PDCD1 and neoplasm: Although the intricacies of immune system recovery following administration of CTLA-4- and PD-1-targeted MAbs, individually or in combination, remain to be fully elucidated, it seems likely that the differentiation and expansion of gut mucosal Th17 cells, which are strongly influenced by components of the gut microbiota (24), play a prominent role in the pathogenesis of some types of IRAEs and possibly the anti-tumor effects of these agents.