Activation of PD-1 may be involved in such impaired activation of Tregs in GVHD and hepatic irAEs, as PD-1-deficient-Tregs display a more enhanced apoptosis than PD-1-intact-Tregs in the presence of low doses of interleukin-2, a crucial cytokine involved in Treg proliferation28. Here, PDCD1 is linked to graft versus host disease.