Identification of Hepatic Progenitors Having No Hematopoietic Potential in Murine Bone Marrow.

We recently reported that when Sca-1⁺ c-Kit⁻ bone marrow cells (BMCs) were co-cultured with fetal liver cells (FLCs) on laminin-coated dishes, alpha-fetoprotein (AFP)-expressing BMCs became completely adherent by day 3 and expressed albumin as assessed with immunochemistry and RNA-PCR (Yamada et al., Exp Hematol. 34: 97, 2006). In the current study, we attempted to further delineate the characteristics of BMCs that differentiate into hepatic-like cells. It was found that AFP-expressing cells were in CD5⁺ or B220⁺ lymphoid lineage, mostly Sca-1⁺CD5⁺ lineage, expressing AFP. When cKit⁺Sca-1⁺ lineage⁻ BMCs (KSLs), which did not express AFP, were cocultured with CD5⁺ BMCs, both from green fluorescence protein (GFP)-expressing transgenic mice, in the presence of FLCs from ROSA26 mice (X-gal⁺ FLCs), fractionated cells gave rise to adherent hepatic-like cells, which expressed albumin and cytokeratin 8 (assessed with immunochemistry) and AFP, albumin, transthyretin and dipeptidylpeptidase IV (examined with RNA-PCR). The hepatic-like cells from KSLs and CD5⁺ BMCs emerged at the frequency of 1 in 50 and 1x10³ as assessed with titration assay. However, CD5⁺ Mac-1⁻ Gr-1⁻ Ter119⁻ BMCs did not differentiate into hepatic-like cells. CD5⁺ CD45⁻ cells differentiated into hepatic-like cells without fusion at the frequency of 1 in 300 but CD5⁺ CD45⁺ cells did not. CD5⁺ CD45⁻ cells hardly produced hematopoietic colonies as compared with CD5⁺ CD45⁺ cells did.

In conclusion, we have shown that KSLs and CD5+ CD45− cells exposed to FLCs are capable of generating hepatic-like cells, which expressed albumin as assessed with immunochemistry and RNA-PCR. We should further explore whether CD5+ CD45− cells emerge from hematopoietic stem cells or mesenchymal stem cells.