T cell migration and activation

• T-cell-mediated immunity is discussed, referencing Chapter 9 of Janeway's Immunobiology.
• The lecture is on "Pharmaceutical Immunology I," course number 535-0830-00L HS 2024, lectured by Prof. Dr. Cornelia Halin Winter at ETH Zurich.

Lymphocyte Circulation and Antigen Encounter

• Circulating lymphocytes encounter antigens in peripheral lymphoid organs, such as lymph nodes.
• Most T lymphocytes constantly recirculate between blood and lymph nodes. Lymphocytes travel: blood => lymph node => efferent lymphatic vessels => thoracic duct => blood
• They circulate several times a day to increase the chances of finding a cognate antigen, referred to as increasing immunesurveillance.
• Once a cognate antigen is found on an antigen-presenting dendritic cell, T cells proliferate and differentiate into effector or memory cells.

T Cell Priming and Differentiation

• T cells encountering pathogen-derived antigens differentiate into specialized effector cells to combat the pathogen.
• Cytotoxic T cells recognize complexes of viral peptides with MHC class I and kill infected cells.
• TH1 cells recognize complexes of bacterial peptides with MHC class II and activate macrophages.
• T follicular helper cells recognize complexes of antigenic peptides with MHC class II and activate B cells.
• T cell priming, also known as activation, occurs in secondary lymphoid organs.

Content Overview

• The lecture will cover the function of secondary lymphoid organs as sites of adaptive immunity initiation.
• The function of antigen-presenting cells will be discussed
• Priming of naïve T cells by pathogen-activated dendritic cells, and general properties of effector T cells and their cytokines will be discussed.
• T-cell-mediated cytotoxicity will be examined

Secondary Lymphoid Organs (SLOs)

• Secondary lymphoid organs (SLOs) are sites of adaptive immunity initiation.
• SLOs serve as anatomical crossroad locations for interactions between antigens and lymphocytes.
• Examples of SLOs include lymph nodes, the spleen, and Peyer's patches.
• SLO anatomy is important.

SLO Similarities and Differences

• SLOs share a similar cellular composition of T and B cells (~90%), dendritic cells, macrophages, stromal cells, and endothelial cells.
• B and T cell zones are similarly defined.
• Positioning is coordinated by chemokines CCL21/CCL19 for T cells and CXCL13 for B cells.
• Distinct antigen arrival routes include afferent lymphatics for lymph nodes, blood for the spleen, and the gut lumen for Peyer's patches..
• Cell entry and exit involve blood and lymphatic vessels.

Naive T Cell Circulation

• Naive T cells recirculate several times daily through secondary lymphoid organs (SLOs) searching for antigens.
• An example T cell circulates in the process blood => lymph node => efferent lymphatic vessels = thoracic duct => blood
• Entry into lymph nodes occurs through High Endothelial Venules (HEVs).
• After entry, the T cells migrate to the T cell area to contact antigen-presenting dendritic cells (DCs).
• If no cognate antigen is encountered, the naïve T cell leaves the lymph node via efferent lymphatics.
• Encountering a cognate antigen causes the T cell to proliferate, lose the ability to exit the node, and differentiate; effector cells exit later via efferent lymphatics.

Antigen Retention and T Cell Activation

• Antigen-specific T cells are transiently retained in lymph nodes when an antigen is present, typically lasting 3-5 days.
• During retention, naive T cells become activated, proliferate, and differentiate into effector cells.
• Cell numbers increase by several thousand-fold during this period.
• This process generates an "army" of cells to combat the infection.

Lymphocyte Entry and Extravasation

• Lymphocyte entry into lymphoid tissues depends on chemokines and adhesion molecules.
• Blood flow velocity in capillaries (1 mm/sec) creates high sheer forces.
• Extravasation involves a multistep adhesion cascade.
• Rolling is mediated by Selectins.
• Activation of T cell is mediated by Chemokines.
• Adhesion is mediated by Integrins.
• Diapedesis is mediated by Chemokines.
• L-selectin is an example of selectins.
• CCL21 is an example of chemokines
• LFA-1 is an example of integrins.

T Cell Extravasation Process

• T cell extravasation via high endothelial venules (HEVs) in lymph nodes involves multiple steps.
• A circulating lymphocyte enters the HEV in the lymph node.
• Binding of L-selectin to GlyCAM-1 and CD34 allows rolling interaction.
• LFA-1 is activated by CCR7 signaling in response to CCL21 bound to the endothelial surface.
• Activated LFA-1 binds tightly to ICAM-1.
• The lymphocyte migrates into the lymph node by diapedesis.
• The same adhesion steps occur during extravasation at sites of inflammation/infection.
• Chemokines/adhesion molecules dictate which leukocytes are recruited into the tissue.

Blocking Leukocyte Extravasation

• Blocking leukocyte extravasation is a therapeutic target in autoimmunity.
• Natalizumab (Tysabri) blocks the α4 integrin subunit on effector T cells.
• Natalizumab inhibits interactions with VCAM-1 and MadCAM-1.
• It reduces leukocyte recruitment to inflamed CNS and intestine.
• It is approved for treating multiple sclerosis and Crohn's disease.

T Cell Recirculation and its Modulation

• Naive T cells recirculate approximately 3-4 times per day through different lymph nodes to aid immunesurveillance.
• Lymphocyte recirculation can be blocked by modulating sphingosine 1-phosphate receptors (S1PRs).
• Blocking S1PRs blocks emigration of T cells out of lymph node into efferent lymphatic vessels, subsequently T cells get "stuck" in the lymph node.
• Fingolimod (FTY720, Novartis) was approved in 2010 for treating multiple sclerosis.

Lymphocyte Egress from Lymph Nodes

• Lymphocyte egress from lymph nodes is mediated by a sphingosine 1-phosphate (S1P) gradient.
• S1P is a sphingolipid with chemotactic activity on T cells.
• S1P levels are low in tissue but high in lymph (efferent lymphatics).
• For T cells to leave, S1PR1-expressing cells sense the gradient and transmigrate across lymphatic endothelial cells.
• FTY720 induces S1PR1 internalization and degradation, causing T cells to no longer "find" their way out.
• Physiologically, S1PR1 downregulation occurs when a T cell is activated by antigen, leading to proliferation and differentiation in the lymph node.

Professional Antigen-Presenting Cells (APCs)

• There are 3 main professional APCs being macrophages, dendritic cells, and B cells .
• These APCs constitutively express MHCII, and can take up antigens
• Dendritic cells, B cells, and Macrophages are professional antigen-presenting cells

Dendritic Cell Antigen Processing and Presentation

• Dendritic cells can uptake, process, and present protein antigens through different routes.
• These routes include receptor-mediated phagocytosis, macropinocytosis, viral infection, cross-presentation after phagocytic uptake, and transfer from incoming cells to resident dendritic cells.
• Cross-presentation is the ability to present phagocytosed antigen on MCH-I rather than MHC-II.
• Cross-presentation can be used for induction of cytotoxic T cells
• Dendritic cells are able to cross-present, unlike macrophages or B-cells.

Langerhans and Dermal Dendritic Cells

• Langerhans cells and dermal dendritic cells take up antigens in the skin, then transport them via afferent lymphatic vessels to the draining lymph node.
• Langerhans cells are a specialized type of dendritic cell (DC) in the epidermis.
• Mature dendritic cells that enter the lymph node can transfer some antigens to resident dendritic cells.
• B7-positive dendritic cells stimulate naive T cells

TLR Signaling in Dendritic Cells

• Microbe-induced TLR-signaling in tissue-resident dendritic-cells induce migration to lymphoid organs and enhances antigen processing.
• PAMP is known as a pathogen associated molecular pattern, which encountering DC's undergo two maturation steps.
• Up-regulation of CCR7 is needed for migration to lymph nodes.Enhanced antigen processing is needed for T cell activation in dLNs.
• Up-regulation of MHC and costimulatory molecules helps to boost T cell activation in dLNs.

B Cell Antigen Presentation

• B cells use surface immunoglobulin to present specific antigens efficiently.
• B cell receptor-mediated uptake allows enrichment of antigen-derived peptides on B cell MHCII molecules.
• B cells are less efficient in priming naïve T cells than dendritic cells due to lower co-stimulatory molecule expression.

APC Comparison

• Dendritic cells are "professional antigen-presenting cells".
• Activation of naive T cells typically requires antigen presentation by dendritic cells.
• Dendritic cells:
  • Use macropinocytosis and phagocytosis for antigen uptake.
  • MHC expression is low on tissue-resident cells, high in lymphoid tissues.
  • High costimulation delivery in lymphoid tissues.
  • They reside ubiquitously throughout the body
  • Activation of naive T cells is caused through these APC cells.
• Macrophages:
  • Use macropinocytosis and phagocytosis for antigen uptake.
  • MHC expression is induced by bacteria and cytokines.
  • Inducible costimulation
  • Mainly in lymphoid tissue and connective tissue, found in body cavities.
  • Activation of macrophages is a result of these APC cells.
• B cells:
  • Utilize antigen-specific receptor (Ig) for antigen uptake.
  • MHC expression is constitutive and increases on activation.
  • Have and inducible costimulation
  • Located in Lymphoid tissues
  • Effect is helps deliver to B cell.

T Cell Priming

• Priming involves the first-time activation of naive T cells, generally occurring upon encountering antigen on a dendritic cell.
• The process takes place in a secondary lymphoid organ.

T Cell and DC Interaction

• T cells and DCs for an immunological synapase
• C-SMAC is the inner ring of peptide-MHC/TCR. Co-stimulatory molecules are required for complexes.
• P-SMAC is the outer ring for adhesion molecules. -Integrin LFA-1 helps with T Cells. ICAM helps -1 on CD4
• Enabling effective signalling and targeted release of cytokines requires T Cell differentiation.

Naive T Cell Activation Signals

• Activation of naive T cells requires 3 different signals
• APCs deliver three kind of signals:
  • Peptide-MHC specificity and activation.
  • B7 molecules (CD80, CD86) are co-stimulation survival molecules.
  • Cytokines are differentiaton molecules specific tot the type of effector T cell (e.g. TH1, TH2, TH17.
• T Cell activation is then caused which gives cell survival -Cell differentation that is specific to the effector type.

Autocrine T-Cell Survival Signal

• Autocrine T-cell signal is caused by : IL-2/IL-2 receptor signaling.
• Upon activation, T cells expression interleukin-2 (IL-2)
• Upregulation of IL-2 ad CD25- T cell receptor co-stimulatory triggering leads to more IL-s.

Activated T Cells & IL-2

• Activated T cells secrete use IL-2 to responds to their target
• Resting T cells expression a moderate IL-2 receptor in it's cycle
• IL-2 is targeted by powerful immuno supressants -Cyclosporin -Basilximab
  • Basiliximab is an antibody directed against IL-2Ra (CD25)

CTLA-4

• CTLA-4 is an inhibitory receptor for B7 protein molecules
• It competes for the connection CD28 protein is also tring to bond with.
• Interaction locks the cell activation by triggering coststimulation
• B7-CTLA interaction blocks T cell activation, by preventing co-stimulation by CD28

Effector T cell Populations

• CD8+ are the killer lymphocytes and are cytotoxic T lymphocytes (CTLs)
• CD4+ are TH1, TH2, TH17, TFH, and Tregs

T Cell Activation

• Activating the cell surface expression changes certain molecules. e.g., less homing lymph less homing lymph is a way to peripherise tissues. This is altered trafficking.

Pathogen categorization

• Categorization and neutrophils, types of cells helps to fight against pathogens.
• Different strategies may need to fight pathogens as well.
• Neutrophils = 10-12 um
• T Cell = Approx 7 um
• Macrophages = Approx 20 um to do.

Pathogens confronted by the Immune system

• The immune system is able to protect against multiple classes of infectious diseases. Diseases include things such as pox, influenza and Varicella.
• Other diseases that are confronted include leprosy, malaria, and toxo plasmin. Fungi like pneumoniae and pneumonia.
• Specialized T cells help to deal with multiple pathogens confronting the immune system.

CD4+ Effector Cells

• CD4 effector cells help fight different pathogens.
• These immune cells can be targeted for increased or enhanced function
• Microbes have different function when targeted

CD4 T cell Differentiation

• CD4 differentiation happens by the variance and signals in order to induce effector type cells.
• Fate-specifying cytokines can have a positive effect or they may inhibit certain things.
• T cells are able to regulate each out and cross each and cross differiation

Cytokine Activation

• Cytokine is a differentiation inducing activation in T cells. Induction results lineage-specific transcription factors
• In certain situations, it can induce into a particular lineage where it needs to be triggered

Cytotoxic T Cells

• The function to kill or cause death to cells containing a peptide of the cell
• CD8 express the killer cells

Cytotoxic Function

• There is a certain order in which it goes to express and stimulate.
• T cells can respond to the certain cells with with certain costimulation

T cell Cytotoxicity

• Stabilization of interaction targeted release for effectors to come into their cells

T-cell mediated cytotoxicity

• Specific recognition of antigen on target cell which induces polarization
• The release can be toards certain target cells
• The cytotoxic t cell will be the correct t cell for the target so to speak

Apoptosis

• Programmed destruction of that cell. Chromatin condensation & DNA fragmentation, cell shrinking, blebbing is the process.
• The reverse of this is necrosis where there can be burst of cells and a release.

Lytic Granules

• The protein triggered into certain types of cell
• Same mechanism as t cell destruction

Granzyme

• Activation can induce DNAse activity
• It can also induce granzyme in certain areas too

Cytotoxic Function Polarized

• After sometime certain cells, the target is removed but it is able to destroy or polarize other aspects of cells.

Cytotoxic T cells

• A cell is infected with a foreign entity and can also spread it's destruction to other T cells
• The synthesis is serial killer

Help From the Immune System

• Cell Types that can contribute as simultaneous and synergystic
• Help Requires simultaneous interaction of three cell types: CD4+ T cell & CD8+ T cell & APC

Chapter 9 Takeaways

• Induction of T cell immunity is started and intitated through secondary lymph as well as coming in contact with antigens.
• This is initiated in secondary lymphoid organs (SLOs) when naïve T cells encounter their cognate antigen on the surface of.
• Typically the activation depends on the dendritic properties
• Activated T cell produce IL-2,which activates and proliferates
• The differiation of activated is depended on t cells
• When naive t cells are continuously recirculate through blood a
• Effector t cells are activation when they recognize cognitive angie
• Cytotoxic cells kill the target cells. and requires the formation of an immunologic synapse and targeted release of cytotoxic granules containing granzyme and perforin.

Explore T cell migration and activation. Understand how antigen arrival routes, T cell recirculation, and adhesion cascades shape the adaptive immune response. Learn about secondary lymphoid organs and immune surveillance.