To evaluate the safety and clinical efficacy of administering an anti–interleukin‐10 (anti–IL‐10) monoclonal antibody (mAb) to systemic lupus erythematosus (SLE) patients with active and steroid‐dependent disease. In addition, we sought to assess the effects of in vivo IL‐10 neutralization on biologic markers of SLE.

Treatment consisted of 20 mg/day intravenous administration of an anti–IL‐10 murine mAb (B‐N10) for 21 consecutive days, with a followup period of 6 months. Six patients were studied.

Treatment was safe and well tolerated. All patients developed antibodies against B‐N10. Cutaneous lesions and joint symptoms improved in all patients beginning during B‐N10 administration and continuing to month 6. The SLE Disease Activity Index decreased from a mean ± SEM of 8.83 ± 0.91 on day 1 to 3.67 ± 0.67 on day 21 (P = 0.001), 1.50 ± 0.84 at month 2, and 1.33 ± 0.80 at month 6 (P < 0.001). At the end of followup, the disease was clinically inactive in 5 of the 6 patients. Prednisone administration was decreased from a mean ± SEM of 27.9 ± 5.7 mg/day on day 1 to 9.6 ± 2.0 mg/day at month 6 (P < 0.005). Activity of immune and endothelial cells rapidly decreased, as assessed by the early evolution of several biologic markers.

This is the first report of IL‐10 antagonist administration to humans. The study shows the involvement of IL‐10 in the pathogenesis of SLE, and indicates that the use of IL‐10 antagonists may be beneficial in the management of refractory SLE.