Donor BM Dendritic Cells Expressing IDO Limit Graft-Versus-Host Disease After Allogeneic Bone Marrow Transplant.

In contrast to the essential role of host dendritic cells (DC) in the initiation of graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) reactions, less is known about the effects of donor DC on T cells in these processes. We have previously reported that adding donor BM plasmacytoid DC (pDC) progenitors to a murine graft composed of purified hematopoietic stem cells (HSC) and T-cells increased donor activation and Th1 polarization leading to enhanced GVL activity without increasing GVHD (Li et al. 2007 Blood 110:2181), while larger numbers of human donor pDC were associated with less GVL activity following allogeneic bone marrow transplant (BMT) (Waller et al. 2001 Blood 97:2948). To explore the dissociation of GVHD from GVL we tested the hypothesis that activation of donor T-cells by donor pDC leads to reciprocal induction of indoleamine 2,3-dioxygenase (IDO) expression and immune counter-regulatory activity by donor DC that limits donor T-cell allo-reactivity.

pDC precursors were purified by high-speed FACS from un-stimulated BM harvested from wild type (WT) and IDO knock-out (IKO) mice. T-cell proliferation and immune polarization in response to indirect antigen presentation by syngenic DC was measured in mixed lymphocyte reaction (MLR) and by recovery of CFSE-labeled donor T-cells from allogeneic transplant recipients. IDO expression in DC was measured by FACS and intracellular staining using pDC from IKO BM as a negative staining control. FACS-purified 5 × 10⁴ pDC either from WT mice or from IKO mice in combination with 3 × 10³ c-kit⁺ Sca-1⁺ hematopoietic stem cells (HSC) and 3 × 10⁵ T-cells were transplanted in MHC mismatched C57BL/6→B10.BR model following lethal irradiation.

FACS-purified lineage⁻CD11c^loCD11b⁻ pDC expressed B220 (72%), CD90 (51%), and CD317 (PDCA-1) (93%), had low levels of MHC-II, partial expression of CD4, and lacked expression of CD24, CD80, CD86 and NK cell or granulocytic markers. IDO expression in purified pDC was up regulated by IFN-γ produced by syngenic T-cells in vitro in one-way MLR. In vivo proliferation of CFSE-labeled donor T-cells was enhanced in mice that received pDC from either WT or IKO mice. Co-transplantation of IKO pDC led to higher proliferation rates of CD8⁺ T-cells but not CD4⁺ T-cells compared with the proliferation of corresponding donor T-cell subset co-transplanted with WT DC. The incidence and severity of GVHD (weight loss and GVHD score) were markedly increased in recipients receiving pDC from IKO mice as compared with mice receiving WT pDC. The enhanced GVL activity of donor T-cells induced by transplanted donor WT pDC was abolished when IKO pDC were transplanted into tumor-bearing recipients. Transplanting WT donor pDC led to larger numbers of donor-derived CD4⁺CD25⁺Foxp3⁺ T-reg cells in the spleens of transplant recipients compared with mice receiving IKO pDC (p<0.01) in combination with purified HSC and T-cells.

Taken together, our data suggest IDO expression in pDC as a critical downstream event that inhibits continued T-cell activation and GVHD. We propose a feedback model in which donor pDC initially induce Th1 polarization of activated donor CD8⁺ T-cells that secret high levels of IFN-γ. IDO expressed by donor pDC in response to local IFN-γ subsequently induces a counter-regulatory effect including the generation of T-reg and down-modulation of CD8⁺ T-cell allo-reactivity and proliferation, limiting GVHD while preserving the GVL activity of donor T-cells.

No relevant conflicts of interest to declare.