CD19 and systemic lupus erythematosus: The MSC-EV-treated groups had a significantly lower proportion of memory B cells (CD19+CD27+CD38–) than the PBS group (PBS: 8.42 ± 1.17 %, FBS-EV: 7.53 ± 0.85 %, Rapa-FBS-EV: 6.22 ± 0.69 %, SLE-EV: 6.47 ± 0.98 %, Rapa-SLE-EV: 5.28 ± 0.53 %).