Host macrophages are a barrier to the engraftment of embryonic stem cell-derived hematopoietic progenitors. (P4544)

Abstract

Abstract Embryonic stem cell-derived hematopoietic progenitors (ES-HPs) have been proposed as an alternative bone marrow (BM) source. ES-HPs display features of adult hematopoietic progenitors, but do not achieve high levels of engraftment after transplantation, even in immunocompromised hosts. We hypothesized that host macrophages (Mϕ) are a barrier to ES-HP engraftment. To test this, we cultured and transplanted CD41+ ES-HPs into sublethally irradiated NOD.SCID.γc (NSG) mice that lack B, T and NK cells, but still possess Mϕ. Less than 5% ES-HP donor chimerism in the BM and spleen was observed in NSG recipients, in contrast to >90% chimerism in adult BM-transplanted controls. In addition, we noticed a 3.5-fold increase in spleen weight and a 1.75-fold increase in splenic host F4/80+ Mϕ in ES-HP recipients. In vitro phagocytosis assays confirmed that allogeneic Mϕ phagocytosed ES-HPs at higher levels compared to adult Lin- BM targets. Interestingly, syngeneic Mϕ also engulfed ES-HPs, but not adult Lin- BM. When Mϕ were eliminated with liposome-encapsulated clodronate, a 2.17-fold increase in ES-HP engraftment in the BM and 5.14-fold increase in the spleen was observed. Taken together, our data demonstrate that F4/80+ Mϕ are a barrier to ES-HP engraftment and suggest that cultured ES-HPs are unable to regulate host Mϕ phagocytosis. In line with this, ES-HPs expressed low levels of the Mϕ inhibitory ligand, CD47. Functional studies using CD47-Fc are in progress.