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Natural killer (NK) cells play important roles in innate immunity by lysing tumor and virally infected cells and by producing cytokines including interferon‐γ. While NK cell progenitors have been described in the fetal thymus, NK cell generation from hematopoietic stem cells (HSC) in the bone marrow (BM) occurs throughout life, and in athymic mice and humans. Interleukin (IL)‐15 promotes NK development in vitro and is essential for the generation of normal numbers of NK cells in vivo. By characterizing BM cells expressing IL‐15 receptor components, we found marked heterogeneity within the IL‐2 receptor β chain+ (CD122+) subset, which included cells uniquely committed to the NK lineage. These CD122+ NK cell precursors (NKP) are negative for markers used to identify mature NK cells, including NK1.1, DX5 and members of Ly‐49 family, and fail to demonstrate natural cytotoxicity against susceptible targetcells. In vitro culture of NKP generates mature lytic NK1.1+ cells at high frequencies, while they do not give rise to T, B, myeloid or erythroid cells under appropriate conditions. NKP lack transcripts associated with early B and T cell differentiation (pTα, λ5 and CD3ϵ), but express a group of genes (IL‐15Rα, Id2, GATA‐3 and Ets‐1) and the 2B4 marker, which may define NK cell committment. We propose that NKP represent the earliest adult BM precursor uniquely restricted to the NK cell lineage.

SummaryInterleukin-10 (IL-10) is a recently described pteiotropic cytokine secreted mainly by type 2 helper T cells. Previous studies have shown that IL-t0 suppresses cytokine expression by natural killer (NK) and type 1 T cells, thus down-regulating cell-mediated immunity and stimulating humoral responses. We here report that injected IL-t0 protein is an efficient inhibitor of tumor metastasis in experimental (B16-F10) and spontaneous (M27 and Lox human melanoma) metastasis models in vivo at doses that do not have toxic effects on normal or cancer cells. Histological characterization after IL-10 treatment confirmed the absence of CD8 + and CD4 + T cells and macrophages at the sites of tumor growth, but abundant NK cells were localized at these sites. This unexpected finding was confirmed by showing that IL-10 inhibits most B16-F10 and Lox metastases in mice deficient in T or B cells (SCID and nu/nu mice), but not in those deficient in NK cells (beige mice or NK cell--depleted mice). However, IL-t0 downregulation of pro-inflammatory cytokine production and/or recruitment of additional eff~ctor cells may also be involved in the anti-tumor effect at higher local concentrations of IL-t0, since transfected B16 tumor cells expressing high amounts of lL-10 were rejected by normal, nu/nu, or SCID mice at the primary tumor stage, and there was still a 33% inhibition of tumor metastasis in beige mice.

We report an in vitro stroma-dependent system for the clonal growth and differentiation of natural killer (NK) cells from lymphoid-restricted bone marrow progenitors or bone marrow NK1.1+ cells. Strikingly, the potential to initiate expression of specific Ly49 receptors becomes increasingly restricted as NK cells develop. Moreover, when NK cells express a Ly49 receptor specific for stromal cell class I MHC, they are less likely to initiate expression of another Ly49 receptor in the clonal culture system. The results indicate multiple roles for stromal cells in NK cell development, in supporting clonal growth, in initiation of Ly49 receptor expression, and in formation of the NK cell receptor repertoire.

Although understanding of the function and specificity of many natural killer (NK) cell receptors is increasing, the molecular mechanisms regulating their expression during late development of NK cells remain unclear. Here we use representational difference analysis to identify molecules required for late NK cell differentiation. Axl protein tyrosine kinase, together with the structurally related receptors Tyro3 and Mer, were essential for NK cell functional maturation and normal expression of inhibitory and activating NK cell receptors. Also, all three receptors were expressed in maturing NK cells, the ligands of these receptors were produced by bone marrow stromal cells, and recombinant versions of these ligands drove NK cell differentiation in vitro. These results collectively suggest that Axl, Tyro3 and Mer transmit signals that are essential for the generation of a functional NK cell repertoire.

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Dengue virus (DENV) induces strong T and B cell responses upon infection. Hence, it is difficult to determine the contribution of cell-mediated immunity alone in the long lasting protection against DENV infection and disease. Numerous CD4+ and CD8+ T cell epitopes have been identified, mainly in the non-structural proteins of DENV. Taking into account the immunogenicity and peptide sequence conservation among the different DENV serotypes, a minimal DENV antigen, called DENV1-NS, has been designed. This antigen is enriched in conserved and highly antigenic epitopes located in the NS3, NS4B, and NS5 regions of DENV1. To evaluate the ability of the DENV1-NS poly-epitope to express the antigenic peptides in the context of different HLA class I molecules, we established its in vivo immunogenicity by measuring, after DNA immunization and electroporation, the activation of DENV-specific CD8 T cells in transgenic mice expressing the human HLA-A * 0201, -A * 2402, -B * 0702, and -B * 3502 class I alleles. We then engineered a lipid nanoparticle (LNP) encapsulated modified mRNA vaccine encoding DENV1-NS and tested immunogenicity and protection in these human HLA class I transgenic mice, after transient blockade of the interferon (IFN) type I receptor. Significant protection was observed, after two injections of the mRNA vaccine. Collectively, these data strongly support the development of T cell-based vaccines targeting immunodominant T cell epitopes that generate potent virus-specific T cell responses conferring immunity against DENV infection.

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