Multi-Tissue Homing of Repopulating Hematopoietic Cells after Bone Marrow Transplantation in Mice.

We wished to assess the tissue distribution of repopulating hematopoietic stem cells after bone marrow transplantation (BMT), since so far only sub-fractions of repopulating cells have been found to home to hematopoietic organs and tissues in BMT models. Two x 10⁵ lineage depleted (lin⁻) bone marrow cells from C57BL/6 Ly5.1 mice were transplanted into 2 x 5.5 Gy irradiated Ly5.2 congenic C57BL/6 recipient mice. On day 1, 7 or 28 post BMT, cell suspensions were prepared from bone marrow, spleen, muscle, brain, lung and liver, and CD45⁺ cells were enriched by magnetic beads separation. Enriched CD45⁺ cells from brain, quadriceps muscle, or other organs and tissues were transplanted into Ly5.2 secondary recipients in groups of three (mean transplanted cell number per mouse: 0.68 x 10⁵; range: 0.26–2.9 x 10⁵) along with 1 x 10⁶ freshly isolated Ly5.2 competitor whole bone marrow cells. A total of 39 primary and 273 secondary recipient mice were analyzed. Here we show that whereas bone marrow displayed substantial donor-derived repopulation potential in non-irradiated and non-transplanted controls (mean contribution of Ly5.1 cells to bone marrow of secondary recipients after 4–7 weeks: 23.5%), muscle, brain, lungs and liver showed little competitive repopulating activity (<2%). In transplanted recipients, however, high donor-derived repopulating activity was detected in bone marrow (38–72%), spleen (7–15%) and, surprisingly, in liver (25–36%), muscle (24–37%), brain (7–14%) and lung (4–12%) at 1, 7 and 28 days post BMT. In contrast, hematopoietic colony-forming cells localized exclusively to hematopoietic tissues and were not found in muscle, brain, or lung. Repopulation potential was present in blood in all groups including non-irradiated, non-transplanted controls (donor contribution, 6–20%). Our data indicate that, following BMT, bone marrow, spleen and additional tissues take up substantial numbers of repopulating cells. This provides an explanation for the published low homing efficiency of repopulating cells to hematopoietic organs.