EFFECTS OF MEGAKARYOCYTE GROWTH AND DEVELOPMENT FACTOR ON SURVIVAL AND RETROVIRAL TRANSDUCTION OF T-LYMPHOID PROGENITOR CELLS

Murine retroviral vectors have the potential to mediate stable gene tr ansfer into hematopoietic progenitor cells. A known drawback to the us e of these vectors is that transduction can only take place in cells a ctively progressing through the cell cycle. Thrombopoietin, the c-mpl ligand, is known to support division of hematopoietic precursors of pr imitive origin. Polyethylene glycol (PEG)-conjugated recombinant human megakaryocyte growth and development factor (MGDF) is a polypeptide r elated to thrombopoietin that stimulates megakaryocyte production. To investigate whether MGDF would also induce stem cell division and supp ort retroviral transduction of CD34(+) cells, we compared the effects of MODE, stem cell factor (SCF), interleukin-3 (IL-3), and IL-6, alone or in combination, using amphotropic and vesicular stomatitis virus ( VSVG) pseudotyped murine retroviral vectors. Similar transduction effi ciency was observed when CD34(+) cells were transduced in the presence of SCF and MODE as compared to SCF, IL-3, and IL-6. Using the SCID-hu mouse model of thymopoiesis, we investigated whether CD34(+) cells tr ansduced in the presence of these cytokines could reconstitute irradia ted thymic implants, and whether vector sequences were present in matu re thymocytes. At early timepoints, no significant differences were ob served on engraftment of donor progenitors incubated with each cytokin e combination. However, a significant difference in the percentage of donor derived CD4(+)/CD8(+) immature thymocytes was observed 9 weeks a fter implantation of CD34(+) cells exposed to the combination of SCF a nd MODE as compared to SCF, IL-3, and IL-6 (p = 0.04), indicating that MGDF/SCF better supported the survival of thymocyte precursor cells. Approximately 4% of thymocytes in both cytokine groups harbored vector sequences. These studies provide evidence that MGDF and SCF in combin ation can mediate transduction of hematopoietic progenitors capable of contributing to long-term thymopoiesis. These results may have import ant applications for the implementation of gene therapy strategies in disorders affecting the T lymphoid system.