Cellular Immune Response PDF

Cellular immune response George Nadăș,Professor of Immunology, [email protected] Cellular immune response Innate responses Aquired responses Cellular immune response Cellular immune response Antigen processing and presentation Exogenous antigens – outside the cells, tissues, fluids - APC Endogenou...

Cellular immune response George Nadăș,Professor of Immunology, [email protected] Cellular immune response Innate responses Aquired responses Cellular immune response Cellular immune response Antigen processing and presentation Exogenous antigens – outside the cells, tissues, fluids - APC Endogenous antigens – originate within the body’s own cells Broken down by proteases bound to MHC Ia surface recognized by Tc not by Th Tc cell destroy virus-infected cells – MHC I and not II cell Cellular immune response DC 1 cells Endogenous – Th1 cell-mediated immune responses Some microbial molecules DC1 IL12 (DNA, LPS, DS viral RNA or B. pertussis toxin) Cellular immune response Processing of endogenous Ag MHC Ia+epitopes recognized by Tc cells MHC Ia ɑ chain is folded a large antigen-binding site is formed on the outermost surface, different from MHC II Bound peptides cannot project out of the end of the site – only bind peptides of nine amino acids Cellular immune response Processing of endogenous Ag Proteins to be recycled linked to ubiquitin Ubiquinated proteins are marked for destruction, recognized by a large tubular protein (proteasome) – regulated by INF-γ The peptide cylinder – broken into 8-15 AA fragments Most fragments are recycled into new proteins Processing of endogenous Ag Cellular immune response 1 in a million molecules is not broken, but saved, by attachement to transporter proteins TAP (transporter for antigen processing) type 1 and 2 select the peptides from cytoplasm endoplasmic reticulum - lumen An aminopeptidase shortens them, one AA at the time unless a 9-AA fits the binding site of an empty MHC I Carried to the surface -displayed Processing of endogenous Ag Cellular immune response A cell can express about 106 MHC-peptide complexes about 200 MHC I loaded with the same viral peptide on a single Tc cell MHC-peptides complete information on all proteins made by a cell Cytotoxic T cells screen peptides and “foreign” bind to their TCRs Cross-priming Cellular immune response Usually endogenous and exogenous pathways are separated, but sometimes exogenous Ag may enter the cytoplasm and be presented on MHC Ia molecules In APC such as macrophages and DC, endocytosed viral Ag is transported from the phagosome cytoplasm proteases and processed as endogenous Ag – degraded This Ag is associated with MHC Ia and recognized by Tc - Importance in immunity to viruses – dead virions may still be able to trigger Tc responses by Initiation of T lymphocytes activation Cellular immune response Lymphocytes populations: T, B and NK CD Receptors: TCR and BCR (bind antigen + signal transduction) 2 T subpopulations α and ß peptide chain TCR α/ß γ and δ peptide chain TCR γ/δ B cells subpopulations: γ, α, µ, ɛ and δ CD3 – on all T cells – signal transducers signal into the cell Cellular immune response Initiation of T lymphocytes activation CD4 – on T cells that recognize processed exogenous Ag CD4 (T helper) – receptor for MHC II, while CD8 (Tc) for MHC I Most human and mouse T cells express either CD4 or CD8 - 65% of human T cells are CD4+CD8- 30% of T cells are CD4-CD8+ - 5% are double negative - CD4-CD8- Cellular immune response Molecules that regulate lymphocytes functions Proteins on cell surface: enzymes, transport proteins and receptors – communication with other cells and environment APC Cytokines, Ab and complement  1. Cytokine receptors: CD25 – IL2 - secreted by Th1 cells CD118 – interferon receptor CD120 – TNF receptor CDw210 – IL 10 receptor Molecules that regulate lymphocytes functions Cellular immune response  2. Antibody receptors = Fc receptors (4 soluble – 3 types) 3 different FcγR: I (CD64), II (CD32) and III (CD16)  CD64 binds IgG with high affinity, not found on T cells, but present on DC, monocytes and macrophages and neutrophils  CD32 – moderate affinity for IgG – only binds immune complexes, found on B, DC and macrophages  CD16 – immune complexes Molecules that regulate lymphocytes functions  3. Complement receptors – CR1 (CD35) CR2 (CD21) Major surface receptors of T cells, ligands and functions C3d and C3bi Cellular immune response C3b and C4b Cellular immune response Functional responses of T cells  Innate – limited number of molecules  Lymphocytes (aquired) - cell surface receptors – specific for Ag and will trigger cell-mediated or an antibody-mediated response  3 major lymphocytes populations: - T helper cells – regulate immune responses - effector or cytotoxic T cells – destroy endogenous Ag - B cells Ab – destroy exogenous Ag Ag receptors on T cells are not designed to react with any foreign antigen of microbe – vast repertoire  On the contrary, are generated randomly newborn Cellular immune response Functional responses of T cells  The immunoglobulin superfamily – all members contain at least one immunoglobulin domain T cell antigen receptor (TCR) Cellular immune response  Each T cell has about 30,000 antigen receptors (TCRs) - all are identical - can only respond to peptides that bind that receptor - six glycoproteins: 2 bind Ag and 4 amplify the signal - 2 different TCR: γ/δ and α/ß cell T cell antigen receptor (TCR) Cellular immune response  The 4 chains (α,ß,γ,δ) are similar in structure – members of the Ig superfamily, but differ in size  Each TCR has 4 domanis: - N-terminal – 100AA - AA sequence does not vary -150AA - T-cell membrane – 20AA - C-terminal – cytoplasm – 5-15 AA - disulfide bonds T cell antigen receptor (TCR)  Ag + TCR send a signal to the T cell Cellular immune response respond The two Ag binding chains of each TCR – associated with CD3 - CD3 = 5 protein chains (γ,δ,ɛ,ζ, η), arranged as 3 dimers (γ-ɛ, δ-ɛ, and ζ-ζ or ζ-η TCR ß chain is directly linked to the γ-ɛ dimer, and TCR α chain is linked to the δ-ɛ dimer  80% - ζ-ζ homodimer Cellular immune response CD4 and CD8  CD4 and CD8 the other important TCR-associated proteins CD4 – single chain glycoprotein of 55 kDa  CD8 – dimer of 68 kDa (CD8α and CD8ß) CD4 or CD8 determines the class of MHC recognized by T cell  CD4 – only on Th (MHC II)  CD8 – only on Tc (MHC Ia) Costimulators  TCR-peptide + MHC Cellular immune response usually insufficient for Th response  Costimulatory signals are needed for optimal T-cell response: - ligands – CD40 – on APC – bind T-cells receptors – amplify responses after T-cell activation - cytokines produced by APCs – determine the T-cell reponse to antigen - cell surface adhesion molecules T-cell + APC – close Costimulatory receptors Cellular immune response  CD40 – surface receptor part of TNF receptor family – on DC and B cells  CD40L = CD154 – surface molecule found on T-cells  Once expressed, it bounds to CD40  CD40-CD154 = signals both ways  T cell is stimulated to express CD 28  APC express CD80 or CD86 Cellular immune response Costimulatory receptors CD40-CD154 TNF-α stimulates APC cytokines (IL-1, 6, 8, 12 and  Prolongs DC survival, B cells respond to Ag and activates macrophages  CD28 ligands are CD80 (DC, macrophages and activated B cells) and CD86 on B cells CD28-CD80/86 CD152 = inactivation Costimulatory cytokines Cellular immune response  Cytokines = short-range signaling proteins  APC cytokines when stimulated by phagocytosis, T-cells signaling by CD40 and CD154  Different DC different cytokine mixtures - Th1 Th1 produces IFN-γ + IL-12 stimulates Th1 IL-18 + IL23 costimulation Complete activation + max IFN-γ release IL-12 Costimulatory cytokines Cellular immune response  DC2 secrete IL-1 and IL-4 stimulates Th2 released into the tissue fluid = IL-1α  APCs IL-1 remain bound to the cell surface = IL-1ß  Cell-bound IL-1α + presented Ag stimulate adherent T-cells Adherence molecules  APC + T-cells stimulate each other effectively if they are held in close contact by adherence molecules - integrins CD2 and CD11a/CD18 on T cells bind their ligands on APC (CD58 and CD54) and lock them in place Cellular immune response Immunological synapse  Synapse = when antigen engages the TCR molecules signals to the T cell TCR + costimulatory strenght of binding  TCR a binary (on/off) switch costimulation amount stimulus duration  MHC receptors can bind a large variety of antigenic peptides, so any individual peptide will be displayed in small amounts  T cells must be able to recognize these few specific peptides-MHC complexes among the vast excess of irrelevant ones MHC-peptide + TCRs Cellular immune response  Recognition by T cells must be highly specific and T cells must recognize endogenous or exogenous Ag response 1 for CD8+ T cell  Number of MHC-peptide complexes 1000 for CD4+ T cell  A CD4+ T cell expresses 30,000 identical TCRs and 8,000 receptors must bind Ag for activation in the absence of costimulation – CD28 or 1,000 in its presence Each MHC-peptide complex can trigger up to 200 TCRs TCRs activation Cellular immune response  Naive T cells must receive sustained signal for at least 10h with costimulation and 30h in its absence costimulatory This costimulation is only provided by DCs adhesion Cellular immune response T helper cell subpopulations  2 CD4+ T helper cells Th1 – stimulated by DC1 (myeloid) B cells CD80  DC1 induce Th1 response by IL-12  Th1 IL-2, IFN-γ and lymphotoxin (TNF-ß) after Ag stimulation and costimulation by IL-12 and IL-18  They promote cell-mediated IR (intracellular + viruses)  Th1 cells lack IL-1 receptors and do not respond to IL-1 Absent IL-12 Th1 response switches to Th2 Cellular immune response T helper cell subpopulations  2 CD4+ T helper cells Th2 – stimulated by DC2 (lymphoid) macrophages – B cells  DC2 induce Th2 response by  Th2 IL-4 + CD86 (IL-1) IL-4, IL-5, IL-10, and IL-13  They - stimulate B-cell proliferation and Ig secretion - enhance B-cell production of IgG and IgA up to 20 fold and IgE up to 1000 fold Cellular immune response T helper cell subpopulations  A third Th cell phenotype cytokine mixture representative of both Th1 and Th2 = Th0  They may be precursors of Th1 and Th2 or cells in transition IL-2, IL-4, IL-5, and IFN-γ Some IL-2 secreting T cells become IL-4 secreting – after exposure to antigen – interleukin switch (Th1 Th2) IL-4 and IL-12 control this switch In the presence of IL-4, Th0 Th2; IL-12 Th0 Th1 Cellular immune response Memory cells  T cells are stimulated Th1 cells are produced, two subpopultions develop IFN-γ+ producing- effector cells IFN-γ- nonproducing- memory cells Cellular immune response Endogenous antigens Viruses, intracellular bacteria – grow inside cells – no antibodies. Destruction by two mechanisms 1 - the infected cell is killed rapidly – no growth of the invader 2 - the infected cell develops the ability to destroy the intracellular organism. Cellular immune response Endogenous antigens Viruses cell cytoplasm/nucleus – killed by cytotoxicity bacteria/parasites cell – protein cytoplasmic vacuoles – cell activation MHC I CD8 Cytotoxic T cells Cellular immune response Programmed cell death Old, surplus, damaged or abnormal cells - removed Membrane blebbing, nuclear fragmentation and caspases Two pathways for apoptosis 1 – cytokines (TNF-ɑ) – extrinsic – death receptor 2 – cytochrome c release from mitochondria – intrinsic or mitochondria-dependent pathway Cellular immune response Programmed cell death Extrinsic – CD 95 – death receptor – ligation DISC inducing signaling complex) containing caspase 8 and 10 (death Intrinsic - cytochrome c formation of apoptosome (polymer) recruits and activates caspase 9 Caspases enzyme cascade degrades cytoplasmic and skeletal proteins endonuclease activation and cell death Apoptotic cells fragment their DNA chromatin condensation Cellular immune response Programmed cell death Cellular immune response Programmed cell death Cells die lipid is exposed binds to macrophages and DC phagocytosis, release of anti-inflammatory cytokines (TGF-ß), inhibition of proinflammatory cytokines (TNF-ɑ) Cellular immune response Programmed cell death DC phagocytose apoptotic cells present Ag-MHC but NO costimulatory molecules, so T cells are not stimulated but turned off tolerance. Anti-inflammatory cytokines Severely damaged cells (trauma, toxicity, microbial invasion) die necrosis (passive) inflammation DC engulfing necrotic cells Ag-MHC + costimulatory molec. T cells that recognize are turned on A cell killed by a virus inflammation and T cell response Cellular immune response Cell cooperation In primary immune responses CD8+ - not respond to infected cells by themselves alone 1013 nucleated cells – possibly several hundred naive T cells for any viral antigen Naive CD8+ cells remain in the lymphoid tissue – Ag Endogenous Ag DCs (MHC I) Costimulated by CD4+ Th1 DCs lymphoid organs CD8+ Cellular immune response Cell cooperation First – Th cell DC through CD40 and its ligand CD 154 Immature DCs molecules low levels of MHC and costimulatory Th cells activate the DCs – up-regulating their MHC expression and stimulating IL-12 and CCL22 Only a fully activated DC response successfully trigger- Tc Cellular immune response Cell cooperation Activated DCs CD8+ Tc (peptides associated with MHC1) They do this if DCs are themselves infected DCs may also present peptides of nonreplicating organisms or dying infected cells Processing dying cells activate T cells endogenous Ag These Tc functions do not require CD40/CD154 interaction Cellular immune response Cell cooperation Activated CD8+ Tc receive 3 key signals IL12 secretion from activated DCs endogenous Ag/MHC I on an abnormal cell CD8+ Tc response IL2 and IFN-γ secreted by Th1 cell Cellular immune response Cell cooperation In CD8+ Tc different responses are triggered by stimuli Tc cytotoxicity induced by threshold – cytokine synthesis Activated Tc – stimulated by brief Ag exposure Naive T cells – stimulated for several hours responsive increasing TCR ocupancy The required stimulation time providing costimulation Cellular immune response CD8+ Tc responses Activated CD8+ Tc leave leave lymphoyd organs and seek infected cells by themselves Recognition of an MHC-Ag on another T cell and its TCR is occupied - kills the target a single peptide-MHC I is sufficient Tc are larger pyroninophilic than unstimulated lymphocytes and CD8+ Tc responses Cellular immune response Activated CD8+ Tc kill their targets by one of 2 ways - contact The perforin pathway – virus CD95/fas – unwanted T cells CD8+ Tc perforin pathway Cellular immune response 3 phases: 1. adhesion, 2. lethal hit and 3. cell death 1. Adhesion CD8-TCR on a Tc cell binds to MHC I on a target cell and an immunological synapse forms around the contact area TCRs + other signaling molecules cluster - center – surrunded by adhesion molecules CD8 inhance the binding between Tc and its target CD8+ Tc perforin pathway Cellular immune response 3 phases: 1. adhesion, 2. lethal hit and 3. cell death 1. In addition to the signal from Ag-MHC-CD8 complexes, CD8+ T cell responses require costimulatory signals (T-target) Similar to CD4, CD8 are optimally activated only – CD28 bind to CD86 (target cell) kill target in the absence of IL-2 – Th1 CD86 (tumor + virus-infected cells) – sensitive to killing by Tc Addintional adhesion Tc+target – CD2-CD58 or CD48 Perforins (P) and granzymes (G) CD8+ Tc perforin pathway Cellular immune response 3 phases: 1. adhesion, 2. lethal hit and 3. cell death 2. Tc orient their microtubule center, Golgi complex and granules target cell After the contact, Tc granules content are secreted – intracellular space of the synapse – P and G are released P aggregate polyperforins – insert – target membrane – transmembrane channels – 12-18 monomers = MAC CD8+ Tc perforin pathway Cellular immune response 3 phases: 1. adhesion, 2. lethal hit and 3. cell death 2. Granzyme B the target cell (injection – perforin or endocytosis and triggers the release of mitochondrial cytocrome c – intrinsic apoptotic pathway – caspase 9 Seconds after the contact between Tc and target cell, the organelles and nucleus apoptotic changes T cell disengages and search another target 52 Natural killer cells 53 Natural killer – recognition of MHC1 54 NK killing mechanisms CD95 (Fas or CD178) pathway Cellular immune response CD95L (CD178 or fas –ligand) and their receptor CD95 (fas) CD95L is a protein expressed on activated CD8+ Tc or NK Tc engage it target CD95L + CD95 and trimerizes – formation of DISC (caspase 8 and 10) caspase 3 – apopt. CD95L-CD95 regulates T-cell development. After completing their task – undergo CD95-mediated apoptosis Other cytotoxic pathways Cellular immune response TNF-ß (lymphotoxin α) – cytotoxin produced by Tc – similar activity to CD95L When produced by antigen stimulate cells acts in 2 ways: Binds to LT-ß in the T cell membrane to form a heterodimeric complex- kill on contact Binds to TNF-ß death receptors on target cells apoptosis 2-3 h – changes 16 h – 90% target cells exposed to TNF-ß -death Other cytotoxic pathways Cellular immune response Granulysin – antibacterial peptide – granules from Tc and NK Produced after T-cell activation kill target and exogenous Ag Similar to other proteins – attack saponins (lipid membranes) Non-pore-forming proteins but activates sphingomyelinases ceramide – apoptosis Listeria and Mycobacterium CD8+ Cellular immune response Tc subsets CD4+ Th subsets – Th1 and Th2 – specific cytokines CD8+ Tc1 and Tc2 (rodents) IL-2 and IFN-γ IL-4 and IL-5 GM-CSF and IL-3 TC0 – third subset CD8+ - strong predilection for Tc1 Other options for cellular cytotoxicity Cellular immune response Antibody receptor FcγRI or FcγRII – bind to targer cells - Ab Monocytes, eosinophyls, neutrophils, B cells and NK cells ADCC – slower and less efficient than Tc direct cytotoxicity Macrophage activating cytokines (IFN-γ and GM-CSF) promote ADCC Cellular immune response Macrophage activation Killing by apoptosis – important – not always necessary L. monocytogenes, M. tubesculosis, B abortus or T. gondii survive and multiply inside macrophages – no antibodies IFN-γ Macrophage activation - 2 signals TNF-α or LPS innate T-cell-mediated mechanisms PAMPS Th1 TNF-α M – CD40 – CD154 Tc NK – IFN-γ IFN-γ (microbial invasion) Cellular immune response Macrophage activation Cellular immune response Memory T-cell Ab response – prolonged vs. T-cell – brief - overproduction Naive T-cells are long-lived resting cells that recirculate – bloodstream lymphoid organs Ag encounter cells respond rapidly and multiply rapidly – 1000 fold within a few days reach peak 5-7 days after infection when pathogen-specific Infection – cleared apoptosis 1-2 weeks after infection Memory T-cell Cellular immune response Excess Tc cells elimination –by CD95L-CD95 system survivors long lived memory cells the In acute viral infections – memory T-cells develop only when the Ag is eliminated. In chronic or persistent infections, Tc cells probably persist 5-10% of the peak number of Tc duration of exposure to antigen memory Tc Cellular immune response Memory Tc are distinguished from Naive Tc by cytokine mixture Memory Tc are CD44+ and express IL-15 IL-2Rß – receptor that binds IL-2 and Express adhesion molecules - connect to APC IL-4 + IFN-γ Enhanced responses – high affinity IL-2 receptors, and divide in the absence of antigen This division requires IL-15 and is inhibited by IL-2 Cellular immune response ©2024 Ross University School of Veterinary Medicine. All rights reserved.