T Cell Activation What is activation? increased transcription, translation cell cycle entry, proliferation increased ‘help’ for B cells (CD40L, cytokines) + CTL (cytokines) increased cell-mediated effector function (granzyme; FasL) How are T cells activated in vivo? Concept of co-stimulation Co-stimulatory molecules, signaling pathways Negative regulation of T cell activation
Overview of TCR/CD3 Signaling Pathways AP-1 NF-kB 1 3 2 IP3 Transcription Factors NF-kB
Global changes in transcription upon T cell activation
Gene Induction after Ag recognition
Gene Induction after Ag recognition
Gene Induction after Ag recognition
Dendritic cells sample antigens in peripheral tissues, mature and migrate to lymph nodes
T Cell Circulation
Selectin Proteins Help Direct T Cell Traffic in vivo Naive T Cells
direct T cell traffic and Integrins also help direct T cell traffic and coordinate binding to different cell types
Strength (Affinity) of Various Receptor/Ligand Systems
Help Stabilize T Cell/APC Interactions Accessory Molecules Help Stabilize T Cell/APC Interactions ‘Immunological synapse’ ‘Supra-molecular activation cluster (SMAC)’
“Inside-out” signaling upregulates T cell adhesion to APC
APC Phenotypes
“Two-signal” model of lymphocyte activation Burnet - clonal selection hypothesis - B cells (1950s) self-reactive cells must be removed during development Bretscher and Cohn (1970) what about hypermutation? ‘helper’ cell (overlapping Ag specif.) for B cell responses Lafferty and Cunningham (1975) second signal for helper cell (from APC) Janeway and Medzhitov (~1989-1992) activation signal for APC (pattern-recog. receptor) PRR’s bind to conserved structures on pathogens
Roles for Co-Stimulation in T Cell Responses Increases efficiency of T cell activation increases proliferation, cytokine production signaling effects both quantitative + qualitative Increases T cell survival Helps ensure activation by appropriate cells i.e. by cells w/ligands for costim. molecules professional APC particularly important for naive cells
Co-stimulation + T cell activation
B Cell activation through surface Ig is aided by a co-receptor complex
Molecules with T cell co-stimulatory activity Mucin domain TIM-1 (upregulated) TIM-4 Yes Yes ? ? No
CD28 44 kD surface glycoprotein Cloned in Brian Seed’s lab (1984) Later shown to augment T cell proliferation Also shown to increase IL-2 production Shown by Allison and colleagues and Jenkins and Schwartz to prevent anergy in T cell clones stimulated through TCR alone Cytoplasmic domain required (signaling)
T Cell Clone Experiments Demonstrating the Importance of Co-stimulation Note: No IL-2 produced
IL-2 is a critical growth factor for expansion of effector T cells and is a target of co-stimulation
Generation of Effector CTL w/T Cell Help
Co-Stimulatory Signaling by CD28 CD28 cytoplasmic domain
CD28 - downstream signaling
Some MAPK pathways are targets for co-stimulatory signals CD28
NF-kB activation by TCR and CD28 PI-3K CARMA1 Akt
Contribution of CD28 to NFAT Activation
Negative Regulation of T Cell Activation CD28 and CTLA-4 After a T cell becomes activated, it up-regulates expression of CTLA-4 on the cell surface. CTLA-4 binds B7 with about 10x higher affinity than does CD28 This appears to act as a damper on activation
Regulation of CTLA-4 Expression
Lack of CTLA-4 Disrupts Normal T Cell Homeostasis wild-type knockout This suggests that there is probably some low-level ‘activation’ happening all the time in vivo, which CTLA-4 normally dampens 1 cm Lymphadenopathy
an inhibitory factor (IDO). CTLA4-Ig Suppresses Immune Responses CTLA4-Ig in the clinic: -autoimmune diseases Also evidence that CTLA4-Ig binding to B7 on APC can result in production of an inhibitory factor (IDO).
CD28 and B7 Family Members
Speculative model for PD-1 Function
Summary T cell activation is aided by accessory receptors Activation results in global changes in gene expression Co-stimulatory molecules are important for activation and function of T cells Related inhibitory molecules play a role in limiting immune responses