Obesity-induced dysregulation of skin-resident PPARγ+ Treg cells promotes IL-17A-mediated psoriatic inflammation.

Obesity is a major risk factor for psoriasis, but how obesity disrupts the regulatory mechanisms that keep skin inflammation in check is unclear. Here, we found that skin was enriched with a unique population of CD4+Foxp3+ regulatory T (Treg) cells expressing the nuclear receptor peroxisome proliferation-activated receptor gamma (PPARγ). PPARγ drove a distinctive transcriptional program and functional suppression of IL-17A+ γδ T cell-mediated psoriatic inflammation. Diet-induced obesity, however, resulted in a reduction of PPARγ+ skin Treg cells and a corresponding loss of control over IL-17A+ γδ T cell-mediated inflammation. Mechanistically, PPARγ+ skin Treg cells preferentially took up elevated levels of long-chain free fatty acids in obese mice, which led to cellular lipotoxicity, oxidative stress, and mitochondrial dysfunction. Harnessing the anti-inflammatory properties of these PPARγ+ skin Treg cells could have therapeutic potential for obesity-associated inflammatory skin diseases.

Differential response of mesenchymal stromal cells (MSCs) to type 1 ex vivo cytokine priming: implications for MSC therapy.

BACKGROUND AIMS: Mesenchymal stromal cells (MSCs) are polymorphic, adherent cells with the capability to stimulate tissue regeneration and modulate immunity. MSCs have been broadly investigated for potential therapeutic applications, particularly immunomodulatory properties, wound healing and tissue regeneration. The exact physiologic role of MSCs, however, remains poorly understood, and this gap in knowledge significantly impedes the rational development of therapeutic cells. Here, we considered interferon γ (IFN-γ) and tumor necrosis factor alpha (TNF-α), two cytokines likely encountered physiologically and commonly used in cell manufacturing. For comparison, we studied interleukin-10 (IL-10) (anti-inflammatory) and interleukin-4 (IL-4) (type 2 cytokine). METHODS: We directly assessed the effects of these cytokines on bone marrow MSCs by comparing RNA Seq transcriptional profiles. Western blotting and flow cytometry were also used to evaluate effects of cytokine priming. RESULTS: The type 1 cytokines (IFN-γ and TNF-α) induced striking changes in gene expression and remarkably different profiles from one another. Importantly, priming MSCs with either of these cytokines did not increase variability among multiple donors beyond what is intrinsic to non-primed MSCs from different donors. IFN-γ-primed MSCs expressed IDO1 and chemokines that recruit activated T cells. In contrast, TNF-α-primed MSCs expressed genes in alternate pathways, namely PGE2 and matrix metalloproteinases synthesis, and chemokines that recruit neutrophils. IL-10 and IL-4 priming had little to no effect. CONCLUSIONS: Our data suggest that IFN-γ-primed MSCs may be a more efficacious immunosuppressive therapy aimed at diseases that target T cells (ie, graft-versus-host disease) compared with TNF-α-primed or non-primed MSCs, which may be better suited for therapies in other disease settings. These results contribute to our understanding of MSC bioactivity and suggest rational ex vivo cytokine priming approaches for MSC manufacturing and therapeutic applications.

Culture Condition of Bone Marrow Stromal Cells Affects Quantity and Quality of the Extracellular Vesicles.

Extracellular vesicles (EVs) released by bone marrow stromal cells (BMSCs) have been shown to act as a transporter of bioactive molecules such as RNAs and proteins in the therapeutic actions of BMSCs in various diseases. Although EV therapy holds great promise to be a safer cell-free therapy overcoming issues related to cell therapy, manufacturing processes that offer scalable and reproducible EV production have not been established. Robust and scalable BMSC manufacturing methods have been shown to enhance EV production; however, the effects on EV quality remain less studied. Here, using human BMSCs isolated from nine healthy donors, we examined the effects of high-performance culture media that can rapidly expand BMSCs on EV production and quality in comparison with the conventional culture medium. We found significantly increased EV production from BMSCs cultured in the high-performance media without altering their multipotency and immunophenotypes. RNA sequencing revealed that RNA contents in EVs from high-performance media were significantly reduced with altered profiles of microRNA enriched in those related to cellular growth and proliferation in the pathway analysis. Given that pre-clinical studies at the laboratory scale often use the conventional medium, these findings could account for the discrepancy in outcomes between pre-clinical and clinical studies. Therefore, this study highlights the importance of selecting proper culture conditions for scalable and reproducible EV manufacturing.

Splenic macrophage phagocytosis of intravenously infused mesenchymal stromal cells attenuates tumor localization.

Mesenchymal stromal cells (MSCs) possess remarkable tumor tropism, making them ideal vehicles to deliver tumor-targeted therapeutic agents; however, their value in clinical medicine has yet to be realized. A barrier to clinical utilization is that only a small fraction of infused MSCs ultimately localize to the tumor. In an effort to overcome this obstacle, we sought to enhance MSC trafficking by focusing on the factors that govern MSC arrival within the tumor microenvironment. Our findings show that MSC chemoattraction is only present in select tumors, including osteosarcoma, and that the chemotactic potency among similar tumors varies substantially. Using an osteosarcoma xenograft model, we show that human MSCs traffic to the tumor within several hours of infusion. After arrival, MSCs are observed to localize in clusters near blood vessels and MSC-associated bioluminescence signal intensity is increased, suggesting that the seeded cells expand after engraftment. However, our studies reveal that a significant portion of MSCs are eliminated en route by splenic macrophage phagocytosis, effectively limiting the number of cells available for tumor engraftment. To increase MSC survival, we transiently depleted macrophages with liposomal clodronate, which resulted in increased tumor localization without substantial reduction in tumor-associated macrophages. Our data suggest that transient macrophage depletion will significantly increase the number of MSCs in the spleen and thus improve MSC localization within a tumor, theoretically increasing the effective dose of an anti-cancer agent. This strategy may subsequently improve the clinical efficacy of MSCs as vehicles for the tumor-directed delivery of therapeutic agents.

Intratumoral Delivery of Interferonγ-Secreting Mesenchymal Stromal Cells Repolarizes Tumor-Associated Macrophages and Suppresses Neuroblastoma Proliferation In Vivo.

Neuroblastoma, the most common extracranial solid tumor in childhood, remains a therapeutic challenge. However, one promising patient treatment strategy is the delivery of anti-tumor therapeutic agents via mesenchymal stromal cell (MSC) therapy. MSCs have been safely used to treat genetic bone diseases such as osteogenesis imperfecta, cardiovascular diseases, autoimmune diseases, and cancer. The pro-inflammatory cytokine interferon-gamma (IFNγ) has been shown to decrease tumor proliferation by altering the tumor microenvironment (TME). Despite this, clinical trials of systemic IFNγ therapy have failed due to the high blood concentration required and associated systemic toxicities. Here, we developed an intra-adrenal model of neuroblastoma, characterized by liver and lung metastases. We then engineered MSCs to deliver IFNγ directly to the TME. In vitro, these MSCs polarized murine macrophages to the M1 phenotype. In vivo, we attained a therapeutically active TME concentration of IFNγ without increased systemic concentration or toxicity. The TME-specific IFNγ reduced tumor growth rate and increased survival in two models of T cell deficient athymic nude mice. Absence of this benefit in NOD SCID gamma (NSG) immunodeficient mouse model indicates a mechanism dependent on the innate immune system. IL-17 and IL-23p19, both uniquely M1 polarization markers, transiently increased in the tumor interstitial fluid. Finally, the MSC vehicle did not promote tumor growth. These findings reveal that MSCs can deliver effective cytokine therapy directly to the tumor while avoiding systemic toxicity. This method transiently induces inflammatory M1 macrophage polarization, which reduces tumor burden in our novel neuroblastoma murine model. Stem Cells 2018;36:915-924.

Concise Review: An (Im)Penetrable Shield: How the Tumor Microenvironment Protects Cancer Stem Cells.

Cancer stem cells (CSCs) are defined by their unlimited self-renewal ability and their capacity to initiate and maintain malignancy, traits that are not found in most cells that comprise the tumor. Although current cancer treatments successfully reduce tumor burden, the tumor will likely recur unless CSCs are effectively eradicated. This challenge is made greater by the protective impact of the tumor microenvironment (TME), consisting of infiltrating immune cells, endothelial cells, extracellular matrix, and signaling molecules. The TME acts as a therapeutic barrier through immunosuppressive, and thereby tumor-promoting, actions. These factors, outside of the cancer cell lineage, work in concert to shelter CSCs from both the body's intrinsic anticancer immunity and pharmaceutical interventions to maintain cancer growth. Emerging therapies aimed at the TME offer a promising new tool in breaking through this shield to target the CSCs, yet definitive treatments remain unrealized. In this review, we summarize the mechanisms by which CSCs are protected by the TME and current efforts to overcome these barriers. Stem Cells 2017;35:1123-1130.

Extracellular vesicles released from mesenchymal stromal cells stimulate bone growth in osteogenesis imperfecta.

BACKGROUND: Systemic infusion of mesenchymal stromal cells (MSCs) has been shown to induce acute acceleration of growth velocity in children with osteogenesis imperfecta (OI) despite minimal engraftment of infused MSCs in bones. Using an animal model of OI we have previously shown that MSC infusion stimulates chondrocyte proliferation in the growth plate and that this enhanced proliferation is also observed with infusion of MSC conditioned medium in lieu of MSCs, suggesting that bone growth is due to trophic effects of MSCs. Here we sought to identify the trophic factor secreted by MSCs that mediates this therapeutic activity. METHODS: To examine whether extracellular vesicles (EVs) released from MSCs have therapeutic activity, EVs were isolated from MSC conditioned medium by ultracentrifugation. To further characterize the trophic factor, RNA or microRNA (miRNA) within EVs was depleted by either ribonuclease (RNase) treatment or suppressing miRNA biogenesis in MSCs. The functional activity of these modified EVs was evaluated using an in vitro chondrocyte proliferation assay. Finally, bone growth was evaluated in an animal model of OI treated with EVs. RESULTS: We found that infusion of MSC-derived EVs stimulated chondrocyte proliferation in the growth plate, resulting in improved bone growth in a mouse model of OI. However, infusion of neither RNase-treated EVs nor miRNA-depleted EVs enhanced chondrocyte proliferation. CONCLUSION: MSCs exert therapeutic effects in OI by secreting EVs containing miRNA, and EV therapy has the potential to become a novel cell-free therapy for OI that will overcome some of the current limitations in MSC therapy.

Hematopoietic stem cell transplantation for infantile osteopetrosis.

We report the international experience in outcomes after related and unrelated hematopoietic transplantation for infantile osteopetrosis in 193 patients. Thirty-four percent of transplants used grafts from HLA-matched siblings, 13% from HLA-mismatched relatives, 12% from HLA-matched, and 41% from HLA-mismatched unrelated donors. The median age at transplantation was 12 months. Busulfan and cyclophosphamide was the most common conditioning regimen. Long-term survival was higher after HLA-matched sibling compared to alternative donor transplantation. There were no differences in survival after HLA-mismatched related, HLA-matched unrelated, or mismatched unrelated donor transplantation. The 5- and 10-year probabilities of survival were 62% and 62% after HLA-matched sibling and 42% and 39% after alternative donor transplantation (P = .01 and P = .002, respectively). Graft failure was the most common cause of death, accounting for 50% of deaths after HLA-matched sibling and 43% of deaths after alternative donor transplantation. The day-28 incidence of neutrophil recovery was 66% after HLA-matched sibling and 61% after alternative donor transplantation (P = .49). The median age of surviving patients is 7 years. Of evaluable surviving patients, 70% are visually impaired; 10% have impaired hearing and gross motor delay. Nevertheless, 65% reported performance scores of 90 or 100, and in 17%, a score of 80 at last contact. Most survivors >5 years are attending mainstream or specialized schools. Rates of veno-occlusive disease and interstitial pneumonitis were high at 20%. Though allogeneic transplantation results in long-term survival with acceptable social function, strategies to lower graft failure and hepatic and pulmonary toxicity are urgently needed.

Severe transplant-associated thrombotic microangiopathy in patients with hemoglobinopathies.

Incidence and severity of transplant-associated thrombotic microangiopathy (TA-TMA) in patients with hemoglobinopathies receiving hematopoietic cell transplant is unknown. We report the outcomes for two patients with TA-TMA who received eculizumab. A 2.5-year-old male with sickle cell disease developed TA-TMA-associated pericardial tamponade, severe hypertension, and acute kidney injury 2 months after transplant. A 7-year-old female with ß-thalassemia major developed TA-TMA-related acute kidney injury, severe hypertension, and seizures at 6 months after transplant. Both patients progressed to chronic kidney disease (CKD). In patients with hemoglobinopathies, preexisting endothelial dysfunction may place them at a greater risk for TA-TMA and subsequent CKD.

Mesenchymal progenitors expressing TRAIL induce apoptosis in sarcomas.

Sarcomas are frequent tumors in children and young adults that, despite a relative chemo-sensitivity, show high relapse rates with up to 80% of metastatic patients dying in 5 years from diagnosis. The real ontogeny of sarcomas is still debated and evidences suggest they may derive from precursors identified within mesenchymal stromal/stem cells (MSC) fractions. Recent studies on sarcoma microenvironment additionally indicated that MSC could take active part in generation of a supportive stroma. Based on this knowledge, we conceived to use modified MSC to deliver tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) targeting different sarcoma histotypes. Gene modified MSC expressing TRAIL were cocultured with different osteosarcoma, rhabdomyosarcoma, and Ewing's Sarcoma (ES) cell lines assessing viability and caspase-8 activation. An in vivo model focused on ES was then implemented considering the impact of MSC-TRAIL on tumor size, apoptosis, and angiogenesis. MSC expressing TRAIL induced significantly high apoptosis in all tested lines. Sarcoma death was specifically associated with caspase-8 activation starting from 8 hours of coculture with MSC-TRAIL. When injected into pre-established ES xenotransplants, MSC-TRAIL persisted within its stroma, causing significant tumor apoptosis versus control groups. Additional histological and in vitro studies reveal that MSC-TRAIL could also exert potent antiangiogenic functions. Our results suggest that MSC as TRAIL vehicles could open novel therapeutic opportunities for sarcoma by multiple mechanisms.

Mesenchymal stem/stromal cells as a delivery platform in cell and gene therapies.

Regenerative medicine relying on cell and gene therapies is one of the most promising approaches to repair tissues. Multipotent mesenchymal stem/stromal cells (MSC), a population of progenitors committing into mesoderm lineages, are progressively demonstrating therapeutic capabilities far beyond their differentiation capacities. The mechanisms by which MSC exert these actions include the release of biomolecules with anti-inflammatory, immunomodulating, anti-fibrogenic, and trophic functions. While we expect the spectra of these molecules with a therapeutic profile to progressively expand, several human pathological conditions have begun to benefit from these biomolecule-delivering properties. In addition, MSC have also been proposed to vehicle genes capable of further empowering these functions. This review deals with the therapeutic properties of MSC, focusing on their ability to secrete naturally produced or gene-induced factors that can be used in the treatment of kidney, lung, heart, liver, pancreas, nervous system, and skeletal diseases. We specifically focus on the different modalities by which MSC can exert these functions. We aim to provide an updated understanding of these paracrine mechanisms as a prerequisite to broadening the therapeutic potential and clinical impact of MSC.

Megakaryocytes promote murine osteoblastic HSC niche expansion and stem cell engraftment after radioablative conditioning.

Successful hematopoietic stem cell (HSC) transplantation requires donor HSC engraftment within specialized bone marrow microenvironments known as HSC niches. We have previously reported a profound remodeling of the endosteal osteoblastic HSC niche after total body irradiation (TBI), defined as relocalization of surviving megakaryocytes to the niche site and marked expansion of endosteal osteoblasts. We now demonstrate that host megakaryocytes function critically in expansion of the endosteal niche after preparative radioablation and in the engraftment of donor HSC. We show that TBI-induced migration of megakaryocytes to the endosteal niche depends on thrombopoietin signaling through the c-MPL receptor on megakaryocytes, as well as CD41 integrin-mediated adhesion. Moreover, niche osteoblast proliferation post-TBI required megakaryocyte-secreted platelet-derived growth factor-BB. Furthermore, blockade of c-MPL-dependent megakaryocyte migration and function after TBI resulted in a significant decrease in donor HSC engraftment in primary and competitive secondary transplantation assays. Finally, we administered thrombopoietin to mice beginning 5 days before marrow radioablation and ending 24 hours before transplant to enhance megakaryocyte function post-TBI, and found that this strategy significantly enhanced donor HSC engraftment, providing a rationale for improving hematopoietic recovery and perhaps overall outcome after clinical HSC transplantation.

Tendon progenitor cells in injured tendons have strong chondrogenic potential: the CD105-negative subpopulation induces chondrogenic degeneration.

To study the cellular mechanism of the tendon repair process, we used a mouse Achilles tendon injury model to focus on the cells recruited to the injured site. The cells isolated from injured tendon 1 week after the surgery and uninjured tendons contained the connective tissue progenitor populations as determined by colony-forming capacity, cell surface markers, and multipotency. When the injured tendon-derived progenitor cells (inTPCs) were transplanted into injured Achilles tendons, they were not only integrated in the regenerating area expressing tenogenic phenotype but also trans-differentiated into chondrogenic cells in the degenerative lesion that underwent ectopic endochondral ossification. Surprisingly, the micromass culture of the inTPCs rapidly underwent chondrogenic differentiation even in the absence of exogenous bone morphogenetic proteins or TGFßs. The cells isolated from human ruptured tendon tissues also showed connective tissue progenitor properties and exhibited stronger chondrogenic ability than bone marrow stromal cells. The mouse inTPCs contained two subpopulations one positive and one negative for CD105, a coreceptor of the TGFß superfamily. The CD105-negative cells showed superior chondrogenic potential in vitro and induced larger chondroid degenerative lesions in mice as compared to the CD105-positive cells. These findings indicate that tendon progenitor cells are recruited to the injured site of tendons and have a strong chondrogenic potential and that the CD105-negative population of these cells would be the cause for chondroid degeneration in injured tendons. The newly identified cells recruited to the injured tendon may provide novel targets to develop therapeutic strategies to facilitate tendon repair.

Genomic and functional comparison of mesenchymal stromal cells prepared using two isolation methods.

BACKGROUND AIMS: Mesenchymal stromal cells (MSCs) have been applied to patients in cell therapy for various diseases. Recently, we introduced a novel MSC separation filter device which could yield approximately 2.5-fold more MSCs from bone marrow in a closed system compared with the conventional open density gradient centrifugation method. MSCs isolated with these two methods were phenotypically similar and met the criteria defining human MSC proposed by the International Society for Cellular Therapy. However, these criteria do not reflect the functional capacity of MSCs. It has been shown that the donor, source, isolation method, culture condition and cryopreservation of MSCs have potential to alter their therapeutic efficacy. To determine the equivalency of MSCs isolated by these two methods, we compared their genomic profiles as an index of their biologic potential and evaluated their growth promoting potential as an index of function. METHODS: The gene expression profiles of human MSCs isolated from 5 healthy donors with two distinct methods were obtained from microarray analyses. The functional activity of freshly expanded/cryopreserved MSCs from these two isolation methods was evaluated using an in vitro chondrocyte proliferation assay. RESULTS: Freshly expanded MSCs isolated by these two methods were found to exhibit similar gene expression profiles and equivalent therapeutic effects, while freshly thawed, cryopreserved MSCs lacked all measureable therapeutic activity. CONCLUSIONS: The MSC separation device generates genomically and functionally equivalent MSCs compared with the conventionally isolated MSCs, although freshly thawed, cryopreserved MSCs, isolated by either method, are devoid of activity in our bioassay.

Going back to class I: MHC and immunotherapies for childhood cancer.

After decades of unfulfilled promise, immunotherapies for cancer have reached a tipping point, with several FDA approved products now on the market and many more showing promise in both adult and pediatric clinical trials. Tumor cell expression of MHC class I has emerged as a potential determinant of the therapeutic success of many immunotherapy approaches. Here we review current knowledge regarding MHC class I expression in pediatric cancers including a discussion of prognostic significance, the opposing influence of MHC on T-cell versus NK-mediated therapies, and strategies to reverse or circumvent MHC down-regulation.

Influence of Posttransplant Lymphoproliferative Disorder on Survival in Children After Heart Transplantation.

The influence of posttransplant lymphoproliferative disorder (PTLD) on long-term survival in children after heart transplantation (HTx) is not well studied. The United Network for Organ Sharing database was queried from 1987 to 2013 for data on PTLD in relation to induction immunosuppression and recipient Epstein-Barr virus status in children (<18 years of age) who underwent HTx. Of 6818 first-time pediatric heart transplants, 5169 had follow-up data on posttransplant malignancy, with 360 being diagnosed with PTLD. Univariate Cox analysis identified diminished survival after PTLD onset using a time-varying measure of PTLD (HR 2.208; 95 % CI 1.812, 2.689; p < 0.001), although Kaplan-Meier survival functions found no difference in survival between the group ever diagnosed with PTLD and the non-PTLD reference group (log-rank test: χ 1 (2)  = 0.02; p = 0.928). A multivariate Cox model found a greater mortality hazard associated with the development of PTLD after adjusting for recipient EBV seronegativity and other covariates (HR 3.024; 95 % CI 1.902, 4.808; p < 0.001). Induction immunosuppression at time of HTx did not significantly influence posttransplant mortality. The development of PTLD adversely influenced long-term survival in children after HTx after adjusting for confounding variables.

Effect of postremission therapy before reduced-intensity conditioning allogeneic transplantation for acute myeloid leukemia in first complete remission.

The impact of pretransplant (hematopoietic cell transplantation [HCT]) cytarabine consolidation therapy on post-HCT outcomes has yet to be evaluated after reduced-intensity or nonmyeloablative conditioning. We analyzed 604 adults with acute myeloid leukemia in first complete remission (CR1) reported to the Center for International Blood and Marrow Transplant Research who received a reduced-intensity or nonmyeloablative conditioning HCT from an HLA-identical sibling, HLA-matched unrelated donor, or umbilical cord blood donor from 2000 to 2010. We compared transplant outcomes based on exposure to cytarabine postremission consolidation. Three-year survival rates were 36% (95% confidence interval [CI], 29% to 43%) in the no consolidation arm and 42% (95% CI, 37% to 47%) in the cytarabine consolidation arm (P = .16). Disease-free survival was 34% (95% CI, 27% to 41%) and 41% (95% CI, 35% to 46%; P = .15), respectively. Three-year cumulative incidences of relapse were 37% (95% CI, 30% to 44%) and 38% (95% CI, 33% to 43%), respectively (P = .80). Multivariate regression confirmed no effect of consolidation on relapse, disease-free survival, and survival. Before reduced-intensity or nonmyeloablative conditioning HCT, these data suggest pre-HCT consolidation cytarabine does not significantly alter outcomes and support prompt transition to transplant as soon as morphologic CR1 is attained. If HCT is delayed while identifying a donor, our data suggest that consolidation does not increase transplant treatment-related mortality and is reasonable if required.

Transplanted bone marrow mononuclear cells and MSCs impart clinical benefit to children with osteogenesis imperfecta through different mechanisms.

Transplantation of whole bone marrow (BMT) as well as ex vivo-expanded mesenchymal stromal cells (MSCs) leads to striking clinical benefits in children with osteogenesis imperfecta (OI); however, the underlying mechanism of these cell therapies has not been elucidated. Here, we show that non-(plastic)-adherent bone marrow cells (NABMCs) are more potent osteoprogenitors than MSCs in mice. Translating these findings to the clinic, a T cell-depleted marrow mononuclear cell boost (> 99.99% NABMC) given to children with OI who had previously undergone BMT resulted in marked growth acceleration in a subset of patients, unambiguously indicating the therapeutic potential of bone marrow cells for these patients. Then, in a murine model of OI, we demonstrated that as the donor NABMCs differentiate to osteoblasts, they contribute normal collagen to the bone matrix. In contrast, MSCs do not substantially engraft in bone, but secrete a soluble mediator that indirectly stimulates growth, data which provide the underlying mechanism of our prior clinical trial of MSC therapy for children with OI. Collectively, our data indicate that both NABMCs and MSCs constitute effective cell therapy for OI, but exert their clinical impact by different, complementary mechanisms. The study is registered at www.clinicaltrials.gov as NCT00187018.

IGF-1-mediated osteoblastic niche expansion enhances long-term hematopoietic stem cell engraftment after murine bone marrow transplantation.

The efficiency of hematopoietic stem cell (HSC) engraftment after bone marrow (BM) transplantation depends largely on the capacity of the marrow microenvironment to accept the transplanted cells. While radioablation of BM damages osteoblastic stem cell niches, little is known about their restoration and mechanisms governing their receptivity to engraft transplanted HSCs. We previously reported rapid restoration and profound expansion of the marrow endosteal microenvironment in response to marrow radioablation. Here, we show that this reorganization represents proliferation of mature endosteal osteoblasts which seem to arise from a small subset of high-proliferative, relatively radio-resistant endosteal cells. Multiple layers of osteoblasts form along the endosteal surface within 48 hours after total body irradiation, concomitant with a peak in marrow cytokine expression. This niche reorganization fosters homing of the transplanted hematopoietic cells to the host marrow space and engraftment of long-term-HSC. Inhibition of insulin-like growth factor (IGF)-1-receptor tyrosine kinase signaling abrogates endosteal osteoblast proliferation and donor HSC engraftment, suggesting that the cytokine IGF-1 is a crucial mediator of endosteal niche reorganization and consequently donor HSC engraftment. Further understanding of this novel mechanism of IGF-1-dependent osteoblastic niche expansion and HSC engraftment may yield clinical applications for improving engraftment efficiency after clinical HSC transplantation.

Transplanted murine long-term repopulating hematopoietic cells can differentiate to osteoblasts in the marrow stem cell niche.

Bone marrow transplantation (BMT) can give rise to donor-derived osteopoiesis in mice and humans; however, the source of this activity, whether a primitive osteoprogenitor or a transplantable marrow cell with dual hematopoietic and osteogenic potential, has eluded detection. To address this issue, we fractionated whole BM from mice according to cell surface immunophenotype and assayed the hematopoietic and osteopoietic potentials of the transplanted cells. Here, we show that a donor marrow cell capable of robust osteopoiesis possesses a surface phenotype of c-Kit(+) Lin(-) Sca-1(+) CD34(-/lo), identical to that of the long-term repopulating hematopoietic stem cell (LTR-HSC). Secondary BMT studies demonstrated that a single marrow cell able to contribute to hematopoietic reconstitution in primary recipients also drives robust osteopoiesis and LT hematopoiesis in secondary recipients. These findings indicate that LTR-HSC can give rise to progeny that differentiate to osteoblasts after BMT, suggesting a mechanism for prompt restoration of the osteoblastic HSC niche following BM injury, such as that induced by clinical BMT preparative regimens. An understanding of the mechanisms that regulate this differentiation potential may lead to novel treatments for disorders of bone as well as methods for preserving the integrity of endosteal hematopoietic niches.