Differentiation of NK1.1(+), Ly49(+) NK cells from flt3(+) multipotent marrow progenitor cells

To delineate factors involved in NK cell development, we established an in vitro system in which lineage marker (Lin)(-), c-kit(+), Sca2(+) bone marro w cells differentiate into lytic NK1.1(+) but Ly49(-) cells upon culture in IL-7, stem cell factor (SCF), and flt3 ligand (flt3L), followed by IL-15 a lone, A comparison of the ability of IL-7, SCF, and flt3L to generate IL-15 -responsive precursors suggested that NK progenitors express the receptor f or flt3L. In support of this, when Lin(-), c-kit(+), flt3(+) or Lin(-), c-k it(+), flt3(-)progenitors mere utilized, 3-fold more NK cells arose from th e flt3(+) than from the flt3- progenitors. Furthermore, NK cells that arose from flt3(-) progenitors showed an immature NK1.1(dim), CD2(-), c-kit+ phe notype as compared with the more mature kNK1.1(bright), CD2(+/-), c-kit(-) phenotype displayed by NK cells derived from flt3(+) progenitors. Both prog enitors, however, gave rise to NK cells that were Ly49 negative. To test th e hypothesis that additional marrow-derived signals are necessary for Ly49 expression on developing MC cells, flt3(+) progenitors were grown in IL-7, SCF, and flt followed by culture with IL-15 and a marrow-derived stromal ce ll line. Expression of Ly49 molecules, including those of which the MHC cla ss I ligands were expressed on the stromal or progenitor cells, as well as others of which the known ligands were absent, was induced within 6-13 days . Thus, we have estabhished an in vitro system in which Ly49 expression on developing NK cells can be analyzed and possibly experimentally manipulated .