C57BL/6 mice genetically deficient in interleukin 15 (IL-15−/− mice) were generated by gene targeting. IL-15−/− mice displayed marked reductions in numbers of thymic and peripheral natural killer (NK) T cells, memory phenotype CD8+ T cells, and distinct subpopulations of intestinal intraepithelial lymphocytes (IELs). The reduction but not absence of these populations in IL-15−/− mice likely reflects an important role for IL-15 for expansion and/or survival of these cells. IL-15−/− mice lacked NK cells, as assessed by both immunophenotyping and functional criteria, indicating an obligate role for IL-15 in the development and functional maturation of NK cells. Specific defects associated with IL-15 deficiency were reversed by in vivo administration of exogenous IL-15. Despite their immunological defects, IL-15−/− mice remained healthy when maintained under specific pathogen-free conditions. However, IL-15−/− mice are likely to have compromised host defense responses to various pathogens, as they were unable to mount a protective response to challenge with vaccinia virus. These data reveal critical roles for IL-15 in the development of specific lymphoid lineages. Moreover, the ability to rescue lymphoid defects in IL-15−/− mice by IL-15 administration represents a powerful means by which to further elucidate the biological roles of this cytokine.

The physiological role of the TNF receptor (TNFR) family member, RANK, was investigated by generating RANK-deficient mice. RANK −/− mice were characterized by profound osteopetrosis resulting from an apparent block in osteoclast differentiation. RANK expression was not required for the commitment, differentiation, and functional maturation of macrophages and dendritic cells from their myeloid precursors but provided a necessary and specific signal for the differentiation of myeloid-derived osteoclasts. RANK −/− mice also exhibited a marked deficiency of B cells in the spleen. RANK −/− mice retained mucosal-associated lymphoid tissues including Peyer's patches but completely lacked all other peripheral lymph nodes, highlighting an additional major role for RANK in lymph node formation. These experiments reveal that RANK provides critical signals necessary for lymph node organogenesis and osteoclast differentiation.

Both naive and memory T cells undergo antigen-independent proliferation after transfer into a T cell–depleted environment (acute homeostatic proliferation), whereas only memory T cells slowly divide in a full T cell compartment (basal proliferation). We show, first, that naive and memory CD8+ T cells have different cytokine requirements for acute homeostatic proliferation. Interleukin (IL)-7 receptor(R)α–mediated signals were obligatory for proliferation of naive T cells in lymphopenic hosts, whereas IL-15 did not influence their division. Memory T cells, on the other hand, could use either IL-7Rα– or IL-15–mediated signals for acute homeostatic proliferation: their proliferation was delayed when either IL-7Rα was blocked or IL-15 removed, but only when both signals were absent was proliferation ablated. Second, the cytokine requirements for basal and acute homeostatic proliferation of CD8+ memory T cells differ, as basal division of memory T cells was blocked completely in IL-15–deficient hosts. These data suggest a possible mechanism for the dearth of memory CD8+ T cells in IL-15– and IL-15Rα–deficient mice is their impaired basal proliferation. Our results show that naive and memory T lymphocytes differ in their cytokine dependence for acute homeostatic proliferation and that memory T lymphocytes have distinct requirements for proliferation in full versus empty compartments.

We have previously implicated TNF-related apoptosis-inducing ligand (TRAIL) in innate immune surveillance against tumor development. In this study, we describe the use of TRAIL gene-targeted mice to demonstrate the key role of TRAIL in suppressing tumor initiation and metastasis. Liver and spleen mononuclear cells from TRAIL gene-targeted mice were devoid of TRAIL expression and TRAIL-mediated cytotoxicity. TRAIL gene-targeted mice were more susceptible to experimental and spontaneous tumor metastasis, and the immunotherapeutic value of α-galactosylceramide was diminished in TRAIL gene-targeted mice. TRAIL gene-targeted mice were also more sensitive to the chemical carcinogen methylcholanthrene. These results substantiated TRAIL as an important natural effector molecule used in the host defense against transformed cells.

Signaling through receptor activator of nuclear factor-B (RANK) is essential for the differentiation and activation of osteoclasts, the cell principally responsible for bone resorption. Animals genetically deficient in RANK or the cognate RANK ligand are profoundly osteopetrotic because of the lack of bone resorption and remodeling. RANK provokes biochemical signaling via the recruitment of intracellular tumor necrosis factor receptor-associated factors (TRAFs) after ligand binding and receptor oligomerization. To understand the RANK-mediated signal transduction mechanism in osteoclastogenesis, we have designed a system to recapitulate osteoclast differentiation and activation in vitro by transfer of the RANK cDNA into hematopoietic precursors genetically deficient in RANK. Gene transfer of RANK constructs that are selectively incapable of binding different TRAF proteins revealed that TRAF pathways downstream of RANK that affect osteoclast differentiation are functionally redundant. In contrast, the interaction of RANK with TRAF6 is absolutely required for the proper formation of cytoskeletal structures and functional resorptive activity of osteoclasts. Moreover, signaling via the interleukin-1 receptor, which also utilizes TRAF6, rescues the osteoclast activation defects observed in the absence of RANK/TRAF6 interactions. These studies are the first to define the functional domains of the RANK cytoplasmic tail that control specific differentiation and activation pathways in osteoclasts.The structural and metabolic integrity of bone is maintained through the dynamic process of bone remodeling that results from the coordinate action of bone resorption by osteoclasts and the formation of new bone by osteoblasts. Osteoclasts are large, multinuclear cells that develop from a hematopoietic progenitor and are highly specialized for the resorptive process (1). Regulation of bone remodeling occurs through multiple mechanisms that ultimately converge on the interaction of osteoclasts or their precursors with osteoblasts and bone marrow stromal cells. Two key factors supplied by the stromal environment are CSF-1 1 and the TNF family member, RANKL (also called TRANCE, ODF, OPGL) (2), as confirmed by the osteopetrotic phenotypes of the op/op mice that are mutated in the CSF-1 gene (3) and the RANKL knockout mice (4).It is now widely accepted that most osteotropic agents including IL-1, IL-6, IL-11, IL-17, TNF-␣, prostaglandin E 2 , parathyroid hormone, and 1,25-dihydroxyvitamin D 3 (5) affect bone resorption primarily by enhancing stromal cell production of RANKL. RANKL affects bone resorption and bone density by influencing the osteoclast population at multiple stages. Not only does RANKL drive the differentiation of osteoclasts from multipotential progenitors, thereby expanding the pool of osteoclasts available for bone resorption (6), RANKL also activates resorption and enhances survival of existing mature osteoclasts in vitro (7,8) and in vivo (9). An essential role for the RANKL receptor, RANK, in osteoclast differenti...