CD56(bright) natural killer cell subsets: Characterization of distinct functional responses to interleukin-2 and the c-kit ligand

Natural killer (NK) cells are bone marrow-derived large granular lymphocytes that express the CD56 surface antigen. The CD56(bright) NK subset represents approximately 10% of all NK cells and is thought to be the least differentiated NK cell component in blood. The most mature NK cell expresses CD56 at low density and CD16 (FcRγIII) at high density, whereas CD56(bright) NK cells either lack CD16 (CD56(bright)CD16^-) or express it at low density (CD56(bright) CD16(dim)). c-kit is a tyrosine kinase receptor which is expressed on both CD34⁺ hematopoietic precursor cells and CD56(bright) NK cells. In the current study, we characterize interleukin (IL)-2 receptor (IL-2R) and c-kit expression in each of the CD56(bright) subsets. Both the CD56(bright) CD16^- and CD56(bright) CD16(dim) NK subsets express the high-affinity IL-2R and the c-kit receptor when isolated from fresh blood. However, each CD56(bright) NK cell subset has distinct functional responses to IL-2, the c-kit ligand (KL), or both. Activation of the high-affinity IL-2R on CD56(bright) CD16^- NK cells induces a proliferative response that is significantly weaker than that observed in the CD56(bright) CD16(dim) NK cell subset. Incubation of the CD56(bright) CD16^- NK cell subset with KL significantly enhances IL-2-induced proliferation, while KL has no such effect on the CD56(bright) CD16(dim) NK subset. Activation of the high-affinity IL-2R in bath CD56(bright) subsets induces lymphokine-activated killer (LAK) activity, but the addition of KL has no effect on LAK activity. Go-stimulation of either CD56(bright) subset with IL-12 and concentrations of IL-2 that only saturate the high-affinity IL-2R induces substantial interferon (IFN-γ) production. The addition of KL to this co-stimulatory signal enhances IFN-γ production in both CD56(bright) NK subsets. The distinct functional responses to IL-2 and KL seen in the CD56(bright) CD16 and CD56(bright) CD16(dim) NK subsets provide insight into IL-2R signaling and suggest that each phenotype identifies a discrete stage of NK cell differentiation.