Rat bone marrow precursors develop into dendritic accessory cells under the influence of a conditioned medium.

Primary in vitro immune responses require the interaction between T lymphocytes and Ia-positive antigen-presenting cells. Recently, techniques were established including discontinuous bovine serum albumin gradients, adherence, irradiation, and Fc-rosetting which led to a positive selection of rat dendritic cells and their identification as the major if not the only accessory cell for rat T lymphocytes treated with the mitogen, sodium periodate, or stimulator cells in mixed lymphocyte reactions. Rat dendritic cells which were found to be present in both lymphoid and non-lymphoid tissues have a low density, are non-adherent, radioresistant, non-specific esterase-negative, Fc-receptor negative and Ia-positive. Dendritic accessory cells were detected in cultures of low-density bone marrow cells and developed from radiosensitive precursors especially in a medium containing 10-20% of a supernatant elaborated by mitogen-preactivated syngeneic spleen cells. When compared to purified lymph node dendritic cells they were found to be identical in potency and phenotype. Adherent cells from bone marrow possessed neither accessory activity nor influenced the development of dendritic accessory cells. The mature dendritic accessory cells which could be enriched 1000-fold were no longer radiosensitive. The production of dendritic accessory cells from bone marrow precursors was influenced by the culture medium. Serum components were found to suppress their development. However, semipurified factor(s) released by mitogen-preactivated spleen cells enhanced the number of dendritic cells considerably. Activity resided in substances with a molecular weight in the range of 40,000 Daltons. This material could be separated on a Sephadex G 75 column from rat Interleukin 2 which had no effect on the formation of dendritic cells but did support proliferation of accessory cell-depleted, mitogen-treated lymph node cells.