CD11b-Donor Dendritic Cells Elevate Donor T-Cell Production of Interferon-Gamma and Graft-Versus-Leukemia Activity in Allogeneic Bone Marrow Transplantation.

Host antigen-presenting cells (APC) persist after high-dose chemotherapy and hematopoietic stem cell transplantation (HSCT) and initiate graft-versus-host disease (GvHD) in mouse models of HSCT. The role for donor APC on transplant outcomes is less clear. In clinical allogeneic HSCT from HLA-matched siblings, larger numbers of donor plasmacytoid dendritic cell (DC) precursors were associated with more relapse, and worse survival. Depletion of CD11b⁺ cells from bone marrow (containing CD11b⁺ DC) modestly augmented graft-versus-leukemia (GvL) activity in murine allogeneic HSCT. In this study, using allogeneic MHC mis-matched HSCT (C57BL/6→B10.BR) of mice bearing a lymphoblastic leukemia (LBRM), recipients of FACS–purified CD11b⁻ donor DC plus FACS–purified HSC and T-cells had dramatically improved long-term survival (45% alive at >100 days) compared to d 5% survival among recipients of HSC and T-cells, or HSC, T-cells and CD11b⁺ DC (p<0.001). Both donor CD11b⁻ and CD11b⁺ DC homed to lymphoid organs, and physically contacted donor T-cells, but only the CD11b⁻ DC subset led to higher levels of interferon-γ (IFN- γ) in serum and higher levels of donor spleen-derived T-cells that synthesized IFN-γ in vivo in the first 10 days post-transplant. In contrast, recipients of CD11b⁺ DC had higher level of sera of IL–10. In CD11b⁻ DC recipients, donor effector-memory CD8⁺ T-cells proliferated more rapidly and expanded in recipients, but did not cause debilitating GvHD in mice transplanted without leukemia. Donor-spleen-derived T-cells among recipients of CD11b⁻ DC were predominately CD8⁺ (CD4:CD8 ratio 0.7:1), while donor spleen-derived T-cells were predominately CD4⁺ (p<0.02; CD4:CD8 ratio 1.3:1) among recipients of CD11b⁺ DC. In vitro co-culture of purified DC with homologous T-cells responding to allo-antigen demonstrated the same pattern of cytokine production as found in vivo, supporting the ability of CD11b⁻ DC to generate Th1 immune responses that are associated with enhanced GvL effects and improved immune reconstitution. In vitro stimulation of DC subsets by CD40L (1μg/mL) and LPS (1μg/mL) showed that CD11b⁺ DC had higher level of PD–L1 expression compared to CD11b⁻ DC. In vitro exposure of LBRM to IFN-γ, at doses of 10–300 pg/ml, similar to those observed in vivo, had no direct cytotoxic effect, and had no long-term growth-inhibitory effect on proliferation during 5 days of culture. CD11b⁺ DC expressed higher levels of PD–L1 and led to higher T-cell synthesis of IL–10 in vivo and in vitro. Transplantation of CD11b⁻ DC resulted in higher levels of donor T-cell production of IFN-γ, and enhanced the GvL effect of donor T-cells without producing debilitating GvHD. These data indicated that purified donor DC can regulate post-transplant immunity via indirect antigen presentation to donor T-cells. CD11b⁺ DC expressed higher level of PD–L1 and led to higher level of IL–10 that resulted in more relapse and light GvHD. CD11b⁻ DC led to higher level of donor IFN-γ that resulted in a stronger GvL effect while no debilitating GvHD. Engineering the DC content of an allograft may augment immune reconstitution and GvL activity without significantly increasing GvHD in allogeneic HSCT.