Mesenchymal stromal cell-derived extracellular vesicles rescue radiation damage to murine marrow hematopoietic cells

Mesenchymal stromal cells (MSCs) have been shown to reverse radiation damage to marrow stem cells. We have evaluated the capacity of MSC-derived extracellular vesicles (MSC-EVs) to mitigate radiation injury to marrow stem cells at 4 h to 7 days after irradiation. Significant restoration of marrow stem cell engraftment at 4, 24 and 168 h post irradiation by exposure to MSC-EVs was observed at 3 weeks to 9 months after transplant and further confirmed by secondary engraftment. Intravenous injection of MSC-EVs to 500cGy exposed mice led to partial recovery of peripheral blood counts and restoration of the engraftment of marrow. The murine hematopoietic cell line, FDC-P1 exposed to 500cGy, showed reversal of growth inhibition, DNA damage and apoptosis on exposure to murine or human MSC-EVs. Both murine and human MSC-EVs reverse radiation damage to murine marrow cells and stimulate normal murine marrow stem cell/progenitors to proliferate. A preparation with both exosomes and microvesicles was found to be superior to either microvesicles or exosomes alone. Biologic activity was seen in freshly isolated vesicles and in vesicles stored for up to 6 months in 10% dimethyl sulfoxide at -80 °C. These studies indicate that MSC-EVs can reverse radiation damage to bone marrow stem cells.

Microvesicular-mediated gene transfer of prostate tumor markers

e16076

Background: Microvesicles have been a subject of research for many years. Recent work has focused on the potential for cancer vaccines via microvesicles. It has also been demonstrated that various cell-specific phenotypes can be transferred from one cell type to another through microvesicle transfer. Studies in our laboratory have demonstrated that co-culture of murine lung tissue with marrow cells across a cell impermeable membrane can induce elevations in lung-specific mRNA expression in human donor marrow stem cells. Our objective is to determine whether there is transfer of genetic or transcriptional factors via microvesicles from human prostate cancer cells to fresh human marrow cells. Methods: Fresh prostate tissue was harvested from surgical specimens following radical retropubic prostatectomy. Samples were histologically confirmed to contain prostatic adenocarcinoma. Co-cultures were established using a transwell system in which 0.05–0.100 grams of prostate tissue was minced and co-cultured with 1–3 million normal, human donor marrow cells for 2–7 days. Marrow not co-cultured with tumorserved as a control. Target cells were collected and total RNA was analyzed for prostate-specific gene expression byReal Time RT-PCR. Fold differences in expression of the genes were analyzed, using TaqMan®, gene assays (Applied Biosystems) and were expressed in relation to the marrow control. Results: We have observed significant increases in gene expression in marrow cells co-cultured with prostate tumor cells (Gleason grades 6–9). Variable increases in expression were seen in 3 patient samples, as high as 7-fold for ERG, greater than 10-fold for ACPP and greater than 100-fold for STEAP, PART, TMPRSS2, PSCA and ETV1. Conclusions: These studies demonstrate that prostate specific genes are present in fresh human marrow cells after co-culture with tumor tissue. This establishes a base to begin evaluating the significance of microvesicle-mediated genetic transfer, mechanisms of transfer and therapeutic options for blocking or manipulating such transfer to influence the disease process.

No significant financial relationships to disclose.

Marrow cell genetic phenotype change induced by human lung cancer cells

11108

Background: Murine lung-derived microvesicles are capable of inducing lung-specific mRNA in marrow cells, when co-cultured across from these cells, but separated from them by a cell-impermeable (0.4 micron) membrane. These converted murine marrow cells showed mRNA elevations, lung-specific protein production and enhanced capacity to convert to lung epithelial cells after in vivo transplantation into irradiated mice. We examine here whether fresh tissue from lung cancer patients would have the same capacity to genetically alter co-cultured human marrow cells. Methods: Lung cancer samples were collected from 5 patients undergoing surgery. Minced tumor tissue at 50–100 mg was co-cultured in a semi-permeable culture plate insert opposite 3.0 ×106 human marrow cells. The marrow cells were harvested after 2–7 days of co-culture. Marrow cell RNA was analyzed for lung specific mRNA using real time RT-PCR. Relative levels of gene expression was expressed a fold increase compared to level in controls. Results: Lung cancers studied were adenocarcinoma, endobronchial alveolar carcinoma, bronchioloalveolar carcinoma, non-small cell carcinoma and squamous cell carcinoma. mRNAs for aquaporin 1–5, specific for type I pneumocytes and surfactant A-D, specific for type II pneumocytes, were measured. Aquaporin I was elevated in marrow cells from co culture with all lung cancers; elevations ranging from 2.15 to 56.7 fold (mean 23 fold). Similarly surfactant B mRNA was induced in marrow cells by all lung cancers with fold elevations ranging from 7.9 to 2164 (mean fold elevation 668). More variable elevations were also seen with aquaporin 3, 4, and 5, surfactant A, surfactant C, and surfactant D. Ultracentrifugation (28,000 g) of conditioned media from these cancers revealed the presence of microvesicles with diameters of 100–180 nm. Conclusions: These observations indicate that the genetic phenotype of cells in the vicinity of lung cancer cells can be altered and that these alterations might be mediated by microvesicle transfer of genetic information.

No significant financial relationships to disclose.

Fluorescence Activated Cell Sorting: A Window on the Stem Cell

Fluorescence activated cell sorting (FACS) in the field of stem cell biology has become an indispensable tool for defining and separating rare cell populations with a high degree of purity. Steady progress has been made in this regard, but the intrinsic lability of the stem cell phenotype presents a different challenge and there are many technical caveats. FACS remains, however, the technology of choice for reporting and characterizing rare cell populations such as stem cells.

The stem cell continuum: considerations on the heterogeneity and plasticity of marrow stem cells

Traditional models of hematopoiesis have been hierarchical. Recent evidence showing that marrow stem cells are a cycling population and that the hematopoietic phenotype of these cells reversibly changes with cycle transit have suggested a continuum model of stem cell regulators. Studies on marrow cell conversion to lung cells have extended this continuum to cycle-related differentiation into nonhematopoietic stem cells. We postulate that stem cells transiting cell cycle continually change their chromatin structure, thus providing different windows of transcriptional opportunity and a continually changing phenotype. Final outcomes with this continuum model would be determined by the specific chromatin state of the cell and the presence of specific differentiation inducers.

The marrow cell continuum: stochastic determinism

Traditional models of hematopoiesis have been hierarchical in nature. Over the past 10 years, we have developed data indicating that hematopoiesis is regulated in a continuum with deterministic and stochastic components. We have shown that the most primitive stem cells, as represented by lineage negative rhodamine(low) Hoechst(low) murine marrow cells are continuously or intermittently cycling as determined by in vivo BrdU labeling. When marrow stem cells are induced to transit cell cycle by in vitro exposure to cytokines, either IL-3, IL-6, IL-11, and steel factor or thrombopoietin, FLT3 ligand, and steel factor, they progress through cycle in a highly synchronized fashion. We have determined that when the stem cells progress through a cytokine stimulated cell cycle the homing, engraftment, adhesion protein, global gene expression, and hematopoietic differentiation phenotypes all change in a reversible fashion. This has led to the continuum model, in which, with cycle transit, chromatin is continually changing altering open transcription areas and providing a continually changing landscape of transcriptional opportunity. More recently, we have extended the changing differentiation profiles to differentiation into lung cells and found that non-hematopoietic differentiation also shows cycle related reversibly modulation. These observations all together support a continuum model of stem cell regulation in which the phenotype of the marrow stem cells is continually and reversibly changing over time.

The marrow stem cell: the continuum

The marrow hematopoietic stem cell is currently being redefined as to all aspects of its phenotype and its total differentiation capacity. This redefinition now includes its plasticity as to production of nonhematopoietic and hematopoietic cell types, the determinants of its in vivo engraftment potential and its expression of stem cell functional characteristics.

Marrow stem cell potential within a continuum

On the basis of our studies of the fluctuation of the hematopoietic stem cell phenotype with cell cycle trnsit, we hypothesize that the ability of marrow stem cells to convert to nonhematopoietic cells will also vary at different points in the cell cycle. The new biology of stem cells has an impact on many fields including developmental biology and stem cell biology and the clinical potential is enormous.

Physical and physiological plasticity of hematopoietic stem cells

Stem cells from a variety of tissues have recently been shown to be capable of differentiating into cells characteristic of a separate tissue, apparently in response to microenvironmental signals. This is hierarchical plasticity. We have shown that both human and murine neurosphere cells with potential for differentiating into neurons, oligodendrocytes, and astrocytes can produce hematopoietic stem cells when engrafted into fetal sheep or murine day 3.5 blastocysts, respectively. We have also demonstrated an alternative form of stem cell plasticity: functional plasticity at different points in cell cycle transit and at different phases of a circadian rhythm. We have shown that long-term engraftment varies reversibly as primitive murine stem cells (lineage-negative rhodamine(low) Hoechst(low)) transit the cell cycle under stimulation by interleukin-3 (IL-3), IL-6, IL-11, and steel factor, with engraftment being defective in late S/early G2. Engraftment also varies markedly with circadian time. Presumptive mechanisms for these phenotypic shifts include alteration in adhesion protein expression with consequent changes in marrow homing. Most recently, we have also demonstrated that stem cell differentiation varies markedly with cell cycle transit. There are other features of the hematopoietic stem cell which suggest that it is a highly plastic cell with the ability to rapidly change its membrane phenotype, while exhibiting extraordinary directed motility. These data suggest that cell cycle and circadian plasticity should be considered additional major features of the hematopoietic stem cell phenotype.

Stem cell engraftment strategies

The donor stem cell phenotype and host microenvironment determine the outcome of a stem cell transplant. In a series of transplant studies in syngeneic male to female or congenic Ly5.1/Ly5.2 models in which hosts have received no or minimal irradiation (100 cGy), evidence overwhelmingly supports the concept that syngeneic engraftment is determined by stem cell competition. These approaches can be extended to H-2 mismatched allogeneic mouse combination when antigen pre-exposure and CD40-CD40 ligand antibody blockage are employed. A human trial in patients with resistant neoplasia infusing pheresed blood with 10(8) CD3 cells/kg showed that tumor responses and complete chimerism occur with very low levels of CD34+ cells/kg and that the extent of previous treatment is a critical factor in determining chimerism. A major feature of transplants is the phenotype of the donor stem cell. This phenotype shows dramatic reversible plasticity involving differentiation, adhesion protein expression, and engraftment with cytokine-induced cell-cycle transit. Homing is probably also plastic. Marked fluctuations in engraftment capacity are also seen at different points in marrow circadian rhythm.

Lymphohematopoietic stem cell engraftment

Traditional dogma has stated that space needs to be opened by cytoxic myeloablative therapy in order for marrow stem cells to engraft. Recent work in murine transplant models, however, indicates that engraftment is determined by the ratio of donor to host stem cells, i.e., stem cell competition. One hundred centigray whole body irradiation is stem cell toxic and nonmyelotoxic, thus allowing for higher donor chimerism in a murine syngeneic transplant setting. This nontoxic stem cell transplantation can be applied to allogeneic transplant with the addition of a tolerizing step; in this case presensitization with donor spleen cells and administration of CD40 ligand antibody to block costimulation. The stem cells that engraft in the nonmyeloablated are in G0, but are rapidly induced (by 12 hours) to enter the S phase after in vivo engraftment. Exposure of murine marrow to cytokines (IL-3, IL-6, IL-11 and steel factor) expands progenitor clones, induces stem cells into cell cycle, and causes a fluctuating engraftment phenotype tied to phase of cell cycle. These data indicate that the concepts of stem cell competition and fluctuation of stem cell phenotype with cell cycle transit should underlie any new stem cell engraftment strategy.

Chiaroscuro hematopoietic stem cell

These observations suggest several immediate clinical strategies. In gene therapy, approaches could be targeted to obtain cycling of hematopoietic stem cells and gene-carrying retrovirus vector integration followed by engraftment at an appropriate time interval which favors engraftment. The same type of approach can be utilized for stem cell expansion approaches. Alternatively marrow or peripheral stem cell engraftment can be obtained with minimal to no toxicity in allochimeric strategies in such diseases as sickle cell anemia or thalassemia. A similar approach could be useful in obtaining cell engraftment with minimal toxicity in therapies employing cellular immune (T-cell and NK-cell) attack against cancer. These areas of clinical application are outline in Table 3.

High levels of engraftment with a single infusion of bone marrow cells into normal unprepared mice

Repetitive infusion of 40 million male murine marrow cells (total 200 million cells) into normal unprepared female BALB/c hosts for 5 consecutive days results in high levels of engraftment at 1-25 months postinfusion, as determined by Southern blot analysis using a Y chromosome-specific probe. We investigated the importance of the schedule of injections in this engraftment model. Surprisingly, a single infusion of 200 x 10(6) male BALB/c bone marrow cells analyzed at 7-14 weeks postinfusion resulted in engraftment levels in individual female mice of over 50% with mean values of 25 +/- 2% for 44 individual transplant points. Engraftment levels in spleen and thymus were 14 +/- 1% and 18 +/- 3%, respectively. Including heparin in the infusion increased engraftment in marrow, spleen, and thymus. Administration of the cells over five or 10 separate infusions, rather than in one injection, did not increase engraftment levels. If the infused bone marrow cells seeded equally between host spleen, thymus, and bone marrow, and if all cells engrafted, the bone marrow engraftment seen here approaches the theoretical maximum. This suggests either a large number of available "niches" or the displacement of host marrow cells by infused marrow. The latter possibility is upheld by cell counts per tibia/femur and total seven-factor HPP-CFC/tibia, which were not increased. These data suggest that a single infusion of marrow homes quantitatively to spleen, thymus, and bone marrow, possibly displacing host cells in the process.