Importance of stromal determinants in the generation of dendritic and natural killer cells in the human spleen

The interaction between stroma and blood cells in the human spleen has received little attention, despite their well-defined roles during blood cell development in bone marrow. We have reported previously that human spleen-derived fibroblasts display a differentiated myofibroblast phenotype and constitutively express a biologically active form of membrane interleukin (IL)-15 that can drive co-cultured CD34⁺ blood cells to differentiate into activated natural killer (NK) cells. Here, we show that, in addition to NK cells, CD34/fibroblast co-cultures also yield myeloid CD1a⁺CD38 ⁺CD68⁺CD86⁺ HLA-DR⁺CD14 ^-CD80^- dendritic cells (DCs) after 3-4 weeks in culture. We found that DC development depended on endogenously secreted stromal macrophage colony-stimulating factor (M-CSF) and CD40/CD40L interaction rather than on fibroblast- and CD34-derived membrane IL-15. CD1a⁺ cells were necessary for co-produced NK cells to acquire lytic functions by a mechanism involving cell-to-cell contact and DC-derived IL-12. This study highlights the importance of spleen myofibroblasts in the in vitro generation of two distinct cell types (DC and NK cells) from the innate immune system and suggests that the human spleen is involved in the generation of NK cells from circulating progenitors.