Transcriptional regulation of IL-15 expression during hematopoiesis.

Dendritic cells (DCs) are the most commonly studied source of the cytokine IL-15. Using an IL-15 reporter transgenic mouse, we have recently shown previously unappreciated differences in the levels of IL-15 expressed by subsets of conventional DCs (CD8⁺ and CD8⁻). In this study, we show that IL-15 promoter activity was differentially regulated in subsets of hematopoietically derived cells with IL-15 expression largely limited to myeloid lineages. In contrast, mature cells of the lymphoid lineages expressed little to no IL-15 activity. Surprisingly, we discovered that hematopoietic stem cells (lineage⁻Sca-1⁺c-Kit⁺) expressed high levels of IL-15, suggesting that IL-15 expression was extinguished during lymphoid development. In the case of T cells, this downregulation was Notch-dependent and occurred in a stepwise pattern coincident with increasing maturation and commitment to a T cell fate. Finally, we further demonstrate that IL-15 expression was also controlled throughout DC development, with key regulatory activity of IL-15 production occurring at the pre-DC branch point, leading to the generation of both IL-15⁺CD8⁺ and IL-15(⁻/low)CD8⁻ DC subsets. Thus, IL-15 expression is coordinated with cellular fate in myeloid versus lymphoid immune cells.