T cells to mediate their key effector functions, which are to help macrophages and B cells. In addition, as discussed earlier, CD40L on the T cells activates dendritic cells to become better APCs, thus providing a positive feedback loop mechanism for amplifying T cell responses.

• CTLA-4 (CD152). The expression of CTLA-4 on T cells also increases within 24 to 48 hours after activation. We have mentioned CTLA-4 earlier as a member of the CD28 family that functions as an inhibitor of T cell activation and thus as a regulator of the response. The mechanism of action of CTLA-4 is described in Chapter 14 (see Fig. 14-5).

• Adhesion molecules and chemokine receptors. After activation, T cells reduce expression of molecules that bring them to the lymphoid organs (such as L-selectin and the chemokine receptor CCR7) and increase the expression of molecules that are involved in their migration to peripheral sites of infection and tissue injury (such as the integrins LFA-1 and VLA-4, the ligands for E- and P-selectins, and various chemokine receptors). These molecules and their roles in T cell migration were described in Chapter 3. Activation also increases the expression of CD44, a receptor for the extracellular matrix molecule hyaluronan. Binding of CD44 to its ligand helps to retain effector T cells in the tissues at sites of infection and tissue damage (see Chapter 10).

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