Paradoxical Effects of Interferon-γ in Allogeneic Bone Marrow Transplant: Donor Dendritic Cells Regulate Graft-Versus-Leukemia and Graft- Versus-Host Disease Activities.

Both host and donor dendritic cells (DCs) have been shown to play a critical role in regulating graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) effect after MHC-mismatched bone marrow transplantation (BMT) (Shlomchik et al. Science 1999, Reddy et al. Nat Med 2005). In contrast to host DCs, much less is known about the precise mechanisms donor DCs may use to modulate donor T-cell activation and GVL activity. A clinical report has suggested an association between the number of donor plasmacytoid DC in the graft and leukemia relapses after allogeneic BMT (Waller et al. Blood 2001). Using allogeneic MHC-mismatched hematopoietic stem cell transplant (HSCT) (C57BL/6→B10.BR) in mice bearing the T lymphoblastic leukemia LBRM, we have previously reported that recipients transplanted with purified CD11b⁻ DC in combination with purified HSC and T-cells had 45% increased long-term leukemia-free-survival, higher numbers of interferon-γ (IFN-γ) producing donor T-cells as well as higher levels of serum IFN-γ (Li et al. Blood 2007). The aim of the present work is to further define whether production of IFN-γ by donor T-cells is necessary for the augmentation of GVL effect seen with CD11b⁻ donor DC and define the mechanism that donor CD11b⁻ DC can augment GVL of donor T-cells without causing fatal GVHD. To evaluate the role for IFN-γ produced by donor T-cells, we used IFN-γ knockout (KO) mice as donors in the C57BL/6→B10.BR transplant model. Recipients of IFN-γ KO donor T-cells in combination with wild-type FACS-purified HSC and CD11b⁻ DC died rapidly with 0% survival at day 80 compared with 65% survival among tumor-bearing recipients of donor CD11b⁻ with wild-type HSC and T-cells and 75% survival in mice transplanted with wild-type cells in the absence of LBRM. Moreover, the addition of donor CD11b⁻ DC to IFN-γ KO donor T-cells did not lead to further augmentation of GVHD. These data supported a role for donor T-cell-derived IFN-γ in the enhanced GVL activity seen among recipients of donor CD11b⁻ DC,but did not explain the lack of increased GVHD. As a potent pro-inflammatory cytokine initiating immune response in GVHD, IFN-γ has also been demonstrated to show a suppressive effect during GVHD as a result of IFN-γ-inducible indoleamine-2,3-dioxygenase(IDO) gene expression. CD11b⁻ DCs were freshly isolated from bone marrow of donor C57BL/6 mice, exposed to 100ng/ml IFN-γ for 18 hours, and the IDO expression was measured by intracellular staining. The results showed that following IFN-γ treatment, IDO levels of CD11b⁻ DCs were up-regulated. Furthermore, in vitro co-culture of FACS-purified CD11b⁻ DC with syngeneic T-cells in the presence of allogeneic antigen also demonstrated increased IDO levels on the co-cultured DCs. Taken together, our data support a model in which donor CD11b⁻ DCs initially induce Th1 polarization of activated donor T-cells that secret high levels of IFN-γ in the lymph node microenvironment. High local levels of IFN-γ subsequently induce IDO expression in DC, resulting in down-modulation of T-cell allo-reactivity and GVHD. Thus, IFN-γ-induced IDO expression on CD11b⁻ donor DCs appears to be a critical downstream event that inhibits continued T-cell activation and leads to less severe GVHD.