Phenotype of peripheral blood and synovial fluid B cells at first arthroscopy. Frequency of B cells in PBMC, SFMCs and single synovial biopsy tissue cells (A). while the percentage of synovial fluid B cells is a fraction of the peripheral blood B cell population, the vast majority of synovial fluid B cells belong to the switched memory B cell compartment (CD27+IgD–). Interestingly, synovial fluid B cells had marked expression of PD-1 and the chemokine receptor CXCR3 compared with peripheral blood B cells. data shown are from paired PBMC and SFMC pretofacitinib (B). SFMC, synovial fluid mononuclear cells; PBMCs, peripheral blood mononuclear cells.