Mice devoid of the IL-15 system lose over 90% of CD8αα+ TCRαβ and TCRγδ intestinal intraepithelial lymphocytes (iIELs). Previous work revealed that IL-15Rα and IL-15 expressed by parenchymal cells, but not by bone marrow-derived cells, are required for normal CD8αα+ iIEL homeostasis. However, it remains unclear when and how the IL-15 system affects CD8αα+ iIELs through their development. This study found that IL-15Rα is dispensable for the thymic stage of CD8αα+ TCRαβ and TCRγδ iIEL development but is required for the maintenance and/or differentiation of the putative lineage marker negative precursors in the intestinal epithelium, especially for the most mature CD8 single positive subset. Moreover, the IL-15 system directly supports the survival of mature CD8αα+ iIEL in vivo. Taken together, this study suggests that regulation of CD8αα+ iIEL homeostasis by the IL-15 system does not occur in the thymus but involves mature cells and putative precursors in the intestine.

NK cell development requires IL-15, which is “trans-presented” to IL-15Rβγ on NK cells by IL-15Rα on other cells. In this study, we report that different levels of IL-15 trans-presentation are required for different NK cell developmental events to reach full maturation status. Because the IL-15Rα intracellular domain has the capacity to recruit signaling molecules, we generated knockin and transgenic (Tg) mice that lack the intracellular domain to assess the role of the IL-15 trans-presentation level independent of the function of this domain. The level of IL-15Rα on various cells of these mice follows the order WT > Tg6 > knockin > Tg1 ≥ knockout. Bone marrow (BM)–derived dendritic cells prepared from these mice induced Stat5 phosphorylation in NK cells. The level of phospho-Stat5 correlated with the level of IL-15Rα on BMDCs, thus offering the opportunity to study quantitative effects of IL-15 trans-presentation on NK cell development in vivo. We found that NK cell homeostasis, mature NK cell differentiation, and acquisition of Ly49 receptor and effector functions require different levels of IL-15 trans-presentation input to achieve full status. All NK cell developmental events examined were quantitatively regulated by the IL-15Rα level of BM-derived and radiation-resistant accessory cells, but not by IL-15Rα of NK cells. We also found that IL-15Rα of radiation-resistant cells was more potent than IL-15Rα of BM-derived accessory cells in support of stage 2 to stage 3 splenic mNK differentiation. In summary, each examined developmental event required a particular level of IL-15 trans-presentation by accessory cells.

NK cell development and homeostasis require IL-15 produced by both hematopoietic and parenchymal cells. Certain hematopoietic IL-15 sources, such as macrophages and dendritic cells, are known, whereas the source of parenchymal IL-15 remains elusive. Using two types of adipocyte-specific Il15−/− mice, we identified adipocytes as a parenchymal IL-15 source that supported NK cell development nonredundantly. Both adipocyte-specific Il15−/− mice showed reduced IL-15 production specifically in the adipose tissue but impaired NK cell development in the spleen and liver in addition to the adipose tissue. We also found that the adipose tissue harbored NK progenitors as other niches (e.g. spleen) for NK cell development, and that NK cells derived from transplanted adipose tissue populated the recipient’s spleen and liver. These findings suggest that adipocyte IL-15 contributes to systemic NK cell development by supporting NK cell development in the adipose tissue, which serves as a source of NK cells for other organs.

Erythroid Krüppel-like factor (EKLF/KLF1) was identified initially as a critical erythroid-specific transcription factor and was later found to be also expressed in other types of hematopoietic cells, including megakaryocytes and several progenitors. In this study, we have examined the regulatory effects of EKLF on hematopoiesis by comparative analysis of E14.5 fetal livers from wild-type and Eklf gene knockout (KO) mouse embryos. Depletion of EKLF expression greatly changes the populations of different types of hematopoietic cells, including, unexpectedly, the long-term hematopoietic stem cells Flk2− CD34− Lin− Sca1+ c-Kit+ (LSK)-HSC. In an interesting correlation, Eklf is expressed at a relatively high level in multipotent progenitor (MPP). Furthermore, EKLF appears to repress the expression of the colony-stimulating factor 2 receptor β subunit (CSF2RB). As a result, Flk2− CD34− LSK-HSC gains increased differentiation capability upon depletion of EKLF, as demonstrated by the methylcellulose colony formation assay and by serial transplantation experiments in vivo. Together, these data demonstrate the regulation of hematopoiesis in vertebrates by EKLF through its negative regulatory effects on the differentiation of the hematopoietic stem and progenitor cells, including Flk2− CD34− LSK-HSCs.