
A tingling sensation in
the lips, fingers, and toes; muscle pain and
cramping; weakness; and fatigue
Adrenal gland insufficiency symptoms
Muscle weakness, loss of appetite, weight loss, low
blood pressure, and changes in skin coloring
Mechanisms of suppression of autoreactive T cells
Anatomical barriers can separate the T cells from the antigen (brain, eyes, testes). Or, the antigen may be at an insufficient concentration to activate these autoreactive T cells. Regulatory T cells develop in the thymus and also act to prevent the activation of autoreactive T cells
Dendritic cells
Dendritic cells in the skin are immature but are specialized in the uptake of pathogens and their antigens. They differentiate into mature dendritic cells in the T-cell areas of the lymph node
Draining lymph node
The lymph node that is close to the site of infection or injury. The lymph enters the lymph node through multiple afferent lymphatic vessels. CCL21 binds to CCR7 on DCs
Role of secondary lymphoid organs in T cell activation
Naive T cells recirculate through secondary lymphoid organs. They leave the blood at a high endothelial venule (HEV) and enter the lymph node cortex, they they counter professional APCs (dendritic cells and macrophages). T cells that don’t find a specific antigen leave the lymph node in the efferent lymph and re-enter the bloodstream. If the T cells do encounter a specific antigen presented by APCs, they are activated to proliferate and differentiate into effector cells. The effector T cells leave the lymph node in the efferent lymph
Dendritic cell migration (6 steps)
- Immature dendritic cells wait in peripheral tissues for pathogens to enter the body
- They have PRRs that allow them to bind to and recognize pathogens, then phagocytose them
- Dendritic cells are activated by the antigens. They start to mature. As a part of this process, they migrate through the tissue and change their behavior to produce immune stimulatory molecules
- Activated dendritic cells migrate from the tissue to lymphatic vessels. Dendritic cells are carried in the lymph to the lymph node
- T cells inspect the dendritic cells for the presence of antigen. T cells that don’t recognize antigen return to the circulation and can go from lymph node to lymph node until they find their antigen. T cells that do are activated and proliferate
- T cells then differentiate into effector cells
Routes for naive T cells to enter a draining lymph node (2)
- From the blood- the cells are transferred from the blood to lymphoid tissue through a high endothelial venule (HEV)
- From the lymph- T cells enter the lymph node in the afferent lymph that comes from an upstream lymph node
Lymphocyte trafficking (5 steps)
- A circulating naive T cell enters the high endothelial venule in the lymph node
- L-selectin binding to GlyCAM-1 and CD34 attaches the T cell to the endothelium
- LFA-1 is activated by a chemokine and binds tightly to ICAM-1
- Extravasation- the lymphocyte leaves the blood
- Extravasation- the lymphocyte enters the lymph node
Homing
The process by which naive T cells leave the bloodstream and enter the T-cell zone of a lymph node. It is guided by chemokines CCL21 and CCL19, which are secreted by stromal cells and dendritic cells
CCL21 and CCL19
Chemokines guiding T cell homing, which are secreted by stromal cells and dendritic cells. They bind to the surface of the high endothelial cells, where they establish a concentration gradient along the surface of the endothelium. Naive T cells express the CCR7 receptor, which binds to CCL21 and CCL19, causing the cells to be guided up the chemokine gradient and toward the source of chemokines in the lymph node
GlyCAM-1
A cellular adhesion molecule found on the high endothelial venules of secondary lymphoid tissues. It’s an important ligand for the L-selectin adhesion molecule on naive lymphocytes, which directs the cells to leave the blood and enter the lymphoid tissues
L-selectin
An adhesion molecule on the T cell surface. It binds to CD34 and GlyCAM-1 on high endothelial venules to initiate the migration of naive lymphocytes into secondary lymphoid tissue
LFA-1
An integrin that strengthens contact between the naive T cell and the endothelium. LFA-1 is found on the T cell surface
ICAM-1 and ICAM-2
Adhesion molecules that are found on the vascular endothelium. They strengthen the interacts between the naive T cell and the endothelium
Conjugate pair
When a CD4 effector T cell is bound to a dendritic cell through its antigen receptor. The T cell proliferates and differentiates under the influence of the dendritic cell.
Importance of adhesive interactions between T cells and dendritic cells
Adhesive interactions allow T
cell to “scan” DC for its cognate
antigen. DCs have extensive surface area and can present peptides to multiple naive T cells
T-cell synapse
The localized area of contact between a T cell and an interacting B cell or macrophage. This is where receptors and their ligands come together, signals are transduced, and cytokines are exchanged. The synapse is a region of contact and communication and is a dynamic structure
What happens if no specific antigen is being presented?
Given the total amount of cells, T cells are actually unlikely to encounter a DC with its specific antigen. The T cells pass from the cortex to the medulla, and then leaves the lymph node through the efferent lymph, similar to how T cells leave once they finish differentiation. Its departure is controlled by sphingosine 1-phosphate
Sphingosine 1-phosphate (S1P)
S1P is a lipid that is made in all cells and has chemotactic activity, like a chemokine. A gradient of S1P is established in the lymph node. The S1P concentration is lowest in the T cell areas, but increases towards the medulla and efferent lymphatic vessel. Using the S1P gradient, T cells with an S1P receptor move out of the lymph node into the efferent lymph to enter the circulation
When a T cell finds its specific antigen, what changes occur? (2)
- Tighter binding between LFA-1
and ICAM-1 form a “seal” - Surface receptors and ligands
on T cell and DC focus to the
area of contact called the synapse
Supramolecular activation complex (SMAC)
Molecules found inside the immunological synapse. Includes an inner structure called the central SMAC (c-SMAC) and an outer structure called the peripheral SMAC (p-SMAC)
c-SMAC
Concentrates TCRs, co-receptors, co-stimulatory receptors, CD2 adhesion molecules, and signaling molecules. This localized concentration of signals is necessary for T cell activation
p-SMAC
Contains the integrin LFA-1, the cell adhesion molecule ICAM-1, and a cytoskeletal protein called talin. Together, they form a tight seal around the c-SMAC
T cell anergy
A state of nonresponsiveness that is induced in naive T cells if their antigen receptor is engaged in the absence of co-stimulation. The T cell can’t respond to any external signal and can’t be revived even if its antigen is presented
Importance of co-stimulation
The first signal in T cell activation is when TCR and CD4 recognize a peptide-MHC class 2 complex. This signal is not enough by itself, so co-stimulation acts as the second signal. This involves CD28 on T cells and B7 on dendritic cells (and B cells and macrophages). It is unregulated by infection and inflammation
Protein kinases
Enzymes that covalently attach phosphate groups to a protein
(phosphorylation). Binding of a ligand to its receptor activates the kinase to phosphorylate intracellular substrates. Examples include growth factors, hormones, cytokines, antigen-receptors
Receptor tyrosine kinases
The protein kinases typically found in immune signaling pathways. Ligand binding to TCR and BCR causes kinases to associate with the cytoplasmic tails of the receptor. When specific tyrosine residues are phosphorylated, it acts as an on switch, causing an initiation of signaling cascades in the cell
Phosphatases
Enzymes that dephosphorylate proteins. They work to regulate signaling, acting as an off switch
Scaffold proteins
Part of the multiprotein signaling complexes assembled by immune receptors. Scaffold proteins lack enzymatic activity but are important because they have many sites for phosphorylation. When they are phosphorylated, many different signaling proteins are brought together. This promotes membrane localization and brings enzymes in closer proximity with their substrates
Adaptor proteins
Part of the multiprotein signaling complexes assembled by immune receptors. These proteins can be membrane anchored or cytoplasmic, and lack enzymatic activity. Their main functions are linking two or more proteins together. They contain SH2 domains that bind to phosphorylated tyrosine residues
TCR complex
A TCR associated with CD3
Immunoreceptor tyrosine-based
activation motifs (ITAMs)
Phosphorylated tyrosine residues in the cytoplasmic tails of CD3 proteins form part of short sequences known as ITAMs. Enzymes and other signaling molecules bind to the phosphorylated tyrosines and they too become activated. Therefore, the extracellular binding of antigen to TCRs triggers the pathways of intracellular signaling that drive T cell differentiation. They are found on CD3 gamma, delta, epsilon, and zeta chains. Phosphorylated ITAMS recruit and activate the kinase ZAP-70
Lck
ITAMs are phosphorylated by Lck, which is tyrosine kinase associated with peptide-MHC complexes. On the formation of the T-cell synapse, Lck phosphorylates and activates another protein kinase, ZAP-70. Lck is attached to CD4 or CD8 by a zinc ion, which bridges cysteine residues in each molecule
ZAP-70
Phosphorylated ITAMS recruit and activate the kinase ZAP-70, which is necessary for initiating major T cell signaling pathways. ZAP-70 is phosphorylated by Lck and transmits the signal onward, along the T-cell signaling pathway- there are 3 signaling pathways that are triggered
What additional molecules does activated ZAP-70 recruit?
LAT (scaffold protein) and SLP76 (adaptor protein); linked by Gads (adaptor protein). Recruitment and activation of other molecules (adaptors and enzymes)
Initial phases of T cell signaling (9 steps)
- Each CD3 chain in the TCR has at least one copy of ITAMs in the cytoplasm
- A phosphorylated tyrosine at one end of Lck is bound to an internal SH2 domain, which locks the kinase domain into an inactive conformation
- CD4 recognizes its antigen on MHC class 2 (MHC class 1 if a CD8 cell)
- Then, CD45 dephosphorylates the terminal tyrosine of Lck and initiates its kinase activity
- CD4 interacts with the MHC, bringing Lck into close proximity with the ITAMs. Activated Lck phosphorylates the ITAMs
- ZAP-70 is recruited to the phosphorylated ITAMs of the TCR, binds using its SH2 domains. Lck then phosphorylates it
- ZAP-70 is in close proximity to the membrane and can phosphorylate transmembrane protein LAT
- Adaptor protein Gads binds to LAT and recruits cytoplasmic protein Slp76 to form a scaffold
- The scaffold is used to activate PLC gamma, which will cleave phosphatidylinositol biphosphate
How is CD45 involved in cell signaling?
The cytoplasmic domain of CD45 has a tyrosine phosphatase enzyme
How are the T cell co-receptors involved in cell signaling?
The T cell co-receptor (CD4 or CD8) have the tyrosine kinase Lck bound to their cytoplasmic domains
Phosphatidylinositol biphosphate is cleaved into
- Diacylglycerol (DAG)
- Inositol triphosphate (IP3)
Next stages of T cell signaling- CD28 costimulation (7 steps)
- CD28 must be engaged by B7 molecules on antigen presenting cells for T cell signaling to proceed further
- Engagement of B7 leads to the phosphorylation of tyrosines in the cytoplasmic domains of CD28. This recruits PI 3-kinase.
- PI 3-kinase gains access to its substrate when it is recruited (PIP2). PIP3 is then formed
- PIP3 binds to the PH domain present in many signaling molecules, including PLC-γ
- PLC-γ localizes to the membrane and can be brought to the scaffold formed by LAT and Slp76
- Itk is also recruited through its PH domain and binds to PLC-γ in the scaffold
- Then, PLC-γ is fully activated due to TCR signaling and co-stimulation
Phospholipase-γ (PLC-γ)
PLC-γ cleaves phosphatidylinositol biphosphate into DAG and IP3. Activated PLC-γ generates second messengers that lead to transcription
factor activation
NFAT
A transcription factor that is activated as a result of signaling from the T cell receptor. It functions as a complex of the NFAT protein with the dimer of Fos and Jun proteins known as AP-1. The Jun component of AP-1 is activated by co-stimulatory signals delivered by CD28
NFAT activation
Which genes are expressed by T cells after activation?
Activation induces genes involved in metabolism, cytoskeletal reorganization,
adhesiveness, cell-to-cell communication
ZAP-70 mutation
An autosomal recessive disorder with a prevalence of 1 in 50,000, only around 20 cases have been identified. Patients are diagnosed by 6 months of age. Although patients survive infancy, they have an increased mortality rate early in life. Treated using antimicrobial drugs and IVIG, as well as stem cell transplant
ZAP-70 mutation symptoms
Similar presentation to SCID- persistent rash, recurrent pulmonary infections, and failure to gain weight. However, patients have normal CD4 T cell numbers and normal levels of IgG. They have reduced CD8 T cells and poor T cell proliferative responses. Antibody titers following vaccination are low/non-protective
IL-2 production
IL-2 production needs signals delivered by the TCR-co-receptor complex and the co-stimulatory receptor. Activation of NFAT, NFκB, and AP-1 induces signals that lead to IL-2 production
IL-2 regulation
IL-2 has powerful effects on T cells, so its production is precisely controlled. Cytokine mRNA is inherently unstable, so sustained production of IL-2 means that its mRNA needs to be stabilized. Stabilization is a consequence of the co-stimulatory signal, which increases T cell IL-2 production by 20-30 fold. Co-stimulation also increases transcription from the IL-2 gene by 3-fold. IL-2 synthesis is increased by 100 fold overall
T cell IL-2 receptor
Made of a heterodimer that consists of a β and γ chain that binds IL-2 with low affinity. On activation, naive T cells synthesize the α chain, which binds with the other 2 chains to form a high affinity IL-2 receptor. This causes T cells to become more responsive to IL-2. When the high-affinity receptor binds to IL-2, it causes T cell division and proliferation. Memory cells respond to lower amounts of IL-2 and require less IL-2 to survive
IL-2 and immunotherapy
IL-2 signaling is a target for immunotherapy. It prevents T cell mediated rejection of organ transplants and can be a treatment for immune mediated disorders such as rheumatoid arthritis, psoriasis, dermatitis, and Sjogren syndrome
NFAT activation (6 steps)
- In an unstimulated cell, NFAT is found in the cytoplasm. It is kept there by serine and threonine residue phosphorylation, which blocks its NLS signal
- NFAT is released from the cytoplasm by the calcium binding protein calmodulin and calcineurin, a calcium dependent phosphatase
- When a T cell is stimulated, the calcium release activated channel (CRAC) opens. It allows extracellular calcium ions to enter the cytoplasm
- The calcium ions bind to calmodulin and alter its conformation it a way that allows calmodulin to activate other cellular proteins. Calcineurin is one of these
- Calcineurin associates with calmodulin and is activated. It acts on its substrate to dephosphorylate NFAT
- NFAT can then enter the nucleus and bind to the promoters of inducible genes and initiate transcription
Cyclosporine
An immunosuppressive drug that blocks the activation of NFAT through the cyclophilin A protein. Cyclophilin normally plays a role in protein folding. Cyclosporine binds to cyclophilin A when it enters the cell, forming a protein complex with a unique function. It prevents inactivated calcineurin from interacting with calmodulin. Even though the CRAC channel opens and activates calmodulin, the 2 proteins can’t bind. NFAT remains phosphorylated and inactive
