CXCL13 and rheumatoid arthritis: Considering that PD-1hiCXCR5−CD4+ T cells are the main source of CXCL13 among synovial T cells in RA2,5, our findings in vitro and in vivo collectively imply that Sox4 expression in combination with the status of the intracellular signals leads to the preferential production of CXCL13 by PD-1hiCXCR5−CD4+ T cells in inflamed joints of RA.