Dendritic cells (DCs) are tissue-resident and circulating cells that detect the presence of microbes and initiate innate immune defense reactions, and they capture microbial proteins for dis play to T cells to initiate adaptive immune responses. These cells are named because of their long membranous projections, reminiscent of the dendrites of neurons. Most DCs, other than plasma cytoid DCs, are widely distributed in lymphoid tissues, mucosal epithelium, and organ parenchyma (Fig. 1). The location of DCs in epithelia and tissues where microbes enter, their ability to capture antigens and take them to lymph nodes where naive T cells circulate, and their rapid responses to microbes all impart to these cells a unique set of roles in the immune system, serving as sentinels of infection that begin the rapid innate response but also link innate responses with the development of adaptive immune responses. We will discuss the role of DCs as mediators of innate immunity and as APCs in Chapters 4 and 6, respectively. Here we will introduce the general properties of DCs.

Fig1. Dendritic cells. (A) Light micrograph of cultured dendritic cells (DCs) derived from bone marrow precursors. (B) A scanning electron micrograph of a DC showing extensive membrane projections. (C) A section of the skin stained with antibodies specific for Langerhans cells, a special type of skin DC found in the epithelial layer (which appear dark brown in this immunoenzyme stain). (D and E) DCs in a lymph node, illustrated schematically (D) and in a section of a mouse lymph node (E) stained with fluorescently labeled antibodies against B cells in follicles (green) and DCs in the T-cell zone (red). A and B, Courtesy of Dr. Y-J Liu, MD, Anderson Cancer Center, Houston, Texas. C, Courtesy of Dr. Jarish Cohen, University of California San Francisco School of Medicine, San Francisco, CA. E, Courtesy of Drs. Kathryn Pape and Jennifer Walter, University of Minnesota School of Medicine, Minneapolis, Minnesota.

Development and Features of Dendritic Cell Subsets. Subsets of DCs can be defined on the basis of different cell surface markers, transcription factors, development from different precursor cells, tissue localization, and functions. We will describe the major subsets that are important in immune responses and are distinguished from one another by their functions and development, and by the expression of different surface molecules and transcription factors (Fig. 2 and Table 1). The common properties of these DC subsets include dependence on the cytokine FLT3L for their development, expression of the CD11c protein, and the ability to present antigens to and activate naive T cells or induce T-cell tolerance.

 • Conventional DCs (cDCs, also called classical DCs) are the major type of DC involved in capturing protein anti gens of microbes that enter through epithelial barriers and presenting the antigens to T cells. Conventional DCs were first identified by their morphology and ability to stimulate strong T-cell responses and are the most numerous DC subset in epithelia and lymphoid organs. They arise from bone marrow HSCs through a developmental pathway that includes a common precursor of both monocytes and conventional DCs, some of which develop into committed precursors for cDCs (called pre-cDCs). All these steps take place in the bone marrow. The pre-cDCs migrate to peripheral tissues, where they mature into cDCs. Similar to tissue macrophages, these DCs constantly sample the environment in which they reside.

• Conventional DCs may be further divided into two main subsets called major, or cDC2, and cross-presenting, or cDC1 (see Fig. 2 and Table 1). cDC2 cells are the most numerous DCs and are potent at capturing exogenous antigens and inducing CD4+ T-cell responses. The cDC1 subset is specialized to present antigens to naive CD8+ T cells by a process called cross-presentation, discussed in Chapter 6; this subset can also present antigens to CD4+ cells.

• Plasmacytoid dendritic cells (pDCs) produce the antiviral cytokines type I interferons (IFNs) in response to viruses and may capture blood-borne microbes and carry their antigens to the spleen for presentation to T cells. These DCs are called plasmacytoid because after activation, they begin to resemble plasma cells morphologically. They develop in the bone marrow from a precursor distinct from that for conventional DCs and are found in the blood and in small numbers in lymphoid organs. Plasmacytoid DCs are the body’s major producers of type I IFNs, which have potent antiviral activities and play an important role in innate host defense against viruses.

• Monocyte-derived DCs include cells with functions similar to those of cDCs but are derived from monocytes that were recruited into tissue inflammatory sites. They express CD11c, like all DCs, and also monocyte markers such as CD11b and CCR2.

• Langerhans cells are DCs found in the epidermis that share functions with cDCs but are developmentally related to tis sue-resident macrophages, arising from embryonic fetal liver and yolk sac precursors (see Fig. 2). They are identified by their location and morphology in the skin, the presence of tennis racket–shaped cytoplasmic organelles called Birbeck granules that are visible using electron microscopy, and the expression of various markers (see Table 1). Langerhans cells may function in the context of skin infections to present antigens to and activate CD4+ T cells, or in the absence of infection, to present self antigens to CD4+ T cells and induce tolerance to these antigens.

Fig2. Maturation of dendritic cells (DCs). DCs arise from a common precursor cell of the myeloid lineage in the bone marrow and further differentiate into subsets, the major ones being conventional DCs (cDCs) and plasmacytoid DCs (pDCs). Monocyte-derived DCs may arise from monocytes in inflamed tissues. Some plasmacytoid DCs may arise from the common DC precursor.

Table1. Human Dendritic Cell (DC) Subsets

Another population of cells called follicular dendritic cells (FDCs) have a dendritic morphology but are not derived from bone marrow precursors, do not present protein antigens to T cells, and should not be confused with DCs. FDCs are stromal cells involved in B-cell activation in lymphoid follicles of secondary lymphoid organs.

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Phagocytes

Major Features of Cell-Mediated Immunity

Major Features of Humoral Immunity

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