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Tumor necrosis factor-alpha (TNF-alpha) is a bifunctional regulator of hematopoiesis, and its cellular responses are mediated by two distinct cell surface receptors. TNF-alpha generally inhibits the growth of primitive murine hematopoietic progenitor cells (Lin-Scal+) in response to multiple cytokine combinations, and the p75 TNF receptor is essential in signaling such inhibition. In the present study we show the reverse phenomenon in that TNF-alpha on the same progenitor cell population in combination with stem cell factor (SCF) and interleukin-7 (IL-7) through the p55 TNF receptor can recruit additional progenitors to proliferate. In contrast, TGF-beta 1, another bifunctional regulator of hematopoietic progenitor cell growth, completely blocked SCF plus IL- 7-induced proliferation. TNF-alpha increased the number of responding progenitors, as well as the size of the colonies formed. The synergistic effects of TNF-alpha were seen at the single cell level, suggesting that its effects are directly mediated. Finally, whereas SCF plus IL-7 promoted primarily granulopoiesis, the addition of TNF-alpha switched the differentiation toward the production of almost exclusively macrophages.

Recombinant human stem cell factor (rhSCF) and recombinant human granulocyte colony-stimulating factor (rhG-CSF) are synergistic in vitro in stimulating the proliferation of hematopoietic progenitor cells and their precursors. We examined the in vivo synergy of rhSCF with rhG-CSF for stimulating hematopoiesis in vivo in baboons. Administration of low-dose (LD) rhSCF (25 micrograms/kg) alone did not stimulate changes in circulating WBCs. In comparison, administration of LD rhSCF in combination with rhG-CSF at 10 micrograms/kg or 100 micrograms/kg stimulated increases in circulating WBCs of multiple types up to twofold higher than was stimulated by administration of the same dose of rhG-CSF alone. When the dose of rhG-CSF is increased to 250 micrograms/kg, the administration of LD rhSCF does not further increase the circulating WBC counts. Administration of LD rhSCF in combination with rhG-CSF also stimulated increased circulation of hematopoietic progenitors. LD rhSCF alone stimulated less of an increase in circulating progenitors, per milliliter of blood, than did administration of rhG-CSF alone at 100 micrograms/kg. Baboons administered LD rhSCF together with rhG-CSF at 10, 100, or 250 micrograms/kg had 3.5- to 16-fold higher numbers per milliliter of blood of progenitors cells of multiple types, including colony-forming units granulocyte/macrophage (CFU-GM), burst-forming unit-erythroid (BFU-E), and colony-forming and burst-forming units-megakaryocyte (CFU- MK and BFU-MK) compared with animals given the same dose of rhG-CSF without rhSCF, regardless of the rhG-CSF dose. The increased circulation of progenitor cells stimulated by the combination of rhSCF plus rhG-CSF was not necessarily directly related to the increase in WBCs, as this effect on peripheral blood progenitors was observed even at an rhG-CSF dose of 250 micrograms/kg, where coadministration of LD rhSCF did not further increase WBC counts. Administration of very-low- dose rhSCF (2.5 micrograms/kg) with rhG-CSF, 10 micrograms/kg, did not stimulate increases in circulating WBCs, but did increase the number of megakaryocyte progenitor cells in blood compared with rhG-CSF alone. LD rhSCF administered alone for 7 days before rhG-CSF did not result in increased levels of circulating WBCs or progenitors compared with rhG- CSF alone. Thus, the synergistic effects of rhSCF with rhG-CSF were both dose- and time-dependent. The doses of rhSCF used in these studies have been tolerated in vivo in humans.(ABSTRACT TRUNCATED AT 400 WORDS).

Recombinant human stem cell factor (rhSCF) and recombinant human granulocyte colony-stimulating factor (rhG-CSF) are synergistic in vitro in stimulating the proliferation of hematopoietic progenitor cells and their precursors. We examined the in vivo synergy of rhSCF with rhG-CSF for stimulating hematopoiesis in vivo in baboons. Administration of low-dose (LD) rhSCF (25 micrograms/kg) alone did not stimulate changes in circulating WBCs. In comparison, administration of LD rhSCF in combination with rhG-CSF at 10 micrograms/kg or 100 micrograms/kg stimulated increases in circulating WBCs of multiple types up to twofold higher than was stimulated by administration of the same dose of rhG-CSF alone. When the dose of rhG-CSF is increased to 250 micrograms/kg, the administration of LD rhSCF does not further increase the circulating WBC counts. Administration of LD rhSCF in combination with rhG-CSF also stimulated increased circulation of hematopoietic progenitors. LD rhSCF alone stimulated less of an increase in circulating progenitors, per milliliter of blood, than did administration of rhG-CSF alone at 100 micrograms/kg. Baboons administered LD rhSCF together with rhG-CSF at 10, 100, or 250 micrograms/kg had 3.5- to 16-fold higher numbers per milliliter of blood of progenitors cells of multiple types, including colony-forming units granulocyte/macrophage (CFU-GM), burst-forming unit-erythroid (BFU-E), and colony-forming and burst-forming units-megakaryocyte (CFU- MK and BFU-MK) compared with animals given the same dose of rhG-CSF without rhSCF, regardless of the rhG-CSF dose. The increased circulation of progenitor cells stimulated by the combination of rhSCF plus rhG-CSF was not necessarily directly related to the increase in WBCs, as this effect on peripheral blood progenitors was observed even at an rhG-CSF dose of 250 micrograms/kg, where coadministration of LD rhSCF did not further increase WBC counts. Administration of very-low- dose rhSCF (2.5 micrograms/kg) with rhG-CSF, 10 micrograms/kg, did not stimulate increases in circulating WBCs, but did increase the number of megakaryocyte progenitor cells in blood compared with rhG-CSF alone. LD rhSCF administered alone for 7 days before rhG-CSF did not result in increased levels of circulating WBCs or progenitors compared with rhG- CSF alone. Thus, the synergistic effects of rhSCF with rhG-CSF were both dose- and time-dependent. The doses of rhSCF used in these studies have been tolerated in vivo in humans.(ABSTRACT TRUNCATED AT 400 WORDS).

In this study, we have compared the ability of recombinant human granulocyte colony-stimulating factor (rhG-CSF) alone and the combination of low doses of recombinant rat pegylated stem cell factor (rrSCF-PEG) plus rhG-CSF to mobilize peripheral blood progenitor cells (PBPCs) with long-term engrafting potential. Female recipient irradiated mice were transplanted with PBPCs from male mice that were mobilized with rhG-CSF alone (group A) or rrSCF-PEG plus rhG-CSF (group B). As previously shown, greater short-term survival resulted in group B compared with group A, with 80% and 40% survival at 30 days posttransplant, respectively. Both groups of animals showed long-term donor-derived engraftment in greater than 95% of animals, as determined by quantitative specific polymerase chain reaction amplification of a Y chromosome sequence from whole blood of the mice at 6 to 12 months posttransplantation. Analysis of individual granulocyte-macrophage colonies, picked up from semisolid methylcellulose culture of bone marrow cells from transplanted mice, resulted in detection of donor- derived DNA in 98% of colonies from group B mice compared with 81% from group A mice. These data show that cells with long-term potential are mobilized by rhG-CSF alone and the combination of rrSCF-PEG plus rhG- CSF. Furthermore, an increased number of cells with short-term and long- term engraftment potential was obtained with rrSCF-PEG plus rhG-CSF compared with rhG-CSF alone.

The effects of recombinant canine granulocyte colony-stimulating factor (rcG-CSF) and recombinant canine stem cell factor (rcSCF), a c-kit ligand, on the circulation of hematopoietic progenitor and stem cells were studied in a canine model. Administration of rcG-CSF (10 micrograms/kg) for 7 days led to a 5.4-fold increase in CFU-GM/mL of blood, while 7 days of rcSCF (200 micrograms/kg) led to an 8.2-fold increase. Although treatment with low-dose rcSCF (25 micrograms/kg) had no effect on the level of peripheral blood progenitors, 7-day exposure to a combination of G-CSF plus low dose SCF led to a 21.6-fold increase (P = .03). To assess the ability of these factors to increase the circulation of cells capable of rescuing animals after lethal total body irradiation (TBI), 1 x 10(8) peripheral blood mononuclear cells (PBMC)/kg were collected and cryopreserved from animals after 7 days of treatment with G-CSF, SCF or a combination of the two. One month later, animals were exposed to 9.2 Gy TBI and transplanted with the previously collected cells. Control animals transplanted with 1 x 10(8) PBMC/kg collected without pretreatment died with marrow aplasia 11 to 29 days after TBI as did animals treated with only low-dose SCF before cell collection. In contrast, all animals given PBMC collected after G-CSF, high-dose SCF, or a combination of G-CSF plus low-dose SCF recovered granulocyte function. Recovery to 500 granulocytes/microL after transplant took 17, 18.8, and 13.6 days, respectively, (P = .056 for the difference between the combination G-CSF-SCF group and the other two groups). In both the G-CSF and SCF groups, 4 of 5 animals completely recovered while 1 of 5 in each group died with prolonged thrombocytopenia. In the combination group, all 5 animals became long- term survivors. These studies demonstrate that both G-CSF and SCF dramatically increase the level of peripheral blood hematopoietic progenitor and stem cells and support the view that these factors can act synergistically.

Human bone marrow-derived mesenchymal stem cell (MSC)-based therapy holds great promise as a new approach for cardiovascular regeneration. In heart failure, there is activation of the renin-angiotensin system and increased formation of angiotensin (Ang) II. We tested the hypothesis that Ang II impairs endothelial differentiation and vasculogenic capacity of human MSCs. MSCs were cultured in endothelial growth media (EGM) and treated with vehicle, 1µM Ang II, or 10µM Ang II for 7 days to assess by real time RT-PCR the expression of two markers of endothelial differentiation, platelet endothelial cell adhesion molecule (PECAM) and von Willebrand factor (vWF). Expression of PECAM and vWF increased similarly in each treatment group at day 7, suggesting endothelial differentiation of MSCs. To assess vasculogenesis, MSCs were cultured in EGM for 7 days and then plated on growth factor reduced Matrigel for 5 hours under the following conditions: vehicle, Ang II (1µM), Ang II + type 1 receptor (AT1R) inhibitor (10µM losartan), or Ang II + type 2 receptor (AT2R) inhibitor (0.1µM PD123177). The vascular index (VI) was determined by multiplying the average number of endothelial tubes formed by the average tube length. MSCs treated for 5 hours with 1µM Ang II exhibited a reduced VI (84.5 ± 2.6% of vehicle treated cells, P

During mouse embryogenesis, hematopoietic development takes place in several distinct anatomic locations. The microenvironment of different hematopoietic organs plays an important role in the proliferation and maturation of the hematopoietic cells. We hypothesized that fetal stromal cells would be distinct to adult bone marrow (BM)-derived stromal cells because the BM contributes mainly to the homeostasis of hematopoietic stem cells (HSCs), while extensive expansion of HSCs occurs during fetal development. Here we report the establishment of stromal cell lines from fetal hematopoietic organs, namely aorta-gonad-mesonephros (AGM), midgestation placenta (PL), and fetal liver (FL) together with adult bone marrow (BM). The growth patterns and hematopoietic supportive potential were studied. Their phenotypic and molecular gene expression profiles were also determined. Stromal cell lines from each tissue were able to support cobblestone area formation of BM c-Kit+Sca-1+ hematopoietic cells: 22 (22/47) from AGM, three (3/4) from PL, three (3/4) from FL, and three (3/3) from BM. There were similar levels of expansion of total mononuclear cells (TMNs) when HSCs were cocultured with fetal stroma and adult BM stroma. However, PL-derived stromal cells supported higher levels of generation of colony-forming progenitor cell (CFU-C), indicated by more colonies and colonies with significantly larger size. Flow cytometric analysis of the PL1 cells demonstrated a phenotype of CD45-, CD105+, Sca-1+, CD34+, and CD49d+, compared to adult BM1 cells, which were CD45-, CD105+, Sca-1+, CD34-, and CD49d-. Using Affymetrix microarray analysis, we identified that genes specifically express in endothelial cells, such as Tie1, Tek, Kdr, Flt4, Emcn, Pecam1, Icam2, Cdh5, Esam1, Prom1, Cd34, and Sele were highly expressed in stroma PL1, consistent with an endothelial phenotype, while BM1 expressed a mesenchymal stromal phenotype. In summary, these data demonstrate distinct characteristics of stromal cells that provide insights into the microenvironmental control of HSCs.

The use of cytokine mobilized peripheral blood progenitor cells (PBPC) in transplantation following chemotherapy has led to enhanced engraftment. Granulocyte-colony stimulating factor (G-CSF) has been shown in a number of clinical studies to be an effective mobilizer of PBPC. Preclinical data in mice and primates have demonstrated a potential role for the use of stem cell factor (SCF) in mobilization of PBPC. In the studies presented here, low doses of SCF are shown to synergize with optimal doses of G-CSF to enhance the number and quality of PBPC compared to G-CSF alone. Phase I studies using r-metHuSCF demonstrated mast cell-related dose limiting effects. The data presented here have led to Phase I/II studies to evaluate the potential use of low doses of SCF in combination with G-CSF for mobilization of PBPC.

Splenectomized mice treated for 7 days with pegylated recombinant rat stem cell factor (rrSCF-PEG) showed a dose-dependent increase in peripheral blood progenitor cells (PBPC) that have enhanced in vivo repopulating potential. A dose of rrSCF-PEG at 25 micrograms/kg/d for 7 days produced no significant increase in PBPC. However, when this dose of rrSCF-PEG was combined with an optimal dose of recombinant human granulocyte colony-stimulating factor (rhG-CSF; 200 micrograms/kg/d), a synergistic increase in PBPC was observed. Compared with treatment with rhG-CSF alone, the combination of rrSCF-PEG plus rhG-CSF resulted in a synergistic increase in peripheral white blood cells, in the incidence and absolute numbers of PBPC, and in the incidence and absolute numbers of circulating cells with in vivo repopulating potential. These data suggest that low doses of SCF, which would have minimal, if any, effects in vivo, can synergize with optimal doses of rhG-CSF to enhance the mobilization of PBPC stimulated by rhG-CSF alone.

Splenectomized mice treated for 7 days with pegylated recombinant rat stem cell factor (rrSCF-PEG) showed a dose-dependent increase in peripheral blood progenitor cells (PBPC) that have enhanced in vivo repopulating potential. A dose of rrSCF-PEG at 25 micrograms/kg/d for 7 days produced no significant increase in PBPC. However, when this dose of rrSCF-PEG was combined with an optimal dose of recombinant human granulocyte colony-stimulating factor (rhG-CSF; 200 micrograms/kg/d), a synergistic increase in PBPC was observed. Compared with treatment with rhG-CSF alone, the combination of rrSCF-PEG plus rhG-CSF resulted in a synergistic increase in peripheral white blood cells, in the incidence and absolute numbers of PBPC, and in the incidence and absolute numbers of circulating cells with in vivo repopulating potential. These data suggest that low doses of SCF, which would have minimal, if any, effects in vivo, can synergize with optimal doses of rhG-CSF to enhance the mobilization of PBPC stimulated by rhG-CSF alone.

A growing number of in vitro studies suggest that recombinant human stem cell factor (SCF) is capable of augmenting the proliferative capacity of human hematopoietic progenitor cells (HPC) and stem cells (HSC). We further evaluated this biologic effect by analyzing the response of bone marrow (BM) HPCs and HSCs to the administration of SCF in eight patients with locally advanced or metastatic breast cancer who were enrolled in an ongoing phase I study. SCF was administered for 14 days by daily subcutaneous injection at dosages of 10, 25, or 50 micrograms/kg/d. BM CD34+ HLA-DR+ and CD34+ HLA-DR- CD15- cells, previously shown by our laboratory to be enriched for various classes of differentiated and primitive HPCs, respectively, were quantitated in BM samples on day 0 (pretreatment) and day 15 (posttreatment). These CD34+ HLA-DR+ and CD34+ HLA-DR- CD15- cells were then isolated by cell- sorting and assayed for several classes of HPCs, including the high-- proliferative potential colony-forming cell (HPP-CFC), the burst- forming unit--megakaryocyte (BFU-MK), and the long-term BM culture-- initiating cell (LTBMC-IC). SCF administration resulted in a 3.3-fold (range, 1.4- to 18.8-fold; P = .018) increase in the absolute numbers of CD34+ cells, a 3.7-fold (range, 1.2- to 8.2-fold; P = .028) increase in the absolute numbers of CD34+ HLA-DR+ cells, and a 2.4-fold (range, 1.1- to 29.3-fold; P = .010) increase in the absolute numbers of CD34+ HLA-DR- CD15- cells. Following the infusion of SCF, a statistically significant increase in the absolute numbers of HPP-CFC (P = .018), BFU- MK (P = .046), CFU-granulocyte, erythrocyte, monocyte, megakaryocyte (CFU-GEMM: P = .043), BFU-erythrocyte (BFU-E; P = .043), CFU- granulocyte, macrophage (CFU-GM; P = .045), and CFU-megakaryocyte (CFU- MK; P = .028) per milliliter of marrow was observed. Stromal cell-free LTBMCs supplemented with SCF and interleukin-3 (IL-3), initiated with CD34+ HLA-DR- CD15- cells obtained on day 0, produced viable cells for 9.6 weeks, compared with 11.5 weeks for LTBMCs initiated with CD34+ HLA- DR- CD15- cells obtained on day 15. Cumulative cellular production by LTBMCs initiated with day 15 CD34+ HLA-DR- CD15- cells was statistically greater than that by day 0 LTBMCs (P = .031). These same cultures produced CFU-GM for 6.3 weeks (day 0) versus 9 weeks (day 15).(ABSTRACT TRUNCATED AT 400 WORDS)

A growing number of in vitro studies suggest that recombinant human stem cell factor (SCF) is capable of augmenting the proliferative capacity of human hematopoietic progenitor cells (HPC) and stem cells (HSC). We further evaluated this biologic effect by analyzing the response of bone marrow (BM) HPCs and HSCs to the administration of SCF in eight patients with locally advanced or metastatic breast cancer who were enrolled in an ongoing phase I study. SCF was administered for 14 days by daily subcutaneous injection at dosages of 10, 25, or 50 micrograms/kg/d. BM CD34+ HLA-DR+ and CD34+ HLA-DR- CD15- cells, previously shown by our laboratory to be enriched for various classes of differentiated and primitive HPCs, respectively, were quantitated in BM samples on day 0 (pretreatment) and day 15 (posttreatment). These CD34+ HLA-DR+ and CD34+ HLA-DR- CD15- cells were then isolated by cell- sorting and assayed for several classes of HPCs, including the high-- proliferative potential colony-forming cell (HPP-CFC), the burst- forming unit--megakaryocyte (BFU-MK), and the long-term BM culture-- initiating cell (LTBMC-IC). SCF administration resulted in a 3.3-fold (range, 1.4- to 18.8-fold; P = .018) increase in the absolute numbers of CD34+ cells, a 3.7-fold (range, 1.2- to 8.2-fold; P = .028) increase in the absolute numbers of CD34+ HLA-DR+ cells, and a 2.4-fold (range, 1.1- to 29.3-fold; P = .010) increase in the absolute numbers of CD34+ HLA-DR- CD15- cells. Following the infusion of SCF, a statistically significant increase in the absolute numbers of HPP-CFC (P = .018), BFU- MK (P = .046), CFU-granulocyte, erythrocyte, monocyte, megakaryocyte (CFU-GEMM: P = .043), BFU-erythrocyte (BFU-E; P = .043), CFU- granulocyte, macrophage (CFU-GM; P = .045), and CFU-megakaryocyte (CFU- MK; P = .028) per milliliter of marrow was observed. Stromal cell-free LTBMCs supplemented with SCF and interleukin-3 (IL-3), initiated with CD34+ HLA-DR- CD15- cells obtained on day 0, produced viable cells for 9.6 weeks, compared with 11.5 weeks for LTBMCs initiated with CD34+ HLA- DR- CD15- cells obtained on day 15. Cumulative cellular production by LTBMCs initiated with day 15 CD34+ HLA-DR- CD15- cells was statistically greater than that by day 0 LTBMCs (P = .031). These same cultures produced CFU-GM for 6.3 weeks (day 0) versus 9 weeks (day 15).(ABSTRACT TRUNCATED AT 400 WORDS)

Tissue homeostasis is dependent on replacement of damaged and aged cells, and stem cells are the primary source of new cell generation. Most tissue and organs of the body contain stromal cells that are part of the stem cell niche and are essential for the maintenance, proliferation and differentiation of stem cells. Hematopoiesis is a well-characterized tissue system in which interactions between hematopoietic stem cells (HSCs) and stromal cells in the bone marrow (BM) have been clearly established (1e3). The BM environment is subject to complex regulation that is critical for hematopoiesis. Many studies have demonstrated the production of both stimulating and inhibitory factors by BM stromal cells (mesenchymal stromal cells, MSCs) (4e6). However, the full potential of MSCs and their role in control of the HSC proliferation and differentiation remains unclear. Critical to sustained hematopoiesis is themaintenance of stem cell quiescence. Without a niche in which a stem cell pool can reside unstimulated by proliferative and differentiation signals, hematopoiesis cannot be sustained life-long. There is evidence that HSC quiescence is at least in part regulated by direct contact between HSCs and MSCs. This contact is mediated through chemokine receptor 4 (CXCR4) on HSCs engaging stromal cellederived factor (SDF)-1 on MSCs (7). With the advent of plerixafor—a CXCR4 antagonist used to mobilize HSCs for stem cell transplantation—it is pertinent to ask how this agent interferes with HSC/MSC interactions. In this issue of Cytotherapy, Ludwig et al. (8) cocultured HSCs and MSCs with and without plerixafor, studying cell division kinetics and stem cell surface marker changes to explore the relationship between HSCs and MSCs and the role of the CXCR4/SDF-1 axis. They found that co-culture of

In 1988, Gluckman and colleagues performed the first successful transplantation of umbilical cord blood (CB) cells.1 The patient was suffering from Fanconi’s anemia, complicated by severe aplastic anemia. The mother was shown to be carrying a sibling that was unaffected by the disease and was human leukocyte antigen (HLA) identical to the patient. The CB was collected and stored frozen in liquid nitrogen. After conditioning with low-dose cyclophosphamide and thoraco-abdominal irradiation, the patient received the thawed CB product. Twenty years later this patient is alive and cured of the anemia.

Regenerative medicines offer the potential for treatment and possibly cure of debilitating diseases including heart disease, diabetes, Parkinson’s disease and liver failure. Approaches using stem cells from various sources are in pre clinical and clinical testing. The goal of these studies is to deliver cellular products capable of replacing damaged tissue and/or cells. However, the balance between cellular proliferation and differentiation is a carefully controlled process involving a range of growth factors and cytokines produced in large part by tissue stromal cells. These stromal cells make up the tissue microenvironment and appear to be essential for normal homeostasis. We hypothesize that tissue damage in many instances involves damage to the microenvironment resulting in a lack of signals through growth factor networks necessary to maintain survival and proliferation of tissue specific stem cells and progenitor cells. Therefore, optimal repair of disease tissue must account for the damage to the stromal environment. We propose that optimal cellular therapies for regenerative medicine will require combination cellular products consisting of a stromal cell population to reconstitute the microenvironment and to support the survival, proliferation and differentiation of the tissue specific stem cells or progenitor cells.

Stem cell therapy for repair of damaged cardiac tissue is an attractive option to improve the health of the growing number of heart failure patients. Mesenchymal stem cells (MSCs) possess unique properties that may make them a better option for cardiac repair than other cell types. Unlike other adult stem cells, they appear to escape allorecognition by the immune system and they have immune-modulating properties, thus making it possible to consider them for use as an allogeneic cell therapy product. There is a large and growing body of preclinical and early clinical experience with MSC therapy that shows great promise in realizing the potential of stem cell therapy to effect repair of damaged cardiac tissue. This review discusses the mechanism of action of MSC therapy and summarizes the current literature in the field.

Stem cell therapy for repair of damaged cardiac tissue is an attractive option to improve the health of the growing number of heart failure patients. Mesenchymal stem cells (MSCs) possess unique properties that may make them a better option for cardiac repair than other cell types. Unlike other adult stem cells, they appear to escape allorecognition by the immune system and they have immune-modulating properties, thus making it possible to consider them for use as an allogeneic cell therapy product. There is a large and growing body of preclinical and early clinical experience with MSC therapy that shows great promise in realizing the potential of stem cell therapy to effect repair of damaged cardiac tissue. This review discusses the mechanism of action of MSC therapy and summarizes the current literature in the field.

Rationale: The regenerative potential of the heart is insufficient to fully restore functioning myocardium after injury, motivating the quest for a cell-based replacement strategy. Bone marrow–derived mesenchymal stem cells (MSCs) have the capacity for cardiac repair that appears to exceed their capacity for differentiation into cardiac myocytes. Objective: Here, we test the hypothesis that bone marrow derived MSCs stimulate the proliferation and differentiation of endogenous cardiac stem cells (CSCs) as part of their regenerative repertoire. Methods And Results: Female Yorkshire pigs (n=31) underwent experimental myocardial infarction (MI), and 3 days later, received transendocardial injections of allogeneic male bone marrow–derived MSCs, MSC concentrated conditioned medium (CCM), or placebo (Plasmalyte). A no-injection control group was also studied. MSCs engrafted and differentiated into cardiomyocytes and vascular structures. In addition, endogenous c-kit + CSCs increased 20-fold in MSC-treated animals versus controls ( P + CSCs in MSC versus control ( P P =0.005). Porcine endomyocardial biopsies were harvested and plated as organotypic cultures in the presence or absence of MSC feeder layers. In vitro, MSCs stimulated c-kit + CSCs proliferation into enriched populations of adult cardioblasts that expressed Nkx2–5 and troponin I. Conclusions: MSCs stimulate host CSCs, a new mechanism of action underlying successful cell-based therapeutics.