Martin Chopra, Marlene Biehl, Tim Steinfatt, Andreas Brandl, Juliane Kums, Jorge Amich, Martin Vaeth, Janina Kuen, Rafaela Holtappels, Jürgen Podlech, Anja Mottok, Sabrina Kraus, Ana-Laura Jordán-Garotte, Carina A. Bäuerlein, Christian Brede, Eliana Ribechini, Andrea Fick, Axel Seher, Johannes Polz, Katja J. Ottmueller, Jeannette Baker, Hidekazu Nishikii, Miriam Ritz, Katharina Mattenheimer, Stefanie Schwinn, Thorsten Winter, Viktoria Schäfer, Sven Krappmann, Hermann Einsele, Thomas D. Müller, Matthias J. Reddehase, Manfred B. Lutz, Daniela N. Männel, Friederike Berberich-Siebelt, Harald Wajant, Andreas Beilhack

• Donor CD4\(^+\)Foxp3\(^+\) regulatory T cells (T reg cells) suppress graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (HCT allo-HCT]). Current clinical study protocols rely on the ex vivo expansion of donor T reg cells and their infusion in high numbers. In this study, we present a novel strategy for inhibiting GvHD that is based on the in vivo expansion of recipient T reg cells before allo-HCT, exploiting the crucial role of tumor necrosis factor receptor 2 (TNFR2) in T reg cell biology. ExpandingDonor CD4\(^+\)Foxp3\(^+\) regulatory T cells (T reg cells) suppress graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (HCT allo-HCT]). Current clinical study protocols rely on the ex vivo expansion of donor T reg cells and their infusion in high numbers. In this study, we present a novel strategy for inhibiting GvHD that is based on the in vivo expansion of recipient T reg cells before allo-HCT, exploiting the crucial role of tumor necrosis factor receptor 2 (TNFR2) in T reg cell biology. Expanding radiation-resistant host T reg cells in recipient mice using a mouse TNFR2-selective agonist before allo-HCT significantly prolonged survival and reduced GvHD severity in a TNFR2-and T reg cell-dependent manner. The beneficial effects of transplanted T cells against leukemia cells and infectious pathogens remained unaffected. A corresponding human TNFR2-specific agonist expanded human T reg cells in vitro. These observations indicate the potential of our strategy to protect allo-HCT patients from acute GvHD by expanding T reg cells via selective TNFR2 activation in vivo.…