PDCD1 and rheumatoid arthritis: Recently, it has been shown that a subset of MP CD4 T cells express high levels of PD-1 (PD-1high MP) and have pathological function in the development of RA and SLE (Rao et al, 2017; Bocharnikov et al, 2019; Caielli et al, 2019; Zhang et al, 2019).