Characterization of accessory cells required for helper T cell induction in vitro: evidence for a phagocytic, Fc-receptor, and Ia-bearing cell type.

The nature of the accessory cell type required for the in vitro induction of antigen-specific helper T (Th) cells to soluble antigen was determined. Two different sources of adherent accessory cells were investigated: peritoneal exudate cells (PEC) and macrophages derived from bone-marrow cells (BMM phi), which were cultured for 8 days in vitro with colony-stimulating factor. PEC and BMM phi were rosetted with EA-SRBC and separated by sedimentation into Fc gamma receptor-positive and negative fractions. Only the Fc gamma receptor-positive fraction was active in Th-cell induction. By means of the fluorescence-activated-cell sorter (FACS-II), PEC or BMM phi were separated either into phagocytic and nonphagocytic, or Ia-positive and Ia-negative cell populations. Only phagocytic and Ia-positive accessory cells induced antigen specific Th cells. In conclusion, the accessory cell is adherent, phagocytic, carries Fc gamma-receptors and Ia products, and thus expresses the most important characteristics of the macrophage lineage.