Activation of T Lymphocytes Chapter 9 Activation of T Lymphocytes

Activation and effector phases of T cell-mediated adaptive immune responses are triggered by antigen recognition by T lymphocytes Naive T lymphocytes home to secondary lymphoid organs, where they may encounter antigens presented by mature dendritic cells on class I or class II MHC molecules and thus become activated Antigen-stimulated T cells that have received both "signal one" through the antigen receptor and "second signals" via co-stimulatory receptors may be induced to secrete cytokines and to express cytokine receptors (like IL-2) T cell responses decline after the antigen is eliminated by effector cells

Activation of Naive and Effector T Cells by Antigen

Phases of T Cell Responses

SIGNALS FOR T LYMPHOCYTE ACTIVATION

The proliferation of T lymphocytes and their differentiation into effector and memory cells require antigen recognition, costimulation, and cytokines

Recognition of Antigen Antigen is always the necessary first signal for the activation of lymphocytes, ensuring that the resultant immune response is specific for the antigen Activation of naive T cells requires recognition of antigen presented by dendritic cells

Role of Costimulation in T Cell Activation

Mechanisms of T cell costimulation by CD28

The major members of the B7 and CD28 families

Role of CD40 in T cell activation

The mechanism of therapeutic costimulatory blockade

FUNCTIONAL RESPONSES OF T LYMPHOCYTES

Changes in surface molecules after T cell activation

Cytokines in Adaptive Immune Responses

Regulation of IL-2 receptor expression

Biologic actions of IL-2

Development of Memory T Cells

Clonal expansion of T cells

Development of memory T cells

DECLINE OF T CELL RESPONSES

Elimination of antigen leads to contraction of the T cell response, and this decline is responsible for maintaining homeostasis in the immune system Costimulation and growth factors like IL-2 stimulate expression of the anti- apoptotic proteins Bcl-2 and Bcl-XL in the activated lymphocytes, and these proteins keep cells viable The inhibitory receptors CTLA-4 and PD-1, apoptosis induced by death receptors of the TNF receptor superfamily (such as TNFRI and Fas), and regulatory T cells