Thirty-five years ago, Siminovitch et al. (Siminovitch L, Till JE, McCulloch EA. J Cell Com Physiol 64:23–32, 1964), using serially transplanted mouse spleens at 14-day intervals, observed a markedly progressive decline in the proliferative capacity of bone marrow (BM) cells, with the loss of cionogenicity by the fourth transplant generation. Using the same protocol, we assessed the proliferative capacity of p53-deficient mouse BM cells transplanted serially at the same 14-day intervals into lethally irradiated mice, which was a useful tool for understanding the characteristics of hemopoletic stem cells lacking solely the p53 gene function. BM cells from p53-deficient homozygous (p53^–/–), p53-heterozygous (p53^+/–), and wild-type (p53^+/+) C57BL/6 mice were transplanted into lethally irradiated C57BL/6 recipients. Fourteen days later, the repopulated spleens were harvested, and 10⁷ cells were retransplanted into secondary recipients. Serial transplantation was continued at 14-day intervals until hemopoietic repopulation failure. The number of heterozygous and homozygous p53-deficient spleen cells increased logarithmically up to the fourth and fifth passages, respectively, whereas wild-type spleen cells ceased to proliferate by the third passage. The number of macroscopic spleen colonies increased logarithmically until the third passage in recipients of heterozygous and homozygous p53-deficient cells, but ceased to grow by the second passage in recipients of wild-type cells. The numbers of heterozygous and homozygous p53-deficient colony forming units in spleen (CFUs-S) remained stable during the first four transplant generations, whereas that of wild-type CFUs-S decreased progressively from the first transplant generation onward. The clonogenicity of p53-deficient cells was lost when the number of CFUs-S per spleen decreased to below 10. This suggests that one out of 10 CFUs-S might be long-term repopulating cells (LTRCs), and that p53-deficient LTRCs may proliferate more rapidly than wild-type LTRCs. Longer passages that were possible in the p53-deficient groups were considered to be due to the faster cell cycle of the p53-deficient hemopoietic progenitor cells, as determined by bromodeoxyuridine incorporation with purging by UV light exposure, followed by hemopoietic colony assay (BUUV assay).

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Serial Transplantation of p53-Deficient Hemopoietic Progenitor Cells to Assess Their Infinite Growth Potential