Cells capable of sustaining secondary long-term hematopoiesis home more efficiently to BM than to spleen by 20 hours after transplantation. Irradiated or nonirradiated 1°ST recipients underwent transplantation with 1 × 105 to 3 × 105 Sca-1+ lin– CD62L– or Sca-1+ lin– CD49e+ ENG cells, were killed 20 hours later, and BM-homed or spleen-homed donor cells were isolated flow cytometrically. Equal numbers of sorted cells (range, 50 to 200 cells per mouse) were transplanted along with 1 × 105 LDBM competitor cells of recipient origin into lethally irradiated 1°LT recipients. Mean ± SEM percent donor-derived chimerism in 1°LT recipients analyzed monthly is shown in panel A; n = 18-22 mice per symbol from 8 separate experiments. In 3 experiments, 1 × 106 to 5 × 106 LDBM cells from 1°LT recipients 6 months after transplantation were transplanted without competition into lethally irradiated 2°LT recipients (B). Data in panel B are pooled data from 1 irradiated and 2 nonirradiated experiments and represent mean ± SEM chimerism; n = 11-14 for BM-homed, and n = 3-7 for spleen-homed donor cells (7 mice in the spleen-homed group failed to survive beyond 1 month of transplantation and therefore are not included in the analysis). Because the relative difference in chimerism between BM-homed and spleen-homed donor cells in 1°LT and 2°LT mice did not differ depending on the conditioning of 1°ST recipients, data from both groups were pooled together. *P < .05 by repeated measures analysis of variance, compared with spleen-homed donor cells.
