[Serum factors and their effect on hematopoiesis]

Human serum contains stimulators of hemopoietic precursor cells. Isolation of "factors" from serum is technically more difficult than from supernatants due to interactions of "factors" with the high molecular weight serum proteins. On the other hand, serum factors are naturally occurring activities which are likely to play a role in regulation of hemopoiesis. Two hemopoietic stimulators which differ from the known colony stimulating factor have been found in human serum. One of them is an indirect "releaser" activity, which induces the release of granulocyte colony stimulating factor (G-CSF) from normal peripheral blood lymphocytes. In patients whose cells release abnormally low amounts of G-CSA, this serum activity is elevated, indicating a feedback mechanism between "releaser" activity and the actual release. G-CSA itself does not circulate in serum. Thus, we assume that releaser activity has a local effect in the bone marrow on the release of G-CSF. These observations suggest that hemopoiesis is regulated by multi-step hormonal mechanisms, in analogy to other known hormone systems.