CO - STIMULATION

CO-STIMULATION IS ESSENTIAL FOR PRIMING OF NAIVE T LYMPHOCYTES The antigen-specific and the co-stimulatory signal has to be induced in concert to induce T lymphocyte activation The antigen-specific and co-stimulatory signals can be delivered simultaneously by professional antigen presenting cells, only The antigen-specific and the co-stimulatory singnals has to be delivered by the same professional antigen presenting cell

Activated APCResting APC T-cell activationT-cell anergy CD4 CD28 CD4 B T CELLS REQUIRE AT LEAST TWO SIGNALS TO GET ACTIVATED ANTIGEN SPECIFIC ACTIVATION, ANERGY OR NEGLECTION No effect CD28 B7 2 APC not presenting antigen

IL-2 IL-2R  1 Antigen Epithelial cell Naïve T cell Signal 1 only Anergy The T cell is unable to produce IL-2 and therefore is unable to proliferate or be clonally selected. Unlike immunosupressive drugs that inhibit ALL specificities of T cell, Signal 1 in the absence of signal 2 causes T cell unresponsiveness to a specific antigen Self peptide epitopes presented by a non-classical APC e.g. an epithelial cell

Helper T cells can be co-stimulated by co-stimulatory molecules expressed by professional APC APC Th Signal 1 antigen & antigen receptor Signal 2 B7 family members (CD80 & CD86)CD28 ACTIVATION Costimulatory molecules are expressed by professional APC including dendritic cells, monocytes, macrophages, and B cells, but not by cells that have no immunoregulatory functions such as muscle, nerves, hepatocytes, epithelial cells etc.

HELPER T CELLS ARE CO-STIMULATED BY VARIOUS CO-STIMULATORY MOLECULES IN AN INDUCIBLE MANNER CD40L B7 NORMAL TISSUE CELLS DO NOT EXPRESS CD40 OR B7 CO-STIMULATORY MOLECULES APC CD40 APC B7 CD28 APC

CO-STIMULATION IS ESSENTIAL FOR PRIMING OF NAIVE T LYMPHOCYTES Dicision making at the first encounter with MHC-peptide complex The antigen-specific and the co-stimulatory signals have to be induced in concert to induce T lymphocyte activation The antigen-specific and co-stimulatory signals can be delivered simultaneously by professional antigen presenting cells, only The antigen-specific and the co-stimulatory singnals has to be delivered by the same professional antigen presenting cell

Affinity low medium high medium no signal no signal signalsignal ααβαβγ βγ αβ γ αβ γγγγ THE IL-2 RECEPTOR FAMILY – hematopoiesis IL-2R IL-15RI IL-7R IL-9R IL-4RI Loss of function mutation of the  -chain results in X-linked inherited severe combined immunodeficiency (X-SCID syndrome)

Gene transcription Proliferation IL-2 CYTOSKELETON βγ α THE HIGH AFFINITY IL-2 RECEPTOR Ligand binding No signaling STAT Signal Transducer and Activatior of Transcription JAK Janus kinase

adhesion costimulation recognition INITIATION OF T CELL PROLIFERATION IL-2R IL-2Rα IL-2R low affinity IL-2R high affinity IL-1 IL-2 transferrin insulin PROLIFERATION IL-2 AUTOCRINE GROWTH FACTOR

IL-2 IL-2R  Express a low affinity IL-2 receptor-  and  chains and produce no IL-2 The role of IL-2 cytokine in helper T-cell activation Signal 1 NFAT binds to the promoter of of the IL-2 receptor-  chain gene. Association of the  chain converts the IL-2R to a high affinity form that is expressed in the cell surface IL-2 IL-2R  1 Antigen Resting T cell

IL-2 IL-2R  1 Antigen 2 Costimulation Signal 2 Activates AP-1 and NF  -B to increase IL-2 gene transcription by 3 fold Stabilises and thus increases the half-life of IL-2 mRNA by fold IL-2 production increased by 100 fold overall Mechanism of co-stimulation in T cells Immunosuppressive drugs illustrate the importance of IL-2 in immune responses Cyclosporin & FK506 inhibit IL-2 by disrupting TCR signalling Rapamycin inhibits IL-2R signalling

FK506 CSA Cytokines, activation molecules IL-2, IL-2R TGF  Cycliphilin A isomerase FKBP2 isomerase Inactive phosphatase Active phosphatase Dephosphorylation of cytoplasmic NF-AT induces translocation to the nucleus MECHANISM OF THE ACTION OF THE IMMUNE SUPPRESSIVE DRUG CYCLOSPORIN A AND FK506/PROGRAPH TCR-CD3 Other receptors NOT ANTIGEN SPECIFIC

Cyclosporin A and Tacrolimus (FK506) inhibit T-cell activation Inhibition is mediated by a threonine phosphatase through inhibition of the calcineurin enzyme Dekapeptide derived from soil fungi Calcineurin enzyme Ca++ activated calmodulin CSA/FKB6 binding to immunophilin inhibits calcineurin activation by calmodulin and prevents dephosphorylation of NF-AT

Co-receptors deliver powerful responses Risk of triggering strong co-stimulatory signals Mice are not humans

Cytokine storm in a phase 1 trial of the anti-CD28 monoclonal antibody TGN1412. Suntharalingam GSuntharalingam G, Panoskaltsis N.Panoskaltsis N N Engl J Med Sep 7;355(10): Six healthy young male volunteers at a contract research organization were enrolled in the first phase 1 clinical trial of TGN1412, a novel superagonist anti-CD28 monoclonal antibody that directly stimulates T cells. Within 90 minutes after receiving a single intravenous dose of the drug, all six volunteers had a systemic inflammatory response characterized by a rapid induction of proinflammatory cytokines and accompanied by headache, myalgias, (pain in multiple muscles) nausea, diarrhea, erythema, vasodilatation, and hypotension. Within 12 to 16 hours after infusion, they became critically ill, with pulmonary infiltrates and lung injury, renal failure, and disseminated intravascular coagulation. Severe and unexpected depletion of lymphocytes and monocytes occurred within 24 hours after infusion. All six patients were transferred to the care of the authors at an intensive care unit at a public hospital, where they received intensive cardiopulmonary support (including dialysis), high- dose methylprednisolone, and an anti-interleukin-2 receptor antagonist antibody. Prolonged cardiovascular shock and acute respiratory distress syndrome developed in two patients, who required intensive organ support for 8 and 16 days. Despite evidence of the multiple cytokine-release syndrome, all six patients survived. Documentation of the clinical course occurring over the 30 days after infusion offers insight into the systemic inflammatory response syndrome in the absence of contaminating pathogens, endotoxin, or underlying disease.

„Extreme response The drug, an antibody called TGN1412, is being developed by a German company TeGenero with the aim of directing the immune system to fight cancer cells, or calm joints inflamed by rheumatoid arthritis. The antibody binds to a receptor molecule called CD28 on the surface of the immune system's infection-fighting T cells. (Nature March ) Scientists who work in the field say there are several possible ways that the drug could have triggered multiple organ failure. It may have stimulated T cells so much that they released an overwhelming flood of inflammatory molecules called cytokines. Or perhaps wayward T cells launched an attack on the body's own tissues, ignoring the safety mechanisms that normally keep this in check. Modulation of the CD28 co-stimulatory pathway.

E-BOOK FOR IMMUNOLOGY From 7th week lectures on Immunpathology start. For studying the clinical aspects of immunology you can use the E-Book online: Mark Peakman, Diego Vergani: Basic and Clinical Immunology ISBN: Elsevier Limited; 2nd Edition, The Proxy and the website where you can reach in the library:

Normal tissue cells do not express MHC class II NO SIGNAL 1. for CD4+ Th activation Normal tissue cells do not express co-stimulatory molecules and do not produce T cell differentiating cytokines NO SIGNAL 2. for CD4+ Th activation Migration of naive T lymphocytes to normal tissues is limited Antigen presenting cells are not activated in normal tissues NO SIGNAL 3. for CD4+ Th activation PERIPHERAL TISSUES TOLERIZE THEMSELVES PERIPHERAL TOLERANCE IMMUNE RESPONSES ARE NOT INITIATED IN THE PERIPHERY EFFECTOR CELLS CAN BE ACTIVATED MORE EASILY THAN NAIVE T- CELLS

Arming of effector T cells APC T Activation of NAÏVE T cells by signal 1 and 2 is not sufficient to trigger effector function, but….. IL-2 Effector T cell Clonal selection and differentiation How can this cell give help to or kill cells that express low levels of B7 family costimulators? the T cell will be activated to proliferate and differentiate under the control of autocrine IL-2 to an effector T cell. These T cells are ARMED

Armed Effector T cell CD28 Co-receptor TcR IL-2 Epithelial cell Naïve T cell Epithelial cell Clonally selected, proliferating and differentiated T cell sees antigen on a B7 negative epithelial cell Epithelial cell Armed Effector T cell Kill The effector programme of the T cell is activated without costimulation This contrasts the situation with naïve T cells, which are anergised without costimulation Effector function or anergy?

ANERGY – Functional unresponsiveness, no IL-2 secretion SIGNAL 1 Recognition of auto-antigen on tissue cell SIGNAL 2 No B7 and CD40 expression, no co-stimulation Tissue resident professional APC are not activated SIGNAL 3 Innate immunity is not activated No inflammation CLONAL DELETION – Activation induced cell death Requires persistant high antigen dose Fas – FasL interaction SUPPRESSION – Activity of other cells Cytokine-mediated balance Effector functions are inhibited by regulatory T cells CLONAL IGNORANCE No contact with the immune system Immunologically privileged sites Central nervous system, eye No recognition in the periphery MECHANISMS OF PERIPHERAL TOLERANCE