Mesenchymal stem cells (MSC) are under investigation as a therapy for a variety of disorders. Although animal models show long term regenerative and immunomodulatory effects of MSC, the fate of MSC after infusion remains to be elucidated. In the present study the localization and viability of MSC was examined by isolation and re-culture of intravenously infused MSC. C57BL/6 MSC (500,000) constitutively expressing DsRed-fluorescent protein and radioactively labeled with Cr-51 were infused via the tail vein in wild-type C57BL/6 mice. After 5 min, 1, 24, or 72 h, mice were sacrificed and blood, lungs, liver, spleen, kidneys, and bone marrow removed. One hour after MSC infusion the majority of Cr-51 was found in the lungs, whereas after 24 h Cr-51 was mainly found in the liver. Tissue cultures demonstrated that viable donor MSC were present in the lungs up to 24 h after infusion, after which they disappeared. No viable MSC were found in the other organs examined at any time. The induction of ischemia-reperfusion injury in the liver did not trigger the migration of viable MSC to the liver. These results demonstrate that MSC are short-lived after i.v. infusion and that viable MSC do not pass the lungs. Cell debris may be transported to the liver. Long term immunomodulatory and regenerative effects of infused MSC must therefore be mediated via other cell types.

The present data confirm previous reports suggesting that MSCs have immunomodulatory properties in vitro. However, their tolerogenic properties in vivo must be questioned, as MSC injections were not only ineffective at prolonging allograft survival, but tended to promote rejection.

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Purpose: Inhibitors of heat-shock protein 90 (Hsp90) may interfere with oncogenic signaling pathways, including Erk, Akt, and hypoxia-inducible factor-1a (HIF-1a). Because insulin-like growth factor-I receptor (IGF-IR) and signal transducer and activator of transcription 3 (STAT3) signaling pathways are implicated in the progression of pancreatic cancer, we hypothesized that blocking Hsp90 with geldanamycin derivates [17-allylamino-geldanamycin (17-AAG), 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG)] would impair IGF-I^and interleukin-6^mediated signaling and thus reduce pancreatic tumor growth and angiogenesis in vivo. Experimental Design: Human pancreatic cancer cells (HPAF-II, L3.6pl) were used for experiments. Changes in signaling pathway activation upon Hsp90 blockade were investigated by Western blotting. Effects of Hsp90 inhibition (17-AAG) on vascular endothelial growth factor were determined by ELISA and real-time PCR. Effects of 17-DMAG (25 mg/kg; thrice a week; i.p.) on tumor growth and vascularization were investigated in a s.c. xenograft model and in an orthotopic model of pancreatic cancer. Results: 17-AAG inhibited IGF-IR signaling by down-regulating IGF-IRh and directly impairing IGF-IR phosphorylation. Hypoxia-and IL-6^mediated activation of HIF-1a or STAT3/STAT5 were substantially inhibited by 17-AAG. Moreover, a novel IL-6/STAT3/HIF-1a autocrine loop was effectively disrupted by Hsp90 blockade. In vivo, 17-DMAG significantly reduced s.c. tumor growth and diminished STAT3 phosphorylation and IGF-IRh expression in tumor tissues. In an orthotopic model, pancreatic tumor growth and vascularization were both significantly reduced upon Hsp90 inhibition, as reflected by final tumor weights and CD31 staining, respectively. Conclusions: Blocking Hsp90 disrupts IGF-I and IL-6^induced proangiogenic signaling cascades by targeting IGF-IR and STAT3 in pancreatic cancer, leading to significant growth-inhibitory effects.Therefore, we suggest that Hsp90 inhibitors could prove to be valuable in the treatment of pancreatic cancer.

The ultimate goal in transplantation medicine is the promotion of operational tolerance. Although Th cells of the Th17 type have been predominantly associated with rejection of allogeneic solid organ grafts, regulatory T (Treg) cells appear to foster operational tolerance. Induced Treg and Th17 cells have a higher lineage plasticity than has been recognized thus far. We found that when mesenchymal stem cells (MSCs) were used to induce long-term acceptance of allogeneic heart grafts in mice, the induction of Treg cells was preceded by development of a CD11bhiGr1int myeloid–derived immunosuppressive cell–mediated Th17 response. Substantial suppression of Foxp3+ Treg cell generation from retinoic acid receptor–related orphan receptor γ−/− T cells by MSCs revealed that retinoic acid receptor–related orphan receptor γ is a common factor in the differentiation of Treg and Th17 cells. Immunosuppressant mycophenolate mofetil treatment of enriched IL-17A+ cells from MSC-primed allograft mouse recipients resulted in a reduction of IL-17A production and an increase in the Foxp3+ Treg cell fraction. Furthermore, identification of IL-17A+ Foxp3+ double-positive and ex–IL-17–producing IL-17AnegFoxp3+ T cells strongly argues for direct conversion of Th17 cells into Treg cells as the underlying mechanism of immune regulation in MSC-mediated allograft survival. The Th17 into Treg conversion identified in this study constitutes an important immunological mechanism by which MSC-induced myeloid-derived immunosuppressive cells mediate operational transplant tolerance. The possibility to create Treg cell–regulated operational tolerance in the absence of complete immune suppression provides strong clinical implications for cell therapy–assisted minimization protocols.

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Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal malignancies. Tumor-derived extracellular vesicles (EVs) induce pre-metastatic niche formation to promote metastasis. We isolated EVs from a highly-metastatic pancreatic cancer cell line and patient-derived primary cancer cells by ultracentrifugation. The protein content of EVs was analyzed by mass spectrometry. The effects of PDAC-derived EVs on natural kill (NK) cells were investigated by flow cytometry. The serum EVs’ TGF-β1 levels were quantified by ELISA. We found that integrins were enriched in PDAC-derived EVs. The expression of NKG2D, CD107a, TNF-α, and INF-γ in NK cells was significantly downregulated after co-culture with EVs. NK cells also exhibited decreased levels of CD71 and CD98, as well as impaired glucose uptake ability. In addition, NK cell cytotoxicity against pancreatic cancer stem cells was attenuated. Moreover, PDAC-derived EVs induced the phosphorylation of Smad2/3 in NK cells. Serum EVs’ TGF-β1 was significantly increased in PDAC patients. Our findings emphasize the immunosuppressive role of PDAC-derived EVs and provide new insights into our understanding of NK cell dysfunction regarding pre-metastatic niche formation in PDAC.