Immunological properties of thymus cell subpopulations: rat thymic dendritic cells are potent accessory cells and stimulators in a mixed leukocyte culture.

Rat thymus cells were fractionated by centrifugation on a discontinuous bovine serum albumin gradient into two subpopulations: one of high density that accounted for greater than 90% of the recovered cells, and a minor low-density subpopulation containing 4 to 10% of the total cells. The high-density subpopulation consisted mainly of uniform small-sized thymocytes, whereas the low-density subpopulation contained mostly larger-sized cells. High-density thymus cells did not function either as stimulators in a mixed leukocyte reaction or as accessory cells required for T-cell response to mitogens, Con A and sodium periodate, as determined by 3H-thymidine incorporation. Dense thymus cells also responded poorly to allogeneic and to mitogenic stimulation, even when accessory cells were added. In contrast, the low-density thymus cells responded well to allogeneic stimulation and to both mitogens. In addition, low-density thymus cells possessed stimulatory activity in mixed leukocyte cultures, as well as accessory activity for mitogenic responses. Both activities were found to reside in dendritic cells that were purified extensively (70-90% of the preparation) with good yield. When tested as accessory cells for T-cell responses to periodate, thymic dendritic cells were as potent as lymph node dendritic cells on a per cell basis. A small number of thymic dendritic cells was able to cause marked enhancement in T-lymphocyte proliferation in response to stimulation. By immunofluorescence thymic dendritic cells were shown to be Ia-positive, but Thy 1.1 -negative.