We conclude that cellular differentiation of pre-BI cells to a pre-BII-like stage, induced by the removal of IL-7, is delayed, but not inhibited by the doxycycline-induced overexpression
of Myc and Pim1, as judged by the retarded loss of c-kit expression, the retarded loss of clonability on stromal cells in the presence of IL-7 and by the slower gain of CD25. Furthermore, CH5424802 cell line acquisition of IgM on the surface or intracellularly is blocked. It appears that the Myc-single and the Pim1/Myc-double-transduced cells are arrested in differentiation before sIgM+ immature B cells. Transplantation of Pim1/Myc-double-overexpressing pre-BI cells in doxycycline-fed Rag1−/− recipient mice (Fig. 3) led to a marked expansion of CD19+ B-lineage cells in selleck chemicals vivo. In two separate experiments, the transplanted pre-B cells were kept either for 4 weeks (Fig. 3A–C) or for 8 weeks (Fig. 3D) in doxycycline-fed mice, followed each by a 4-week period without doxycycline in the drinking water. At 4 weeks, high numbers of transplanted cells overexpressing Pim1 and Myc were detected in BM, spleen and
peritoneum. At 8 weeks, the transplanted pre-B cells could also be detected in the swollen lymph nodes of the animals (data not shown). FACS analysis of the phenotypes of B lineage cells showed that spleens of doxycycline-induced mice, which harbored Pim1/Myc overexpressing B cells contained 100-fold higher numbers of pre-B cells, up to 6-fold higher numbers of immature IgM+ B cells, and up to twice the numbers of mature B cells than spleens of doxycycline-uninduced mice (Fig. 3B and C). The expanded number of cells detected after 8 weeks in BM, spleen, peritoneum and lymph nodes in the presence of doxycycline were, in majority, CD93+IgM− pre-B
cells (data not shown). Removal of doxycycline from the drinking water from transplanted mice 4 or 8 weeks after transplantation resulted in the disappearance of the previously expanded numbers of pre-B-, immature, and the slightly increased numbers of mature B cells from the spleen to normal numbers seen in uninduced mice (Fig. 3A, B and D). In a separate experiment, the capacities of Pim1/Myc-overexpressing pre-B cells to proliferate ex MycoClean Mycoplasma Removal Kit vivo after expansion in vivo were tested (Fig. 3E and F). These Pim1/Myc-overexpressing IgM− pre-B cells isolated from spleen and LNs of mice fed for 8 weeks with doxycycline could be propagated in vitro without IL-7 and OP9 cells in the presence, but not in the absence of doxycycline. Upon removal of doxycycline from these ex vivo cultures, the cells terminated proliferation and acquired IgM on their surface (Fig. 3F). The reasons for this oncogene-dependent inhibition of IgM expression are presently under detailed investigation.