Reductions in circulating B cell subsets and immunoglobulin G levels with long-term belimumab treatment in patients with SLE.

Belimumab treatment up to 312 weeks (6 years) resulted in substantial decreases in several circulating B cell subsets and IgG levels. Higher baseline naïve B cell counts and IgG levels were associated with improved SRI-4 response and increased infection rates, respectively.