To target or not to target APRIL in systemic lupus erythematosus: that is the question!

A proliferation inducing ligand (APRIL) promotes IL-10 production and regulatory functions of human B cells

B cells may have a negative regulatory role, mainly mediated by interleukin 10 (IL-10). We recently showed that regulatory B-cell functions are impaired in patients with rheumatoid arthritis (RA) and that mice transgenic for a proliferation-inducing ligand (APRIL) are protected against collagen-induced arthritis. We aimed to explore the effect of APRIL on human B-cell IL-10 production, in healthy subjects and in patients with RA. The IL-10 production of B-cell was greater with APRIL than with BLyS or control medium, in a dose dependent manner. TACI expression was greater in IL-10 producing B cells (B10) than non-IL-10-producing B cells whereas BAFF-R expression was lower. TNF-α and IFN-γ secretion of T-cells were decreased by APRIL-stimulated B cells. APRIL stimulated STAT3 and STAT3 inhibition decreased B10 cells. APRIL also promoted B10 cells in RA patients. In conclusion, APRIL but not BLyS promotes IL-10 production by CpG-activated B cells and enhances the regulatory role of B cells on T cells. B10 cells in RA patients are responsive to APRIL, which suggests a possible therapeutic application of APRIL to expand B10 cells. This could also explain the difference of clinical efficacy observed between belimumab and atacicept in RA.

B-cell survival factors in autoimmune rheumatic disorders

Autoimmune rheumatic disorders have complex etiopathogenetic mechanisms in which B cells play a central role. The importance of factors stimulating B cells, notably the B-cell activating factor (BAFF) and A proliferation inducing ligand (APRIL) axis is now recognized. BAFF and APRIL are cytokines essential for B-cell proliferation and survival from the immature stages to the development of plasma cells. Their levels are increased in some subsets of patients with autoimmune disorders. Several recent biologic drugs have been developed to block this axis, namely belimumab [already licensed for systemic lupus erythematosus (SLE) treatment], tabalumab, atacicept and blisibimod. Many clinical trials to evaluate the safety and efficacy of these drugs in several autoimmune disorders are ongoing, or have been completed recently. This review updates the information on the use of biologic agents blocking BAFF/APRIL for patients with SLE, rheumatoid arthritis, Sjögren's syndrome and myositis.

Profile of atacicept and its potential in the treatment of systemic lupus erythematosus

The importance of B cell activating factors in the generation of autoantibodies in patients with systemic lupus erythematosus (SLE) is now recognized. The two key factors, known as BAFF and APRIL, produced by a variety of cells including monocytes, dendritic cells and T cells, also help to regulate B cell maturation, function and survival. Biologic agents that block these factors have now been developed and tried out in large scale clinical trials in SLE patients. Benlysta which blocks BAFF has met some of its end points in clinical trials and is approved for use in patients with skin and joint disease who have failed conventional drugs. In contrast, clinical trials using atacicept which blocks both BAFF and APRIL have been more challenging to interpret. An early study in lupus nephritis was, mistakenly, abandoned due to serious infections thought to be linked to the biologic when in fact the dramatic fall in the immunoglobulin levels took place when the patients were given mycophenolate, prior to the introduction of the atacicept. Likewise the higher dose arm (150 mgm) of a flare prevention study was terminated prematurely when 2 deaths occurred. However, the mortality rate in this study was identical to that seen in the Benlysta studies and a post hoc analysis found a highly significant benefit for the 150mgm arm compared to the lower dose (75 mgm) and placebo arms. Other trials with both Benlysta and atacicept are on-going.

APRIL levels strongly correlate with IL-17 in systemic lupus erythematosus

Introduction

Activated self-reactive B cells play an important part in systemic lupus erythematosus (SLE). A proliferation-inducing ligand (APRIL) and B cell-activating factor (BAFF) are B-cell specific stimulators, but activate B cells through different receptors. We investigated the reciprocal association between serum APRIL (s-APRIL), serum BAFF (s-BAFF) and immunological and clinical findings in SLE patients.

Methods

A cross-sectional case-control study was performed in 100 SLE patients (87% female, age 49 years, disease duration 12 years). APRIL and BAFF levels were measured by sandwich ELISA, compared with healthy controls and correlated with autoantibody, cytokine (IL-6 and IL-17) and clinical findings through nonparametric and multivariate regression analyses.

Results

Both median s-APRIL (478 vs. 0 pg/ml, p = 0.01) and s-BAFF (1720 vs. 0.9 pg/ml, p < 0.001) were higher in SLE patients than controls. Increased s-BAFF was observed in 86% of patients, while s-APRIL was increased only in 17% ( p < 0.01). S-APRIL correlated with s-BAFF in controls ( p = 0.04), but not in SLE ( p = 0.8). Increased s-APRIL was strongly and independently associated with IL-17 activation ( p < 0.001), while increased s-BAFF levels were associated with anti-nucleosome antibody presence ( p = 0.001). Disease activity and organ damage were associated with s-BAFF but not s-APRIL.

Conclusions

While both s-BAFF and s-APRIL levels are elevated in SLE patients, they reflect different immunologic and clinical pathways. The strong association between s-APRIL and IL-17 activation supports a role for Th17 helper cells in B cell activation in SLE.