B Lymphocyte Stimulator Levels in Systemic Lupus Erythematosus: Higher Circulating Levels in African American Patients and Increased Production after Influenza Vaccination in Patients with Low Baseline Levels

B lymphocyte stimulator (BLyS, also known as TNFSF13B and BAFF) is a type II transmembrane protein and a member of the TNF superfamily (1, 2). BLyS is produced by several different cell types, including monocytes, activated neutrophils, T cells, and dendritic cells (3–5), and is expressed as a cell surface protein which can be furin-cleaved and released into the circulation. Although BLyS has been shown to be constitutively expressed, certain inflammatory cytokines such as IL-2, TNF-α, IFN-γ, and IFN-α can enhance its production and secretion (3, 4, 6, 7). BLyS binds to 3 receptors found primarily on B cells (8). Activation of these receptors leads to B cell proliferation, differentiation, survival, and IgG class switching (1, 4, 9). BLyS has been shown to play an important role in primary immune responses, as anti-BLyS treated mice show profoundly reduced numbers of naive B cells with accompanying attenuated responses to both T-dependent and T-independent antigens (10). Transgenic mice that overexpress BLyS display a myriad of autoimmune features, such as high levels of rheumatoid factor, anti-DNA, circulating immune complexes, and immunoglobulin deposition in the kidneys (9). Mice treated with exogenous BLyS develop an increase in B cell numbers, particularly those directed against chromatin (11), and autoreactive cells encountering transitional B cell checkpoints in the spleen require higher concentrations of BLyS to survive than do non-autoreactive B cells (8, 9, 12). Mouse models have also demonstrated that deletion of either BLyS or its receptor severely impairs B cell development beyond the transitional stage with a resulting decrease in peripheral B cell populations (9, 13–15). Elevated circulating BLyS levels have been found in patients with systemic autoimmune disorders such as SLE, rheumatoid arthritis (RA), and Sjoren’s syndrome (16–18). Early reports found increased serum BLyS levels in SLE patients compared to healthy individuals that correlated with anti-dsDNA titers, but not disease activity (16, 17, 19). BLyS levels did, however, decrease following high-dose corticosteroid treatment (19). Other studies found that peripheral blood leukocyte BLyS mRNA levels correlated with SLE disease activity (20, 21). While most of these initial studies, which were performed in mainly Hispanic SLE patients, failed to demonstrate a correlation between serum BLyS levels and disease activity, a later longitudinal study undertaken at 4 different clinical centers that contained ancestral diversity, found an association between increases in plasma BLyS levels from a previous visit and increases in SELENA-SLEDAI scores at the next visit (22). Additionally, another study involving Norwegian SLE patients, also found a correlation with serum BLyS levels and SLEDAI scores (23). A study examining plasma BLyS protein levels in pediatric SLE also demonstrated an association between elevated BLyS levels and increased disease activity (24). IFN-α has proven to be a key cytokine in SLE pathogenesis, and has been shown to increase BLyS expression in antigen-presenting cells (4). Therefore, it is possible that IFN-α plays a role in driving BLyS production in SLE, although a direct correlation between circulating BLyS levels and serum IFN-α activity has yet to be demonstrated in SLE patients. Additionally, several environmental factors have been implicated in the pathogenesis of SLE (25), including vitamin D deficiency (26). As vitamin D has been shown to suppress the expression of the IFN signature in myeloid-derived dendritic cells, as well as suppress B cell activation and immunoglobulin production (27, 28), it is of interest to assess the relationship between vitamin D and BLyS levels. Belimumab is a fully human monoclonal antibody that binds BLyS and inhibits its activity. Two recent large randomized, double-blind, placebo-controlled, multicenter phase 3 trials, BLISS-52 and BLISS-76, evaluated the efficacy of belimumab plus standard of care compared to placebo plus standard of care; positive results were obtained with both studies reaching their primary efficacy endpoints of reductions in disease activity without ancillary organ flares (29, 30). Taken together, these data suggest that excessive BLyS may be an important mechanism underlying SLE pathogenesis and that BLyS targeted therapies seem promising. However, normal B cell function requires some BLyS activity. To date no studies have evaluated whether specific levels of BLyS are required to mount appropriate vaccination responses or whether BLyS levels change after vaccination. The primary objective of this study is to investigate the relationship between circulating BLyS levels and humoral responses made to influenza vaccination, as well as the effect of influenza vaccination on BLyS production in SLE. The current study also examines select demographic, environmental, and clinical features of SLE for association with elevated BLyS levels in an effort to better understand the mechanisms of elevated BLyS in a heterogeneous SLE population.