Peripheral Blood Stem Cell Grafts Mobilized by G-CSF and Plerixafor in Comparison to G-CSF Alone: Phenotypic and Functional Characterization,
Authors: Gaugler Béatrice , Arbez J , Frikeche J , Le Gouill Steven , Gastinne Thomas , Saas Philippe , Moreau Philippe , Derenne S , Mohty Mohamad 
Published: October 2019

Abstract
Abstract 4045

Background.
Plerixafor is a CXCR4 antagonist that is currently approved for use in combination with G-CSF to mobilize autologous peripheral blood stem cells in patients with lymphoma and myeloma candidate to autologous transplantation. Previous studies showed that the drug can efficiently mobilize higher numbers of CD34+ stem cells and early hematopoietic progenitors. However, immune factors contained in the graft may also be important in terms of post-transplant outcome. Thus, this study aimed to characterize the phenotype and function of the different immune effectors contained in aphaeresis samples obtained from patients mobilized with Plerixafor and G-CSF (P+G group) in comparison to grafts mobilized with G-CSF alone (G group).

Patients and Methods.
Aliquots of aphaeresis products were obtained from 36 lymphoma and myeloma patients following treatment with 5 days of G-CSF (10 μg/kg/day; n=18) or following sequential mobilization with G-CSF and Plerixafor (240 μg/kg; n=18), according to Plerixafor label. The phenotype and cytokine secretion profile of the different T cell and dendritic cell (DC) subsets were characterized by multicolor flow cytometry including intracellular cytokine staining.

Results.
In samples derived from grafts collected after P+G, there was a significantly higher % of total CD3+ T cells (median, 81%) as compared to samples collected after G alone (median 71%; p=0.01). However, the CD4/CD8 ratio was comparable between both groups (p=0.56). The % of CD19+ B cells and CD3-CD56+ NK cells were not significantly different between both groups. When considering the different T cell subsets (naïve, central memory, terminally differentiated and memory), there was no significant differences in the distribution of T cells between the 2 groups. However, from a functional level, there was a significant increase of CD8+ IFN-gamma and TNF-alpha secreting T cells in the P+G group as compared to the G group (median, 12.3% vs. 5.3%, p=0.01; and 5.9% vs. 2.8%, p=0.02, respectively). IFN-gamma and TNF-alpha secreting CD4+ T cells were also increased, but to a lesser extent, in the P+G group (median, 4.4% vs. 2.5%, p=0.04; 8.6% vs. 5.3%, p=0.07; respectively). Grafts mobilized with P+G contained a similar percentage of CD4+ regulatory T cells (Treg; characterized by CD25 and Foxp3 expression; median 5.9% vs. 5.0%, p=0.21). Nevertheless, the Treg compartment in the P+G grafts contained a lower proportion of ICOS+Foxp3+ cells (36.6% vs. 55%, p=0.09). Also, Treg from the P+G grafts displayed a significantly higher expression of CD127 (median MFI, 387 vs. 240, p=0.002) suggesting that Treg mobilized with P+G likely exhibit different functional properties.
In terms of DC subsets, grafts mobilized with P+G contained similar % of myeloid (MDC, Lin-CD11c+HLA-DR+CD123-) and BDCA3+ DCs. In contrast, the % of plasmacytoid DCs (PDC; CD123+BDCA2+HLADR+) was significantly increased in the P+G grafts (median, 0.87% vs. 0.30%; p=0.002), leading to a significantly higher PDC/MDC ratio in the P+G group, 2.08 vs. 1.01, p<0.0001). PDCs mobilized by P+G displayed different functional markers in comparison to PDCs mobilized with G alone: higher % of ILT7+ PDCs (65.5 vs. 44.4, p=0.03), and decreased expression of CD86 (5.8% vs. 11.4%, p=0.007; MFI, 787 vs. 944, p=0.01) suggesting a potential regulatory capacity of PDCs mobilized by P+G.

Conclusion.
This comprehensive comparative analysis showed that grafts mobilized with P+G exhibited major different phenotypic and functional features in comparison to grafts mobilized with G alone, highlighting that such grafts may have a significant impact on patients' outcome after autologous and allogeneic stem cell transplantation. We are currently testing the in vivo relevance of these differences in a mouse transplant model.

Disclosures:
Mohty: Genzyme: Honoraria, Membership on an entity's Board of Directors or advisory committees.