Development of CD8+ T cells in the thymus Double negative T cell Lymphoid progenitor Double positive T cell Cortical epithelial cell Death by Neglect due to Weak signalling CORTEX MEDULLA CD8 committed T cell CD4 committed T cell THYMUS Medullary epithelial cell Or Dendritic cell CD8 T cell Negative selection and death Positive selection TO THE PERIPHERY! Hematopoietic precursor cells arrive to the thymus and become lymphoid progenitor cells in the cortex. These cells are CD3-/CD4-/CD8-, double negative cells, but soon start to express CD3 and T cell receptor (TCR). Next these cells begin to express both CD4 and CD8 molecules and become double positive T cells. At this stage double positive T cells interact with cortical epithelial cells. Some cells undergo apoptosis due to weak TCR-signalling. Depending if the double positive T cells recognize MHC I or MHC II via their TCR, they become committed double positive T cells. The committed T cells migrate to the medulla of thymus and stop expressing CD4 or CD8, depending on the initial recognition of MHC I or MHC II. CD8 cells recognize MHC I and stop expressing CD4. After they have become single positive CD8 cells they interact with medullary epithelial cells and either undergo apoptosis (negative selection) due to self-reactivity, or develop to mature CD8 T cells which exit from the thymus to the periphery as naïve CD8+ T cells. Raine Toivonen

Activation of CD8+ T cells in the periphery Ag TCR CD28 IL-2 IL2R CD8 Dendritic cell MHC I CD80 / CD86 CD8+ T cell encounters an activated APC The APC presents antigen on its surface within MHC I Antigens can be bound to MHC I either when they are synthesized within the cell (endogenous or viral antigens) or transferred from phagosomes to cytosolic compartment (cross-presentation of exogenous antigens) CD8 T cell produces IL-2 to drive its own proliferation CD8 T cells can be activated when they encounter an activated professional antigen presenting cell (dendritic cell, DC) within organized lymphoid tissue (such as a lymph node). In order to become activated by a DC, the DC must present an antigen to the CD8 T cell within MHC I. Antigens are presented within MHC I when they are synthetized by the cell (self antigen, viral or tumor antigen), or phagocytosed by the cell and transferred from phagosomes to the cytosolic compartment (exogenous antigens, especially membrane bound or particulate). The CD8 T cell recognizes the antigen and MHC I with its specific T-cell receptor. CD8 molecule interacts with the MHC I molecule of the APC to stabilize T-cell receptor ligation (”trimolecular complex”). CD80 and/or CD86 interact with CD28 to provide costimulation during activation. The interaction between CD28 and CD80 / CD86 helps the CD8 T cell e.g. to produce IL-2 and to drive its own proliferation. Raine Toivonen

The DC is activated by CD4+ T cell: license to kill Ag TCR CD8 IL12R DENDRITIC CELL MHC I CD4 CD28 B7 (CD80 / CD86) CD4+ T HELPER CELL CD40L CD40 In many cases, activation of CD8+ T cells to become cytotoxic effector cells (cytotoxic T lymphocytes, CTL) requires CD4+ T-cell help. The CD4+ T cell recognizes its specific antigen within MHC II on the same dendritic cell. While becoming activated itself, the CD4+ T cell augments activation of the DC e.g. by CD40-CD40L signalling. CD40 pathway enhances the expression of both MHC I and B7 costimulatory ligands, as well as induces the production of cytokines. Raine Toivonen

Killing mechanisms Perforin/Granzymes DNA CASPASE Raine Toivonen CTL Ag CTL Infected cell CASPASE DNA Activated CD8+ T cells differentiate to effector cells called Cytotoxic T Lymphocytes (CTL). CTL contain perforins and granzymes within their cytotoxic granules. After recognition of the target cell expressing its cognate antigen whithin MHC I, the CTL makes a contact with its target cell and releases its cytotoxic granules to the membrane of the target cell. Perforins attach to the plasma membrane of the target cell and enable the granzymes to enter the target cell. Granzymes are serine proteases, which cleave procaspases to biologically active caspases. Active caspases lead to cell death by apoptosis, which involves multiple mechanisms including cleavage of DNA by activated DNAses. Raine Toivonen

Killing mechanisms 2. FAS / FAS ligand DAXX RIP1 FADD RAIDD DISC CTL Infected cell DISC DAXX RIP1 FADD RAIDD CASPASE 2 ASK1 ProCASPASE 8 CASPASE 8 Another mechanism of target-cell apoptosis which can be executed by an activated CTL is activation of receptor-mediated apoptosis. Activated T-cells including CTL express FAS Ligand (CD95L) which binds and multimerizes Fas receptors on the target cell. The intracellular part of each Fas receptor contains a domain (death domain) which interacts with intracellular apoptosis machinery, including FADD and Caspase 8/10. Clustering of FADD and Caspase 8/10 creates death-inducing signaling complexes (DISC), which lead to activation of downstream executive caspases and subsequent apoptosis of the target cell. Ag CASPASE 3 JNK APOPTOSIS Raine Toivonen