Abatacept statistically significantly reduced disease activity in patients with rheumatoid arthritis and an inadequate response to methotrexate. Longer treatment in different patient populations is needed to establish its appropriate role in rheumatoid arthritis.

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Objective To assess the effects of the B-lymphocyte stimulator (BLyS)-specific inhibitor belimumab on immunologic biomarkers, including B- and T-cell populations, and maintenance of antibody titers to prior vaccines in autoantibody-positive systemic lupus erythematosus (SLE) patients. Methods Pooled data from two phase 3 trials—BLISS-52 and -76—comparing belimumab 1 or 10 mg/kg vs placebo (each plus standard SLE therapy) were analyzed for changes in autoantibodies, immunoglobulin (Ig), and complement (C); BLISS-76 patients were analyzed for changes in B- and T-cell populations, and effects on prior vaccine-induced antibody levels. Results Belimumab-treated patients experienced significant sustained reductions in IgG and autoantibodies, and improvement in C3/C4, resulting in greater positive-to-negative conversion rates for IgG anti–double-stranded DNA (anti-dsDNA), anti-Smith, anticardiolipin, and antiribosomal P autoantibodies, and normalization of hypergammaglobulinemia and low C3/C4. Belimumab-treated patients experienced significant decreases in naïve and activated B cells, as well as plasma cells, whereas memory B cells and T-cell populations did not decrease. Belimumab did not substantially affect pre-existing antipneumococcal or antitetanus antibody levels. Post-hoc analysis showed greater reductions in SLE disease activity and the risk of severe flares in patients treated with belimumab 10 mg/kg (P ≤ 0.01) who were anti-dsDNA positive with low C3/C4 at baseline. Normalization of C3 or anti-dsDNA by 8 weeks, irrespective of therapy, was predictive of a reduced risk of severe flare over 52 weeks. Conclusion Belimumab appears to promote normalization of serologic activity and reduce BLyS-dependent B-cell subsets in serologically and clinically active SLE. Greater serologic activity may predict a better treatment response to belimumab.