PDCD1 and metastatic melanoma: However, a negative correlation between the Treg frequency and irAEs was discovered by preclinical models of irAEs (48), and the frequency of peripheral Tregs between irAEs group and non-irAEs group showed no significant differences in patients with advanced metastatic melanoma who were receiving PD-1 inhibitors (44), which implied the controversial role that Tregs played in irAEs.