ABSTRACT
A common set of multipotent stem cells, mostly located in bone marrow, gives rise to the different lineages of blood cells which are mostly short-lived and thus need to be continually replaced. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-3, like other cytokines and polypeptide hormones, initiate their biological activities by binding to specific receptors on the plasma membrane of responding target cells. The availability of iodinated purified recombinant GM-CSF and IL-3 has made possible the characterization of such receptors. Activated T-lymphocytes seem to be the unique normal cellular source of both GM-CSF and IL-3. Numerous studies have demonstrated that the secretion of these molecules requires stimulation of the cells with an appropriate antigen, a lectin, or antireceptor antibodies. Large amounts of histamine are present in culture supernatants of in vitro stimulated spleen cells, as first demonstrated in conditioned medium from mixed lymphocyte culture between donor and recipient of a skin allograft in the mouse.
