Production by spleen and lymph node cells of conditioned medium with erythroid and other hemopoietic colony‐stimulating activity

Pokeweed mitogen‐stimulated suspension cultures of mouse spleen cells produced conditioned medium able to stimulate granulocyte‐macrophage, eosinophil and megakaryocyte colony formation in agar cultures of C57BL marrow cells and granulocyte‐macrophage and erythroid colony formation in agar cultures of CBA fetal liver cells. Medium conditioned by other mouse tissues stimulated only granulocyte‐macrophage colony formation and this activity was not increased by the addition of pokeweed mitogen. Spleen cells stimulated by mixed leucocyte culture or concanavalin‐A had a weak capacity to stimulate erythroid colony formation. Production of the factors stimulating the four types of hemopoiesis was T‐lymphocyte dependent and nu/nu spleen cells were inactive. Factor production was also dependent on adherent cells but evidence from rat‐mouse combinations suggested that the T‐lymphocytes actually produced the active factors. Production of the four types of colony stimulating factors was radiosensitive (D 0 120–238 rads) and spleen cell populations of lighter buoyant density than 1.075 g/cm 3 and sedimenting at 3.5–5.0 mm/hour were able to produce active conditioned media. Fractionation experiments failed to segregate spleen populations able to produce only one of the four stimulating factors. Production of factors stimulating hemopoiesis by mitogen‐stimulated lymphoid populations could be a process contributing to the control of hemopoiesis in vivo.