Long-term maintenance of the ability of STAT3-C to enhance the regenerative activity of transplanted bone marrow cells. (A) Regenerative activity of transduced bone marrow cells in mice that received a transplant of limiting numbers of transduced CRUs. Cells (4-5 × 10⁴) from 5-FU–pretreated Pep3b mice were transduced with MIG or Stat3-C and then transplanted into each B6 recipient in 3 independent experiments (5-6 mice per group per experiment). Assays for the frequency of GFP⁺ CRUs in the cells injected showed that the primary recipients were given either 0.3 MIG or 0.5 to 0.8 Stat3-C–transduced CRUs each. Shown are the percent GFP⁺ WBCs present in the blood of individual mice at the times shown. In the third experiment, the mice were analyzed after 6 and 16 weeks only, and mean values, indicated by the bars, are based on values from both positively and negatively engrafted mice. (B) Long-term follow-up of mice repopulated with Stat3-C–transduced cells. Individual mice repopulated with Stat3-C–transduced cells from the experiments shown in panel A were monitored for up to 54 weeks for the continued presence of GFP⁺ WBCs. Also shown for comparison is the mean ± SEM of values obtained for the 5 positively engrafted mice that received a transplant of MIG-transduced cells. (C) Normal appearance of bone marrow of mice that received a transplant of each type of transduced cells. Shown are representative touch smears of pelvic bone marrow 40 weeks after transplantation of mice with transduced cells. (D) Reactivation of the enhancing effect of STAT3-C following the transfer of Stat3-C–transduced cells from primary recipients into secondary mice. Marrow cells from the primary recipients shown in panel B were pooled, and aliquots containing the indicated numbers of GFP⁺ CRUs (determined in independent CRU assays) were transplanted into secondary irradiated B6 recipients. Values shown are the mean ± SEM of the percentage of GFP⁺ WBCs detected in the secondary mice at the times shown (a total of 12 secondary mice for each source of primary recipient cells assayed).