Fibroblasts from human spleen regulate NK cell differentiation from blood CD34+ progenitors via cell surface IL-15

Besides a structural role in tissue architecture, fibroblasts have been shown to regulate the proliferation and differentiation of other neighboring specialized cell types, but differently according to the anatomic site and pathologic status of their tissue of origin. In this study we report a novel regulatory function of human spleen-derived fibroblasts in the development of NK cells from adult resting blood progenitors. When CD34(+) cells were cocultured with spleen-derived fibroblasts in monolayers, nonadherent CD56(+)CD3(-) NK cells were predominantly produced after 2-3 wk of culture in the absence of exogenous cytokines. Most NK cells expressed class I-recognizing CD94 and NK p46, p44, and p30 receptors as well as perforin and granzyme lytic granules. Moreover, these cells demonstrated spontaneous killing activity. Cell surface immunophenotyping of spleen-derived fibroblasts revealed a low and consistent expression of IL-15, Flt3 ligand, and c-kit ligand. Additionally, low picogram amounts of the three cytokines were produced extracellularly. Neutralizing Abs to IL-15, but not the other two ligands, blocked NK cell development. Additionally, suppressing direct contacts of CD34+ progenitors and fibroblasts by microporous membrane abrogated NK cell production. We conclude that stromal fibroblasts within the human spleen are involved via constitutive cell surface expression of bioactive IL-15 in the development of functional activated NK cells under physiologic conditions.