TNFRSF9 and acute graft versus host disease: As for the three MUD recipients in whom SARS-CoV-2-specific CD137+ T cells remained below or at the assay detection limit (0.1 T cells/μl: R036, R040, R049), potent suppression of T-cell proliferation and function resulted from treatment of acute graft-versus-host disease (aGVHD).