PDCD1 and neoplasm: These authors also proposed that hyperprogressive disease (HPD), a type of rapid tumor progression that occurs in some patients after immune checkpoint inhibitor treatment [21], could be due to blockade of the PD-1/PD-L1 axis in PD-1 + “exhausted” Tregs, which are present at a relatively high frequency in the microenvironment of tumors undergoing HPD [20].