The miR-17∼92 cluster family is composed of three members encoding microRNAs that share seed sequences. To assess their role in cerebellar and medulloblastoma (MB) development, we deleted the miR-17∼92 cluster family in Nestin-positive neural progenitors and in mice heterozygous for the Sonic Hedgehog (SHH) receptor Patched 1 (Ptch1+/−). We show that mice in which we conditionally deleted the miR-17∼92 cluster (miR-17∼92floxed/floxed; Nestin-Cre+) alone or together with the complete loss of the miR-106b∼25 cluster (miR-106b∼25−/−) were born alive but with small brains and reduced cerebellar foliation. Remarkably, deletion of the miR-17∼92 cluster abolished the development of SHH-MB in Ptch1+/− mice. Using an orthotopic transplant approach, we showed that granule neuron precursors (GNPs) purified from the cerebella of postnatal day 7 (P7) Ptch1+/−; miR-106b∼25−/− mice and overexpressing Mycn induced MBs in the cortices of naïve recipient mice. In contrast, GNPs purified from the cerebella of P7 Ptch1+/−; miR-17∼92floxed/floxed; Nestin-Cre+ animals and overexpressing Mycn failed to induce tumors in recipient animals. Taken together, our findings demonstrate that the miR-17∼92 cluster is dispensable for cerebellar development, but required for SHH-MB development.

To study gene functions specifically in NKp46 + cells we developed novel Cre mice allowing for conditional gene targeting in cells expressing Ncr1 (encoding NKp46). We generated transgenic Ncr1 greenCre mice carrying an EGFPcre fusion under the control of a proximal Ncr1 promoter that faithfully directed EGFPcre expression to NKp46 + cells from lymphoid and nonlymphoid tissues. This approach allowed for direct detection of Cre-expressing NKp46 + cells via their GFP signature by flow cytometry and histology. Cre was functional as evidenced by the NKp46 + cell-specific expression of RFP in Ncr1 greenCre RosadtRFP reporter mice. We generated Ncr1 greenCre Il2rg fl/fl mice that lack NKp46 + cells in an otherwise intact hematopoietic environment. Il2rg encodes the common gamma chain (γ c ), which is an essential receptor subunit for cytokines (IL-2, -4, -7, -9, -15, and -21) that stimulate lymphocyte development and function. In Ncr1 greenCre Il2rg fl/fl mice, NK cells are severely reduced and the few remaining NKp46 + cells escaping γc deletion failed to express GFP. Using this new NK-cell-deficient model, we demonstrate that the homeostasis of NKp46 + cells from all tissues (including the recently described intraepithelial ILC1 subset) requires Il2rg. Eur. J. Immunol. 2014. 44: 3380-3391 Innate immunity 3381 in vivo and their capacity to promptly kill target cells (including tumors and infected cells) without prior sensitization [1][2][3]. NK cells have been shown to participate in immune responses to various microbial pathogens, including viruses, bacteria, and parasites (reviewed in [4]) through secretion of type 1 cytokines that recruit and activate hematopoietic effector cells. In the mouse, NK cells can be identified as CD3-NKp46 + cells that coexpress NK1.1 on appropriate genetic backgrounds (including C57BL/6). NK cells do not represent a homogeneous cell population but can be phenotypically separated into several subpopulations. Commonly used cell surface markers include CD27 and CD11b that divide NK-cell populations into CD27 + CD11b − NK cells (representing the least differentiated cells) that give rise to CD27 − CD11b + NK cells via CD27 + CD11b + NK-cell intermediates [5,6]. CD11b + NK cells can be further subdivided on the basis of KLRG1 expression with KLRG1 + marking terminally differentiated NK cells [7]. NK cells have been detected in lymphoid tissues, including bone marrow (BM), spleen, lymph nodes (LNs), Peyer's patches, and thymus, but also in nonlymphoid tissues, including liver, lungs, uterus, pancreas, skin, and intestine (reviewed in [8]).Many studies aiming at deciphering gene functions for NK cells make use of germ-line gene targeted mice where all cells lack the gene in question. Given that the activity and differentiation of NK cells is intimately regulated by other immune cells (reviewed in [9,10]), the consequences of gene deletions that intrinsically affect NK cells may be difficult to distinguish from indirect effects that occur in non-NK cells. Furthermore, studying the role of N...