CD41⁺ Progenitors Are Critical for the Engraftment of Hematopoietic Cells Derived From Embryonic Stem Cells.

Abstract 2548

Poster Board II-525

Bone marrow cells are by far the most transplanted cells. However, their high immunogenicity requires severe immunosuppressive regimens that can be toxic and lethal to patients. Previous studies from our lab have shown the potential of embryonic stem cells to generate hematopoietic progenitor cells (HPCs), which can reconstitute bone marrow. These cells are uniquely qualified for transplantation due to their poor immunogenicity. Murine embryonic stem cells were differentiated in vitro over 26 days using embryoid body formation and a cocktail of hematopoietic cytokines and growth factors. After differentiation, the HPCs are a mixture of progenitor subpopulations that express different surface markers. The majority of these cells are, however, CD45⁺. Recent evidence in the literature has pointed to CD41⁺ cells as an earlier progenitor population in both the bone marrow and HPCs. Here, we hypothesize that CD41 expressing progenitors generated from murine embryonic stem cells engraft more efficiently in immunodeficient mice than the CD45⁺CD41⁻ cells. This concept was borne out of our observations that CD41 expression peaks at day 14 during the differentiation process decreasing thereafter. In contrast, CD45 peaks at the 26-day mark, when the differentiation process ends. Indeed when these cell populations were transplanted respectively in Rag2^−/−γ_c^−/− mice, higher degrees of chimerism were achieved with the CD41⁺ cells. To study this phenomenon more definitively, Rag2^{−/−_gc−/−} mice were transplanted with either the CD41⁺ or the CD45⁺ cells. The CD41⁺ cells achieved a higher level of mixed chimerism, 35.1%±18.1%, compared with only 5.1%±0.1% observed in animals transplanted with CD45⁺ cells. We further observed that CD41⁺CD45⁻ cells are a transient cell population that quickly becomes CD45⁺ in culture. Surprisingly, the CD45⁺ cells formed many more colony forming units as those detected in the CD41⁺ cell cultures, challenging the long-held paradigm that CFUs are a true reflection of hematopoiesis potential. Thus, our data here allow the conclusion that transplanting CD41 expressing derived from embryonic stem cells might be preferable to achieving more robust bone marrow reconstitution.