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T he cytokines B lymphocyte stimulator (BLyS) and a proliferation-inducing ligand (APRIL) enhance autoimmune disease by sustaining B cell activation. In RA, B cells contribute to the formation of 3 functionally distinct types of lymphoid microarchitectures in the inflamed synovium: ectopic GCs; T cell–B cell aggregates lacking GC reactions; and unorganized, diffuse infiltrates. We examined 72 tissues representing the 3 types of synovitis for BLyS and APRIL production and for expression of APRIL/BLyS receptors. Biologic effects of BLyS and APRIL were explored by treating human synovium–SCID mouse chimeras with the APRIL and BLyS decoy receptor transmembrane activator and CAML interactor:Fc (TACI:Fc). GC⁺ synovitis had the highest levels of APRIL, produced exclusively by CD83⁺ DCs. BLyS was present in similar levels in all tissue types and derived exclusively from CD68⁺ macrophages. In GC⁺ synovitis, treatment with TACI:Fc resulted in GC destruction and marked inhibition of IFN-γ and Ig transcription. In contrast, inhibition of APRIL and BLyS in aggregate and diffuse synovitis left Ig levels unaffected and enhanced IFN-γ production. These differential immunomodulatory effects correlated with the presence of TACI⁺ T cells in aggregate and diffuse synovitis and their absence in GC⁺ synovitis. We propose that BLyS and APRIL regulate B cell as well as T cell function and have pro- and antiinflammatory activities in RA.