To study the expression of blood dendritic cell antigen 2 (BDCA‐2) and BDCA‐4 molecules by plasmacytoid dendritic cells (PDCs) in the blood of patients with systemic lupus erythematosus (SLE), and to study PDC production of interferon‐α (IFNα) and its inhibition by anti–BDCA‐2 and anti–BDCA‐4 antibodies.

Peripheral blood mononuclear cells (PBMCs) from SLE patients (SLE PBMCs) and from healthy controls were induced to produce IFNα in vitro by SLE serum containing an endogenous IFNα‐inducing factor (SLE‐IIF) or by herpes simplex virus type 1 (HSV‐1). The frequencies and numbers of BDCA‐2–, BDCA‐3–, and BDCA‐4–expressing cells were analyzed by flow cytometry, and the effects of anti–BDCA‐2 and anti–BDCA‐4 monoclonal antibodies (mAb) on IFNα production were investigated.

IFNα production by SLE PBMCs induced by SLE‐IIF or HSV‐1 was decreased compared with that of healthy control PBMCs (P = 0.002 and P = 0.0007, respectively). The proportions of BDCA‐2– and BDCA‐3–expressing cells in SLE PBMCs were reduced compared with those in PBMCs from healthy controls (P = 0.01 and P = 0.004, respectively). IFNα producers in culture, especially among SLE PBMCs, displayed reduced BDCA‐2 expression and constituted only a minority of the BDCA‐2–positive cells, at least in healthy control PBMCs (median 18%). IFNα production by both SLE and healthy control PBMCs stimulated by SLE‐IIF or HSV‐1 was markedly reduced by anti–BDCA‐2 mAb (median 81–98% inhibition). Anti–BDCA‐4 mAb only partially inhibited SLE‐IIF–induced IFNα production.

SLE patients had a reduced number of BDCA‐2–expressing PDCs, also termed natural IFNα‐producing cells, and their IFNα production could be inhibited by anti–BDCA‐2/4 mAb. Such mAb may be a therapeutic option for inhibiting the ongoing IFNα production in SLE patients.