Bone marrow and intrathymic precursors of T-cells produce a factor which enhances colony formation in the spleen.

Bone marrow Ig-Thy-1-SC-1- stem cells (precursors of T-lymphocytes, PTL, containing the SC-1 antigen) spontaneously secrete a humoral factor. When bone marrow Ig-Thy-1-SC-1- cells were treated with this factor they became able to form haemopoietic colonies in the spleens of lethally irradiated mice. This new colony-stimulating factor (CSF) is thermostable and has a molecular mass of about 25-30 kDa. Production of the CSF by bone marrow SC-1+Thy-1- cells was stimulated by treatment with the thymus preparation Thymoptin. Such treatment of bone marrow SC-1+Thy-1- cells also induced the production of a suppressor factor (thermolabile, molecular mass about 45 kDa), that reversed the effect of the CSF. The CSF and the suppressor factor are both also produced by cortisone-resistant radioresistant L3T4-Lyt-2-SC-1+ thymocytes (i.e. intrathymic PTL). The regulation of haemopoiesis appears to be a normal function of bone marrow PTL.