Ron Hoffman

Ex vivo-expanded mesenchymal stem cells (MSCs) were transduced with a green fluorescent protein (GFP) retroviral construct and subsequently infused into 3 adult baboons following lethal total body irradiation and hematopoietic support or without any prior conditioning. To study the long-term fate of these MSCs, necropsies were performed between 9 and 21 months following MSC infusion, and an average of 16 distinct tissues were recovered from each recipient and evaluated for the presence of the GFP transgene in purified genomic DNA by sensitive real-time polymerase chain reaction (PCR). Two baboons received autologous and one allogeneic GFP-transduced MSCs. Both allogeneic and autologous MSCs appeared to distribute in a similar manner. Gastrointestinal tissues harbored high concentrations of transgene per microgram of DNA. Additional tissues including kidney, lung, liver, thymus, and skin were also found to contain relatively high amounts of DNA equivalents. Estimated levels of engraftment in these tissues ranged from 0.1% to 2.7%. The nonconditioned recipient appeared to have less abundant engraftment. These data suggest that MSCs initially distribute broadly following systemic infusion and later may participate in ongoing cellular turnover and replacement in a wide variety of tissues.

The hematopoietic system is derived from ventral mesoderm. A number of genes that are important in mesoderm development have been identified including members of the transforming growth factor-beta (TGF-beta) superfamily, the fibroblast growth factor (FGF) family, and the Wnt gene family. Because TGF-beta plays a pleiotropic role in hematopoiesis, we wished to determine if other genes that are important in mesoderm development, specifically members of the Wnt gene family, may play a role in hematopoiesis. Three members of the Wnt gene family (Wnt-5A, Wnt-2B, and Wnt-10B) were identified and cloned from human fetal bone stromal cells. These genes are expressed to varying levels in hematopoietic cell lines derived from T cells, B cells, myeloid cells, and erythroid cells; however, only Wnt-5A was expressed in CD34(+)Lin- primitive progenitor cells. The in vitro biological activity of these Wnt genes on CD34(+)Lin- hematopoietic progenitors was determined in a feeder cell coculture system and assayed by quantitating progenitor cell numbers, CD34(+) cell numbers, and numbers of differentiated cell types. The number of hematopoietic progenitor cells was markedly affected by exposure to stromal cell layers expressing Wnt genes with 10- to 20-fold higher numbers of mixed colony-forming units (CFU-MIX), 1.5- to 2. 6-fold higher numbers of CFU-granulocyte macrophage (CFU-GM), and greater than 10-fold higher numbers of burst-forming units-erythroid (BFU-E) in the Wnt-expressing cocultures compared with the controls. Colony formation by cells expanded on the Wnt-expressing cocultures was similar for each of the three genes, indicating similar action on primitive progenitor cells; however, Wnt-10B showed differential activity on erythroid progenitors (BFU-E) compared with Wnt-5A and Wnt-2B. Cocultures containing Wnt-10B alone or in combination with all three Wnt genes had threefold to fourfold lower BFU-E colony numbers than the Wnt-5A- or Wnt-2B-expressing cocultures. The frequency of CD34(+) cells was higher in Wnt-expressing cocultures and cellular morphology indicated that coculture in the presence of Wnt genes resulted in higher numbers of less differentiated hematopoietic cells and fewer mature cells than controls. These data indicate that the gene products of the Wnt family function as hematopoietic growth factors, and that they may exhibit higher specificity for earlier progenitor cells.

Within the bone marrow stroma are multipotential cells which are capable of differentiation into a number of mesenchymal cell lineages. These cells, termed mesenchymal stem cells, have recently been identified and characterized in humans. Many studies indicate that the bone marrow stroma is damaged following bone marrow transplantation. Since the marrow stroma is critical for the maintenance of hematopoiesis, its ability to support hematopoiesis following stem cell transplantation may be impaired. Animal models suggest that the transplantation of healthy stromal elements, including mesenchymal stem cells, may enhance the ability of the bone marrow microenvironment to support hematopoiesis after stem cell transplantation. Here the authors review recent data that suggest that mesenchymal stem cells may possess therapeutic value not only for the repair of damaged mesenchymal tissues following hematopoietic stem cell transplantation, but also as potential vectors for the delivery of corrective genes.

Standard myeloablative conditioning prior to allogeneic hematopoietic stem cell (HSC) transplantation has been associated with significant toxicity in patients older than 45 years of age with myelofibrosis with myeloid metaplasia (MMM). We sought to evaluate the efficacy of a reduced-intensity conditioning regimen for allogeneic HSC transplantation in this setting. A regimen consisting of fludarabine (30 mg/m(2) intravenously daily for 5 days) and melphalan (70 mg/m(2) intravenously daily for 2 days) followed by transplantation of filgrastim-mobilized peripheral blood cells from HLA-identical siblings was administered to 4 older patients (median age, 56 years; range, 48-58 years) with advanced MMM. All patients achieved prompt neutrophil and platelet engraftment and have experienced a significant regression of splenomegaly and bone marrow fibrosis. All now have normal bone marrow cellularity. With a median follow-up of 13 months (range, 11-19 months), all 4 patients are alive with stable full-donor hematopoietic chimerism. These results support the feasibility and effectiveness of reduced-intensity conditioning prior to allogeneic HSC transplantation for older patients with advanced MMM.