ABSTRACT
Dendritic cells (DCs) are antigen-presenting cells (APCs), unique in their ability to control immune responses. DC progenitors in the bone marrow give rise to circulating precursors that home to the tissue where they reside as immature cells with high phagocytic capacity. Recent studies suggest that immature DCs are important in the establishment/maintenance of peripheral tolerance. Upon microbial invasion, immature DCs capture the microbe, become activated either by microbial components or the proinflammatory molecules released by the damaged tissue, and then migrate to the draining lymphoid organs. Meanwhile, DCs undergo maturation, that is, they express a novel set of surface molecules, particularly costimulatory molecules, and digest/process the microbes to present their derivatives (peptides, oligosaccharides, and lipids) on MHC class I and II as well as CD1 molecules. These complexes allow selection and activation of rare Ag-specific T cells, thereby initiating immune responses. DCs present Ag to CD4–T cells, which further activate the DC and also stimulate other immune effectors including Ag-specific CD8 T cells and B cells as well as non-Ag-specific macrophages, eosinophils, and natural killer (NK) cells. Just as lymphocytes are composed of different subsets with specific effector functions (B, NK, and T cells), DCs are composed of distinct subsets with specific regulatory functions. Given this central role in controlling immunity, DCs
Palucka and Banchereau282
represent logical targets for many clinical situations, including resistance to tumors and infectious agents.
