T-Cell Activation and Differentiation

• Chapter 10 focuses on T-cell activation, helper subset differentiation, and memory

Main topics

• T-cell activation and the two-signal hypothesis
• Helper CD4+ T-cell differentiation
• T-cell memory

Naive T Cells

• T cells are naive when they exit the thymus and must be activated before they can perform their functions
• These cells each express a unique T-cell receptor (TCR) and either CD4 or CD8
• Naive T cells circulate the body, actively searching for antigens
• Naive T cells feature condensed chromatin, very little cytoplasm, limited transcriptional activity
• When a T cell encounters an antigen, it stops migrating, initiates a response, and is no longer considered naive

T-Cell Activation

• Activation needs three signals, though it is often referred to as the two-signal hypothesis
• Antigen presentation is the first signal for T cells
• Certain molecular interactions act as the second activation signal
• Local cytokines will provide the third signal
• Following activation, T cells differentiate into their functional forms
• CD8+ T cells differentiate into killer or cytotoxic T cells
• CD4+ T cells differentiate into helper T cells
• Discussions focus on general activation events and the differentiation of CD4+ T cells

T-Cell Activation and Differentiation Steps

• Antigen recognition occurs when antigen-presenting cells (APCs) present antigen to the T-cell receptor (TCR)
• Co-stimulatory molecules like CD28 on T cells and CD80/86 on APCs amplify the signal
• Local cytokines enhance the T-cell activation
• Once activated, T cells proliferate clonally
• CD8+ T cells proceed to become cytotoxic T cells, which kill infected or abnormal cells directly
• CD4+ T cells differentiate into subsets based on the cytokine environment
  • The subsets include Th1 cells which promote cell-mediated immunity and activate macrophages
  • Th2 cells which stimulate B cells to produce antibodies and enhance humoral immunity
  • Th17 cells which are involved in inflammatory responses and defense against extracellular pathogens
  • T regulatory (Treg) cells which suppress immune responses and maintain tolerance to self-antigens

Costimulatory Signals

• Costimulatory signals that are required for T-cell activation and proliferation include, signal 1, signal 2, signal 3
• Signal 1 is the antigen-specific TCR engagement, which is presented by the MHC on APCs
• Signal 2 involves receptors on T cells, such as CD28 and ICOS, that contact costimulatory ligands
• Costimulatory ligands are, for example, CD80, CD86, and ICOS-L on the APC, signaling the presence of infection/danger
• Signal 3 consists of cytokines, which are secreted by various immune cells in response to conditions like infection, inflammation, or other stimuli
  • They direct T-cell differentiation into distinct effector cell types

Immunological Synapses

• Successful T cell-APC interactions organize signaling molecules into an immunological synapse
• The TCR/MHC-peptide complexes and coreceptors centralize, forming the central supramolecular activating complex (cSMAC)
  • Co-receptors like CD4 or CD8 may also be present
• Adhesion molecules and bound ligands localize peripherally, forming the peripheral supramolecular activating complex (pSMAC)
  • An example is LFA-1 on the T cell interacting with ICAM-1 on the APC

Initiation of TCR Signaling

• T-cell activation starts with the tyrosine kinase Lck, which is specific to lymphocytes
• CD4 and CD8 cytoplasmic tails guide Lck (lymphocyte-specific protein tyrosine kinase) to the TCR-MHC complex
• Lck phosphorylates ITAMs (immunoreceptor tyrosine-based activation motifs) on CD3
• Phosphorylated ITAMs become docking sites for ZAP-70 via tandem SH2 domains
• Lck phosphorylates and activates ZAP-70
• ZAP-70 phosphorylates adaptor proteins like LAT (Linker for Activation of T cells) and SLP-76

PLCy Signaling and Activation of Transcription

• PLCY action activates NF-кВ and NFAT transcription factors
• The kinase ZAP-70 activates PLCY
• PLCY binds LAT, near membrane phospholipids
• PLCY catalyzes the split of PIP2 into soluble IP3 and membrane-DAG
• IP3 induces calcium (Ca2+) release
• Calcium binds calcineurin phosphatase, then dephosphorylates and activates NFAT
• NFAT enters the nucleus
• DAG activates PKC-Ө, which leads to NF-kB translocation into the nucleus

Ras-ERK Signaling and Activation of AP-1

• Ras-ERK signaling leads to transcription factor AP-1 activation
• Guanine nucleotide exchange factors (GEFs) promote GDP exchange for GTP on the Ras protein, which leads to Ras activation
• The Ras pathway activates MAP Kinase (MAPK)
• The MAPK cascade phosphorylates RAF, MEK, and ERK sequentially
• ERK translocates to the nucleus, phosphorylating transcription factors and co-regulators, including AP-1
• AP-1 phosphorylation enhances its transcriptional activity, leading to the expression of genes that are involved in cell proliferation, differentiation, and survival

Signal 2

• Co-stimulatory receptors like CD28 and ICOS facilitate T-cell activation
  • CD28 binds to CD80 and CD86 on APCs, amplifying T-cell activation/proliferation
  • ICOS plays a similar co-stimulatory role in T cells, promoting activation/effector functions
• Co-inhibitory/checkpoint receptors turn activation off
  • CTLA-4, PD-1, and BTLA are examples
  • CTLA-4 competes with CD28 for CD80/86 binding, inhibiting T-cell activation/proliferation
  • PD-1 interacts with PD-L1/PD-L2, inhibiting effector functions and enhancing tolerance
  • BTLA regulates T-cell activation/tolerance via HVEM on APCs, dampening responses post initial threat
• Co-inhibitory/checkpoint receptors help maintain T-cell tolerance and reduce inflammation

Positive Costimulatory Receptors

• Positive co-stimulatory receptors facilitate activation, like CD28 and ICOS
  • CD28 is involved in initial activation events in T cells
  • CD28 is a 44 kDa glycoprotein expressed as a homodimer on most T cells and significantly enhances TCR-induced proliferation and survival
  • CD28 binds to CD80 (B7-1) and CD86 (B7-2), which are expressed by APCs
  • ICOS is expressed on memory and effector T cells, plus a subset of regulatory T cells (Tregs)
  • ICOS is an inducible co-stimulator that binds ICOS-ligand (ICOS-L) on activated APCs
  • ICOS may help maintain activity of already differentiated cells
  • ICOS is important for promoting T-cell effector functions and supporting long-term immunity, particularly in chronic infections or inflammatory conditions

Coinhibitory Receptors

• This is negative co-stimulatory receptors and help turn activation off.
• CTLA-4 (CD152) works by inducing within 24 hours after activation and peaks 2-3 days post-stimulation
  • It binds to CD80 (B7-1)/CD86 (B7-2) on APCs, has higher affinity over CD28, it then shuts down signaling pathways
  • It helps regulate intensity and duration of immune responses, preventing excessive activation and immune-mediated tissue damage
• PD-1 (programmed death-1, CD279) has cell surface receptor ligands that binds two ligands PD-L1 (B7-H1) and PD-L2 (B7-DC), expressed on various cell types, including APCs and nonlymphoid tissues
  • It may help mediate peripheral T-cell tolerance in nonlymphoid tissues and limits immune responses to self-antigens or harmless environmental stimuli
• BTLA (B- and T-lymphocyte attenuator, CD272) may downregulate inflammatory and autoimmune responses
  • It binds to its ligand herpes virus entry mediator (HVEM), expressed on APCs and other immune cells

Clonal Anergy

• When a T-cell receives its signal one (via TCR engagement with antigen-MHC complex) without the necessary costimulatory signal 2, nonresponsiveness occurs
  • This response is known as clonal anergy
• The cell is rendered nonresponsive if only signal 1 is received
  • This might happen if a T cell has not been screened against a peripheral self-antigen during development
• The action of co-inhibitory receptors can also induce anergy

Signal 3

• Cytokines provide Signal 3
• Depending on which cytokines are present as the T cell is becoming activated, different outcomes can occur
• IL-2 is an autocrine type of cytokine response, where T cells produce the cytokine and its receptor
  • The binding of this ligand induces a very strong proliferation signal during activation stages, driving clonal expansion and the generation of a large pool of effector T cells
• Polarizing cytokines can direct T cells down different subset development pathways
  • These subsets exhibit distinct cytokine secretion profiles and effector functions

Cells That Can Provide Signal 1 and Signal 2

• APCs have characteristic co-stimulatory properties.
• These include dendritic cells, macrophages, and B cells
• Dendritic cells are the best activators of naive T cells as they have high levels of MHC and costimulatory molecules

Superantigens

• Superantigens are a special class of T-cell activators
• These are viral or bacterial proteins that bind to specific Vβ regions of TCRs and the α chain of class II MHC molecules
  • This effectively "short circuits” the need for co-stimulation and produces dramatic cytokine secretion
• This is done by a disproportionate amount of activated T cells
• Exogenous superantigens are soluble proteins secreted by bacteria.
• Endogenous superantigens are cell membrane proteins generated by viral genes that are integrated into mammalian genomes

Helper CD4+ T-cell Differentiation

• T-Cell differentiation happens when initial activation signals 1 and 2 induce upregulation of pro-survival genes, for example, Bcl-2
  • Bcl-2 helps prevent apoptosis in activated T cells
• Transcription of IL-2 and IL-2R genes then occurs
• IL-2 is a key cytokine involved in T cell proliferation and differentiation, while its receptor (IL-2R) allows for autocrine signaling
• This leads to the differentiation of long lived memory cells and clonal effector cells

Polarizing Cytokines

• This is the regulation of differentiation of helper T cells subsets
• This happens with signal 3, where cytokines are nearby when the T cell is activated
  • APCs may bind PAMPS via PRRs, inducing cytokine secretion
  • Engaging different PRRs (via different antigens) results in production of different cytokines
• Viruses stimulate IL-12 to induce TH1 subsets, where worms will stimulate IL-4 to induce TH2 subsets

Helper T-Cell Subsets

• Helper T cells can be divided into at least five distinct subsets
  • TH regulates immunity to intracellular bacteria and viruses
  • TH2 regulate immunity to worms
• TH17 regulate immunity to extracellular bacteria and fungi
• TREG are inhibitory in terminating immune responses and inhibiting autoimmunity
• TFH regulate humoral immunity (B cells)
• Each produces a distinct cytokine profile and regulates distinct activities within the body

Distinguishing Helper T Subsets

• Helper T (TH) subsets are distinguished by three properties:
• A distinct set of polarizing cytokines induces expression of
• A master gene regulator regulates expression of
• A signature set of effector cytokines is produced by that subset

Th17 Cells

• Activation occurs in the presence of IL-1β, IL-6, IL-23, and TGF-β
• IL-23 finalizes the subset commitment
• The master regulator RORyt differentiates activating T cells into this subset
• IL-17F, IL-21, and IL-22 production fight off fungal and extracellular bacterial infections
• IL-21 regulates immune responses and B cell activation/differentiation in this process
• IL-22 maintains barrier integrity and homeostasis at mucosal surfaces
• IL-17A production links to chronic inflammatory and autoimmune responses

Peripheral T Reg Cells

• Peripheral TREG (pTREG) were formerly known as (induced) iTREG cells
• These are similar to natural TREG in the thymus and arise from activation by T cells with CD4+ in the presence of TGF-β
• TGF-B induces the FoxP3 master regulator in this subset
• FoxP3 expression is essential for the suppressive function of both nTREG and pTREG cells
• pTREG cells secrete IL-10 and TGF-β to downregulate inflammation and suppress T-cell subsets
• They play a crucial role in immune homeostasis and prevent autoimmunity

Follicular Helper T (TFH) Cells

• These cells are polarized by IL-6 and IL-21
• IL-6 and IL-21 activate Bcl-6 as master regulator and inhibit T-Bet, GATA3, and RORyt expression
• IL-4 and IL-21 are secreted cytokines that promote B-cell differentiation
• They express CD40L for B-cell help, also, they express CXCR5 for chemokine that attracts them to B-cell follicles

TH9 Cells

• This cells produce IL-9, which helps to expel worms from host intestine and contributes to antitumor response
• IL-2, TGFβ, and IL-4 play roles in polarizing naïve T cells to TH9 cells
• Transcription factors IRF-4 and PU.1 drive TH9 Differentiation
• Dysregulated TH9 responses have been associated with allergic inflammation and autoimmune diseases
• Potential in cancer Immunotherapy, although further research needed

TH22 Cells

• These cells contribute to inflammation associated with multiple autoimmune disorders
• IL-6, IL-23, and TNF polarize naïve T cells to this subset by upregulating the AHR (aryl hydrocarbon receptor) transcription factor
• The TH22 effector cytokine is IL-22, which has pleiotropic effects on various cell types, plays a role in tissue homeostasis, inflammation, and repair
• IL-22 is involved in the regulation of epithelial barrier function, antimicrobial defense, and the production of pro-inflammatory mediators
• They do not express RORyt or make IL-17
• They secrete IL-13, have homing receptors for skin, and thus suggest a role in allergic/inflammatory, and immune responses

Helper T-Cell Lineage Commitment

• Helper T cells can shift from being irrevocably committed
  • When exposed to IL-12, young TH2 cells produce IFN-γ and if young TH1 cells will produce IL-4 in TH2 polarizing conditions, they don't reach TH17 or pTREG traits
• The commitment to a particular lineage then becomes rigid, as it specializes it is more difficult to fully differentiated helper T cells
• Fluiditiy makes it difficult establisment of helper cell lineages, due to their expression of cytokines or transcription factors
• They may expibit phenotypic and functional charactersitics or transition bewteen subsets to react to certain stimuli

Helper T-Cell Subsets in Health and Disease

• Helper T-cell subsets play critical roles in immune health and disease, but genetic differences in subset differentiation potential may lead to different outcomes
• Leprosy is an example
  • Tuberculoid stimulates it with the TH1, with high circulating levels of IL-2, IFN-y, and lymphotoxin-a and if lepromatous stimulates TH2 responses
  • There are high circulation of IL-4, IL-5, and IL-10 from other subsets, mainly cells in TREG. The response is not as effective at clearing the disease
• HIV progresses to AIDS as TREG responses may be favored over time and the TREG plays a role in suppressing immune responses as the activity goes up and contributes to dysfunction
• During the virus, EBV, produces viral produced homologous to the virus, which reduces help in T-cell and macrophage
• VH-10 may add to immunosuppression, which then dampens immune responses contributing to EBV persistence TH17 can control infections of extracellular bacteria and fungi and enhances the barriere fuction for the mucosal

T-Cell Memory

• Naive, effector, and memory T cells are differentiated
  • Each surface have surface marker to show their differences
• There is T cell help which must be provided with CD4+ help
• May have signaling of costim and cytokine production that causes diferrentiation of memory T cells and play important role of TsCM. IL-7, L-15 and WnT may play role. IIL-2 is important generatibg to effector cells
• Do memory cells reflect heterogeneity of effector or a primary? The answers appears diverse.

T-Cell Memory Distinguishing

• Tcm cells reside and travel at the secondary area. Longer to come and rapid exposure
• Tem reside in the circulate , for immediate service and defense
• Tru resides the none -lypmioid and long lasting re infection. CD8 tru can release cytokins and reciryt cytotix cell to tissue

Types of Memory T Cells Cont.

• Tcm is centeral and Teu effetctor, Trm tissur resident as they locate in the region
• This all function to keep up immume system