Derivation of SSEA4-/CD73+ Mesenchymal Stem Cells from Human Embryonic Stem Cells.

Human embryonic stem cells (hESCs) could potentially provide a renewable source of different types of cells for cell therapy applications. Recently, mesenchymal stem cells (MSCs) have been derived from hESCs either through co-culturing with murine OP9 bone marrow stromal cell line or directly from hESCs without co-culturing with OP9 cells. Although the latter methodology is clinically advantageous over co-culturing with an animal cell layer those mesenchymal cells were reported to be positive for SSEA4. SSEA4 is a marker of undifferentiated hESCs and thus the presence of this marker on hESC-derived cells could potentially be problematic for clinical applications. We have recently achieved a novel and reproducible methodology for deriving a pure population of SSEA4-/CD73+ MSCs from federally approved hESC lines H1 and H9. To initiate the differentiation of hESCs to MSCs, we cultured undifferentiated hESCs on matrigel plates in murine embryonic fibroblast conditioned media with media changes every 3 days. Under these culture conditions a portion of embryonic stem cells differentiated into fibroblast looking cells. Through a multi-step process which involved the use of a culture methodology similar to what is being used to culture bone marrow (BM)-derived MSCs, and passaging cultured cells at defined time points we were able to derive a pure population of cells that were uniformly positive for MSC marker CD73 in about a 4-weeks period. These cells had fibroblast/mesenchymal looking morphology, and expressed cell surface marker antigens similar to what has been reported for adult human BM-derived MSCs: they are positive for CD29, CD44, CD54, CD71, CD90, glycophorin A, CD105, and were negative for hematopoietic markers such as CD34 and CD45. Similar to adult BM-derived MSCs these cells express HLA class-I antigens but not class-II antigens. Using established differentiation protocols we could differentiate the hESC-derived CD73+ MSCs into adipocytes, osteocytes, and chondrocytes as verified by immunohistochemistry and RT-PCR assays. So far we have grown these CD73+ MSCs up to passages 15–18. These cells retained their differentiation potential, and were karotypically normal when tested at passage 12. Most importantly, we did not observe any MSCs that were double positive for CD73 and SSEA4 antigen at any time point during our experiments. MSCs from a variety of fetal and adult sources are in various stages of clinical trials with some encouraging preliminary results. Our hESC-derived MSCs that are very similar to adult BM-derived MSCs regarding their growth and morphologic properties, immunophenotypic characteristics, differentiation potential, and importantly are devoid of hESC marker SSEA4 could potentially provide a novel source of MSCs for clinical applications.