Jakub Tolar

To study the biodistribution of MSCs, we labeled adult murine C57BL/6 MSCs with firefly luciferase and DsRed2 fluorescent protein using nonviral Sleeping Beauty transposons and coinfused labeled MSCs with bone marrow into irradiated allogeneic recipients. Using in vivo whole-body imaging, luciferase signals were shown to be increased between weeks 3 and 12. Unexpectedly, some mice with the highest luciferase signals died and all surviving mice developed foci of sarcoma in their lungs. Two mice also developed sarcomas in their extremities. Common cytogenetic abnormalities were identified in tumor cells isolated from different animals. Original MSC cultures not labeled with transposons, as well as independently isolated cultured MSCs, were found to be cytogenetically abnormal. Moreover, primary MSCs derived from the bone marrow of both BALB/c and C57BL/6 mice showed cytogenetic aberrations after several passages in vitro, showing that transformation was not a strain-specific nor rare event. Clonal evolution was observed in vivo, suggesting that the critical transformation event(s) occurred before infusion. Mapping of the transposition insertion sites did not identify an obvious transposon-related genetic abnormality, and p53 was not overexpressed. Infusion of MSC-derived sarcoma cells resulted in malignant lesions in secondary recipients. This new sarcoma cell line, S1, is unique in having a cytogenetic profile similar to human sarcoma and contains bioluminescent and fluorescent genes, making it useful for investigations of cellular biodistribution and tumor response to therapy in vivo. More importantly, our study indicates that sarcoma can evolve from MSC cultures.

This review of the mechanism of osteopetrosis incriminates molecular defects in osteoclasts as the cause of the imbalance between bone formation by osteoblasts and bone resorption by osteoclasts. Numerous kinds of defects of osteoclast function can cause osteopetrosis in experimental animals, but only one class of defect — the inability to acidify — has been identified in osteopetrosis in humans.

Myeloid-derived suppressor cells (MDSCs) are a well-defined population of cells that accumulate in the tissue of tumor-bearing animals and are known to inhibit immune responses. Within 4 days, bone marrow cells cultured in granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor resulted in the generation of CD11b(+)Ly6G(lo)Ly6C(+) MDSCs, the majority of which are interleukin-4Rα (IL-4Rα(+)) and F4/80(+). Such MDSCs potently inhibited in vitro allogeneic T-cell responses. Suppression was dependent on L-arginine depletion by arginase-1 activity. Exogenous IL-13 produced an MDSC subset (MDSC-IL-13) that was more potently suppressive and resulted in arginase-1 up-regulation. Suppression was reversed with an arginase inhibitor or on the addition of excess L-arginine to the culture. Although both MDSCs and MDSC-IL-13 inhibited graft-versus-host disease (GVHD) lethality, MDSC-IL-13 were more effective. MDSC-IL-13 migrated to sites of allopriming. GVHD inhibition was associated with limited donor T-cell proliferation, activation, and proinflammatory cytokine production. GVHD inhibition was reduced when arginase-1-deficient MDSC-IL-13 were used. MDSC-IL-13 did not reduce the graft-versus-leukemia effect of donor T cells. In vivo administration of a pegylated form of human arginase-1 (PEG-arg1) resulted in L-arginine depletion and significant GVHD reduction. MDSC-IL-13 and pegylated form of human arginase-1 represent novel strategies to prevent GVHD that can be clinically translated.

Mesenchymal stromal cells or mesenchymal stem cells (MSCs) have captured considerable scientific and public interest because of their potential to limit physical and immune injury, to produce bioactive molecules and to regenerate tissues. MSCs are phenotypically heterogeneous and distinct subpopulations within MSC cultures are presumed to contribute to tissue repair and the modulation of allogeneic immune responses. As the first example of efficacy, clinical trials for prevention and treatment of graft-versus-host disease after hematopoietic cell transplantation show that MSCs can effectively treat human disease. The view of the mechanisms whereby MSCs function as immunomodulatory and reparative cells has evolved simultaneously. Initially, donor MSCs were thought to replace damaged cells in injured tissues of the recipient. More recently, however, it has become increasingly clear that even transient MSC engraftment may exert favorable effects through the secretion of cytokines and other paracrine factors, which engage and recruit recipient cells in productive tissue repair. Thus, an important reason to investigate MSCs in mechanistic preclinical models and in clinical trials with well-defined end points and controls is to better understand the therapeutic potential of these multifunctional cells. Here, we review the controversies and recent insights into MSC biology, the regulation of alloresponses by MSCs in preclinical models, as well as clinical experience with MSC infusions (Table 1) and the challenges of manufacturing a ready supply of highly defined transplantable MSCs.