After activation, there are characteristic changes in the expression of various surface molecules in T cells, which are best defined in CD4+ helper cells (Fig. 9-8). Many of the molecules that are expressed in activated T cells are also involved in the functional responses of the T cells. Some of the functionally important molecules induced on activation are the following.
CD69. Within a few hours after activation, T cells increase their expression of CD69, a plasma membrane protein. This protein binds to and reduces surface expression of the sphingosine 1-phosphate receptor S1PR1, which we described in Chapter 3 as a receptor that mediates egress of T cells from lymphoid organs. The consequence of decreased S1PR1 expression is that activated T cells are retained in lymphoid organs long enough to receive the signals that initiate their proliferation and differentiation into effector and memory cells. After cell division, CD69 expression decreases, the activated T cells re-express high levels of S1PR1, and therefore effector and memory cells can exit the lymphoid organs (see Chapter 3). CD25 (IL-2Ra). The expression of this cytokine receptor enables activated T cells to respond to the growth-promoting cytokine IL-2. This process is described later.
CD40 ligand (CD40L, CD154). Within 24 to 48 hours after activation, T cells express high levels of the ligand for CD40. The expression of CD40L enables activated
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