Although dendritic cells (DC) originate from bone marrow, they were not observed in fresh preparations of bone marrow cells (BMC). Likewise, accessory activity was barely measurable in a sensitive assay for this potent function of DC. However, both DC and accessory activity developed when BMC were cultured for 5 d. Based on fractionation before culture, nearly all of the accessory activity could be attributed to only 5% of the total BMC recovered in a low-density (LD) fraction. The LD-DC precursors differed from mature DC in a number of important respects. Removal of Ia+ cells from the LD fraction by panning did not decrease the production of DC when the nonadherent cells were cultured. Thus, the cell from which the DC is derived does not express or minimally expresses Ia antigens, in contrast to the strongly Ia+ DC that is produced in bone marrow cultures. Irradiation of LD cells before culture prevented the development of DC. When irradiation was delayed by daily intervals, progressive increases in the number of DC resulted, up to the fifth day. These findings, together with preliminary autoradiographic data, indicate that cell division has occurred, in contrast to the DC, which does not divide. We conclude that bone marrow-derived DC arise in culture from the division of LD, Ia- precursors.

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\s=b\The physiologic measurements of a subpopulation of mononuclear cells derived from head and neck lymphoid tissues are similar to those of dendritic cells are described. Dendritic cells are a subpopulation of bone marrow-derived leukocytes that were originally identified in rodents and now described in man as having central control of T-lymphocyte functions. We describe a technique for the enrichment of dendritic cells obtained from tonsils utilizing a bovine serum albumin (BSA) gradient and note that they have the light and electron microscopic appearance of dendritic cells. The measured oxidative mitogenic response and interferon-\g=g\production in complete leukocyte cultures was compared with BSA gradient-separated preparations. The denser cells, comprised mostly of normal appearing lymphocytes, would not undergo a mitogenic response nor produce normal amounts of interferon when stimulated unless the dendritic cell-rich, lessdense fraction, was added back. The dendritic cells derived from tonsils seem to behave as a potent accessory cell for these T-lymphocyte-associated functions. (Arch Otolaryngol 1984;110:701-706) During the last several years, Steinman and his colleagues'·6 at Rockefeller University, New York, have described a novel cell in the peripheral lymphoid organs of mice.These cells are called dendritic cells because of their irregularly shaped dendritic processes, but are bone mar¬ row-derived leukocytes and are not part of the nervous system. The gen¬ eral group of dendritic cells are leuko¬ cytes referred to in other works by a variety of names, including dendritic cells and interdigitating cells in lym¬ phoid tissues and veiled cells in lymph.' These are similar to but can be differentiated from epidermal Langerhans' cells and follicular ger¬ minal center dendritic cells.The recent major effort has been to distinguish dendritic cells from cells within the macrophage group.8 Den¬ dritic cells lack active endocytotic capacities, which is a cardinal feature of other mononucler cells.

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