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We have developed a novel cotransplantation system in which gene- transduced human CD34+ progenitor cells are transplanted into immunodeficient (bnx) mice together with primary human bone marrow (BM) stromal cells engineered to produce human interleukin-3 (IL-3). The IL- 3-secreting stroma produced sustained circulating levels of human IL-3 for at least 4 months in the mice. The IL-3-secreting stroma, but not control stroma, supported human hematopoiesis from the cotransplanted human BM CD34+ progenitors for up to 9 months, such that an average of 6% of the hematopoietic cells removed from the mice were of human origin (human CD45+). Human multilineage progenitors were readily detected as colony-forming units from the mouse marrow over this time period. Retroviral-mediated transfer of the neomycin phosphotransferase gene or a human glucocerebrosidase cDNA into the human CD34+ progenitor cells was performed in vitro before cotransplantation. Human multilineage progenitors were recovered from the marrow of the mice 4 to 9 months later and were shown to contain the transduced genes. Mature human blood cells marked by vector DNA circulated in the murine peripheral blood throughout this time period. This xenograft system will be useful in the study of gene transduction of human hematopoietic stem cells, by tracing the development of individually marked BM stem cells into mature blood cells of different lineages.

A retroviral vector (N2-SV-GC) was constructed by inserting a normal human glucocerebrosidase (GC) cDNA under control of the SV40 early region promoter into the Moloney murine leukemia virus-derived N2 vector. N2-SV-GC produced human GC in murine 3T3 fibroblasts at levels in the range of the endogenous murine GC as determined by enzymatic assay and Western blot analysis. The N2-SV-GC retroviral vector was used for studies of gene transduction of murine hematopoietic progenitor cells (HPC). Infection of bone marrow cultured for 2 to 10 days in medium containing hematopoietic growth factors was significantly more efficient than infection of freshly isolated marrow cells (24% to 32% G418-resistant CFU-GM v 15%, respectively). The marrow infected by N2-SV-GC was maintained in long-term bone marrow culture (LTBMC) and had a stable level of G418-resistant HPC over 2 months of serial assays. The human GC gene of the vector was persistently expressed in the nonadherent cell fraction of the murine LTBMC as determined by Northern blotting, Western blotting, and immunohistochemical staining using a monoclonal antibody specific for human GC. N2-SV-GC also expressed the human GC gene in day 12 CFU-S. LTBMC represents a novel system for retroviral vector-mediated gene transduction of HPC and may accurately predict the activities of vectors in vivo.

We sought to define optimal conditions for retroviral-mediated transduction of long-lived human hematopoietic progenitors from bone marrow and peripheral blood. CD34+ cells were transduced by the LN and G2 retroviral vectors in the presence or absence of stromal support and with or without cytokine addition. After transduction, a portion of the cells was plated in methylcellulose colony-forming assay, with or without G418, to assess the extent of gene transfer into committed progenitors. The remaining cells from each experiment were transplanted into immunodeficient mice to allow analysis of transduction of long- lived progenitors. Human colony-forming cells contained within the murine bone marrow were analyzed after engraftment periods of 2 to 11 months. Cells were plated in a human-specific colony-forming assay with and without G418 to assess the extent of transduction of primitive progenitors. Individual human colonies were also analyzed by polymerase chain reaction for the presence of provirus. Bone marrow progenitors were efficiently transduced only when stroma was present, whereas mobilized peripheral blood progenitors were effectively transduced in the presence of either stroma or cytokines. Inclusion of the cytokines interleukin-3, interleukin-6, and stem cell factor did not further augment the extent of gene transfer in the presence of a stromal support layer. Additionally, human CD34+ progenitors from bone marrow or mobilized peripheral blood that had been transduced for 3 days in the absence of stroma failed to produce sustained, long-term engraftment of bnx mice. Mice transplanted with the same pools of human progenitors that had been transduced in the presence of stroma for 3 days had significant levels of human cell engraftment at the same timepoints, 7 to 11 months after transplantation. Our data show loss of long-lived human progenitors during 3-day in vitro transduction periods in the absence of stromal support. Therefore, the presence of bone marrow stroma has dual benefits in that it increases gene transfer efficiency and is essential for survival of long-lived human hematopoietic progenitors.