PDCD1 and Autoimmunity: First identified as a marker of developing thymocytes, co-inhibitory receptor programmed cell death protein 1 (PD-1) plays a well-recognised role in restraining T cell responses in autoimmunity, infection or cancer (Chamoto et al., 2017; Dai et al., 2014; Sharpe and Pauken, 2018; Sun et al., 2018).