Background: SB-728-T is a zinc finger nuclease-mediated, CCR5-modified autologous CD4 T-cell product that can provide a reservoir of CD4 T-cells that are resistant to HIV entry. Studies in ART-treated CCR5 Δ32 heterozygote HIV subjects showed VL reductions from peak during treatment interruption that correlated with estimated circulating bi-allelic CCR5-modified CD4 T-cells (maximal at ~200 cells/ul), supporting the importance of achieving this level of transferred T cells. We have now evaluated cyclophosphamide (CTX) conditioning prior to infusion of SB-728-T in HIV subjects. CTX enhances the engraftment and activity of adoptively transferred anti-cancer T-cells by homeostatic proliferation of the transferred T cells, an increase in endogenous cytokines (e.g. IL-7 and 15) and promotion of homing to lymphoid organs. Methodology: A dose escalation study of IV CTX, 200 mg (n=3); 500 mg/m2 (n=6) and 1 g/m2 (n=3) administered 1-3 days prior to SB-728-T infusion was performed in aviremic, ART treated HIV subjects with CD4 T cell counts > 500. Results: CTX was well tolerated at all doses, except for low grade GI side-effects which were treated with anti-emetics. No clinically significant decreases in neutrophils, WBC, platelets or Hgb/Hct were seen. A dose-related increase in total CD4 count and engraftment of CCR5 modified cells was observed (median estimated bi-allelic modified CD4 cells and median increase in total CD4 at six weeks post infusion of 14/μL and 299/μL at 200 mg, 74/μL and 771/μL at 500 mg/m2 and 148/μL and 2077/μL at 1 g/m2, respectively). By comparison, total CD8 cell count at wk 6 was not affected by the CTX dose administered (median of 783/μL at 200mg, 536/μL at 500 mg/m2 and 679/μL at 1 g/m2). Engraftment of estimated bi-allelic CCR5 modified CD4 cells with the 1 g/m2 CTX dose was near levels associated with anti-viral effects in CCR5Δ 32 heterozygote HIV subjects. A 1.1 to 1.5-log VL reduction from peak was seen in 2 subjects at the 500mg/m2 dose level and in another subject at the 1 gram/m2 dose level during a 16 week TI. The latter subject had an increase of CD4 T-cells of 3496 cells /μl at wk 1, and estimated bi-allelic modified CD4 T-cells of 148 cells/μL at six weeks post infusion with VL reduction from 235K to 7.9 K (VL set pt 35,500). Conclusions: CTX pre-treatment is well tolerated up to 1 g/m2 in ART treated HIV subjects with no adverse effects on hematologic parameters. A doserelated increase in engraftment of SB-728 T cells occurred to levels obtained in CCR5Δ 32 heterozygote HIV subjects who have achieved unmeasurable VL during TI. CTX conditioning may be a useful strategy to 1) maximize the engraftment and anti-viral effects of SB-728-T adoptive T cell therapy in HIV subjects and 2) may be an important immunomodulatory chemotherapeutic agent for immunotherapy in HIV.