Objective. To report on the long-term follow-up, clinically and serologically, of 98 patients with SLE treated with B cell depletion (BCD) over a 12 year period, focusing on the duration of the depletion. Methods. A retrospective review of clinical and serological features of all SLE patients treated with BCD from January 2000 until December 2012 in the Centre for Rheumatology, University College London Hospital. Clinical activity was assessed by the classic BILAG score at baseline and 6 and 12

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... months after the treatment. Results. The period of depletion is extremely variable between patients and within the same patient on different occasions. The patients were divided into two groups according to the duration of depletion and a defined threshold of 12 months was utilized. The group with longer duration of depletion was associated with a better outcome, with a decrease in BILAG score at 6 and 12 months. This group was also associated with lymphopenia present at any time during the course of the patient's disease. No other clinical or serological feature was associated with longer duration of BCD. Conclusion. Cycles of BCD that induce longer duration of BCD are associated with better outcome. Lymphopenia may help to predict longer duration of the depletion and better outcome, although the mechanism is unclear. Rheumatology key messages . We report the largest single-centre experience of treating SLE patients with B cell depletion. . The longer the duration of the B cell depletion, the better the outcome overall in SLE. . Considerable variation in the response of individual SLE patients to B cell depletion is evident.