Human Pancreatic Mesenchymal Stem Cells Produce Blood in the Non-Injury Fetal Sheep Model.

Mesenchymal stem cells (MSC) are known for their relatively easy isolation, expansion by culture, and low immunogenicity. Here we report the isolation and characterization of a human fetal pancreas-derived MSC population (pMSC), and its long-term engraftment and differentiation into hematopoietic elements. We started a three step in-vitro culture system with freshly digested human fetal pancreatic cells cultured on uncoated plastic in RPMI media supplemented with 2% FBS for 48 hours. Next, adherent cells were further cultured in restrictive media (0.5% FBS) for about 4 weeks where the ensued starvation depleted contaminating cells. In the third phase, human pMSC were expanded in the presence of 10% FBS, bFGF, and EGF. The pMSC maintained their proliferative capacity even after repeated freeze-thaw cycles. Characterization of the pMSC was done by FACS analyses, RT-PCR, and differentiation studies. Cultured cells became CD44⁺, CD29⁺, CD90⁺, CD105⁺, CD34⁻, and CD45⁻, a phenotype similar to the bone marrow-derived MSC. RT-PCR analyses revealed that the pMSC from our culture system did not express pancreatic tissue-specific markers including insulin, PDX1 and PAX6, but expressed the general stem cell marker Nestin and also the c-Met marker attributed to primitive endodermal and pancreatic stem cells. Using specific differentiation inductive media, the pMSC were able to differentiate in-vitro into adipocytes and osteocytes. Thus, the cultured cells could be labeled as pMSC as confirmed by their multi-lineage differentiation when induced, as well as their ability to remain undifferentiated in restrictive media with the expression of characteristic markers listed above. To investigate the engraftment and in-vivo differentiation potential of human pMSC we transplanted these cells into pre-immune sheep (1–2x10e6 cells/fetus) (n=8) at 55–60 days of gestation by intra-peritoneal injections. Repeated evaluation of the recipients up to 1 year of age confirmed the engraftment of human cells in the bone marrow of animals by PCR for the human-specific GAPDH gene and by flow cytometry using human-specific antibodies. Human hematopoietic markers including CD47, CD45, CD133, and CD15 were found in both the bone marrow and peripheral blood of chimeric animals. Sargramostim (human specific GM-CSF) treatment of one animal (5 micrograms/ kg /day for three days) induced up-regulation of the human CD47, CD45, CD36, and CD56 expressing human cells. Methylcellulose culture of a bone marrow sample from this animal generated colonies that tested positive for human GAPDH by PCR thereby confirming the presence of human hematopoietic progenitors. In summary, we established a three step in vitro culture system for the isolation and expansion of human fetal pMSCs. Using the non-injury sheep model we were able to demonstrate the plasticity of human pMSC which gave rise to hematopoietic elements for up to one year following engraftment. These animals will be further evaluated for the pMSC contribution in other tissues including the pancreas.