Arellano and Lonial transplantation. A study by the European working group for blood and marrow transplant reported similar rates of overall survival, leukemia-free survival, and similar incidence of graft versus host disease (GVHD) when comparing bone marrow with cytokine-mobilized peripheral blood grafts. They did, however, observe improved platelet recovery with cytokine- mobilized peripheral graft compared to BMT (Schmitz et al 1998). A randomized, multi-center trial of cytokine-mobilized peripheral blood progenitor cell grafts versus BMT reported by Schmitz et al (2002), found an increased risk of acute and chronic GVHD among recipients of cytokine-mobilized peripheral blood grafts, but no difference in survival compared to recipients of bone marrow transplants. In contrast, in another randomized trial comparing peripheral blood hematopoietic progenitor cell and bone marrow transplantation, Bensinger et al found more rapid neutrophil and platelet recovery and similar rates of acute and chronic GVHD. In that trial, the duration of chronic GVDH was longer among recipients of blood progenitor cell grafts. Furthermore, recipients of blood progenitor cell grafts had a higher estimated probability of overall survival and 2-year disease-free survival (Bensinger et al 2001). Interestingly, Urbini and colleagues found a higher number of CD34+ cells in peripheral blood grafts mobilized with G-CSF compared to bone marrow, and the dose of CD34+ cells infused correlated with the number of DC1 in peripheral blood grafts and DC2 in bone marrow allografts. In addition, among recipients of cytokine mobilized grafts, a signiﬁcantly shorter overall survival and a trend toward lower disease free survival was noted among recipients of larger numbers of CD34+ cells (Urbini et al 2003). The fact that the incidence of acute GVHD in recipients of peripheral blood progenitor cell grafts was similar to, or only slightly higher than in BMT recipients, despite the higher content of T cells in the peripheral blood grafts, suggests that there may be quantitative differences in the other immune cellular components of peripheral blood grafts; such as, the presence of increased numbers of immunosuppressive DC2 in G-CSF mobilized peripheral blood grafts compared to bone marrow grafts (Arpinati et al 2000). Clinical outcomes after hematopoietic progenitor cell transplantation: contribution of dendritic cells Recent growing interest in the role of accessory cells in hematopoietic progenitor cell grafts has led to further studies of the immune-modulatory effects of CSF on the 16 constituents of the peripheral blood progenitor cell graft. Flt3 and GM-CSF administration led to mobilization of increased numbers of DC1 in the cellular apheresis product (Gasparetto et al 2002). In contrast, administration of G-CSF lead to mobilization of increased numbers of DC2 cells in the grafts (Arpinati et al 2000). The clinical consequences of mobilizing more DC1 with GM-CSF and more DC2 with G-CSF remain unknown, and are the subject of current investigation. Waller et al tested the hypothesis that the cellular con- stituents of the graft could affect clinical outcomes after bone marrow transplantation (Waller et al 2001). They performed a retrospective study of 113 patients with hematological malignancies who received non-T cell-depleted bone marrow grafts from HLA-matched siblings. After evaluating patient and disease-speciﬁc characteristics, characteristics of the graft constituents, and clinical outcomes, they reported that patients who received larger numbers of donor DC2 had signiﬁcantly worse clinical outcomes, with lower event-free survival, less chronic GVHD, and an increased incidence of relapse than their counterparts who received fewer numbers of DC2 cells (Waller et al 2001). The signiﬁcance of absolute numbers of DC in post- transplant outcomes was also evaluated in a clinical study by Reddy and colleagues (Reddy et al 2004). Fifty patients undergoing allogeneic transplantation for hematological disorders were evaluated. After evaluating the constituents of the grafts, as well as the type and number of DC in the peripheral blood early after engraftment, they noted that patients with lower absolute numbers of DC ( 4.97 cells/μL) at engraftment had worse clinical outcomes compared to patients with higher numbers of DC at engraftment, with lower overall survival (p = 0.002), increased incidence of relapse (p = 0.002), and a higher incidence of acute GVHD (p = 0.0005). Multivariable analysis conﬁrmed that low DC count was independently associated with death (hazard ratio [HR], 3.8; p = 0.02), time to relapse (HR, 11.6; p = 0.001), and acute GVHD (HR, 3.3; p = 0.04). The effect was similar when DC1 were analyzed separately. However, when DC2 were analyzed separately, the effect was only signiﬁcant for increased incidence of acute GVHD among patients with lower numbers of DC2 at engraftment. The independent effect of DC1 and DC2 was not conﬁrmed in the multivariate analysis (Reddy et al 2004). Subsequently, Lonial and colleagues hypothesized that the combination of G-CSF and GM-CSF administered for the mobilization of stem cells after chemotherapy would reduce the content of DC2 in the autologous blood hematopoietic Biologics:Targets & Therapy 2008:2(1)