Long-term hematopoietic transfer of the anti-cancer and lifespan-extending capabilities of a genetically engineered blood system by transplantation of bone marrow mononuclear cells.

A causal relationship exists among the aging process, organ decay and disfunction, and the occurrence of various diseases including cancer. A genetically engineered mouse model, termed Klf1K74R/K74R or Klf1(K74R), carrying mutation on the well-conserved sumoylation site of the hematopoietic transcription factor KLF1/EKLF has been generated that possesses extended lifespan and healthy characteristics, including cancer resistance. We show that the healthy longevity characteristics of the Klf1(K74R) mice, as exemplified by their higher anti-cancer capability, are likely gender-, age-, and genetic background-independent. Significantly, the anti-cancer capability, in particular that against melanoma as well as hepatocellular carcinoma, and lifespan-extending property of Klf1(K74R) mice, could be transferred to wild-type mice via transplantation of their bone marrow mononuclear cells at a young age of the latter. Furthermore, NK(K74R) cells carry higher in vitro cancer cell-killing ability than wild-type NK cells. Targeted/global gene expression profiling analysis has identified changes in the expression of specific proteins, including the immune checkpoint factors PDCD and CD274, and cellular pathways in the leukocytes of the Klf1(K74R) that are in the directions of anti-cancer and/or anti-aging. This study demonstrates the feasibility of developing a transferable hematopoietic/blood system for long-term anti-cancer and, potentially, for anti-aging.

SENP2 restrains the generation of pathogenic Th17 cells in mouse models of colitis.

The molecular mechanisms contributing to the regulation of Th17-mediated inflammation remain underexplored. We here report a SUMO-specific protease (SENP)2-mediated pathway induced in pathogenic Th17 cells that restricts the pathogenesis of inflammatory colitis. SENP2 regulates the maturation of small ubiquitin-like modifiers (SUMO) and recycles SUMO from the substrate proteins. We find higher levels of SENP2 in pathogenic Th17 cells. By deleting Senp2 in T-cell lineages in mice, we demonstrate that the lack of Senp2 exacerbates the severity of experimental colitis, which is linked to elevated levels of GM-CSF+IL-17A+ pathogenic Th17 cells and more severe dysbiosis of the intestinal microbiome. Adoptive transfer experiments demonstrate the cell-autonomous effect of Senp2 in restraining Th17 differentiation and colitis. The enzymatic activity of SENP2 is important for deSUMOylation of Smad4, which reduces Smad4 nuclear entry and Rorc expression. Our findings reveal a SENP2-mediated regulatory axis in the pathogenicity of Th17 cells.

Inactivation of mitochondrial pyruvate carrier promotes NLRP3 inflammasome activation and gout development via metabolic reprogramming.

The nucleotide-binding and oligomerization domain, leucine-rich repeats, and pyrin domain-containing protein 3 (NLRP3) inflammasome plays a crucial role in innate immunity and is involved in the pathogenesis of autoinflammatory diseases. Glycolysis regulates NLRP3 inflammasome activation in macrophages. However, how lactic acid fermentation and pyruvate oxidation controlled by the mitochondrial pyruvate carrier (MPC) affect NLRP3 inflammasome activation and autoinflammatory disease remains elusive. We found that the inactivation of MPC with genetic depletion or pharmacological inhibitors, MSDC-0160 or pioglitazone, increased NLRP3 inflammasome activation and IL-1ß secretion in macrophages. Glycolytic reprogramming induced by MPC inhibition skewed mitochondrial ATP-associated oxygen consumption into cytosolic lactate production, which enhanced NLRP3 inflammasome activation in response to monosodium urate (MSU) crystals. As pioglitazone is an insulin sens MSDC-itizer used for diabetes, its MPC inhibitory effect in diabetic individuals was investigated. The results showed that MPC inhibition exacerbated MSU-induced peritonitis in diabetic mice and increased the risk of gout in patients with diabetes. Altogether, we found that glycolysis controlled by MPC regulated NLRP3 inflammasome activation and gout development. Accordingly, prescriptions for medications targeting MPC should consider the increased risk of NLRP3-related autoinflammatory diseases.

Thymic Epithelial Cell-Derived IL-15 and IL-15 Receptor α Chain Foster Local Environment for Type 1 Innate Like T Cell Development.

Expression of tissue-restricted antigens (TRAs) in thymic epithelial cells (TECs) ensures negative selection of highly self-reactive T cells to establish central tolerance. Whether some of these TRAs could exert their canonical biological functions to shape thymic environment to regulate T cell development is unclear. Analyses of publicly available databases have revealed expression of transcripts at various levels of many cytokines and cytokine receptors such as IL-15, IL-15Rα, IL-13, and IL-23a in both human and mouse TECs. Ablation of either IL-15 or IL-15Rα in TECs selectively impairs type 1 innate like T cell, such as iNKT1 and γδT1 cell, development in the thymus, indicating that TECs not only serve as an important source of IL-15 but also trans-present IL-15 to ensure type 1 innate like T cell development. Because type 1 innate like T cells are proinflammatory, our data suggest the possibility that TEC may intrinsically control thymic inflammatory innate like T cells to influence thymic environment.

Chinese herbal medicine SS-1 inhibits T cell activation and abrogates TH responses in Sjögren's syndrome.

BACKGROUND/PURPOSE: Sjögren's syndrome (SS) is an autoimmune disease and its conventional treatment has exhibited limited therapeutic efficacy. Traditional Chinese medicine has been demonstrated to ameliorate the sicca symptoms of SS by decreasing the level of TH1 and TH2 cytokines and increasing salivary flow rate. A newly designed traditional Chinese medicine, SS-1, showed improved efficacy in alleviating the dryness symptoms of SS patients in the National Taiwan SS cohort investigation. Here, we investigated the effect of SS-1 on T cell responses. METHODS: SS-1 was authenticated and its major compounds were verified by high-performance liquid chromatography. We examined the effects of SS-1 on the activation and TH1, TH2, and TH17 polarization of murine T cells. We also determined the level of TH1, TH2, and TH17 cytokine RNA in peripheral blood mononuclear cells of SS patients before and after SS-1 treatment. RESULTS: SS-1 treatment inhibits the activation and TH1, TH2, and IL-17A+IFNγ+ TH polarization of murine T cells. SS-1 treatment also significantly reduces IFN-γ, IL-4, and IL-13 expression, and moderately reduces IL-17A expression in peripheral blood mononuclear cells of SS patients. CONCLUSION: Our results suggest that SS-1 inhibits T cell activation and diminishes TH1, TH2, and IL-17+IFN-γ+ TH responses in SS patients.

CD8+CD122+ T cell homeostasis is controlled by different levels of IL-15 trans-presentation.

The homeostasis of CD8+CD122+ T cell requires IL-15 trans-presentation. We use Il15ra mutant mice and bone marrow chimeras to assess the role of IL-15 trans-presentation level in CD8+CD122+ T cells homeostasis. We demonstrate that CD8+CD122+ T cells require different levels of IL-15 trans-presentation to support their homeostasis.

Bmal1 integrates mitochondrial metabolism and macrophage activation.

Metabolic pathways and inflammatory processes are under circadian regulation. Rhythmic immune cell recruitment is known to impact infection outcomes, but whether the circadian clock modulates immunometabolism remains unclear. We find that the molecular clock Bmal1 is induced by inflammatory stimulants, including Ifn-γ/lipopolysaccharide (M1) and tumor-conditioned medium, to maintain mitochondrial metabolism under metabolically stressed conditions in mouse macrophages. Upon M1 stimulation, myeloid-specific Bmal1 knockout (M-BKO) renders macrophages unable to sustain mitochondrial function, enhancing succinate dehydrogenase (SDH)-mediated mitochondrial production of reactive oxygen species as well as Hif-1α-dependent metabolic reprogramming and inflammatory damage. In tumor-associated macrophages, aberrant Hif-1α activation and metabolic dysregulation by M-BKO contribute to an immunosuppressive tumor microenvironment. Consequently, M-BKO increases melanoma tumor burden, whereas administering the SDH inhibitor dimethyl malonate suppresses tumor growth. Therefore, Bmal1 functions as a metabolic checkpoint that integrates macrophage mitochondrial metabolism, redox homeostasis and effector functions. This Bmal1-Hif-1α regulatory loop may provide therapeutic opportunities for inflammatory diseases and immunotherapy.

Interleukin 15 blockade protects the brain from cerebral ischemia-reperfusion injury.

Acute ischemic stroke is followed by a complex interplay between the brain and the immune system in which ischemia-reperfusion leads to a detrimental inflammatory response that causes brain injury. In the brain, IL-15 is expressed by astrocytes, neurons and microglia. Previous study showed that ischemia-reperfusion induces expression of IL-15 by astrocytes. Transgenic over-expression of IL-15 in astrocytes aggravates ischemia-reperfusion brain damage by increasing the levels and promoting the effector functions of CD8+ T and NK cells. Treatment of neonatal rats with IL-15 neutralizing antibody before hypoxia-ischemia induction reduces the infarct volume. However, as stroke-induced inflammatory responses differ between neonate and adult brain, the effects of IL-15 blockade on the injury and immune response arising from stroke in adult animals has remained unclear. In this study, we examined the effect of post-ischemia/reperfusion IL-15 blockade on the pathophysiology of cerebral ischemia-reperfusion in adult mice. Using a cerebral ischemia-reperfusion model, we compared infarct size and the infiltrating immune cells in the brain of wild type (WT) mice and Il15-/- mice lacking NK and memory CD8+ T cells. We also evaluated the effects of IL-15 neutralizing antibody treatment on brain infarct volume, motor function, and the status of brain-infiltrating immune cells in WT mice. Il15-/- mice show a smaller infarct volume and lower numbers of activated brain-infiltrating NK, CD8+ T, and CD4+ T cells compared to WT mice after cerebral ischemia-reperfusion. Post-ischemia/reperfusion IL-15 blockade reduces infarct size and improves motor and locomotor activity. Furthermore, IL-15 blockade reduces the effector function of NK, CD8+ T, and CD4+ T cells in the ischemia-reperfusion brain of WT mice. Ablation of IL-15 responses after cerebral ischemia-reperfusion ameliorates brain injury in adult mice. Therefore, targeting IL-15 is a potential effective therapy for ischemic stroke.

Interleukin 15 activates Akt to protect astrocytes from oxygen glucose deprivation-induced cell death.

Astrocytes play a pivotal role in neuronal survival under the condition of post-ischemic brain inflammation, but the relevant astrocyte-derived mediators of ischemic brain injury remain to be defined. IL-15 supports survival of multiple lymphocyte lineages in the peripheral immune system, but the role of IL-15 in inflammatory disease of the central nervous system is not well defined. Recent research has shown an increase of IL-15-expressing astrocytes in the ischemic brain. Since astrocytes promote neuron survival under cerebral ischemia by buffering excess extracellular glutamate and producing growth factors, recovery of astrocyte function could be of benefit for stroke therapy. Here, we report that IL-15 is the pro-survival cytokine that prevents astrocyte death from oxygen glucose deprivation (OGD)-induced damage. Astrocytes up-regulate expression of the IL-15/IL-15Rα complex under OGD, whereas OGD down-regulates the levels of pSTAT5 and pAkt in astrocytes. IL-15 treatment ameliorates the decline of pAkt, decreases the percentage of annexin V+ cells, inhibits the activation of caspase-3, and activates the Akt pathway to promote astrocyte survival in response to OGD. We further identified that activation of Akt, but not PKCα/ßI, is essential for astrocyte survival under OGD. Taken together, this study reveals the function of IL-15 in astrocyte survival via Akt phosphorylation in response to OGD-induced damage.

Quantitative PPARγ expression affects the balance between tolerance and immunity.

PPARγ modulates energy metabolism and inflammation. However, its specific functions in the balance of immunity in vivo have been explored incompletely. In this study, by the age of 14 mo, Pparg(C/-) mice with PPARγ expression at 25% of the normal level exhibited high autoantibody levels and developed mesangial proliferative glomerulonephritis, which resembled systemic lupus erythematosus (SLE)-like autoimmune disease. These symptoms were preceded by splenomegaly at an early age, which was associated with increases in splenocyte accumulation and B-cell activation but not with relocation of hematopoiesis to the spleen. The mechanism of splenic lymphocyte accumulation involved reduced sphingosine-1-phosphate receptor 1 (S1P1) expression and diminished migration toward S1P in the Pparg(C/-) splenocytes, which impeded lymphocyte egression. Mechanistically, increased Th17 polarization and IL-17 signaling in the Pparg(C/-) CD4(+) T cells contributed to B-cell hyperactivation in the spleen. Finally, the activation of the remaining PPARγ in Pparg(C/-) mice by pioglitazone increased S1P1 levels, reduced the Th17 population in the spleen, and ameliorated splenomegaly. Taken together, our data demonstrated that reduction of Pparg expression in T-helper cells is critical for spontaneous SLE-like autoimmune disease development; we also revealed a novel function of PPARγ in lymphocyte trafficking and cross talk between Th17 and B cells.

Skeletal muscle interleukin 15 promotes CD8(+) T-cell function and autoimmune myositis.

BACKGROUND: Interleukin 15 (IL-15) is thought to be abundant in the skeletal muscle under steady state conditions based on RNA expression; however, the IL-15 RNA level may not reflect the protein level due to post-transcriptional regulation. Although exogenous protein treatment and overexpression studies indicated IL-15 functions in the skeletal muscle, how the skeletal muscle cell uses IL-15 remains unclear. In myositis patients, IL-15 protein is up-regulated in the skeletal muscle. Given the supporting role of IL-15 in CD8(+) T-cell survival and activation and the pathogenic role of cytotoxic CD8(+) T cells in polymyositis and inclusion-body myositis, we hypothesize that IL-15 produced by the inflamed skeletal muscle promotes myositis via CD8(+) T cells. METHODS: Expression of IL-15 and IL-15 receptors at the protein level by skeletal muscle cells were examined under steady state and cytokine stimulation conditions. The functions of IL-15 in the skeletal muscle were investigated using Il15 knockout (Il15 (-/-) ) mice. The immune regulatory role of skeletal muscle IL-15 was determined by co-culturing cytokine-stimulated muscle cells and memory-like CD8(+) T cells in vitro and by inducing autoimmune myositis in skeletal-muscle-specific Il15 (-/-) mice. RESULTS: We found that the IL-15 protein was not expressed by skeletal muscle cells under steady state condition but induced by tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ) stimulation and expressed as IL-15/IL-15 receptor alpha (IL-15Rα) complex. Skeletal muscle cells expressed a scanty amount of IL-15 receptor beta (IL-15Rß) under either conditions and only responded to a high concentration of IL-15 hyperagonist, but not IL-15. Consistently, deficiency of endogenous IL-15 affected neither skeletal muscle growth nor its responses to TNF-α and IFN-γ. On the other hand, the cytokine-stimulated skeletal muscle cells presented antigen and provided IL-15 to promote the effector function of memory-like CD8(+) T cells. Genetic ablation of Il15 in skeletal muscle cells greatly ameliorated autoimmune myositis in mice. CONCLUSIONS: These findings together indicate that skeletal muscle IL-15 directly regulates immune effector cells but not muscle cells and thus presents a potential therapeutic target for myositis.

The interleukin-15 system suppresses T cell-mediated autoimmunity by regulating negative selection and nT(H)17 cell homeostasis in the thymus.

The interleukin-15 (IL-15) system is important for regulating both innate and adaptive immune responses, however, its role in autoimmune disease remained unclear. Here we found that Il15(-/-) and Il15ra(-/-) mice spontaneously developed late-onset autoimmune phenotypes. CD4(+) T cells of the knockout mice showed elevated autoreactivity as demonstrated by the induction of lymphocyte infiltration in the lacrimal and salivary glands when transferred into nude mice. The antigen-presenting cells in the thymic medullary regions expressed IL-15 and IL-15Rα, whose deficiency resulted in insufficient negative selection and elevated number of natural IL-17A-producing CD4(+) thymocytes. These findings reveal previously unknown functions of the IL-15 system in thymocyte development, and thus a new layer of regulation in T cell-mediated autoimmunity.

TNFR1-JNK signaling is the shared pathway of neuroinflammation and neurovascular damage after LPS-sensitized hypoxic-ischemic injury in the immature brain.

BACKGROUND: Hypoxic-ischemia (HI) and inflammation are the two major pathogenic mechanisms of brain injury in very preterm infants. The neurovascular unit is the major target of HI injury in the immature brain. Systemic inflammation may worsen HI by up-regulating neuroinflammation and disrupting the blood-brain barrier (BBB). Since neurons and oligodendrocytes, microvascular endothelial cells, and microglia may closely interact with each other, there may be a common signaling pathway leading to neuroinflammation and neurovascular damage after injury in the immature brain. TNF-α is a key pro-inflammatory cytokine that acts through the TNF receptor (TNFR), and c-Jun N-terminal kinases (JNK) are important stress-responsive kinases. OBJECTIVE: To determine if TNFR1-JNK signaling is a shared pathway underlying neuroinflammation and neurovascular injury after lipopolysaccharide (LPS)-sensitized HI in the immature brain. METHODS: Postpartum (P) day-5 mice received LPS or normal saline (NS) injection before HI. Immunohistochemistry, immunoblotting and TNFR1- and TNFR2-knockout mouse pups were used to determine neuroinflammation, BBB damage, TNF-α expression, JNK activation, and cell apoptosis. The cellular distribution of p-JNK, TNFR1/TNFR2 and cleaved caspase-3 were examined using immunofluorescent staining. RESULTS: The LPS + HI group had significantly greater up-regulation of activated microglia, TNF-α and TNFR1 expression, and increases of BBB disruption and cleaved caspase-3 levels at 24 hours post-insult, and showed more cortical and white matter injury on P17 than the control and NS + HI groups. Cleaved caspase-3 was highly expressed in microvascular endothelial cells, neurons, and oligodendroglial precursor cells. LPS-sensitized HI also induced JNK activation and up-regulation of TNFR1 but not TNFR2 expression in the microglia, endothelial cells, neurons, and oligodendrocyte progenitors, and most of the TNFR1-positive cells co-expressed p-JNK. Etanercept (a TNF-α inhibitor) and AS601245 (a JNK inhibitor) protected against LPS-sensitized HI brain injury. The TNFR1-knockout but not TNFR2-knockout pups had significant reduction in JNK activation, attenuation of microglial activation, BBB breakdown and cleaved caspase-3 expression, and showed markedly less cortical and white matter injury than the wild-type pups after LPS-sensitized HI. CONCLUSION: TNFR1-JNK signaling is the shared pathway leading to neuroinflammation and neurovascular damage after LPS-sensitized HI in the immature brain.

Adipocyte IL-15 regulates local and systemic NK cell development.

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.

Thymic epithelial ß-catenin is required for adult thymic homeostasis and function.

The role of ß-catenin in thymocyte development has been extensively studied, however, the function of ß-catenin in thymic epithelial cells (TECs) remains largely unclear. Here, we demonstrate a requirement for ß-catenin in keratin 5 (K5)-expressing TECs, which comprise the majority of medullary TECs (mTECs) and a progenitor subset for cortical TECs (cTECs) in the young adult thymus. We found that conditionally ablated ß-catenin in K5(+)-TECs and their progeny cells resulted in thymic atrophy. The composition of TECs was also aberrantly affected. Percentages of K5(hi)K8(+)-TECs, K5(+)K8(-)-TECs and UEA1(+)-mTECs were significantly decreased and the percentage of K5(lo)K8(+)-TECs and Ly51(+)-cTECs were increased in ß-catenin-deficient thymi compared with that in the control thymi. We also observed that ß-catenin-deficient TEC lineage could give rise to K8(+)-cTECs more efficiently than wild-type TECs using lineage-tracing approach. Importantly, the expression levels of several transcription factors (p63, FoxN1 and Aire), which are essential for TEC differentiation, were altered in ß-catenin-deficient thymi. Under the aberrant differentiation of TECs, development of all thymocytes in ß-catenin-deficient thymi was impaired. Interleukin-7 (IL-7) and chemokines (Ccl19, Ccl25 and Cxcl12) levels were also downregulated in the thymic stromal cells in the mutants. Finally, introducing a BCL2 transgene in lymphoid lineages, which has been shown to rescue IL-7-deficient thymopoiesis, partially rescued the thymic atrophy and thymocyte development defects caused by induced ablation of ß-catenin in K5(+)-TECs. Collectively, these findings suggest that ß-catenin is required for the differentiation of TECs, thereby contributing to thymocyte development in the postnatal thymus.

IL-15 modulates the balance between Bcl-2 and Bim via a Jak3/1-PI3K-Akt-ERK pathway to promote CD8αα+ intestinal intraepithelial lymphocyte survival.

IL-15 is an essential survival factor for CD8αα(+) intestinal intraepithelial lymphocytes (iIELs) in vitro and in vivo. However, the IL-15-induced survival signals in primary CD8αα(+) iIELs remains elusive. Although Bcl-2 level in CD8αα(+) iIELs positively correlates with IL-15Rα expression in the intestinal epithelial cells, overexpression of Bcl-2 only moderately restores CD8αα(+) γδ iIELs in Il15(-/-) mice. Here, we found that IL-15 promptly activated a Jak3-Jak1-PI3K-Akt pathway that led to the upregulation of Bcl-2 and Mcl-1. This pathway also induced a delayed but sustained ERK1/2 activation, which not only was necessary for the maintenance of Bcl-2 but also resulted in the phosphorylation of extra-long Bim at Ser(65) . The latter event facilitated the dissociation of Bim from Bcl-2 without affecting Bim abundance in IL-15-treated CD8αα(+) iIELs. Using an adoptive cell transfer approach, we found that either overexpression of Bcl-2 or removal of Bim from CD8αα(+) iIELs promoted their survival in Il15ra(-/-) mice. Taken together, IL-15 promotes CD8αα(+) iIEL survival by both increasing Bcl-2 levels and dissociating Bim from Bcl-2 through activation of a Jak3-Jak1-PI3K-Akt-ERK1/2 pathway, which differs from a previously reported IL-15-induced survival signal.

Deficiency of interleukin-15 enhances susceptibility to acetaminophen-induced liver injury in mice.

Hepatocytes have a direct necrotic role in acetaminophen (APAP)-induced liver injury (AILI), prolonged secondary inflammatory response through innate immune cells and cytokines also significantly contributes to APAP hepatotoxicity. Interleukin 15 (IL-15), a multifunction cytokine, regulates the adaptive immune system and influences development and function of innate immune cells. To better understand the role of IL-15 in liver injury, we treated wild-type (WT) and IL-15-knockout (Il15⁻/⁻) mice with a hepatotoxic dose of APAP to induce AILI and evaluated animal survival, liver damage, APAP metabolism in livers and the inflammatory response. Production of pro-inflammatory cytokines/chemokines was greater in Il15⁻/⁻ than WT mice. Subanalysis of hepatic infiltrated monocytes revealed greater neutrophil influx, along with greater hepatic induction of inducible nitric oxide synthase (iNOS), in Il15⁻/⁻ than WT mice. In addition, the level of hepatic hemeoxygenase 1 (HO-1) was partially suppressed in Il15⁻/⁻ mice, but not in WT mice. Interestingly, elimination of Kupffer cells and neutrophils did not alter the vulnerability to excess APAP in Il15⁻/⁻ mice. However, injection of galactosamine, a hepatic transcription inhibitor, significantly reduced the increased APAP sensitivity in Il15⁻/⁻ mice but had minor effect on WT mice. We demonstrated that deficiency of IL-15 increased mouse susceptibility to AILI. Moreover, Kupffer cell might affect APAP hepatotoxicity through IL-15.

Different NK cell developmental events require different levels of IL-15 trans-presentation.

NK cell development requires IL-15, which is "trans-presented" to IL-15Rßγ 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.

IL-15Rα of radiation-resistant cells is necessary and sufficient for thymic invariant NKT cell survival and functional maturation.

The development of invariant NKT (iNKT) cells depends on the thymus. After positive selection by CD4(+)CD8(+)CD1d(+) cortical thymocytes, iNKT cells proceed from CD44(low)NK1.1(-) (stage 1) to CD44(high)NK1.1(-) (stage 2), and then to CD44(high)NK1.1(+) (stage 3) cells. The programming of cytokine production occurs along the three differentiation stages, whereas the acquisition of NK receptors occurs at stage 3. Stage 3 thymic iNKT cells are specifically reduced in Il15ra(-/-) mice. The mechanism underlying this homeostatic deficiency and whether the IL-15 system affects other thymic iNKT cell developmental events remain elusive. In this study, we demonstrate that increased cell death contributed to the reduction of stage 3 cells in Il15ra(-/-) mice, as knockout of Bim restored this population. IL-15-dependent upregulation of Bcl-2 in stage 3 cells affected cell survival, as overexpression of hBcl-2 partially restored stage 3 cells in Il15ra(-/-) mice. Moreover, thymic iNKT cells in Il15ra(-/-) mice were impaired in functional maturation, including the acquisition of Ly49 and NKG2 receptors and the programming of cytokine production. Finally, IL-15Rα expressed by radiation-resistant cells is necessary and sufficient to support the survival as well as the examined maturation events of thymic iNKT cells.