The activation of specific B and T cells by antigen is essential for their functional interaction and brings them into proximity to enhance the possibility that the antigen-specific B and T cells will locate one another (Fig. 11-8). The frequency of naive B cells or T cells specific for a given epitope of an antigen is as low as 1 in 105 to 1 in 106 lymphocytes, and both populations have to be activated and the specific B and T cells have to find each other and physically interact to generate strong antibody responses. Helper T cells that have been activated by antigen and costimulation are induced to proliferate, express CD40L, and secrete cytokines. They also down-regulate the chemokine receptor CCR7 and increase the expression of CXCR5 and as a result leave the T cell zone and migrate toward the follicle. As mentioned earlier, CXCL13, the ligand for CXCR5, is secreted by follicular dendritic cells and other follicular stromal cells, and it contributes to the migration of activated CD4+ T cells toward the follicle.
Although protein antigens, being monovalent, typically do not provide strong enough signals to induce much B cell proliferation and differentiation, they can activate B cells and initiate a number of events. BCR engagement by these antigens results in reduced cell surface expression of the chemokine receptor CXCR5 and increased expression of CCR7, which is normally expressed on T cells. As a result, activated B cells migrate toward the T cell zone drawn by a gradient of CCL19 and CCL21, the ligands for CCR7. B cells activated by protein antigens can also express CD69, which blocks surface expression of sphingosine 1-phosphate receptors, causing retention of activated B cells in lymph nodes (see Chapter 3). Protein antigens are endocytosed by the B cell and presented in a form that can be recognized by helper T cells, and this represents the next step in the process of T-dependent B cell activation.
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