AMD3100 Enhances Engraftment treated mice occurred mainly within 2 weeks, a period before any blood cell count differences were apparent between AMD3100- treated and PBS-injected mice. One possible explanation for this time gap could be that AMD3100 promotes earlier hematopoietic progenitor recovery. To determine whether post transplant admin- istration of AMD3100 would affect hematopoietic progenitor cell recovery, we measured CFUs-spleen at day +9. As shown in Fig. 3A, AMD3100-treated mice demonstrated a trend to higher number of CFUs-spleen compared to PBS- injected control mice. Another possible mechanism may be related to AMD3100 mediated- inhibition of a cytokine storm. Transplant preparative conditioning induces massive cell death and tissue damage, resulting in a dramatic increase in the levels of secreted chemokines, cytokines and proteolytic enzymes[16]. This massive release of cytokines/chemokines (a ‘‘cytokine storm’’) exacerbates inflammation and vascular/tissue injuries, and leads to multi- organ toxicities. CXCR4 plays an important role in the cytokine storm by recruiting inflammatory cells to the injury sites[31,32]. We reasoned that post transplant administration of AMD3100 would attenuate the cytokine storm, and hence reduce multi-organ toxicities. We measured a total of 23 serum cytokines at Day +7 post transplantation (Fig.3B). We found that post transplant Figure 3. Post transplant administration of AMD3100 increases CFUs-spleen and reduces cytokine, chemokine, and growth factor levels. A: CFUs-spleen assay. Lethally irradiated CD45.2 mice (10.5 Gy) were injected with whole marrow cells from CD45.1 mice (16105 marrow cells per mouse). The recipient mice were injected subcutaneously with PBS buffer or AMD3100 at 5 mg/kg every other day beginning at day +2. At day +9, the mice were sacrificed and CFUs-spleen measured (n = 5–10 mice per group). B: Plasma cytokine/chemokine measurement. Lethally irradiated CD45.2 mice (10.5 Gy) were injected with sorted KTLS stem cell from CD45.1 mice (7000 KTLS cells per mouse). The recipient mice were injected subcutaneously with PBS buffer or AMD3100 at 5 mg/kg every other day beginning at day +2. The mice were bled at Day +7 and plasma prepared. Plasma cytokines and chemokines (pg/ml) were measured as per manufacturer’s instruction (Bioplex, Bio-Rad Laboratories) (n = 5 mice per group). G- CSF level was shown in a separate panel. C: 9 cytokines/chemokines that were significantly reduced in AMD3100-treated mice. D: Survival rate in lethally irradiated mice without HCT. CD45.2 mice were lethally irradiated (10.5 Gy) but not transplanted with HSCs. The mice were then administered subcutaneously with PBS or AMD3100 at 5 mg/kg every other day beginning at day +2 post irradiation until the end of experiments. Animal survival was monitored daily (data representative of 3 separate sets of experiments with 10 mice in each group of each experiment). doi:10.1371/journal.pone.0011316.g003 PLoS ONE | www.plosone.org 4 June 2010 | Volume 5 | Issue 6 | e11316