Hematopoietic Stem Cells in the Mouse Spleen

The spleen is a hematopoietic organ in mice. Hematopoietic stem cells (HSCs) migrate into the spleen around embryonic day 14, and then migrate into the bone marrow (BM) around embryonic day 17. Thereafter, HSCs reside in both BM and spleen throughout the life of a mouse. The spleen is the major site of extramedullary hematopoiesis in pathological conditions. The spleen serves as an active hematopoietic organ in lethally irradiated mice for a while after transplantation with BM cells. Osteoblasts are considered to be one of the stem cell niche components. Because there are no osteoblasts in the spleen, niches in the spleen possibly functions differently from ones in the BM. The regulation of HSCs likely differs between the BM and spleen. In order to understand a role of spleen HSCs in physiological conditions, we have characterized HSCs in the spleen as compared with those in the BM.

BM and spleen cells were obtained from 8–10 week-old C57BL/6 mice. Competitive repopulation showed that the repopulating activity per 10⁶ BM cells was significantly greater than that per 10⁶ spleen cells (about 10-fold). Limiting analysis showed that the frequency of long-term repopulating cells in BM cells was significantly higher than that in spleen cells (about 10-fold). As a result, the mean activity per BM stem cell was similar to that per spleen stem cell. Similarly to BM, CD34-negative, c-Kit-positive, Sca-1-positive, lineage markers-negative (CD34⁻KSL) cells were highly enriched in HSCs in the spleen. The frequency of CD34⁻KSL cells in the spleen was significantly lower than that in the BM. These data indicate that functionally equivalent HSCs exist in the spleen but at a low frequency. Data from single cell-transplantation supported this notion. The proportion of pyronin Y-negative G₀ cells among BM CD34⁻KSL cells was greater than that among spleen CD34⁻KSL cells at any one time. BrdU-uptake analysis showed that spleen CD34⁻ KSL cells were cycling more rapidly than BM CD34⁻KSL cells. These data suggest that spleen HSCs contribute to hematopoiesis to some extent under physiological conditions. BM and spleen HSCs may be interchangeable via the circulation. When BM HSCs are in the spleen, they are possibly under the control of spleen niches different from BM niches.