Activation of T Lymphocytes

At what two stages do T lymphocytes need to recognize the same antigen?

1. To initiate the immune response
2. To perform effector functions

What are the steps of T cell activation and response?

• Dendritic cells carry microbes or their antigens to the lymph nodes from the site of infection.

• Naïve T cells circulate through lymph nodes and recognize the antigen, leading to activation.

• Activated T cells develop into effector cells.

• Effector T cells migrate to the site of infection.

• At the infection site, effector T cells recognize the antigen again and eliminate the microbe.

What are the three signals required for the differentiation of naïve T cells into effector and memory T cells? (CD4+ T cells and CD8+ T cells)

1. Antigen recognition (lymphoid organ)

2. Costimulation

3. Cytokines

What happens during the differentiation of naïve T cells?

• Antigen recognition – APC presents antigen to naïve CD4+ or CD8+ T cells in lymphoid organs.

• Lymphocyte activation – IL-2R and cytokines (e.g., IL-2) activate T cells.

• Proliferation – Activated T cells multiply in lymphoid organs.

• Differentiation – T cells develop into effector or memory cells in lymphoid organs.

• Effector function in peripheral tissues –

  • CD4+ T cells become effector CD4+ or memory CD4+ T cells, leading to macrophage activation, B cell activation, and inflammation.

  • CD8+ T cells become cytotoxic T lymphocytes (CTLs) or memory CD8+ T cells, leading to the killing of infected or tumor cells.

Q: What is Signal 1 in the immunological synapse?

antigen recognition, which is the first and necessary signal for lymphocyte activation, ensuring an antigen-specific immune response.

What happens when an unactivated APC (costimulator-deficient) binds to a naïve T cell? (@ immunological synapse - signal 1)

T cell does not respond or becomes tolerant, failing to activate.

What is Signal 2 in the immunological synapse?

costimulation, which is required for the proliferation and differentiation of naïve T cells. APCs activated by microbes increase the expression of costimulators.

How does costimulation occur between APCs and naïve T cells?

• Activated APCs express B7 molecules.

• B7 binds to CD28 on naïve T cells.

• This interaction leads to the release of IL-2, promoting T cell survival, proliferation, and differentiation into effector T cells.

How does CD28 contribute to T cell activation in Signal 2 of the immunological synapse?

CD28 on naïve T cells binds to costimulatory molecules B7-1 (CD80) and B7-2 (CD86) on activated APCs. This interaction, along with antigen recognition via the TCR complex, leads to:

1. Cell survival – Increased production of BCL-Xl and BCL-2.

2. Proliferation – Increased secretion of IL-2, expression of IL-2R, increased cyclins, and decreased cell cycle inhibitors.

3. Differentiation – Activation of signaling pathways (PI-3 kinase/AKT, RAS/MAP kinase) promoting the formation of effector and memory T cells.

What is the function of B7-1 (CD80) binding to CD28 on T cells?

Activation of naïve T cells and induction of immune responses.

What happens when B7-2 (CD86) binds to CTLA-4 on regulatory and activated T cells?

Inhibition of T cell activation.

What is the function of ICOS-L (CD275) binding to ICOS on T cells?

Generation of T follicular helper (Tfh) cells.

What is the effect of PD-L1 (CD274, B7-H1) and PD-L2 (CD275, B7-DC) binding to PD-1 on activated T cells?

Inhibition of T cell activation.

What cytokine is responsible for T cell activation?

IL-2, are crucial for T cell activation. IL-2 promotes growth, survival, and differentiation of T lymphocytes

What is the role of IL-2 in T cell survival and function?

• IL-2 promotes survival by inducing the anti-apoptotic protein BCL-2.

• IL-2 stimulates cell cycle progression.

• IL-2 increases the production of effector cytokines, such as IFN-γ and IL-4.

Describe the process of T cell activation involving antigen and costimulatory signals.

1. APC with B7 costimulator binds to CD28 on the resting naïve T cell.

2. The IL-2Rβγ complex on the naïve T cell is initially unoccupied.

3. The secretion of IL-2 binds to the unoccupied IL-2Rβγ complex, forming the high-affinity IL-2Rαβγ complex.

4. This results in IL-2-induced T cell proliferation.

What happens when an activated T cell releases IL-2?

IL-2 released by the activated T cell binds to regulatory T cells, causing their proliferation and maintenance.

What are the components of the immunological synapse and the supramolecular activation cluster (SMAC)?

• APC components:

  • iCAM-1, CD80/CD86, and pMHC.

• T lymphocyte components:

  1. cSMAC: Contains TCR/CD3 and CD28.

  2. pSMAC: Contains LFA-1 and MTOC.

  3. dSMAC: Contains F-actin, TCR, and MCs.

movement from outer dSMAC towards inner cSMAC

What are the characteristics of Th1 effector T cells?

• Defining cytokine: IFN-gamma

• Principal target cells: Macrophages

• Major immune reactions: Macrophage activation

• Host defense: Intracellular pathogens

• Role in disease: Autoimmunity; chronic inflammation

What are the characteristics of Th2 effector T cells?

• Defining cytokines: IL-4, IL-5, IL-13

• Principal target cells: Eosinophils

• Major immune reactions: Eosinophil and mast cell activation, alternative macrophage activation

• Host defense: Helminths

• Role in disease: Allergy

4o mini

What are the characteristics of Th17 effector T cells?

• Defining cytokines: IL-17, IL-22

• Principal target cells: Neutrophils

• Major immune reactions: Neutrophil recruitment and activation

• Host defense: Extracellular bacteria and fungi

• Role in disease: Autoimmunity; inflammation

What are the characteristics of Tfh effector T cells?

• Defining cytokine: IL-21 (and IFN-gamma or IL-4)

• Principal target cells: B cells

• Major immune reactions: Antibody production

• Host defense: Extracellular pathogens

• Role in disease: Autoimmunity (autoantibodies)

How do naive CD4+ T cells differentiate into Th1 cells?

• Naive CD4+ T cells are exposed to IL-12 and IFN-gamma.

• These signals activate T-bet and STAT4, key transcription factors.

• T-bet and STAT4 drive the differentiation of the T cells into Th1 cells.

• Th1 cells produce IFN-gamma and TNF.

• Th1 cells are involved in defending against intracellular pathogens and are linked to chronic inflammation.

How do naive CD4+ T cells differentiate into Th2 cells?

• Naive CD4+ T cells are exposed to IL-4.

• IL-4 activates the transcription factors GATA-3 and STAT6.

• GATA-3 and STAT6 drive the differentiation of the T cells into Th2 cells.

• Th2 cells produce IL-4, IL-5, and IL-13.

• Th2 cells are involved in defense against extracellular parasites and contribute to allergic disorders.

How do naive CD4+ T cells differentiate into Th17 cells?

• Naive CD4+ T cells are exposed to IL-1, IL-6, TGF-beta, and IL-23.

• These signals activate RoRγt and BLIMP-1, key transcription factors.

• RoRγt and BLIMP-1 drive the differentiation of the T cells into Th17 cells.

• Th17 cells produce IL-17, IL-21, and IL-22.

• Th17 cells are involved in defense against extracellular pathogens and contribute to autoimmunity.

How do naive CD4+ T cells differentiate into Th22 cells?

• Naive CD4+ T cells are exposed to TNF-alpha and IL-6.

• These cytokines activate the transcription factor AHR (Aryl hydrocarbon receptor).

• AHR drives the differentiation of the T cells into Th22 cells.

• Th22 cells produce IL-22.

• Th22 cells are involved in wound healing and contribute to inflammatory diseases.

How do naive CD4+ T cells differentiate into T regulatory cells?

• Naive CD4+ T cells are exposed to TGF-beta and IL-2.

• These signals activate the transcription factors Foxp3 and STAT5.

• Foxp3 and STAT5 drive the differentiation of the T cells into T regulatory cells (Tregs).

• T regulatory cells produce IL-10 and TGF-beta.

• Tregs play a crucial role in maintaining self-tolerance and preventing autoimmunity.

What is the role of IL-2 in immune responses?

• IL-2 promotes T cell proliferation.

• Supports regulatory T cell survival.

• Activates activated T cells.

What is the role of interferon-gamma in immune responses?

• activates macrophages via the classical pathway.

• Produced by CD4+ Th1 cells, CD8+ T cells, and natural killer (NK) cells.

What is the role of IL-4 in immune responses?

• induces B cell switching to IgE.

• Activates alternative macrophage activation.

• Produced by CD4+ Th2 T cells and mast cells.

What is the role of IL-5 in immune responses?

• activates eosinophils.

• Produced by CD4+ Th2 T cells, mast cells, and innate lymphoid cells.

What is the role of IL-13 in immune responses?

• induces B cell switching to IgE.

• Activates alternative macrophage activation.

• Produced by CD4+ Th2 T cells, mast cells, and innate lymphoid cells

What is the role of IL-17 in immune responses?

• stimulates acute inflammation.

• Produced by CD4+ Th17 T cells and other cells.

What is the role of IL-21 in immune responses?

• activates B cells.

• Supports Tfh differentiation.

• Produced by CD4+ Tfh T cells.

What is the role of IL-22 in immune responses?

• maintains epithelial barrier function.

• Produced by CD4+ Th17 T cells, NK cells, and innate lymphoid cells.

What is the function of Th1 cells?

• Bacteria are presented by APCs.

• Naive T cells differentiate into Th1 cells.

• Th1 cells produce IFN-gamma.

• IFN-gamma activates macrophages, leading to classical macrophage activation.

• This enhances microbial killing by macrophages.

How do Th1 cells activate macrophages?

• Activation of effector cells: CD4+ effector T cells (Th1 cells) bind to macrophages that have ingested bacteria.

• Activation of macrophage:

  • CD40L on the Th1 cell binds to CD40 on the macrophage.

  • The Th1 cell releases IFN-gamma, which binds to the IFN-gamma receptor on the macrophage.

• Responses of activated macrophages:

  • Macrophage releases cytokines such as TNF, IL-1, IL-12, and chemokines.

  • Production of ROS, NOS

  • Increases expression of MHC and costimulators (B7 molecules). = increased T cell activation

  • Killing of phagocytosed bacteria is enhanced.

TNF, IL-1 and chemokines role in cell-mediated immunity

leukocyte recruitment (inflammation)

IL-12 role in cell-mediated immunity

Th1 differentiation and IFN-gamma production

How do Th1 cells develop in response to intracellular microbes like mycobacteria?

• Dendritic cell presents antigen and releases IL-12. // or NK cells produce IFN-gamma.

• IL-12 activates T-bet, STAT4, and STAT1 in antigen-activated T cells, leading to the differentiation into Th1 cells.

How do Th2 cells develop and what are their roles in immune responses?

• Initiation:

  • Helminths or protein antigens are recognized by APCs, which activate naive CD4+ T cells.

  • Naive CD4+ T cells undergo proliferation and differentiation into Th2 cells.

• 4 Pathways of Th2 Cells:

  1. Tfh cells release IL-4, promoting B cell activation, leading to IgE production for mast cell degranulation or IgG4 (humans) / IgG1 (mice) for antibody production.

  2. Th2 cells release IL-4 and IL-13, stimulating intestinal mucus secretion and peristalsis.

  3. Th2 cells release IL-5, activating eosinophils, which attach to helminths.

  4. Th2 cells release IL-4 and IL-13, leading to macrophage activation for alternative activation, enhancing fibrosis and tissue repair.

How do Th2 cells respond to heminths?

• Helminths bind to either dendritic cells, mast cells, or eosinophils.

• These cells activate CD4+ T cells and release IL-4.

• IL-4 activates GATA-3 and STAT6 in the antigen-activated T cell, leading to differentiation into Th2 cells.

What is the function of classical activated macrophages (M1)?

• Monocytes are activated by microbial TLR-ligands and IFN-gamma.

• This activation leads to the differentiation of macrophages into M1 macrophages.

• M1 macrophages produce:

  • ROS, NO, and lysosomal enzymes for microbicidal actions.

  • Phagocytosis and killing of bacteria and fungi.

• Additionally, M1 macrophages release IL-1, IL-12, IL-23, and chemokines, which cause inflammation.

What is the function of alternatively activated macrophages (M2)?

• Monocytes are activated by IL-13 and IL-4, which enter the macrophage.

• M2 macrophages release IL-10 and TGF-beta.

• These factors lead to:

  • Anti-inflammatory effects.

  • Wound repair and fibrosis.

  • Stopping inflammation.

What is the balance between Th1 and Th2 cell activation and its impact on immune responses?

• Th1 pathway:

  • Naive CD4+ T cell differentiates into Th1 cells, which release IFN-gamma and TNF.

  • This results in macrophage activation and promotes cell-mediated immunity.

  Th2 pathway:

  • Th2 cells release IL-4 and IL-13, which inhibit the microbicidal activity of macrophages.

  • This leads to macrophage activation but can impair cell-mediated immunity.

Infection with Leishmania major:

• most mouse strains, Th1 response leads to recovery.

• In BALB/c mice, Th2 response leads to disseminated infection.

Infection with Mycobacterium

• some patients, Th1 response leads to tuberculoid leprosy.

• In some patients with defective Th1 response or dominant Th2 response, it leads to lepromatous leprosy with high bacterial count.

How do Th17 cells develop in response to extracellular fungi and bacteria?

• Extracellular fungi or bacteria activate dendritic cells.

• Dendritic cells release IL-1, IL-6, IL-23, and TGF-beta.

• These signals lead to activation of RORgammaT and STAT3 (transcription factors) in the antigen-activated T cell, resulting in differentiation into Th17 cells.

What are the functions of Th17 cells?

• Th17 cells respond to extracellular fungi and bacteria by differentiating from naive CD4+ T cells through activation by APCs.

• After proliferation and differentiation, Th17 cells can follow two pathways:

  • Pathway 1 (IL-17 production):

    • IL-17 is released, which acts on leukocytes and tissue cells, leading to the release of chemokines, TNF, IL-1, IL-6, and colony-stimulating factors (CSFs).

    • This triggers inflammation and a neutrophil response.

  • Pathway 2 (IL-22 production):

    • IL-22 acts on epithelial cells, enhancing barrier integrity.

  • Combined action:

    • Both IL-17 and IL-22 can act on epithelial cells, leading to the production of anti-microbial peptides, strengthening the defense against pathogens.

What are the mechanisms of killing of infected cells by CD8+ cytotoxic T lymphocytes (CTLs)?

• Activation of CTLs:

  • Differentiated CD8+ CTLs do not require costimulation or T cell help for activation. They are already primed to target infected or abnormal cells.

• Recognition and Binding:

  • The target cell expresses specific antigens that are recognized by the TCR (T cell receptor) on the CTL.

  • ICAM-1 on the target cell binds to LFA-1 on the CTL, and CD8 on the CTL interacts with MHC Class I on the target cell, strengthening the connection. This leads to antigen recognition and binding of the CTL to the target cell.

• Granule Exocytosis and Cytotoxicity:

  • Upon activation, the CTL undergoes granule exocytosis, releasing cytotoxic molecules such as perforin and granzymes.

  • Perforin facilitates the entry of granzymes into the target cell's cytosol by forming pores in the target cell membrane.

  • Granzymes then activate apoptosis in the target cell, leading to the death of the infected or abnormal cell through programmed cell death (apoptosis).

How do activated CTLs induce cell apoptosis without granule exocytosis?

• Fas Ligand (FasL): Activated CD8+ CTLs express Fas ligand (FasL) on their surface.

• Binding to Fas (CD95): FasL binds to Fas (CD95), a death receptor on the target cell.

• Caspase Activation: This binding activates the death domain of Fas, which recruits Fas-associated death domain (FADD) proteins.

• Caspase Cascade: FADD activates a cascade of caspases, which are enzymes that drive the process of apoptosis (programmed cell death) in the target cell.

• No Granule Exocytosis Required: This pathway of inducing apoptosis does not require granule exocytosis (i.e., no perforin or granzymes are needed).

Are CTLs injured during the process of killing target cells?

• No: CD8+ CTLs (cytotoxic T lymphocytes) are not injured during the killing process.

• Reusability: After killing a target cell, the CTL can detach and go on to kill additional target cells, as it does not undergo injury in the process.

How does dendritic cell activation of CTLs occur? and the process of effector CTLs on killing cancer or virus-infected cells?

dendritic cell presents antigen bound to MHC-I to the TCR of a naive T cell. also expresses CD80/CD86, which binds to CD28 on the naive T cell, providing the costimulatory signal for activation.

leads to the activation, proliferation, and migration of the naive T cell into an effector CTL.

____________

• Granule-mediated Killing:

  • CTLs release Perforin (PNF) and Granzyme B (GzmB), leading to the death of the target cell (cancer or virus-infected cell).

• Cytokine-mediated Killing:

  • CTLs produce IFN-gamma and TNF-alpha, which also lead to the death of the target cell (cancer or virus-infected cell).

How does Mycobacteria inhibit the immune response?

Inhibition of phagolysosome fusion: Mycobacteria prevent the fusion of phagosomes with lysosomes in macrophages, allowing them to evade degradation and persist within host cells.

How does Herpes Simplex Virus (HSV) evade immune detection?

interferes with the TAP transporter, preventing the transport of viral peptides into the endoplasmic reticulum, thereby inhibiting antigen presentation via MHC-I molecules.

How does Cytomegalovirus (CMV) evade the immune system?

inhibits proteasomal activity and removes class I MHC molecules from the endoplasmic reticulum (ER), preventing effective antigen presentation on the cell surface.

How does Epstein-Barr Virus (EBV) evade immune response?

1. inhibits proteasomal activity, preventing the proper processing of antigens for presentation on MHC-I molecules.

2. Production of IL-10: EBV produces IL-10, which inhibits the activation of macrophages and dendritic cells, further evading immune detection.

How does Pox Virus evade immune activation?

Inhibition of effector cell activation: Pox viruses produce soluble cytokine receptors that bind and neutralize immune system cytokines, preventing effective immune responses and reducing the activation of effector cells.

How does T cell activation differ between acute and chronic infections?

Acute Infection:

• Viral load: Peaks briefly and then drops.

• T cell response: High initially, then slightly decreases, stabilizing at a mid level.

Chronic Infection:

• Viral load: Increases at first, then stabilizes at a consistently high but stable level.

• T cell response: Initially high, then declines due to T cell exhaustion (PD-1, CTLA-4 upregulation), with a prolonged state of low response.

What is T cell exhaustion and how does it affect immune responses?

T cell exhaustion is a phenomenon where T cells initially mount a response against a virus or tumor, but the response is prematurely terminated. This happens due to the increase of inhibitory receptors like PD-1.

PD-L1 on tumor cells binds to PD-1 on T cells, which impedes T cell function and prevents the T cell from destroying the tumor cell.

Blocking PD-L1 or PD-1 can reverse this inhibition and allow T cells to effectively destroy the tumor cells.