T cell (T Lymphocyte) – Definition, Structure, Types, Development, Functions

T cell or T lymphocyte is a type of white blood cell which plays an important role in the adaptive immune system.

It is mainly involved in the recognition of foreign antigen and destruction of infected cells. It also helps in antibody production and regulation of immune response.

T cells are produced from progenitor cells in the bone marrow. After this, these cells move to the thymus gland for maturation. Because of this thymus maturation, they are called T cells.

After maturation, T lymphocytes enter into the blood circulation. They also move through lymphoid tissues and detect the specific foreign antigen present in the body.

The following are the major functions of T cells–

• Cytotoxic T cells directly kill the virus infected cells and cancerous cells.
• Helper T cells help B cells to produce antibodies against antigen.
• T cells produce chemical substances called cytokines, which help in immune reaction and inflammation.
• Regulatory T cells suppress the immune response and prevent attack against own body tissues.

So, T lymphocytes are very important cells of immune system. They act as controlling and protective cells during immune response. They recognize antigen, activate other immune cells and also maintain immune balance in the body.

Properties of T cell (T Lymphocyte)

The following are the properties of T cell–

• Origin and maturation– T cells are formed from progenitor cells in bone marrow, but maturation is completed in the thymus gland. In thymus, selection and development of T cells takes place.
• Adaptive immunity– T lymphocyte is an important cell of adaptive immune system. It mainly gives cell mediated immune response.
• Action against infected cells– T cells are mainly active against virus infected cells, intracellular pathogens and tumour cells.
• Antigen recognition– T cells cannot recognize free antigen directly. They recognize antigen only when peptide antigen is presented with Major Histocompatibility Complex (MHC) on the surface of other cell.
• T cell receptor– Antigen is recognized by T cell receptor (TCR). It is present on the surface of T lymphocyte.
• TCR-CD3 complex– TCR is generally made of alpha-beta (αβ) heterodimer. It remains associated with CD3 complex, which helps to send activation signal inside the T cell.
• Activation requirement– Naive T cell needs three signal for complete activation. First is TCR binding with peptide-MHC complex. Second is co-stimulation like CD28-B7 binding. Third is cytokine signal which decides the responder type of T cell.
• Major types– Conventional T cells are mainly of two types. CD4⁺ helper T cell and CD8⁺ cytotoxic T cell.
  • CD4⁺ helper T cell– CD4⁺ T cell recognize antigen with MHC class II molecule. It secretes cytokines and helps macrophages, neutrophils and B cells in immune reaction.
  • CD8⁺ cytotoxic T cell– CD8⁺ T cell recognize antigen with MHC class I molecule. It kills virus infected cells and malignant cells by perforin, granzymes and Fas-FasL pathway.
• Functional specialization– Activated helper T cells may form different types like Th1, Th2, Th17 and Tfh. These types are formed according to antigen and cytokine signal.
• Regulatory property– Regulatory T cells (Tregs) suppress the immune response. It maintains immune tolerance and prevents autoimmune reaction.
• Memory formation– After infection, most effector T cells die. Some cells remain as memory T cells, which give quick and strong response during second entry of same pathogen.
• Unconventional T cells– Some unusual T cells are also present like gamma-delta (γδ) T cells, NKT cells and MAIT cells. These cells act between innate and adaptive immunity and may recognize non-peptide antigen also.

Origin and Development Process of T Cells

The following are the origin and development process of T cells–

• T cells are originated from common lymphoid progenitor cells present in the bone marrow, but their complete maturation is not done in bone marrow.
• The immature T cell precursors leave the bone marrow and migrate through blood circulation to the thymus, which is a specialized lymphoid organ present above the heart.
• After entering the thymus, these immature cells are called thymocytes and they start their early development inside the thymic region.
• In the early stage, thymocytes do not have CD4 and CD8 surface markers, so this stage is called double-negative (DN) stage.
• The double-negative stage has four steps, DN1, DN2, DN3 and DN4, where the cells become fixed for T cell lineage and TCR β-chain gene rearrangement begins.
• During DN3 stage, the newly formed TCR β-chain joins with pre-Tα chain and forms pre-TCR, which checks whether the β-chain is functional or not.
• This checking stage is called β-selection checkpoint, and successful cells stop further β-chain rearrangement and then multiply in number.
• After this, the thymocytes start expressing both CD4 and CD8 markers on their surface, so this stage is called double-positive (DP) stage.
• In the double-positive stage, TCR α-chain gene rearrangement takes place and a complete αβ T cell receptor (TCR) is formed on the surface of thymocytes.
• The DP thymocytes then undergo positive selection in the thymic cortex, where they interact with cortical thymic epithelial cells (cTECs) and self peptide-MHC complex.
• In positive selection, those T cells which can recognize self MHC properly get survival signal, but those cells which cannot recognize self MHC die by apoptosis, called death by neglect.
• After this, the selected cells undergo negative selection, where the T cells reacting very strongly with self antigen are removed by apoptosis.
• Negative selection is important for self tolerance, because it prevents formation of T cells which may attack own body tissue and cause autoimmune disease.
• Due to these strict selection process, most developing T cells die inside the thymus, and only a small number of cells become mature.
• The surviving double-positive T cells then lose either CD4 or CD8 marker and become mature single-positive (SP) T cells.
• If the cell keeps CD4 marker, it becomes CD4⁺ helper T cell, and this development is controlled by transcription factor ThPOK.
• If the cell keeps CD8 marker, it becomes CD8⁺ cytotoxic T cell, and this development is controlled by transcription factor Runx3.
• Fully mature naive CD4⁺ and CD8⁺ T cells leave the thymus and enter into peripheral blood circulation and lymphoid tissues.
• These mature naive T cells then circulate in the body and remain ready to recognize foreign antigen during immune response.
• New T cell production is highest before puberty, but after puberty the thymus slowly shrinks and production of new T cells becomes reduced.
• In adult stage, body maintains T cell number mainly by division of already existing mature T cells in peripheral tissues.

Maturation process of T Cells in the Thymus

The following are the maturation process of T cells in thymus–

• Early thymic progenitor cells come from bone marrow and enter into the thymus, where these cells are now called thymocytes.
• At first, these thymocytes do not have CD4 and CD8 co-receptors on their surface, so it is called double negative (DN) stage.
• The double negative stage has four stages, DN1, DN2, DN3 and DN4, and in these stages early growth and commitment of T cell occurs.
• In DN2 stage, Notch1 signaling from Delta-like 4 (DL4) ligand of thymic stromal cells makes the progenitor cell to follow T cell lineage.
• In DN3 stage, TCR β-chain gene rearrangement occurs and newly formed β-chain is joined with pre-Tα chain to make pre-TCR.
• This stage is called β-selection checkpoint, because here the cell checks whether β-chain is useful or not, and successful cell then multiply and stops further β-chain rearrangement.
• After β-selection, the thymocyte starts to express both CD4 and CD8, so the cell becomes double positive (DP) thymocyte.
• In DP stage, TCR α-chain gene rearrangement takes place and complete αβ TCR is formed on the surface of thymocyte.
• Then positive selection occurs in thymic cortex, where DP thymocytes are tested by self peptide-MHC complex present on cortical thymic epithelial cells (cTECs).
• Those cells which can recognize self MHC with proper binding get survival signal, but those cells which cannot bind properly die by apoptosis, called death by neglect.
• In this positive selection, large number of DP thymocytes die, nearly 75-80%, because they fail to recognize self MHC.
• The selected thymocytes then adjust their sensitivity by CD5 expression, where more self reactivity causes more CD5, and it reduces strong future response to self antigen.
• After this, negative selection takes place, where thymocytes which bind too strongly with self peptide-MHC complex are killed by apoptosis.
• Negative selection is carried out in cortex and medulla by medullary thymic epithelial cells (mTECs) and dendritic cells.
• This step is important for central tolerance, because it removes autoreactive T cells which may attack own body tissue.
• The surviving DP thymocytes then lose one co-receptor and become single positive (SP) T cells.
• If signal is strong and long through MHC class II, ThPOK is formed and cell becomes CD4⁺ helper T cell.
• If signal is weak and short through MHC class I, Runx3 acts and cell becomes CD8⁺ cytotoxic T cell.
• Only small number of thymocytes survive this whole process, about 2-5% of original thymocytes.
• Finally, mature single positive naive T cells leave the thymus and enter into blood and lymphoid tissues for immune response.

Classification of T Cells

The following are the classification of T cells–

1. Naive T cells– These are mature T cells which comes out from thymus. They have not met with their specific foreign antigen.
2. CD4⁺ helper T cells– These cells recognize antigen with MHC class II molecule. They mainly help and control immune reaction by secretion of cytokines.
   • Th1 cells– These cells act against intracellular pathogens. Example are viruses and mycobacteria. They activate macrophage and cytotoxic T cells.
   • Th2 cells– These cells act against large extracellular parasites like helminths. They recruit eosinophils, basophils and mast cells. They also help B cells to produce IgE antibody.
   • Th9 cells– These cells help in expansion of CD4⁺ T cells. They also recruit mast cells.
   • Th17 cells– These cells act against extracellular bacteria and fungi. It is important in mucosal surface. They recruit neutrophils.
   • T follicular helper cells (Tfh)– These cells are present in follicles of spleen and lymph nodes. They directly interact with B cells and help in high affinity antibody formation.
3. CD8⁺ cytotoxic T cells– These cells recognize antigen with MHC class I molecule. They directly destroy virus infected cell and malignant cell by perforin, granzymes and apoptotic pathway.
   • Tc1 and Tc2 cells– These are the subtypes of cytotoxic T cells. They are divided on the basis of cytokine profile.
4. Regulatory T cells (Tregs)– These are special CD4⁺ T cells. They suppress the immune response and maintain self tolerance.
   • Foxp3– It is the main transcription factor present in Tregs.
   • IL-10 and TGF-β– These are suppressive cytokines secreted by Tregs.
   • Th3 and Tr1 cells– These are included under regulatory type cells.
5. Memory T cells– These cells remain in the body for long time after infection is removed. They give quick response when same antigen enters again.
   • Stem cell memory T cells (Tscm)– These are stem like memory cells. They have high self renewal power and can form other memory and effector T cells.
   • Central memory T cells (Tcm)– These cells remain and circulate through blood and secondary lymphoid organs. They can multiply rapidly when antigen comes again.
   • Effector memory T cells (Tem)– These cells circulate in blood and non lymphoid tissues. They give fast defence but their proliferation is less than Tcm cells.
   • Tissue resident memory T cells (Trm)– These cells stay permanently in tissues like skin, gut and lungs. They do not recirculate normally and act as local watch cells.
   • Virtual or innate memory T cells– These cells have not seen antigen, but they show memory like character due to cytokine effect.
6. Unconventional T cells– These cells act between innate and adaptive immunity. They recognize non-peptide antigen quickly and usually do not need normal priming in lymph node.
   • Gamma-delta (γδ) T cells– These cells have γδ TCR instead of αβ TCR. They recognize phosphoantigen, lipid and metabolite without strict classical MHC restriction.
   • Natural killer T cells (NKT cells)– These cells recognize lipid or glycolipid antigen presented by CD1d molecule. They are of invariant NKT (iNKT) and diverse NKT (dNKT) type.
   • Mucosal-associated invariant T cells (MAIT cells)– These cells recognize vitamin B2 (riboflavin) pathway metabolite made by bacteria and yeast. It is presented by MR1 molecule.

Structure and Morphology of T Cells

The following are the structure and morphology of T cells–

• T cell is generally small rounded lymphocyte type cell, with large nucleus and thin rim of cytoplasm.
• The surface of T cell contains T cell receptor (TCR), which is the main structure for antigen recognition.
• TCR is commonly made up of two protein chains, one alpha (α) chain and one beta (β) chain.
• Each TCR chain has variable part, constant part, transmembrane part and very small cytoplasmic tail.
• The variable part of TCR forms antigen binding site, and it helps to recognize peptide antigen with MHC molecule.
• The cytoplasmic tail of TCR is very short, so it cannot send signal properly by itself.
• For signaling, TCR remains attached with CD3 complex on the membrane of T cell.
• CD3 complex is made up of CD3γϵ, CD3δϵ and CD3ζζ chains, and it acts as signaling part of TCR.
• The full TCR-CD3 complex is an octameric structure. It contains TCRαβ, CD3γϵ, CD3δϵ and CD3ζζ in fixed arrangement.
• The inner tail of CD3 complex contains ITAMs. These ITAMs help in starting and increasing signal inside the T cell after antigen binding.
• A single TCR-CD3 complex has total 10 ITAMs, which act as docking site for signaling proteins.
• Mature T cells also show surface co-receptors, either CD4 or CD8.
• CD4 binds with MHC class II molecule, and CD8 binds with MHC class I molecule.
• These co-receptors help to hold the T cell with target cell and bring Lck kinase near the TCR for signal starting.
• When T cell attaches with antigen presenting cell, the contact area becomes organized and this is called immunological synapse.
• The immunological synapse has SMAC arrangement. It gives a bull’s eye like pattern at the contact region.
• The central part is called central SMAC (cSMAC), where many TCR-CD3 complexes are gathered.
• The middle ring is called peripheral SMAC (pSMAC), where adhesion molecules like LFA-1 are present and hold two cells together.
• The outer part is called distal SMAC (dSMAC), where actin and large surface glycoproteins like CD45 and CD44 are present.
• CD8⁺ cytotoxic T cells contain special intracellular vesicles called lytic granules.
• These lytic granules are modified lysosome like structures and they store perforin and granzymes.
• During killing, cytotoxic T cell releases these granules toward the synapse and destroy virus infected cell or malignant cell.

T Cell Receptor (TCR)

The following are the important points of T Cell Receptor (TCR)–

Types of T Cell Receptor (TCR)

1. Alpha-beta (αβ) TCR– Alpha-beta TCR is the common type of T cell receptor. It is found on about 95% of circulating T cells. It is made up of one alpha (α) chain and one beta (β) chain.
   • Conventional αβ TCR is present on ordinary T cells like CD4+ helper T cells and CD8+ cytotoxic T cells. It recognizes peptide antigen presented with MHC class I or MHC class II molecules.
   • Invariant αβ TCR is present on Type I natural killer T cells (iNKT cells). It has conserved alpha chain, mainly TRAV10 in humans. It recognizes glycolipid antigen presented by CD1d.
   • Semi-invariant αβ TCR is present on Mucosal-associated invariant T cells (MAIT cells). It uses TRAV1-2 alpha chain in humans. It recognizes vitamin B2 metabolite intermediates presented by MR1.
   • Diverse αβ TCR is found on Type II NKT cells. It uses variable alpha and beta chains. It recognizes different lipid antigens presented by CD1d.
2. Gamma-delta (γδ) TCR– Gamma-delta TCR is less common type of T cell receptor. It is found on about 5% of circulating T cells. It is made up of one gamma (γ) chain and one delta (δ) chain.
   • This receptor is found only on γδ T cells. These cells are related with both innate and adaptive immunity.
   • It does not require classical MHC restriction like ordinary αβ TCR. It can recognize non-peptide antigens such as phosphoantigens, lipids and polar metabolites.
   • The antigen may be presented by butyrophilin (BTN), CD1 and MR1 molecules. Its binding is more flexible and not fixed like usual αβ TCR-MHC binding.

Expression rule– A single T cell expresses either αβ TCR or γδ TCR. Both receptors are not present together on the same T cell at same time.

T Cell Receptor Loci

• TCR Beta (β) Locus:
  • Timing of rearrangement: It is the first TCR locus to rearrange. It occurs early during T cell development in thymus. The stage is Double-negative 3 (DN3) stage.
  • Gene segments: It contains Variable (V), Diversity (D) and Joining (J) gene segments. So it has V-D-J structure. It is similar with heavy chain locus of B cell antibody.
• TCR Alpha (α) Locus:
  • Timing of rearrangement: It starts rearrangement after successful β locus rearrangement. This occurs later in small Double-positive (DP) stage.
  • Gene segments: It contains only Variable (V) and Joining (J) gene segments. D segment is absent here. So it is similar with antibody light chain locus.
  • Successive attempts: The α locus can do repeated rearrangement. If one V-J joining is not functional, another joining may occur. This gives more chance to form proper receptor.
• TCR Gamma (γ) and Delta (δ) Loci:
  • Alternative lineage: These loci are rearranged in a small group of early T cell progenitors. They form γδ T cell lineage. It is different from common αβ T cell lineage.
• Mechanism of Recombination:
  • Somatic rearrangement: All TCR loci undergo somatic gene rearrangement. Different V, D and J segments are joined in random way. This produces large diversity of T cell receptors.
  • RAG genes: The cutting and joining of gene segments is controlled by recombination activating genes. These genes are RAG1 and RAG2.

TCR Downstream Signaling

• Step 1: Receptor engagement– The process starts when T cell receptor (TCR) binds with peptide-MHC (pMHC). This pMHC is present on antigen presenting cell (APC). This is the first contact step.
• Step 2: ITAM phosphorylation– TCR itself has no enzyme activity. It is attached with CD3 complex and ζ chains. After binding, CD4 or CD8 brings Lck kinase near the receptor. Lck and Fyn phosphorylate ITAMs of CD3 and ζ chains.
• Step 3: ZAP-70 recruitment– Phosphorylated ITAMs become binding site. ZAP-70 comes and binds by SH2 domains. Then Lck phosphorylates ZAP-70. Now ZAP-70 becomes active.
• Step 4: Signalosome assembly– Active ZAP-70 phosphorylates LAT and SLP-76. LAT is Linker for Activation of T cells. It works as scaffold. Gads, Grb2 and other proteins collect here and form signalosome.
• Step 5: Second messengers– The LAT signalosome recruits VAV1, ITK and PLCγ1. PLCγ1 becomes active. It breaks PIP2 of membrane. From this, IP3 and DAG are formed.
• Step 6: Calcium and NFAT– IP3 releases Ca2+ from ER. Calcium store of ER becomes low. Then STIM opens CRAC channels and more Ca2+ enters from outside. Increased calcium activates calcineurin. Calcineurin dephosphorylates NFAT and NFAT enters nucleus.
• Step 7: AP-1 pathway– DAG stays in membrane. It recruits RasGRP. RasGRP activates Ras. Then MAPK pathway starts through Raf, Mek1/Mek2 and Erk1/Erk2. At the end AP-1 is formed by Fos and Jun.
• Step 8: NF-κB pathway– DAG also recruits PKCθ. Then CARMA1, BCL10 and MALT1 form CBM signalosome. This activates IKK complex. IκB is degraded. Then NF-κB enters nucleus.
• Step 9: Gene transcription– NFAT, AP-1 and NF-κB reach nucleus. They bind with DNA promoter region. New genes become active. IL-2 is produced and T cell starts proliferation and clonal expansion.

Surface Markers of T Cells

The following are the important surface markers of T cells–

• Antigen recognition markers–
  • T cell receptor (TCR)– It is the main antigen recognizing receptor of T cell. It recognize peptide antigen only with MHC molecule. Most TCR are αβ type, but some are γδ type also.
  • CD3 complex– It is attached with TCR on the surface of T lymphocyte. It is made up of γ, δ, ϵ and ζ chains. It sends activation signal inside the cell.
• Lineage markers–
  • CD4– It is marker of helper T cell and many regulatory T cells. It binds with MHC class II molecule on antigen presenting cells.
  • CD8– It is marker of cytotoxic T cell. It binds with MHC class I molecule on infected cell or target cell.
• Co-stimulatory markers–
  • CD28– It binds with CD80/CD86 on antigen presenting cell. It gives second signal for activation of naive T cell.
  • ICOS, 4-1BB (CD137) and OX40– These are activation supporting receptors. They help in survival, proliferation and further activation of T cell after antigen recognition.
• Inhibitory markers–
  • CTLA-4 (CD152)– It is inhibitory receptor of activated T cell. It competes with CD28 and decreases immune response.
  • CD5– It is negative regulator of TCR signaling. It reduces strong response against self antigen and helps to prevent autoimmunity.
• Memory and activation markers–
  • CD45– It is surface phosphatase marker needed for signaling. CD45RA is mostly present on naive T cells. CD45RO is mostly present on memory T cells.
  • CD25– It is IL-2 receptor alpha chain. It is present on activated T cells and also important marker of regulatory T cells (Tregs).
• Homing markers–
  • CD62L and CCR7– These are lymph node homing markers. They help naive T cells and central memory T cells to enter into lymph node and other lymphoid organs.
• Tissue resident markers–
  • CD69 and CD103– These are markers of tissue resident memory T cells. CD69 keeps the cell in tissue. CD103 helps attachment with epithelial cells.
• Adhesion marker–
  • CD44– It is adhesion molecule. It is high on antigen experienced T cells, mainly effector and memory T cells.
• Exhaustion markers–
  • PD-1, LAG-3, TIM-3 and TIGIT– These are inhibitory markers. They are high on exhausted T cells in chronic infection and cancer.
• Effector marker–
  • Fas ligand (FasL/CD178)– It is present on activated cytotoxic T cells and Th1 cells. It binds with Fas receptor on target cell and causes apoptosis.

Antigen Recognition by T Cells

The following are the steps of antigen recognition by T cells–

• At first, foreign protein is taken by antigen presenting cell (APC). This cell may be dendritic cell or macrophage. The protein is digested inside the cell and small peptide fragments are formed.
• These peptide fragments are combined with Major Histocompatibility Complex (MHC) molecule. Then peptide-MHC complex is expressed on the surface of APC.
• T cell receptor (TCR) present on T cell now recognize the specific peptide-MHC complex. This binding is referred to as first signal of T cell activation.
• During this process, CD4 or CD8 co-receptor also bind with MHC molecule. CD4 bind with MHC class II and CD8 bind with MHC class I molecule.
• The co-receptor binding helps in stable attachment of T cell with APC. It also bring Lck kinase near the TCR-CD3 complex for starting the signal.
• TCR has very short cytoplasmic part. So, signal is not carried by TCR alone. The main signal is carried by CD3 complex and zeta (ζ) chain.
• Lck and Fyn phosphorylate ITAMs present on CD3 and ζ-chain. After phosphorylation, ITAMs become the binding site for ZAP-70 kinase.
• ZAP-70 bind with phosphorylated ITAMs. It is then activated by Lck and after activation it phosphorylate LAT (Linker for Activation of T cells).
• LAT acts as a platform for different signaling proteins. Gads, SLP-76, VAV1 and ITK collect there and form a signaling complex called signalosome.
• This signalosome activates PLCγ1 enzyme. PLCγ1 breaks membrane lipid and produces two second messenger, IP3 and DAG.
• Complete activation of T cell need second signal also. This signal is produced when CD28 of T cell bind with B7 molecule (CD80/CD86) of APC.
• In absence of this second signal, T cell does not become properly active. It may enter into inactive condition called anergy.
• IP3 causes release of calcium ion from endoplasmic reticulum. Calcium activate calcineurin, and calcineurin activate NFAT.
• DAG activate PKCθ and RasGRP pathway. This results in activation of NF-κB and AP-1 transcription factors.
• NFAT, NF-κB and AP-1 enter into nucleus. These factors start new gene expression required for activation of T cell.
• After this, IL-2 and other cytokines are produced. The T cell then divide rapidly and this is called clonal expansion.
• The activated T cells finally become effector T cells. They perform helper function or cytotoxic killing function according to their type.

Activation of T Cells

The following are the steps of activation of T cells–

• Activation of T cell starts when T cell receptor (TCR) bind with specific peptide antigen present with MHC molecule on the surface of antigen presenting cell (APC). This is called first signal or signal 1.
• During this binding, CD4 or CD8 co-receptor also bind with MHC molecule. CD4 bind with MHC class II and CD8 bind with MHC class I. This makes the attachment more stable.
• The binding of TCR and co-receptor brings Lck kinase near the inner part of TCR-CD3 complex. Lck is attached with the co-receptor and it starts the early signaling process.
• Lck along with another kinase Fyn phosphorylate ITAMs present on CD3 and zeta (ζ) chain. These ITAMs are important signaling region of TCR-CD3 complex.
• After phosphorylation, these ITAMs act as docking place for ZAP-70 kinase. ZAP-70 bind with phosphorylated ITAMs and then it is activated by Lck.
• Activated ZAP-70 phosphorylate adapter proteins like LAT and SLP-76. These proteins now form platform for collection of other signaling proteins.
• On this platform, proteins such as Gads, VAV1 and ITK are collected. This multi-protein signaling group is called signalosome.
• The signalosome recruit and activate PLCγ1 enzyme. PLCγ1 breaks membrane phospholipid and produces two second messengers, IP3 and DAG.
• For complete activation, second signal is required. This signal occurs when CD28 of T cell bind with B7 molecule (CD80/CD86) present on APC.
• This CD28-B7 binding gives co-stimulatory signal and increases the activation signal. If this signal is absent, T cell may become unresponsive or anergic.
• IP3 causes release of stored calcium ion from endoplasmic reticulum. Calcium activate calcineurin, and calcineurin then activate NFAT transcription factor.
• DAG activate PKCθ and RasGRP pathway. This leads to activation of NF-κB and AP-1 transcription factors.
• NFAT, NF-κB and AP-1 enter into nucleus. These transcription factors bind with DNA and start new gene expression needed for active T cell.
• After gene expression, T cell produce IL-2 and other cytokines. IL-2 helps in growth and rapid multiplication of T cells.
• The activated T cell then undergoes clonal expansion. Many same type T cells are formed from one selected T cell.
• Third signal or signal 3 is given by cytokines from APC and surrounding cells. Example are IL-12, IL-4 and IL-6.
• These cytokines decide the final type of activated T cell. According to cytokine signal, T cell may become Th1, Th2, Th17 or other effector type.
• Finally the activated T cells become functional effector cells. They perform helper activity, cytokine secretion or killing of infected cells according to their type.

Clonal Expansion and Differentiation of T Cells

The following are the steps of clonal expansion and differentiation of T cells–

• After proper activation of T cell, transcription factors like NFAT and AP-1 enter into nucleus and start the genes needed for growth and division of T cell.
• In this stage, activated T cell starts production of IL-2, which is an important growth cytokine for T lymphocyte.
• IL-2 acts on the same activated T cell and nearby T cells, and it helps in rapid multiplication of antigen specific T cells.
• The activated naive T cell divide again and again, and large number of same antigen specific T cells are formed. This process is called clonal expansion.
• During clonal expansion, all newly formed T cells have same specificity for the same antigen, because they are derived from one activated T cell.
• At the same time, the surrounding environment also affect the final nature of T cell. This environment contains different cytokines released from APC and tissue cells.
• These cytokines bind with specific cytokine receptors present on dividing T cells, and after binding they activate internal signaling pathways.
• The main pathways involved are JAK-STAT pathway and SMAD pathway, which carry the cytokine signal into the nucleus.
• These signaling pathways induce different master transcription factors. These factors decide what type of effector T cell will be formed.
• In CD4⁺ T cells, STAT1 and STAT4 induce T-bet, which helps in formation of Th1 cells.
• STAT6 induce GATA3, which helps in formation of Th2 cells.
• SMAD2/3 induce Foxp3, which helps in formation of regulatory T cells (Tregs).
• According to these transcription factors, expanding T cells now become specialized effector cells.
• CD4⁺ helper T cells may differentiate into Th1, Th2, Th17 or Tfh cells. These cells secrete different cytokines and control different immune response.
• Th1 cells mainly activate macrophages and help in response against intracellular pathogens.
• Th2 cells help in response against helminths and also recruit eosinophils, basophils and mast cells.
• Th17 cells recruit neutrophils and protect against extracellular bacteria and fungi.
• Tfh cells help B cells in lymphoid follicle and help in production of high affinity antibody.
• CD8⁺ cytotoxic T cells differentiate into killing cells and produce perforin and granzymes.
• These toxic proteins are stored in lytic granules, and later they are released toward infected cell or malignant cell to cause apoptosis.
• After pathogen is removed, large number of effector T cells are not needed. So most of these cells die by programmed cell death or apoptosis.
• This reduction of effector T cell number is called immune contraction. It helps to restore immune balance and prevent tissue damage.
• Fas-FasL interaction also help in death of extra effector T cells during this contraction phase.
• Some T cells do not die after immune contraction. These cells remain for long time as memory T cells.
• Memory T cells are of different types like central memory T cells (Tcm), effector memory T cells (Tem) and tissue resident memory T cells (Trm).
• Tcm cells remain mainly in blood and lymphoid organs. Tem cells circulate in blood and peripheral tissues. Trm cells stay in tissue like skin, lung and gut.
• These memory T cells give rapid and strong immune response when same pathogen enter into the body again.

Cytokines Produced by T Cells

The following are the cytokines produced by T cells–

• CD4⁺ helper T cells– These cells produce different cytokines according to their subtype. They mainly control other immune cells.
  • Th1 cells– They produce Interferon-gamma (IFN-γ) and Interleukin-2 (IL-2). These are used against intracellular pathogen like virus.
  • Th2 cells– They produce Interleukin-4 (IL-4), Interleukin-5 (IL-5), Interleukin-10 (IL-10) and Interleukin-13 (IL-13). These are used against helminth parasite and also help B cells.
  • Th17 cells– They produce Interleukin-17 (IL-17) and Interleukin-22 (IL-22). These help in mucosal defence against extracellular bacteria and fungi.
  • T follicular helper cells (Tfh)– They produce Interleukin-21 (IL-21). It helps B cells in germinal centre and high affinity antibody formation.
• Regulatory T cells (Tregs)– These cells produce suppressive cytokines. They reduce excess immune reaction and maintain self tolerance.
  • IL-10– It suppress inflammatory response and decrease activation of other immune cells.
  • TGF-β– It helps in immune suppression and prevention of autoimmune reaction.
• CD8⁺ cytotoxic T cells– These cells are killing type T cells. They also produce cytokines along with perforin and granzymes.
  • IFN-γ– It inhibits viral replication and activate macrophage.
  • TNF-α and TNF-β– These cytokines help in inflammation and killing of infected or malignant cells.
• Memory T cells– These cells produce cytokines rapidly when same antigen enter again.
  • Tissue resident memory T cells (Trm)– They produce IFN-γ, TNF-α and IL-2 after local antigen recognition.
  • CCL3 and CCL4– These are chemokines produced by Trm cells. They call other immune cells at the tissue site.

T Cell Tolerance and Regulation

Central Tolerance (In the Thymus)

• Negative selection– During development in the thymus, immature T cells are tested against self antigens. If T cell receptor (TCR) bind too strongly with self antigen, the cell is eliminated by programmed cell death or apoptosis.
• Role of thymic cells– This screening is done with the help of medullary thymic epithelial cells (mTECs) and dendritic cells. Special mTECs use Aire regulator to present many tissue specific self antigens to developing T cells.
• Functional tuning– Positively selected T cells which have little higher self reactivity increase CD5 on their surface. CD5 acts as negative regulator and reduce future TCR sensitivity.
• Cytokine and receptor mediation– Molecules like IL-23 and receptors like Nur77 and RORγt helps in signaling and apoptosis of self reactive thymocytes.

Peripheral Tolerance and Regulatory T Cells (Tregs)

• Regulatory T cells (Tregs)– Some self reactive T cells may escape from negative selection. These cells are controlled in periphery by Tregs. They are special CD4⁺ T cells expressing CD25 and Foxp3.
• Immunosuppressive cytokines– Tregs maintain self tolerance by secretion of suppressive cytokines. The main cytokines are Interleukin-10 (IL-10) and Transforming Growth Factor-beta (TGF-β).

Inhibitory Receptors and Immune Checkpoints

• ITIM-containing receptors– T cells express inhibitory receptors having Immunoreceptor Tyrosine-based Inhibitory Motifs (ITIMs). These motifs recruit phosphatases like SHP-1 and SHP-2, which remove phosphate groups from signaling molecules and reduce activation signal.
• CTLA-4– It is an inhibitory receptor induced after T cell activation. It bind with same co-stimulatory molecule as CD28, but gives inhibitory signal and wind down the T cell response.
• T cell exhaustion– During chronic antigen exposure like persistent viral infection or cancer, T cells express PD-1, CTLA-4, LAG-3, TIM-3 and TIGIT continuously. These cells become exhausted and lose effector function. It also protect host from severe chronic tissue damage.

Homeostatic Self-Regulation (Negative Feedback)

• Self-limiting cytotoxicity– CD8⁺ cytotoxic T cells regulate their own response by killing the APCs which stimulate them. This is done by perforin dependent and Fas-FasL dependent pathway. It removes the source of activation and limits prolonged TCR stimulation, hyper proliferation and excess cytokine synthesis.

Thymus-dependent Antigens Induce T-dependent B Cell Responses

• Step 1: Antigen recognition by B cells– B cell first comes in contact with thymus-dependent antigen. These are mainly protein antigens. BCR on B cell surface binds with the specific antigen.
• Step 2: Antigen uptake– After binding, antigen is taken inside the B cell. The BCR-antigen complex enters into the cell. This is the internalization step.
• Step 3: Antigen processing– Inside the cell, antigen is broken into small peptide pieces. This is done by proteolytic enzymes. Now the antigen is in processed form.
• Step 4: MHC-II presentation– The peptide fragments are joined with MHC class II molecules. This peptide-MHC-II complex comes to the surface of B cell. Now B cell acts as APC.
• Step 5: T cell contact– CD4+ helper T cell binds with this B cell. Mainly Th2 cell or T follicular helper cell (Tfh) is involved. TCR of T cell recognizes the peptide-MHC-II complex.
• Step 6: Cognate interaction– This is direct contact between same antigen-specific B cell and T cell. It mostly occurs in follicles and germinal centers of secondary lymphoid organs.
• Step 7: Cytokine help– Helper T cell now releases cytokines. Th2 cells produce IL-4, IL-5, IL-10 and IL-13. Tfh cells mainly produce IL-21.
• Step 8: B cell activation– TCR-MHC-II binding and cytokines together activate the B cell. This help is needed. Without T cell help, response is not strong.
• Step 9: B cell proliferation– Activated B cell starts rapid division. Many same type B cells are formed from one selected cell. This is clonal expansion.
• Step 10: B cell differentiation– The divided B cells change into plasma cells and memory B cells. Plasma cells are antibody secreting cells. Memory cells remain for long period.
• Step 11: Antibody maturation– Class switch recombination occurs. IgM changes to IgG, IgA or IgE. Affinity maturation also occurs, so antibody becomes stronger binding.
• Step 12: Antibody production– Plasma cells secrete high affinity antibodies. These antibodies bind with pathogen antigen. Then pathogen is neutralized or removed.

Tumor-Specific T Cell Induction and Function

Induction of Tumor-Specific T Cells

• Step 1: Tumor antigen presentation– Antigen presenting cells (APCs), mainly dendritic cells, take tumor proteins. These proteins are processed into small peptides. Then these peptides are shown with MHC molecules on APC surface.
• Step 2: Antigen recognition– Naive tumor-specific T cell comes near the APC. Its T cell receptor (TCR) binds with tumor peptide-MHC complex. This gives first signal to the T cell.
• Step 3: Co-stimulation– Second signal is also needed. CD28 on T cell binds with B7 (CD80/CD86) on APC. Without this signal, activation remains poor.
• Step 4: Clonal expansion and differentiation– The selected T cell starts rapid division. Many same type cells are formed. These cells become effector cells, mainly CD8+ cytotoxic T lymphocytes (CTLs).

Function of Tumor-Specific T Cells

• Step 5: Tumor recognition and synapse formation– CTLs move to tumor area. They bind with tumor cell showing same peptide with MHC class I. A close contact area is formed. This is called immunological synapse.
• Step 6: Cytotoxic granule exocytosis– The CTL releases lytic granules into the synapse. Granules contain perforin and granzymes. Perforin makes pores in tumor cell membrane. Granzymes enter through these pores.
• Step 7: Granzyme action– Granzymes act inside tumor cell. They start caspase mediated death pathway. The tumor cell undergoes apoptosis.
• Step 8: Death ligand-mediated apoptosis– Activated CTLs also have Fas ligand (FasL) and TRAIL on their surface. These bind with Fas or TRAIL-R on tumor cell. Then another apoptotic pathway starts.
• Step 9: Cytokine secretion– CTLs secrete IFN-γ and TNF during attack. These cytokines act on tumor and surrounding cells. They help in stopping tumor growth.
• Step 10: Tumor arrest– IFN-γ increases MHC class I expression on tumor cells. It also stops cell cycle and makes tumor cell more sensitive to death signal. Tumor blood vessels may also get damaged.

Disorders Associated with T Cells

Congenital Immunodeficiencies (Developmental Defects)

• Severe Combined Immunodeficiency (SCID)– It is a group of severe genetic disorders where mature T cells and B cells are not formed properly. It may occur due to RAG1 or RAG2 gene mutation. Infants become highly susceptible to opportunistic infections.
• DiGeorge Syndrome (DGS)– It is commonly caused by 22q11.2 chromosomal deletion. In this disorder, development of thymus is abnormal. In complete DGS, thymus is absent and T cell number becomes very low like SCID. In partial DGS, thymus is underdeveloped and mild to moderate T cell deficiency occurs.
• CHARGE syndrome– It is linked with CHD7 gene mutation. In this condition, thymus is hypoplastic and early T cell progenitor colonization is affected. This leads to immunodeficiency.

Autoimmune Diseases (Loss of Tolerance)

• Multiple sclerosis and Type 1 diabetes– These are autoimmune diseases where self reactive T cells attack own body tissues. In some cases, these autoreactive T cells cannot form proper immunological synapse and central SMAC also not assembled properly. So TCR signaling does not switch off normally and chronic hyper reactivity occurs.
• Inflammatory bowel disease (Colitis)– It may occur when regulatory mechanism of T cells fails. Autoreactive T cells then enter into peripheral tissue and cause intestinal inflammation.
• Myasthenia gravis– It is also associated with loss of tolerance. Failure of thymic negative selection or poor regulation allows harmful T cells to act against self tissue.

T Cell Malignancies (Cancers)

• T cell acute lymphoblastic leukemia (T-ALL)– It is a cancer of T cell lineage. It may occur due to abnormal early expression of master regulator genes during T cell development.
• Thymic lymphoma– It is malignant condition related with developing T cells in thymus. Aberrant expression of transcription factors like ThPOK may disturb normal development and support cancer formation.

Acquired Dysfunctions (T Cell Exhaustion)

• Chronic viral infections– In infections like HIV-1, Hepatitis B and Hepatitis C, CD8⁺ cytotoxic T cells are exposed to antigen for long time. These cells become exhausted and lose cytokine production like IL-2 and IFN-γ.
• Checkpoint receptor expression– Exhausted T cells show high expression of inhibitory receptors. These include PD-1, CTLA-4 and LAG-3. Due to this, killing capacity and effector function becomes reduced.
• Solid tumors and malignancies– In cancer, continuous tumor antigen exposure makes local T cells exhausted. The tumor microenvironment becomes immunosuppressive and body cannot clear the malignant cells properly.

Functions of T Cells

The following are the functions of T cells–

• Direct killing of target cells– CD8⁺ cytotoxic T cells kill virus infected cells and malignant cells. This killing is done by perforin and granzymes present in lytic granules. Perforin makes pore on target cell membrane and granzymes enter inside the cell. It causes apoptosis. Fas ligand (FasL) of T cell also bind with Fas receptor and cell death occurs.
• Helping in antibody production– CD4⁺ helper T cells help B cells for antibody production. Mainly Th2 cells and Tfh cells give help to B lymphocytes. They secrete cytokines like IL-4 and IL-21. As a result B cells divide, form plasma cells and produce high affinity antibody.
• Macrophage activation– Th1 cells activate macrophage by secreting Interferon-gamma (IFN-γ). Activated macrophage kill the engulfed intracellular bacteria more effectively. This function is important in infection like Mycobacterium tuberculosis.
• Recruitment of immune cells– Helper T cells recruit different immune cells at infection site. Th2 cells recruit eosinophils, basophils and mast cells for helminth infection. Th17 cells recruit neutrophils for extracellular bacteria and fungi. Th9 cells also helps in mast cell recruitment.
• Regulation of immune response– Regulatory T cells (Tregs) suppress immune response. They produce IL-10 and TGF-β. These cytokines inhibit autoreactive T cells. It helps to maintain self tolerance and prevent autoimmune reaction.
• Prevention of excess inflammation– Tregs reduce immune reaction after infection is removed. CD8⁺ T cells also limit their own response by killing the APCs which stimulate them. So long TCR stimulation and excess cytokine production becomes reduced.
• Immunological memory– Some T cells remain in the body after pathogen is cleared. These are called memory T cells. Important types are central memory T cells (Tcm), effector memory T cells (Tem) and stem cell memory T cells (Tscm). They give fast response when same pathogen enters again.
• Local tissue surveillance– Tissue resident memory T cells (Trm) remain in tissues like skin, gut, lung and brain. They act as local sentinel cells. On second antigen entry, they release IFN-γ and TNF-α quickly.
• Non-cytolytic viral control– In sensitive tissue like brain, some Trm cells control latent virus without killing the important cells. They release granzymes which block viral replication protein. Neurons are not destroyed in this process.
• Link between innate and adaptive immunity– Some unusual T cells like gamma-delta (γδ) T cells, NKT cells and MAIT cells give rapid immune response. They recognize lipid, phosphoantigen and vitamin B2 metabolite. These cells act early without long priming process.

Applications of T Cells

The following are the applications of T cells–

• Chimeric Antigen Receptor T-cell therapy (CAR T-cell therapy)– It is used in cancer treatment by taking patient own T cells and genetically modifying them in laboratory, so that they can recognize and destroy malignant cells. It is successful in blood cancers like B-cell leukemia and also being developed for solid tumour and brain tumour.
• Immune checkpoint blockade (ICB)– It is used to restore the function of exhausted T cells in cancer. In this therapy inhibitory receptors like PD-1, CTLA-4 and LAG-3 are blocked, so the T cells can again attack tumour cells.
• Functional cure research for HIV-1– T cells are used in advanced CAR T-cell design for targeting HIV-1 Envelope (Env) protein. These HIV-specific CAR T cells are also combined with checkpoint inhibitors to reduce viral resistance and T cell exhaustion.
• Tumour-infiltrating lymphocyte (TIL) therapy– This therapy uses patient own T cells which are already present inside tumour tissue. These cells are taken from tumour, expanded in laboratory and again given to patient for tumour destruction.
• Organ transplant rejection prevention– T cells are main cells involved in transplant rejection. So drugs like Cyclosporin A and FK506 (Tacrolimus) are used to inhibit calcineurin-NFAT pathway and prevent activation of alloreactive T cells.
• Vaccine design and testing– T cells are used in vaccine studies because T cell memory gives long term protection. T cell responses are studied in vaccines for COVID-19, Malaria, Tuberculosis and HIV.

References

1. 5. Overview of T cell subsets. (n.d.). Immunopaedia.
2. Zheng, L., Lin, J., Zhang, B., Zhu, Y., Li, N., Xie, S., Wang, Y., Gao, N., & Huang, Z. (2019). Structural basis of assembly of the human T cell receptor-CD3 complex. Nature, 573, 546-552. https://doi.org/10.1038/s41586-019-1537-0
3. A new look at TCR signaling to NF-κB. (n.d.). PubMed Central (PMC).
4. AP-1–independent NFAT signaling maintains follicular T cell function in infection and autoimmunity. (n.d.). PubMed Central (PMC).
5. Janeway, C. A., Jr., Travers, P., Walport, M., & Shlomchik, M. J. (2001). Antigen receptor structure and signaling pathways. In Immunobiology: The immune system in health and disease (5th ed.). Garland Science.
6. CD1 and MR1 recognition by human γδ T cells. (n.d.). PubMed Central (PMC).
7. CD4 T Helper Cell Subsets and Related Human Immunological Disorders. (n.d.). PubMed Central (PMC).
8. Decision checkpoints in the thymus. (n.d.). PubMed Central (PMC).
9. Decoding and overcoming T cell exhaustion: Epigenetic and transcriptional dynamics in CAR-T cells against solid tumors. (n.d.). PubMed Central (PMC).
10. Detailed analysis of gene expression during development of T cell lineages in the thymus. (n.d.). PubMed Central (PMC).
11. shreddit13. (2015). Do Cytotoxic T cells implement the Fas (CD95) pathway and the perforin/granzyme pathway under different circumstances, or are they just redundant to ensure apoptosis? [Online forum post]. Reddit.
12. Functional Anatomy of T Cell Activation and Synapse Formation. (n.d.). PubMed Central (PMC).
13. Janeway, C. A., Jr., Travers, P., Walport, M., & Shlomchik, M. J. (2001). Generation of lymphocytes in bone marrow and thymus. In Immunobiology: The immune system in health and disease (5th ed.). Garland Science.
14. Granzymes: The Molecular Executors of Immune-Mediated Cytotoxicity. (n.d.). PubMed Central (PMC).
15. HIV-1-Specific CAR-T Cells With Cell-Intrinsic PD-1 Checkpoint Blockade Enhance Anti-HIV Efficacy in vivo. (n.d.). PubMed Central (PMC).
16. HIV-specific CAR T cells return to the clinic. (n.d.). PubMed Central (PMC).
17. Human T Cell Reconstitution in DiGeorge Syndrome and HIV-1 Infection. (n.d.). PubMed Central (PMC).
18. INTEGRIN FUNCTION IN T CELL HOMING TO LYMPHOID AND NON-LYMPHOID SITES: GETTING THERE AND STAYING THERE. (n.d.). PubMed Central (PMC).
19. Immunodeficiency in DiGeorge Syndrome and Options for Treating Cases with Complete Athymia. (n.d.). PubMed Central (PMC).
20. Medaphysics Repository. (n.d.). Immunology: T cell receptor structure and function [Video]. YouTube.
21. Key links in the physiological regulation of the immune system and disease induction: T cell receptor -CD3 complex. (n.d.). PubMed.
22. Living rent free in your head: resident memory T cells. (n.d.). PubMed Central (PMC).
23. Lung dendritic cells imprint T cell lung homing and promote lung immunity through the chemokine receptor CCR4. (n.d.). PubMed Central (PMC).
24. Mechanisms of T cell organotropism. (n.d.). PubMed Central (PMC).
25. Memory T cells: strategies for optimizing tumor immunotherapy. (n.d.). PubMed Central (PMC).
26. Memory T-Cell Heterogeneity and Terminology. (n.d.). PubMed Central (PMC).
27. Memory T-cell trafficking: new directions for busy commuters. (n.d.). PubMed Central (PMC).
28. Modeling the Dynamics of T-Cell Development in the Thymus. (n.d.). PubMed Central (PMC).
29. Molecular Mechanisms of T Helper Cell Differentiation and … (n.d.). PubMed Central (PMC).
30. NF-κB: Roles and Regulation In Different CD4+ T cell subsets. (n.d.). PubMed Central (PMC).
31. Nuclear Factor of Activated T Cells (NFAT) Proteins as Targeted Molecules in Diseases: A Narrative Review. (n.d.). PubMed Central (PMC).
32. Perforin and Fas killing by CD8+ T cells limits their cytokine synthesis and proliferation. (n.d.). PubMed Central (PMC).
33. Peripheral Tissue Homing Receptor Control of Naïve, Effector, and Memory CD8 T Cell Localization in Lymphoid and Non-Lymphoid Tissues. (n.d.). PubMed Central (PMC).
34. Peripheral tissue homing receptors enable T cell entry into lymph nodes and affect the anatomical distribution of memory cells. (n.d.). PubMed Central (PMC).
35. Regulation of Human Helper T Cell Subset Differentiation by Cytokines. (n.d.). PubMed Central (PMC).
36. Lowe, R. M., Li, H., Hsu, H.-C., & Mountz, J. D. (2018). Regulation of negative selection in the thymus by cytokines: Novel role of IL-23 to regulate RORγt. In J. Soboloff & D. J. Kappes (Eds.), Signaling mechanisms regulating T cell diversity and function. CRC Press/Taylor & Francis.
37. Regulation of T cell exhaustion and stemness: molecular mechanisms and implications for cancer immunotherapy. (n.d.). PubMed Central (PMC).
38. Role of memory T cell subsets for adoptive immunotherapy. (n.d.). PubMed Central (PMC).
39. Structural Biophysics, Signaling Networks, and Homing Topography of T-Lymphocyte Subsets in Physiology and Immunotherapy. (n.d.). [Markdown Document].
40. Janeway, C. A., Jr., Travers, P., Walport, M., & Shlomchik, M. J. (2001). Summary to chapter 7. In Immunobiology: The immune system in health and disease (5th ed.). Garland Science.
41. T Cell Activation. (n.d.). PubMed Central (PMC).
42. Frank Lectures. (n.d.). T Cell Receptor(TCR) and CD3 (FL-Immuno/28) [Video]. YouTube.
43. T Helper Cell Differentiation, Heterogeneity, and Plasticity. (n.d.). PubMed Central (PMC).
44. Janeway, C. A., Jr., Travers, P., Walport, M., & Shlomchik, M. J. (2001). T cell-mediated cytotoxicity. In Immunobiology: The immune system in health and disease (5th ed.). Garland Science.
45. T cells at the interface of neuro-immune communication. (n.d.). PubMed Central (PMC).
46. Cavanagh, M., & Gwyer Findlay, E. (2022). T-cell activation. British Society for Immunology.
47. TOX transcriptionally and epigenetically programs CD8+ T cell exhaustion. (n.d.). PubMed Central (PMC).
48. Mookerjee-Basu, J., Chemmannur, S. V., Qin, L., & Kappes, D. J. (2018). ThPOK, a key regulator of T cell development and function. In J. Soboloff & D. J. Kappes (Eds.), Signaling mechanisms regulating T cell diversity and function. CRC Press/Taylor & Francis.
49. The Emerging Roles and Therapeutic Potential of Unconventional T Cells in Sepsis. (n.d.). PubMed Central (PMC).
50. The NF-κB signaling network in the life of T cells. (n.d.). PubMed Central (PMC).
51. The current state and future of T-cell exhaustion research. (n.d.). PubMed Central (PMC).
52. The immunological synapse. (n.d.). PubMed Central (PMC).
53. Rossy, J., Williamson, D. J., Benzing, C., & Gaus, K. (2012). The integration of signaling and the spatial organization of the T cell synapse. Frontiers in Immunology, 3, 352. https://doi.org/10.3389/fimmu.2012.00352
54. Yates, A. J. (2014). Theories and quantification of thymic selection. Frontiers in Immunology, 5, 13. https://doi.org/10.3389/fimmu.2014.00013
55. Tissue-Resident Memory T Cells. (n.d.). PubMed Central (PMC).
56. Transcriptional programming of tissue-resident memory CD8+ T cells. (n.d.). PubMed Central (PMC).
57. Unconventional T Cells’ Role in Cancer: Unlocking Their Hidden Potential to Guide Tumor Immunity and Therapy. (n.d.). PubMed Central (PMC).
58. Unconventional T cells in anti-cancer immunity. (n.d.). PubMed Central (PMC).
59. Understanding subset diversity in T cell memory. (n.d.). PubMed Central (PMC).
60. Understanding the HIV-specific T-cell response to immune checkpoint blockade: what can we learn from cancer immunotherapy? (n.d.). PubMed Central (PMC).
61. γδ, NKT and MAIT cells during evolution: redundancy or specialized functions? (n.d.). PubMed Central (PMC).