CD4 and rheumatoid arthritis: However, despite the suppressive function of the PD-1–PD-L1 pathway on TCR-mediated proliferation, recently it has been discovered that PD-1high MP CD4 T cells are pathogenic in Rheumatoid Arthritis (RA) and systemic lupus erythematosus (SLE) patients and are not only inflammatory but also promote the responses of autoimmune B cells (Rao et al, 2017; Bocharnikov et al, 2019; Caielli et al, 2019; Zhang et al, 2019), indicating that regulatory mechanisms in these cells control their homeostasis in the steady state.