TRERF1 and systemic lupus erythematosus: Compared to the PBS group, all MSC-EV treatment groups exhibited significantly reduced the proportions of plasma cells (PBS group: 27.25 ± 5.18 %, FBS-EV group: 18.5 ± 1.74 %, Rapa-FBS-EV group: 15.28 ± 1.75 %, SLE-EV group: 15.52 ± 2.21 %, Rapa-SLE-EV group: 11.2 ± 4.38 %).