Pluripotent hemopoietic stem cells are c-kit(<low)

Pluripotent hemopoietic stem cells (P-HSCs) were thought to be c-kit(+), but recent reports indicate that they are c-kit(low). In the present report, we provide evidence using Ly5 congenic mice that P-HSCs are c-kit(<low). Lineage-negative (Lin(-))/CD71(-) cells among bone marrow cells (BMCs) from C57BL/6 Ly5.1 mice were separated into major histocompatibility complex class I-high (class I-high)/C-kit(low) and class I-high/c-kit(<low) populations, Each population (500 cells) was transplanted into lethally (9.0 Gy) irradiated C57BL/6 Ly5.2 congenic mice along with Ly5.2 (2 x 10(5)) compromised cells. Donor-derived Ly5.1(+) cells were detected 6 months after transplantation in primary recipients reconstituted with either class I-high/c-kit(low) or class I-high/C-kit(<low) cells. BMCs (1 x 10(6)) from the primary recipients were further transplanted into secondary recipients (Ly5.2 mice) to assess their long term repopulating activity. Six months after bone marrow transplantation, Ly5.1(+) cells in all lineages were detected only in secondary recipients that had been given BMCs from the primary recipients reconstituted with class I-high/C-kit(<low) cells but not in cells that were class I-high/C-kit(low). When the BMCs (1 x 10(6)) of these secondary recipients were further transplanted into tertiary recipients, all tertiary recipients that had been given BMCs from the secondary recipients originally reconstituted with Lin(-)/CD71(-)/class I-high/c-kit(low) cells died within 10 days whereas all six tertiary recipients originally reconstituted with Lin(-)/CD71(-)/class I-high/c-kit(<low) cells showed donor (Ly5.1(+))-derived cells in their peripheral blood. In the single tertiary recipient that was killed, donor-derived T cells, B cells, macrophages, and granulocytes also were detected in several major hematolymphoid organs. The remaining five mice continue to survive more than 6 months after the tertiary bone marrow transplantation.