Hemoglobin vesicles (HbVs), liposomal oxygen carriers containing human hemoglobin, are candidates for development of a clinically useful transfusion alternative. Our previous in vivo animal studies of massive HbV dosages demonstrated the lack of any suppressive effect on hematopoiesis. Before starting clinical trials, we aimed to examine the details of the biocompatibility of HbVs with human hematopoietic stem/progenitor cells. We investigated the effect of HbVs at a concentration of up to 3 vol/vol (%) on expansion of human umbilical cord (CB) hematopoietic stem/progenitor cells, using a coculture system of human TERT-transfected bone marrow stromal cells and CD34 cells in vitro. The exposure of CB CD34 cells to HbVs up to 14 days suppressed the expansion of total cells and the CD34 cells themselves, whereas the empty liposomes, that did not contain Hb, had modestly inhibitory effects on the expansion of these cells. As a result, the number of colonies obtained from the expanded CD34 cells was inhibited by the exposure to HbVs. In contrast, exposure to HbVs for 3 days had no effect on the expansion of CD34 cells and only slightly decreased the number of total cells. Our in vitro experimental condition does not fully recreate the physiological condition, and the effect of the direct contact of HbV would be magnified because of the absence of shielding by the vasculature and the lack of the reticuloendothelial system and blood stream. However, the present data raise some concern regarding hematopoiesis, and one has to pay attention to this in future human clinical trials.
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