Production of a negative regulator of IL-3 by bone marrow cells in response to the supernatant of IL-3-producing STIL-3 leukemia cells.

When spleen cells of mice grafted with STIL-3 C5 cells, a leukemic T-cell line producing IL-3, are cultured in vitro, a high IL-3 activity is detectable in the culture supernatant. However, when bone marrow cells of the same grafted mice are cultured under similar conditions, hardly any IL-3 activity is detectable. To elucidate the mechanism of this difference, we examined whether the bone marrow cells either suppress IL-3 production by STIL-3 C5 cells or produce an IL-3 inhibitor. When STIL-3 C5 cells were cultured in the presence of normal bone marrow cells, the culture supernatant showed a significantly reduced IL-3 activity as assessed by growth stimulatory effects on IL-3-dependent DA-1 cells and mast cells. The conditioned medium (CM) did not inhibit the growth of IL-3-independent cell lines. Heat treatment of the CM resulted in a recovery of the IL-3 activity, indicating that the effect was mediated by a heat-labile inhibitor rather than by suppression of IL-3 production. CM of bone marrow cells alone did not inhibit the IL-3 activity. The inhibitor was produced by a stem cell-enriched fraction of the bone marrow, and not by fractions of T cells, granulocytes, or adherent cells including macrophages. Stimulation of the stem cell-enriched fraction of bone marrow with STIL-3 C5-CM also induced the production of the IL-3 inhibitor, which was recovered in a MW 50-100 kD fraction after ultrafiltration. These results suggest a possible presence of a feedback mechanism against the IL-3 effect on hemopoietic stem cells and progenitors in the bone marrow.