CXCR4 and rheumatoid arthritis: Previous flow cytometric studies have reported a presence of hyper-activated neutrophils in RA-SF, with a high expression of activation-associated surface markers, including CD14, CD64 (a.k.a. FcγRI), PD-L1, ICAM-1, CXCR4, HLA-DR or DB, and lower expression of CXCR1 (31–34).