AMD3100 (AMD) is a bicyclam compound that rapidly mobilizes hematopoietic progenitor cells into circulation by inhibiting stromal cell derived factor-1 binding to its cognate receptor CXCR4 present on CD34+ cells. Preliminary data in healthy donors and cancer patients show large numbers of CD34+ cells are mobilized following a single injection of AMD3100. To determine whether AMD3100 mobilized cells would be suitable for allografting, we performed a detailed phenotypic analysis using 6 color flow cytometry (CYAN Cytometer MLE) of lymphocyte subsets mobilized following the administration of AMD3100, given as a single 240mcg/kg injection either alone (n=4) or in combination with G-CSF (n=2: G-CSF 10 mcg/kg/day x 5: AMD3100 given on day 4). Baseline peripheral blood (PB) was obtained immediately prior to mobilization; in recipients who received both agents, blood was analyzed 4 days following G-CSF administration as well as 12 hours following administration of AMD3100 and a 5th dose of G-CSF. AMD3100 alone significantly increased from baseline the PB WBC count (2.8 fold), Absolute lymphocyte count (ALC: 2.5 fold), absolute monocyte count (AMC: 3.4 fold), and absolute neutrophil count (ANC: 2.8 fold). Subset analysis showed AMD3100 preferentially increased from baseline PB CD34+ progenitor counts (5.8 fold), followed by CD19+ B-cells (3.7 fold), CD14+ monocytes (3.4 fold), CD8+ T-cells (2.5 fold), CD4+ T-cells (1.8 fold), with a smaller increase in CD3−/CD16+ or CD56+ NK cell counts (1.6 fold). There was no change from baseline in the % of CD4+ or CD8+ T-cell expressing CD45RA, CD45RO, or CD56, CD57, CD27, CD71 or HLA-DR. In contrast, there was a decline compared to baseline in the mean percentage of CD3+/CD4+ T-cells expressing CD25 (5.5% vs 14.8%), CD62L (12.1% vs 41.1%), CCR7 (2.1% vs 10.5%) and CXCR4 (0.5% vs 40.9%) after AMD3100 administration; similar declines in expression of the same 4 surface markers were also observed in CD3+/CD8+ T-cells. A synergistic effect on the mobilization of CD34+ progenitors, CD19+ B cells, CD3+ T-cells and CD14+ monocytes occurred when AMD3100 was combined with G-CSF (Figure). In those receiving both AMD3100 and G-CSF, a fall in the % of T-cells expressing CCR7 and CXCR4 occurred 12 hours after the administration of AMD3100 compared to PB collected after 4 days of G-CSF; no other differences in the expression of a variety activation and/or adhesion molecules on T-cell subsets were observed. Whether differences in lymphocyte subsets mobilized with AMD3100 alone or in combination with G-CSF will impact immune reconstitution or other either immune sequela (i.e. GVHD, graft-vs-tumor) associated with allogeneic HCT is currently being assessed in an animal model of allogeneic transplantation.