The curative Graft-vs-Tumor effect (GvT) of allogeneic Stem cell transplantation (SCT) is frequently complicated with life threatening Graft-vs-Host Disease (GvHD). In mice, prevention of GvHD, without abrogation of GvT is possible by co-transplantation of naturally occurring regulatory T cells (Tregs) with SC grafts. Consistent with these murine studies, we recently demonstrated that also human Tregs possess potent GvHD-downregulatory capacities in a xenogeneic(x) model, where x-GvHD is induced by infusion of autologous human T cells in RAG2−/−γc−/− mice (

Mutis et al.
Clin. Cancer Res.
). Towards clinical application of Tregs, we now explored the impact of human Treg-administration on GvT in a bioluminescence imaging (BLI) based human-GvT model in the RAG2−/−γc−/− mice. In this model, mice inoculated with luciferase (LUC)-transduced human myeloma (MM) cell lines developed BLI-detectable, progressive, MM-like multifocal tumors exclusively in the bone marrow (BM). Full blown tumors were effectively eliminated by infusion of allogeneic human PBMC. This treatment also caused lethal x-GvHD as expected. In this setting, co-infusion of human PBMC with autologous, in vitro cultured Tregs at a 1:1 Treg: T effector cell ratio had no adverse effects on the development of GvT while significantly reducing the lethality of x-GvHD. In vitro analyses of sacrificed mice at day 21 revealed that administered Tregs homed to BM and spleen, significantly downregulated the total numbers of IFN-γ-producing CD4+ and CD8+ T cells responding to CD3 mediated signals, but had no downregulatory effect on the frequencies of IFN-γ-producing T cells responding to tumor cells. There was also no downregulation of cytotoxic activity against tumor cells in Treg-treated mice. Conclusively, these results showed that Tregs, at doses which are inhibitory for x-GvHD-inducing T cells, could maintain the GvT effect by allowing T cell reactivity against tumor cells. Human Tregs thus still hold promise as attractive cellular tools for separating GvT from GvHD.

Author notes

Disclosure:Research Funding: This work is in part funded by IMF ( International Myeloma Foundation).