BAFF System in Rheumatoid Arthritis: from Pathobiology to Therapeutic Targets BAFF System in Rheumatoid Arthritis: from Pathobiology to Therapeutic Targets

Recent advances in understanding the multifaceted pathobiology of rheumatoid arthritis have highlighted the pivotal role and continuing crosstalk between activated immune cells, pro-inflammatory cytokines, and matrix-degrading mediators, promoting chronic inflammation as well as irreversible tissue damage within an autoimmune background. B cells are widely recognized as leading players in immune-mediated pathology based on their ability to produce not only different patterns of autoantibodies and driving cyto kine synthesis but also as independent antigen-presenting cells and by modulating the specific activation of T cells. Overwhelming evidence emphasized the role of BAFF, a B-cell-activating factor, and BAFF receptors (TACI, BCMA, BAFF-R) in promoting B-cell homeostasis, proliferation, and survival under normal and autoimmune systemic disorders. We systematically reviewed data from literature focusing on BAFF, its homo - log molecule APRIL, and BAFF-binding receptors biology, dysregulation of BAFF/BAFF receptor signaling in autoimmune settings, and current status of targeting BAFF/BAFF receptor pathway for rheumatoid arthritis.


Introduction
Rheumatoid arthritis (RA) is a chronic immuno-inflammatory disease characterized by a multifaceted pathobiology, where a complex cytokine and cellular network contribute to excessive and extensive articular and systemic inflammatory events, accompanied by progressive tissue damage [1][2][3][4].
B cells are widely recognized as leading players in the mechanisms underlying the pathogenesis of RA based on their ability to produce not only different subsets of autoantibodies but also as independent antigen-presenting cells, cytokine synthesis, and modulators of T-cell activation. Moreover, their differentiation and survival are driven by positive feedback loops induced by cytokines, especially members belonging to the tumor necrosis factor (TNF) family [1][2][3][4].
Recent data highlighted the role of signaling crosstalk in B-lymphocytes, particularly of B-cell-activating factor (BAFF) and its receptors in early steps of the disease, advancing clinical development of BAFF antagonists for the treatment of RA [2][3][4][5]. Acting as an innate cytokine mediator, BAFF is known to modulate peripheral B-and T-cell homeostasis, promoting specific downstream signaling events through three different types of receptors [2,4,5].
Abnormal BAFF/BAFF receptor-signaling pathways were reported in several autoimmune disorders including systemic lupus erythematosus, Sjogren's syndrome, ANCA-associated vasculitis, and RA [2,4,5]. Of interest, elevated BAFF levels were detected in synovial fluid, serum, and saliva in very early stages of RA, suggesting its involvement in cell-cell interactions network in the synovial microenvironment, as well as B-cell activation and the development of autoreactive B cells [2][3][4]. Furthermore, the overexpression of BAFF receptors, as well as disturbed autocrine and paracrine BAFF network, seems to be related to inflammatory events and RA progression [1][2][3][4][5][6].
Although clinical development of BAFF antagonists as potential therapeutic target for systemic autoimmune conditions is promising, the benefit of specific agents such as belimumab, atacicept, or tabalumab in RA is controversial [2,4,6].
We systematically reviewed data from the literature focusing on the biology of BAFF, its homologue molecule APRIL and BAFF-binding receptors, dysregulation of BAFF/BAFF receptor signaling in RA and current status of targeting BAFF/BAFF receptor pathway.
BAFF is recognized under two isoforms, a biologically active full-length isoform and the alternatively spliced one, 4BAFF, meaning a protein with a small peptide deletion which does not bind to BAFF receptors, but has the ability to form heterotrimers with the original isoform [2][3][4][5][6]. Additionally, BAFF is expressed as a membrane-binding homotrimer and released as a soluble, biologically active molecule in peripheral blood after cleavage by a dedicated furinprotease [2,4].
APRIL, a proliferation-inducing ligand known as the homolog molecule of BAFF or TNFSF13 (TNF ligand superfamily member 13), is also a key cytokine for B-cell activation and maturation; APRIL prompts B-cell proliferation, antibody class switching and survival, but is not required for the normal B-cell development [12][13][14].
TACI remains the predominant receptor on marginal zone B cells and short-lived plasma cells, while BCMA is typically expressed by long-lived plasma cells being essential for their optimal generation [2,4]. The ability of TACI to act as a sink for BAFF, preventing the attachment of BAFF to its BAFF-R, may also reflect mixed regulatory functions on B cells [2,4,5,[10][11][12][13][14].
Each of the three receptors has a different pattern of expression and mediates distinct functions [2,4].

BAFF and BAFF receptors functioning
It is widely accepted that BAFF plays a crucial role in B-cell homeostasis, adjusting their maturation, proliferation, and survival under different backgrounds. BAFF may also indirectly interfere with T-cell functioning, providing several co-stimulatory signals in conjunction with T-cell receptor, mainly related to cellular proliferation and synthesis of mediators [2,4].
RA is thought to be the result of an interplay between multiple cells and their products (cytokines and mediators), from both innate and adaptive immunity that lead to systemic inflammatory and tissue-damaging events [1][2][3][4].
Persistent immune cell trafficking into the inflamed joints typically focuses on B-and T-lymphocytes, with special polarization for TCD4+ subpopulation, neutrophils, macrophages, and dendritic cells, which actively infiltrate the RA synovium and orchestrate inflammation and cartilage damage [1][2][3][4]. At least three different histological subtypes of immune infiltrates are actually recognized within the RA synovitis, meaning diffuse, nodular infiltrates, and lymphoid aggregates with germinal centers [2][3][4].
As innate cytokines, BAFF and APRIL are potentially involved in the dysregulated immunoinflammatory synovial microenvironment, affecting both autocrine and paracrine feedback [1][2][3][4][5][6]. In addition, BAFF and APRILL are involved in an amplification loop which is locally activated by inflammation: B-and T-lymphocytes, together with plasmacytoid and myeloid dendritic cells, are interconnected through a continuing crosstalk [1][2][3][4]. Thus, B cells migrated to the inflamed tissues and activated produce pro-inflammatory and destructive cytokines and chemokines, but also exert their potent effector function by presenting self-antigens to and activating T cells [2][3][4]. Moreover, immune complexes as a result of aberrant functioning of B cells induce the activation of different subtypes of dendritic cells within the IFN direct supervision and further enhance B-and T-participation [4].
Overall, various residents as well as recruited cells in rheumatoid synovium are responsible for BAFF synthesis, especially dendritic cells, macrophages and fibroblast-like macrophages, neutrophils, and CD4+ T cells. Further, BAFF interventions on the effector cell network (dendritic cells, macrophages and fibroblast-like macrophages, neutrophils, TCD4+ lymphocytes, and B cells) are able to promote by positive feedback their differentiation, proliferation, activation with subsequent cytokine production, and survival [2,4].

Targeting BAFF and BAFF receptors for rheumatoid arthritis
Accumulating data on the importance of B cells in various autoimmune diseases have reshaped the therapeutic armamentarium, specifically directed toward B-lymphocytes [2,4,25,26].
While playing a pivotal role in B-cell survival and functioning, BAFF/BAFF receptor system recently emerged as a reasonable target for different autoimmune conditions [2,4,25,26]; furthermore, several BAFF antagonists are already under development exploring the appropriate therapeutic intervention based on BAFF blockade in RA [2,4,25,26].
Generally, therapeutic BAFF antagonism accounts not only for direct B-cell depletion and indirect impairment of B-cell-mediated processes such as antigen presentation, cytokine synthesis, and humoral immune response, but may also influence T-cell biology based on costimulatory signals [2,4,25,26].
A simplified view of BAFF/APRIL/BAFF-binding receptor pathway and their targeted therapy in immune mediated is presented (Figure 1).
We reviewed the current status of targeting BAFF/BLyS, APRIL and their receptors in RA.
The efficacy and tolerability of a novel, fully human variant of anti-BLyS monoclonal antibody was further evaluated in patients with active RA non-responsive to standard therapy in different clinical trials [2,4,[26][27][28][29][30]. While safety data are convincing across all the studies and belimumab seems to be a promising agent for a specific RA population, optimal clinical efficacy needs further evaluation [2,4,[26][27][28][29][30]. The majority of trials have demonstrated positive outcomes under belimumab BAFF-blockade, meaning significant response rates according to American College of Rheumatology (ACR) improvement ≥20% criteria but not ACR50 or ACR70, specifically for RA patients classified as having high disease activity (disease activity score DAS28>5.1), RF and ACPA-positive status, naïve to other biologics including TNF antagonists, previous methotrexate failure or low baseline BAFF levels [2,4,[26][27][28][29][30].
To summarize, although tabalumab showed clinical and biological efficacy in phase 2 clinical trials irrespective of prior exposure to synthetic remissive drugs (methotrexate) or biologics, phase 3 trials fail to promote clinical benefit in patients with moderate-to-severe RA with prior inadequate response to TNF antagonists. Despite demonstrating biological improvement as supported by substantial change in B-cell count and decline in serum immunoglobulin levels, it is obvious that targeting the BAFF pathway alone is not a feasible strategy in RA [2,4,25,[31][32][33][34][35].
Although the primary efficacy end point (ACR20 response) was not achieved in none of phase 2 studies, atacicept significantly reduced the immunoglobulin (IgM, IgG, and IgA) and rheumatoid factor levels in a dose-dependent manner. The safety profile was acceptable as atacicept did not show an increased susceptibility to infections, although the overall rate of adverse events was somewhat higher than placebo and lupus [2,4,25,[36][37][38][39][40][41].

Other BAFF antagonists (AMG-623 and BR3-Fc)
Other BAFF-blockade agents are under development, targeting different BAFF receptorbinding molecules. A good example is A-623, previously known as AMG-623, a polypeptide fusion protein containing both IgG and the ligand-binding section of the BAFF-R, able to block both membrane and soluble BAFF and, therefore, to impair normal B-cell functioning [43]. Briobacept or BR3-Fc is another distinct BAFF-blocking molecule acting as a homodimeric fusion glycoprotein including the extracellular ligand-binding portion of BAFF-R and the Fc portion of an IgG. This new drug was typically designed to induce further B-cell apoptosis interfering with BAFF/BAFF-R signaling [2,4,43].

Conclusions
Abnormal BAFF signaling (throughout either overexpression of B-cell-related activation and survival genes, or BAFF receptors) represents an important step in the pathobiology of rheumatoid arthritis, particularly by promoting the development of autoreactive B cells in early disease, but also by supporting disease progression.
Although BAFF/APRIL/BAFF receptors targeting therapy seems to be an attractive option for systemic autoimmune conditions, particularly systemic lupus erythematosus, considerable response heterogeneity and safety concerns are reported in rheumatoid arthritis.