Response to Letter of C. W. Song, F. M. Uckun, S. H. Levitt and T. H. Kim

Distribution of hematopoietic stem cells in the bone marrow according to regional hypoxia

“…This drug was initially developed as a therapeutic agent to target hypoxic cells in tumors (9) but its effect on the primitive HSC population in the bone marrow had yet to be investigated. By discriminating shortand long-term repopulating cells in the well-validated CAFC assay (27)(28)(29)(30) and establishing that TPZ is only toxic to these cells under hypoxia in vitro, we now show that HSCs are the most depleted by the in vivo effects of this drug (Fig. 4), although this needs to be confirmed by using the in vivo long-term repopulation (LTR) assay.…”

“…This is in apparent contrast to preclinical studies showing only moderate, if any, early hematological toxicity of this drug when administered as a single agent (31)(32)(33). Comparisons among a number of different chemotherapy drugs and radiation regimens have, however, shown that the loss of progenitor non-stem cell populations predominantly contributes to their acute hematological side-effects (27,28). As the early-forming CAFC subsets (representing committed short-term repopulating CFU-C and CFU-S progenitor populations) were relatively spared by TPZ treatment then it is perhaps not surprising that the high toxicity to real long-term repopulating HSCs have been previously overlooked.…”

“…Cells from six different regions of the Ho gradient were isolated and evaluated for hematopoietic activity in the cobblestone area-forming cell (CAFC) assay. CAFC frequencies at shorter (days [7][8][9][10][11][12][13][14] and longer (days [28][29][30][31][32][33][34][35] times in culture were used to measure relatively mature and primitive hematopoietic cell populations, respectively. As shown in Fig.…”