Chapter 11 T-cell Activation and Differentiation Immunology

Initiating event of adaptive immune response Interaction between niave T cell and an antigen-presenting cell Signal 1 – provided by antigen-specific TCR engagement Signal 2 – provided by contact with costimulatory ligand expressed by APC Example - CD28 Signal 3 - cytokines

T cell Activation Initiated by TCR-CD3 complex with processed antigen on MHC molecule CD8+ cells with Class I CD4+ cells with Class II Initiates cascade of biochemical events Inducing resting T cell to enter cell cycle, proliferate, differentiate into memory and effector T cells

T cell Activation Cascade of biochemical events leading to gene expression: Interaction of signal and molecule (example: TCR + MHC and antigen) Generation of “second messenger” that diffuses to other areas of cell Protein kinases and protein phosphatases are activated or inhibitied Signals are amplified by enzyme cascades https://www.youtube.com/watch?v=tMMrTRnFdI4 Click on link to see example

T cell Activation Gene products after activation Immediate genes (1/2 hour of recognition) Transcription factors (c-Myc, NFAT, NF-κB) Early genes (1-2 hours from recognition) IL-2, IL2R, IL-6, IFN-γ Late genes (more than 2 days later) Encode adhesion molecules

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Superantigens Exogeneous Endogeneous Bind to BOTH the TCR and MHC Can cause over-activation Overproduction of TH-cell cytokines, leading to systemic toxicity Exogeneous Variety of exotoxins secreted by some Gram+ bacteria Endogeneous Cell membrane proteins encoded by viruses

T-Cell Differentiation CD4+ and CD8+ cells leave thymus and enter circulation in G0 phase Naïve cells (condensed chromatin, little cytoplasm) About twice as many CD4+ Naïve cell recognized MHC-antigen complex Initiated primary response After 48 hours, enlarges into blast cell and undergoes repeated rounds of cell division Differentiate into: Effector cells – cytokine secretion, B-cell help Memory cells – long lived, respond with heightened activity (secondary response)

Treg Cells Shown to inhibit proliferation of other T cells in vitro CD4+CD25+ Shown to inhibit development of autoimmune diseases

Cell Death and T Cell Populations Apoptosis plays critical role Deletion of potentially autoreactive thymocytes Deletion of T cell populations after activation Fas and FasL pathway to induce self death