Macrophages, which are found in most tissues including lymphoid tissue, and B cells, which are located primarily in lymphoid tissue, can be similarly activated by pathogen-recognition receptors to express co-stimulatory molecules and act as antigen-presenting cells. The distribution of dendritic cells, macrophages, and B cells in a lymph node is shown schematically in Fig. 9.13. Only these three cell types express co-stimulatory molecules required to efficiently activate T cells, and they express these molecules only when activated in the context of infection. However, these cells activate T-cell responses in distinct ways. Dendritic cells take up, process, and present antigens from all types of sources, and are present mainly in the T-cell areas where they drive the initial clonal expansion and differentiation of naive T cells into effector T cells. By contrast, B cells and macrophages specialize in processing and presenting soluble antigens and antigens from intracellular pathogens, respectively; they interact mainly with effector CD4 T cells already primed by dendritic cells to recruit helper functions of those T cells.
9-9 Dendritic cells process antigens from a wide array of pathogens.
Dendritic cells primarily arise from myeloid progenitors within the bone marrow (see Fig. 1.3). They emerge from the bone marrow to migrate via the blood to tissues throughout the body, or directly to secondary lymphoid organs. There are two major classes of dendritic cells: conventional dendritic cells, and plasmacytoid dendritic cells (Fig. 9.14). The cell-surface markers and subset-specific transcription factors that distinguish these two classes, and the interferon-producing functions of plasmacytoid dendritic cells in the innate
Fig. 9.14 Conventional and plasmacytoid dendritic cells have different roles in the immune response. Mature conventional dendritic cells (left panel) are primarily concerned with the activation of naive T cells. There are several subsets of conventional dendritic cells, but these all process antigen efficiently, and when they are mature they express MhC proteins and co-stimulatory molecules for priming naive T cells. The cell-surface proteins expressed by the mature dendritic cell are described in the text. Immature dendritic cells lack many of the cell-surface molecules shown here but have numerous surface receptors that recognize pathogen molecules, including most of the Toll-like receptors (TLrs). plasmacytoid dendritic cells (right panel) are sentinels primarily for viral infections, and secrete large amounts of class I interferons. This category of dendritic cell is less efficient in priming naive T cells, but they express the intracellular receptors TLr-7 and TLr-9 for sensing viral infections.
immune response, are discussed in Chapter 3. In this chapter we shall focus on the role of conventional dendritic cells in the adaptive immune response— presenting antigens to and activating naive T cells.