THE IN-VIVO EFFECTS OF STEEL FACTOR ON NATURAL-KILLER LINEAGE CELLS IN MURINE SPLEEN AND BONE-MARROW

Steel factor (SlF), also known as stem cell factor, is a potent growth stimulator of hemopoietic progenitor cells. In the context of transpl antation of hemopoietic cells to irradiated allogeneic hosts, natural killer (NK) cells exert restrictive control on hemopoietic cell prolif eration, and are regularly found in elevated concentration in areas of intense hemopoiesis.The present study was designed to examine the eff ects with time of SlF in vivo on the numbers of NK cells, identified b y the presence of the NK 1.1 surface molecule, in the spleen and bone marrow. Throughout the first 3 days of SlF exposure, NK cell numbers, in spite of rapid (1 day) and significant increases in the other hemop oietic cell lineages, did not change in either the spleen or the bone marrow. However, NK cells were increased 2-fold in both organs by 7 da ys of SlF exposure. At this time, immature granuloid and erythroid cel ls and the large lymphoid cells in the spleen had more than doubled th eir respective control numbers and in the bone marrow, immature granul oid cells increased by 47% of control levels. The presence of a late, but not early, influence of SlF on NK cells of the spleen and bone mar row suggests an indirect effect of SlF on this lineage, occurring only when SlF-stimulated hemopoiesis becomes sufficiently intense, providi ng, thus, an abundance of NK cell targets.