Dendritic Cells

Jonathan M Austyn Nuffield Department of Surgery, University of Oxford, John Radcliffe Hospital, Oxford, UK

Dendritic cells (DCs) are a lineage of specialized anti-gen-presenting cells (APCs) that play a central role in the initiation of adaptive immune responses. Since the first description by Steinman and Cohn in 1973 of the lymphoid DC as a trace cell type isolated from lymphoid tissues of mice, it has become clear that cells of this leukocyte lineage are widely distributed throughout lymphoid and most nonlymphoid tissues of all mammalian species studied. DCs have or can acquire a phenotype distinct from that of other leukocyte lineages (Table 1).

It is convenient to distinguish between different stages of DC according to the cells' specialized functions in different anatomical compartments. 1) DC progenitors originate from the bone marrow (BM) of adult mammals and enter the blood to seed the tissues. 2) Within nonlymphoid tissues the cells develop into the so-called 'immature' or, perhaps better, 'processing' stage, with optimal capabilities for the uptake and processing of foreign antigens, MHC (major histocompatibility complex) production, and the formation of foreign peptide-MHC complexes. 3) The cells begin to mature, and migrate from nonlymphoid tissues into secondary lymphoid tissues. 4) In secondary lymphoid tissues, DCs at the 'mature' or 'costimulatory' stage present foreign peptide-MHC complexes to resting T cells and have acquired the capacity to deliver specialized costimulatory signals for T cell activation, a crucial step in the induction of most adaptive immune responses. 5) DCs within the thymus may present self peptide-MHC complexes to developing thymocytes for tolerance induction.

In contrast to cells of the DC leukocyte lineage, the follicular dendritic cell (FDC) probably represents a distinct cell type that is not BM derived, can express membrane-bound native antigens in immune complexes for prolonged periods, and plays a role in B cell (humoral) responses.

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