A stimulator of mouse stem cell proliferation produced by human regenerating bone marrow.

We examined CFU-S proliferation stimulator, which recruits stem cells in DNA synthesis, in conditioned media prepared from bone marrow cells of patients with regeneration hemopoiesis after chemotherapy induced hypoplasia. This activity was estimated by hydroxyurea sensitivity of CFU-S in mice, under conditions of incubation with human bone marrow conditioned medium (BMCM). We found that CFU-S proliferation stimulator was present to a considerable extent in human regenerating BMCM, but less so in normal BMCM and that the production fluctuated with change of hemopoietic states, in the same patient. This stimulator was heat-labile, trypsin-sensitive and mainly produced by adherent cells. This factor may possibly be involved in regulation of proliferation of stem cells in regenerating bone marrow in humans.