Unfractionated human peripheral blood lymphocytes seeded at 5 × 105 cells per 35-mm Petri dish with phytohemagglutinin (PHA) can be induced to form T cell colonies with a cloning efficiency of 1%. Fractionated cells with T cell markers did not form any colonies at this seeding level. T cells were induced to form colonies either by increasing the number of cells seeded, or by adding conditioned medium (CM) from syngeneic or allogeneic T cells cultured with PHA. This CM induced a cloning efficiency of 10% at a seeding level of 5 × 103 cells per Petri dish. The induction of T cell colonies by CM did not require the presence of phagocytic or B cells. The T cell colony-inducing activity in the CM appears to be a trypsin-sensitive protein(s) and was not substituted by thymopoietin. Incubation of T lymphocytes with this inducing activity for 30 min was sufficient to induce colony formation.

Colony-inducing activity for human T cells was also found in CM from normal human unfractionated lymphocytes incubated with concanavalin A, pokeweed mitogen, or lipopolysaccharide, human peripheral blood cells adherent to a Petri dish, Krebs mouse ascites tumor cells, and MGI+D+ mouse myeloid leukemia cells cultured with PHA. The results indicate that the colony-inducing activity for normal human T cells (TCI) can be found in CM from normal human T lymphocytes, human adherent cells, and some malignant mouse cells.

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