Immunology V Helper T-Cell Activation and Polarization BMS150 Spring 2023 PDF

Immunology V Helper T-cell Activation and Polarization BMS 150 Week 2 T-lymphocyte Biology - Objectives Recall the process of Th activation and the mechanism of T-lymphocyte class restriction Compare the function and location of costimulator, co-...

Immunology V Helper T-cell Activation and Polarization BMS 150 Week 2 T-lymphocyte Biology - Objectives Recall the process of Th activation and the mechanism of T-lymphocyte class restriction Compare the function and location of costimulator, co-receptor, and adhesion molecules during APC-Th interaction cSMAC vs. pSMAC Adhesion vs. recognition of PRR activation Describe the role of IL-2 in lymphocyte activation and its mechanism of release Compare and contrast the following for Th1, Th2, Th17, Tfh, and Treg helper T- cells: Activating signal, polarizing cytokines, and key determining transcription factors Effector cytokines and binding proteins, site of Th effector function How each Th type contributes to immune responses to different types of pathogens Briefly describe the ways that Treg cells and Tfh cells are unique from other types of Th cells Define the concept of cross-regulation between Th cells and briefly describe the mechanism by which it occurs For a CD8+ cytotoxic T-cell, describe the process of activation, antigen recognition, and destruction of foreign cells Recall All CD8+ T cells are Class I MHC-restricted All CD4+ T cells are Class II MHC-restricted Recall – T-cell activation For the next two slides: How does signal 1a relate to the specificity of adaptive immunity? What is the importance of signal 1b? How does signal 2 aid regulation of Th cells? How does the role of the pSMAC differ from that of the cSMAC? T-cell activation 1a. T-cell receptor interacting with HLA-2 TCR HLA-2 CD4 CD28 CD80 2. Co-stimulatory interaction: CD28 1b. CD4 co-receptor interacting with CD80 interacting with HLA-2 (or 86) T Cell Activation—2 Step process Costimulators are key to Th activation Major costimulator – CD80/86 on the APC ▪ Binds to CD28 on the Th ▪ FYI – CD80/86 used to be known as B7 Costimulators are not usually expressed at high levels on APCs ▪ HLA-2 and costimulator expression increase only when the APC recognizes a DAMP or a PAMP Advantage? Interleukin 2 (IL-2) and Lymphocyte Activation (Slide 8) IL-2 is a major T cell and B cell growth factor ▪ Proliferation and differentiation of activated naïve T cells ▪ Proliferation and differentiation of B-cells (more later) Once activated, the naïve T cell begins to synthesize IL-2 and the high-affinity form of the IL-2 receptor ▪ IL-2 binding to the Th IL-2R acts in an autocrine fashion ▪ Activates the Th cell and causes it to enter the cell cycle, as well as avoid apoptosis (upregulates Bcl-2 – apoptosis inhibitor) Synthesis of both IL-2 and the high-affinity IL-2R is induced after CD28-CD80/86 binding and recognition of antigen by the TCR (via HLA-2) Ag recognition in absence of co-stimulation causes T cell ANERGY ▪ Anergic T cells CANNOT produce IL-2 ▪ Prevents anergic T cells from proliferating and differentiating into effector cells when they encounter Ag - even if presented by APC that express co-stimulatory molecules T Cell Activation—Interleukin 2 (IL-2) How Do Helper T-cells Aid Immunity? CD4+ helper T-lymphoctes (Th) never kill pathogens or other “foreign-looking” entities (like cancer cells) directly Instead, they: ▪ Activate or inhibit other cells through direct contact (i.e. the “immunologic synapse) CD8+ cytotoxic T-cells, B-cells, macrophages ▪ Activate or inhibit other cells through secretion of cytokines Can influence a wide range of effectors Th cells will differentiate after they are activated so that they “help” in a specific way – this is known as T-cell polarization Concepts in Th Polarization T-helper cells become polarized after they are activated ▪ Polarization = specialized Th phenotype → the Th secretes a “profile” of cytokines that mediate distinct effector cell functions Th type is determined by the environment it is found in ▪ In particular the types of cytokines that are present in high concentrations in the immediate vicinity of the newly- activated Th – these are known as polarizing cytokines Key information about Th polarization (need to know): ▪ Inducing cytokines and the transcription factors that they activate in the Th ▪ Cytokines secreted by the Th ▪ The microenvironment that the polarized Th “works in” and the effects of the cytokines it secretes A great overview: How does Th polarization work? Example – what is the story of the polarization of the Th1 subtype? 1. An infection occurs – likely viral or an intracellular bacterium 2. The infection causes the emergence of PAMPs (or DAMPs, if damage) ▪ What sorts of PRRs would be activated by a viral or intracellular bacterium? 3. Dendritic cells phagocytose the pathogen → PRR activation in dendritic cells → DCs enter the lymphatics and migrate to a lymph node or other secondary lymphatic organ (SLO) ▪ Any dendritic cell? All of them? No one is sure FYI: It is possible that certain dendritic cells migrate to the LN in response to certain stimuli – for a Th1 response it may be a dendritic cell type known as the conventional dendritic cell, type 1 (cDC1) How does Th polarization work? 4. When the dendritic cell arrives at the SLO, it presents the phagocytosed antigen via its HLA-2 to a naïve Th cell ▪ PRR activation → increased expression of HLA-2 and CD 80/86 on the dendritic cell 5. If the TCR on the Th recognizes the antigen it becomes activated (no longer naïve) ▪ IL-2 secretion → division (activated clones express the same TCR) 6. The dendritic cell secretes Th1 polarizing cytokines, and these bind to receptors on the activated Th cells ▪ Major Th1 polarizing cytokines: IL-12, IL-18 IL-12, IL-18, IFN- How does Th polarization work? 7. IL-12 and IL-18 bind to their receptors on the Th cell → activation/production of the Th1 transcription factor ▪ Th1 transcription factor → Tbet ▪ This transcription factor is what polarizes the Th – it causes expression of genes that “do Th1 things” 8. Tbet causes the Th to secrete IFN-γ → effective macrophage and APC activator, can also tailor some B-cell responses: ▪ Secretion of (more) IL-12, increased phagocytosis ▪ Stimulation of dendritic cells to aid cytotoxic T-cell activation (if CD8+ TCR recognizes the antigen) ▪ Stimulates B-cells to secrete antibody types that are effective opsonins IL-12, IL-18, IFN- IFN-, TNF- How does Th polarization work? 9. The activated Th1 cell might stay in the LN or migrate into a site of inflammation – hopefully the same place the dendritic cell came from ▪ In the lymph node – continued activation of Tc and B- cells that recognize the antigen ▪ At the site of infection – local activation of macrophages that fight infection TH1 Cells Macrophage activation is one of the the principal effector actions of Th1 cells Macrophages require 2 signals for activation: ▪ IFN- ▪ Cell contact → CD40- CD40L (more later) Armed effector Th1 cells can deliver both signals TH1 Cells Useful Generalities in Th Physiology Th cells usually first activated and polarized in secondary lymphatic tissue/organs (SLO) ▪ HLA-2-TCR + costimulation + IL2 (usually) production ▪ Usual APC is a dendritic cell (sometimes macrophage), and the Th cell remains in the lymph node or other SLO, dividing and becoming activated, for several days Dendritic cells migrate to the secondary lymphatic tissue via the lymphatics Polarization type (Th1, Th2, Th17, Tfh, Treg) usually seems to be dependent on the APC ▪ Dendritic cell type may influence polarization “choice” ▪ What activated the dendritic cell likely influences polarization “choice” Useful Generalities in Th Physiology Th cells can function as effectors in SLOs or in peripheral tissue ▪ If they migrate to peripheral tissue, then it is likely at the same (or similar) site to where the APC migrated from in the first place After a Th cell is polarized, it tends to stay as that cell type ▪ i.e. it is uncommon for a Th1 cell to become a Th2 cell (though it can happen) ▪ Exception – Tfh cells often “re-polarize” to another Th type after the antibody response they help is completed Signals generated by a polarized Th cell tend to prevent other local Th cells from being polarized into a different type ▪ Known as cross-regulation Th17 – an “Innate Activating” Th Type Th17 cells were discovered relatively recently ▪ One of the polarizing cytokines, IL-23, was mistaken for IL-12 (so they were confused with Th1 cells) ▪ Key Th type implicated in protection from infection and many autoimmune disorders Bacterial and fungal disorders of the skin and mucosal surfaces Rheumatoid arthritis, inflammatory bowel disease, psoriasis, MS Three major polarizing cytokines: ▪ “Pro-inflammatory” → IL-6 and IL-23 (IL-23 produced by activated dendritic cells) ▪ “Pro-fibrotic” → TGF- enhances wound healing, sometimes reduces inflammation, can inhibit growth and activation of lymphocytes and macrophages (if found in isolation) Th17 – an “Innate Activating” Th Type Naïve Th become Th17 cells: ▪ In secondary lymphoid organs (SLOs), stimulated by DCs secreting IL-6, TGF-, IL-23 ▪ Likely some are generated outside of SLOs as well – many APCs can secrete these cytokines and the pattern of cytokines is typical of chronically-inflamed tissue Th17 effector cells produce IL-22 and IL-17 when they are activated and polarized ▪ IL-22 and IL-17: Induce release of IL-6, IL-1, TNF-, GM-CSF Cause release of chemokines that recruit neutrophils and macrophages Cause secretion of anti-microbial proteins from cells in inflamed tissue (especially barrier tissues like skin or mucosa) ▪ Thus Th17 cells do their major “jobs” outside of SLOs Th17 – an “Innate Activating” Th Type Transcription factor that polarizes a Th17 cell after it is exposed to IL-6, TGF-, and IL-23: ROR-t ▪ Presence of this TF tends to prevent the Th17 cell from secreting non-Th17 cytokines In many ways, the activity of Th1 and Th17 cells overlap ▪ Activation of macrophages and increased phagocytosis (recruitment of neutrophils and macrophages into inflamed tissue) ▪ Less likely to enhance antibody secretion Th1 cells only encourage one particular antibody subtype, Th17 cells do not stimulate B-lymphocytes to produce antibodies ▪ Some Th cells may secrete cytokines that overlap between Th1 and Th17 subtypes ▪ Therefore, immunologists often talk about Th1 and Th17 responses as a general “Type 1 response”  In-Class Assignment: From the posted description of a Th2 response, “fill in the blanks” for the Th2 summary diagram ▪ Try yourself, then partner up (or group up in larger groups) to check your work Follicular Th Cells – Key Cells in Antibody Production Many Th types aid antibody production, but they don’t usually stay in SLOs to optimally stimulate B-cells – they migrate to non-lymphatic tissue ▪ Th2, Th1, Th9 (not covered today) Th1, Th2, and Th9 cells will influence B-cells to make particular types of antibodies that accomplish particular tasks, but cannot aid other important B-cell activities ▪ i.e. Th1 cells cause B-cells to form antibodies that are good at opsonizing targets ▪ They don’t help B-cells divide very well, nor can they aid “antibody refinement” in germinal centers of SLOs Follicular Th cells (Tfh) are the main Th cell that aids full B- cell development in the SLOs Follicular Th Cells – Key Cells in Antibody Production What do Tfh cells do? Stay in the SLOs for the duration of their activation Remain associated with B-cells and induce the formation of germinal centres in the SLOs ▪ We make “2 rounds” of antibodies when we experience an infection that our immune system has trouble clearing Round 1 – lower-affinity antibodies, many epitopes Round 2 – B-cells “reshuffle” their antibodies and the ones that are very high affinity are selected to reproduce in germinal centres ▪ The “2 rounds” of B-cell antibody production is known as affinity maturation and is crucial – without Tfh, it does not happen B-cell function will be discussed in more detail next day A model of Tfh production Tfh have only been recently discovered and characterized (about 14 years ago) because their development is complicated Likely 2 major steps to Tfh polarization: 1. Naive Th interacts with dendritic cells in SLOs and becomes a Tfh effector cell TCR-HLA2 and CD28-CD80/86 interaction, but limited IL-2 production ▪ IL-2 is inhibitory to Tfh development, and likely drives the naïve Th to other types (i.e. Th17, Th1) 2. Tfh migrates to the B-cell region of the SLO and interacts with B-cells that present antigen via their HLA-2 At this point, the Tfh stays in the follicle and “prefers” the B-cell as an APC versus a dendritic cell A model of Tfh production Step 1 – naïve T-cell and DC interaction: Occurs in the main T-cell/DC zone of a SLO Typical TCR-HLA2 interaction with the DC expressing costimulators CD80/86 that bind to CD28 on the Th Polarizing cytokine in humans – likely IL-12 (secreted by DCs) ▪ FYI - IL-6 is the polarizing cytokine in mice At this stage, the new Tfh produces little IL-2 IL-12 and a weak IL-2 signal → expression of the main lineage- determining transcription factor for Tfh – Bcl6 ▪ FYI – if a naïve Th experiences a strong IL-2 signal, it develops into the other Th cell types, and will not express Bcl6 ▪ It’s like a decision is made early in naïve Th cell development – to be a Tfh, or to be any other Th cell type A model of Tfh production Step 2 – Tfh and B-cell interactions The new Tfh expresses a chemokine that allows it to migrate to the B-cell zone of an SLO (FYI - CXC5) Tfh interacts with a B-cell (remember, B-cells are APCs) via TCR-HLA2 and two other new costimulator interactions: ▪ iCOSL (iCOS ligand) on B-cells interacts with iCOS on Tfh This interaction stimulates cytokine production by the Tfh ▪ CD40L (CD40 ligand) on Tfh interacts with CD40 on B- cells This interaction stimulates the B-cell to increase antibody production A model of Tfh production Step 2 – Tfh and B-cell interactions continued… After this second Th-APC interaction, the Tfh will secrete typical Tfh effector cytokines ▪ Major Tfh cytokine: IL-21 IL-21 and CD40-CD40L are key for full B-cell maturation and development of the germinal centres where most antibodies are produced Without these two signals, most B-cells die prematurely after antigen recognition ▪ Tfh can secrete other cytokines that influence antibody type or B-cell development: IL-4, TNF, IL-2… there are many others, and they are currently being researched Tfh can provide IL-2 stimulation to promote B-cell division  Tfh – Step 1 Step 1– in the more “peripheral” parts of the SLO DC presents Ag to the T-cell via HLA-2 (signal #1) & CD28 with CD80/86 (signal #2) If the dendritic cell secretes IL-12 (or IL- 12 is in high concentrations) AND IL-2 secretion by the naïve Th is low → Bcl6 expressed by the Th Bcl6+ Th is now a Tfh  Tfh – Step 2 Step 2 – in the B-cell regions of the SLO B-cell presents Ag to the T-cell via HLA-2 (signal #1) & iCOSL costimulation (signal #2) The Tfh stimulates B- cells via IL-21 secretion and CD40L- CD40 interactions The B-cells will divide, produce more Ab (plasma cells) and eventually undergo affinity maturation due to IL-21, IL-4, CD40 stimulation Tfh activity is unique Follicular helper T-cells differ from other polarized Th cells: They stay in the SLO, interacting continually and closely with B- cells ▪ Without Tfh, we do not produce germinal centres in our SLOs and our “antibody effectiveness” is poor They undergo two separate rounds of interactions with APCs ▪ First round – polarizes the naïve Th so it becomes a Tfh, the APC is a dendritic cell ▪ Second round – provides specific help to B-cells, (ALSO APCs) Specific help means that the Tfh only helps B-cells that have had their receptors (antibodies) bound to antigen When the B-cell/antibody response to the infection is concluded, Tfh cells often gain the ability to switch types later ▪ i.e. – same cell could become a Th1 cell later, under the right circumstances  Cell-mediated or antibody response? It’s usually not an either/or decision For this virus, both strategies can be taken at the same time The best immunity for a viral infection is production of antibodies that prevent invasion AND killing of cells that are infected (CD8+ T- cell)  Th1 or Th2? The type of response hinges on the 3rd signal from the APC (usually a polarizing cytokine) In general, this is determined by type of PRR and ensures Th help is appropriate to the invader T-regulatory cells The development of regulatory T-cells is unique in that their effector activities down-regulate effector activities of other immune cells ▪ Macrophages and other APCs ▪ Th cells They arise in environments where APCs present an antigen in a cytokine environment that is predominantly “anti- inflammatory” – no inflammatory cytokines ▪ Lots of TGF- ▪ Very little or no IL-6, IL-23, IL-12, IL-1… etc. This is interpreted as an environment where “self” is being presented, but there are no other signs of “foreign invasion” ▪ Animals or humans that cannot produce effective Tregs usually develop severe autoimmune disease T-regulatory cells – quick breakdown Inducing/polarizing cytokine: TGF- ▪ And no inflammatory cytokines like IL-6, IL-23, etc. Transcription factor that “causes” a Th to become a Treg: FoxP3 Effector functions of the Treg ▪ Secretion of anti-inflammatory cytokines: TGF-, IL-10 ▪ FYI – there are other anti-inflammatory cytokines, not a complete list ▪ Down-regulation of CD80/86 signaling by binding to Treg CTLA-4 (more later, see picture next slide) ▪ Soaking up and “stealing” IL-2 from other effector T-cells T-regulatory cells – quick breakdown In this model, binding of: Treg TCR to HLA2 CTLA-4 to CD80/86 results in a dendritic cell that generates “anti-inflammatory signals” We will discuss more later as we go through tolerance Th cells – cross-regulation When a naïve Th has “made the decision” to become polarized, the key event is expression of the determining transcription factor (TF) ▪ i.e. FoxP3 in a Treg, Tbet in a Th1 The determining transcription factor prevents: ▪ Production of other determining TFs i.e. Tbet prevents the expression of GATA-3, FoxP3 prevents the expression of RORt ▪ Production of cytokines other than those that are typical of that Th type Plus, cytokines from one Th type can prevent the proliferation or survival of other Th types ▪ i.e. IFN-gamma can negatively regulate Th2 cell types This process is known as cross-regulation  Know – and understand – this overview Note – IL-6 does not cause induction of Tfh in humans (it does in mice) IL-21 and IL- Not for 12 are the Not for key today today cytokines for Tfh polarization in humans A quick segue – cytotoxic T-cells Cytotoxic T-cells kill cells that express abnormal intracellular antigens on HLA-1 (virally-infected cells or malignant cells) ▪ The mechanism of killing is very similar to NK cells – granzyme/perforin and Fas/FasL interactions with the infected or abnormal cell Cytotoxic T-cells are activated by Th1 cells in lymphatic tissues and are dependent on two signals: ▪ An activating signal from a dendritic cell that has been licensed to present antigen Often these dendritic cells engage in cross-presentation – i.e. extracellular antigens displayed on HLA-1, intracellular antigens displayed on HLA-2 Dendritic cells activate naïve CD8+ T-cells via HLA-1 interactions ▪ An activating signal from a Th1 cell in the near vicinity of the cytotoxic T-cell and its activating dendritic cell Activation of a naïve CD8+ cytotoxic T-cell Note the 3-cell communication at the top Ensures that a specific Th1 activates a specific CD8+ Tc Th cells express CD40L This is a stimulating cell surface molecule that activates: ▪ Lymphocytes – B-cells and cytotoxic T-cells ▪ Macrophages during a Th1 response ▪ All of these cells express CD40 Cytotoxic T-cell activation Once a CD8+ T cell has differentiated into an armed effector cell, encounter with its specific Ag results in immune attack without the need for co-stimulation i.e. – no CD80/86 and CD28 interaction necessary HLA-1 and TCR interaction is still needed, since HLA-1 is necessary to present Ag to CD8+ cytotoxic T-cells