CXCL13 and rheumatoid arthritis: Recently, PD-1high MHC-II+ (major histocompatibility complex class II) peripheral helper T cells (TPH) were identified in the synovial fluid and tissue of seropositive RA patients7–9 where they are proposed to facilitate B cell recruitment and activation through CXCL13 and IL-21 production; however, there is still no satisfying marker that defines the TPH subset.