Study on the Mechanism of Selective Interaction of BR3 and BCMA with BAFF and APRIL.

BACKGROUND: B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) can activate signaling pathways by binding to specific receptors. BR3 (BAFF receptor) shows a unique selectivity for BAFF ligand, while B-cell maturation antigen (BCMA) exhibits a stronger interaction between APRIL-BCMA rather than BAFF-BCMA interaction. OBJECTIVE: The combined domains were fused with IgG1 Fc to better understand which domain affects the selective interaction of the receptor with BAFF and APRIL. METHODS: Since BR3 and BCMA both contain cysteine-rich repeat domains (CRD) with DxL motif, the binding domains of BR3 and BCMA were segmented into two parts in this study. BR3-1 (CFDLLVRHGVAC) and BCMA-1 (YFDSLLHACIPC) contained the conservative DxL motif, while BR3-2 (GLLRTPRPKPA) and BCMA-2 (QLRCSSNTPPLT) were adjacent to the CRDs yet still joined with BR3-1 and BCMA-1. Affinity between all possible combinations was then tested. RESULTS: The affinity of BR3-1-BCMA-2-Fc and BR3-1-BR3-2-Fc for BAFF was higher than BCMA-1-BR3-2-Fc and BCMA-1-BCMA-2-Fc. Moreover, BR3-1-BCMA-2-Fc and BCMA-1-BCMA- 2-Fc had affinity for APRIL, while BR3-1-BR3-2-Fc and BCMA-1-BR3-2-Fc hardly interacted with APRIL. CONCLUSION: BR3-1 region played a key role for interaction with BAFF, while BCMA-1 region exhibited weaker binding with BAFF. BCMA-2 region having an α-helix might contribute towards selectivity of APRIL-BCMA binding and BR3-2 rigid region had deleterious effects on the APRIL-BR3 interaction. These results provide comprehensive insights of the mechanism of selective interactions, and may promote specific antagonist design in the future.

A loop region of BAFF controls B cell survival and regulates recognition by different inhibitors.

The B cell survival factor (TNFSF13B/BAFF) is often elevated in autoimmune diseases and is targeted in the clinic for the treatment of systemic lupus erythematosus. BAFF contains a loop region designated the flap, which is dispensable for receptor binding. Here we show that the flap of BAFF has two functions. In addition to facilitating the formation of a highly active BAFF 60-mer as shown previously, it also converts binding of BAFF to TNFRSF13C (BAFFR) into a signaling event via oligomerization of individual BAFF-BAFFR complexes. Binding and activation of BAFFR can therefore be targeted independently to inhibit or activate the function of BAFF. Moreover, structural analyses suggest that the flap of BAFF 60-mer temporarily prevents binding of an anti-BAFF antibody (belimumab) but not of a decoy receptor (atacicept). The observed differences in profiles of BAFF inhibition may confer distinct biological and clinical efficacies to these therapeutically relevant inhibitors.

Characterization of Lamprey BAFF-like Gene: Evolutionary Implications.

BAFF (TNF superfamily [TNFSF] 13B/Blys) and APRIL (TNFSF13) are important regulatory factors for lymphocyte activation and survival in mammals. A BAFF/APRIL-like relative called BAFF- and APRIL-like molecule (BALM) has also been identified in cartilaginous and bony fishes, and we report in this study a BAFF-like gene in lampreys. Our phylogenetic analysis of these genes and a related TNFSF12 gene called TNF-like weak inducer of apoptosis (TWEAK) suggest that, whereas an ancestral homolog of BAFF and APRIL was already present in a common ancestor of jawed and jawless vertebrates, TWEAK evolved early on in the jawed vertebrate lineage. Like mammalian BAFF and APRIL, the lamprey BAFF-like gene is expressed in T-like, B-like, and innate immune cells. The predicted protein encoded by this BAFF-like gene in lampreys exhibits higher sequence similarity with mammalian BAFF than APRIL. Correspondingly, we find BAFF orthologs in all of the jawed vertebrate representatives that we examined, although APRIL and/or BALM orthologs are not identifiable in certain jawed vertebrates. For example, BALM is not identifiable in tetrapods, and APRIL is not identifiable in several bony fishes or in birds, the latter of which also lack a TWEAK-like gene. Our analysis further suggests that a hybrid molecule called TWE-PRIL, which is a product of an in-genomic fusion between APRIL and TWEAK genes evolved early in mammalian evolution.

Survival of Igα-Deficient Mature B Cells Requires BAFF-R Function.

Expression of a functional BCR is essential for the development of mature B cells and has been invoked in the control of their maintenance. To test this maintenance function in a new experimental setting, we used the tamoxifen-inducible mb1-CreER(T2) mouse strain to delete or truncate either the mb-1 gene encoding the BCR signaling subunit Igα or the VDJ segment of the IgH (H chain [HC]). In this system, Cre-mediated deletion of the mb-1 gene is accompanied by expression of a GFP reporter. We found that, although the Igα-deficient mature B cells survive for >20 d in vivo, the HC-deficient or Igα tail-truncated B cell population is short-lived, with the HC-deficient cells displaying signs of an unfolded protein response. We also show that Igα-deficient B cells still respond to the prosurvival factor BAFF in culture and require BAFF-R signaling for their in vivo maintenance. These results suggest that, under certain conditions, the loss of the BCR can be tolerated by mature B cells for some time, whereas HC-deficient B cells, potentially generated by aberrant somatic mutations in the germinal center, are rapidly eliminated.

Murine BAFF-receptor residues 168-175 are essential for optimal CD21/35 expression but dispensable for B cell survival.

BAFF-R (B cell-activating factor belonging to the tumor necrosis factor family receptor) regulates B lymphocyte survival, maturation, homeostasis, and self-tolerance through signaling mechanisms that are not completely understood. A spontaneous BAFF-R mutation, Bcmd-1, disrupts BAFF-R signaling. However, it is not clear why the Bcmd-1-encoded BAFF-R fails to adequately support B cell survival, optimal CD21/35 expression, and B-cell tolerance to dsDNA, since it is 95% identical to the wild-type (wt) BAFF-R and retains the only known signaling motif. A retrotransposon insertion in A/WySnJ strain mice generated the Bcmd-1 allele, replacing the eight C-terminal BAFF-R residues with 21 retrotransposon-encoded residues. New data reported here show that the displaced residues, previously thought to have no signaling role, are essential for optimal CD21/35 expression but contribute little to B cell survival signaling. Analysis of wt Baffr or Bcmd-1 homozygous (A/WySnJ X B6.BCL2)F2 mice confirmed that BCL2 complemented Bcmd-1 for B cell survival but not CD21/35 expression. Through in vivo retroviral transduction experiments, we show that Baffr complemented Bcmd-1 for B cell survival but not CD21/35 expression, whereas the BaffrDelta103-175 deletion mutant lacking the BAFF-R cytoplasmic domain failed to support these functions. Importantly, we show that the BaffrDelta168-175 deletion mutant lacking the retrotransposon-displaced residues, and a BaffrT170A mutant lacking a critical threonine, supported B cell survival but failed to support optimal CD21/35 expression. These data provide the first evidence for a possible bifurcation at the receptor level in the BAFF-R signaling pathway. We suggest that discrete BAFF-R cytoplasmic domains may interact with distinct downstream pathways to provide fine control over B cell survival, maturation, and tolerance induction.

Evidence for an asialoglycoprotein receptor on nonparenchymal cells for O-linked glycoproteins.

B cell-activating factor receptor 3 (BR3)-Fc is an IgG1-receptor dimeric fusion protein that has multiple O-linked glycosylation sites and sialylation levels that can vary in the manufacturing process. Increased sialic acid levels resulted from increased site occupancy with the O-linked N-acetylgalactosamine (GalNAc-Gal), but because the ratio of sialic acid per mole of oligosaccharide remained approximately 1, this led to increased asialo terminal GalNAc. Previous studies have demonstrated an effect of terminal asialo Gal or GalNAc on the clearance of glycoproteins due to uptake and degradation by lectin receptors in the liver. However, the previous studies examined N-linked oligosaccharides, and there are less data regarding O-linked oligosaccharides. The objective of these studies was to determine the effects on the pharmacokinetics and distribution of the asialo terminal GalNAc and varying amounts of sialic acid residues on BR3-Fc. The results of the data presented here suggest that exposed Gal on the desialylated BR3-Fc led to rapid clearance due to uptake and degradation in the liver that was associated with nonparenchymal cells. It is interesting to note that the data indicated a decreased clearance and increased exposure of BR3-Fc as the sialic acid levels increased, even though increased sialic acid was associated with increased asialo GalNAc. Therefore, the exposed GalNAc did not seem to play a role in the clearance of BR3-Fc; although the Gal linked to the hydroxyl group at position 3 may have prevented an interaction. Because we did not see uptake of desialylated BR3-Fc in hepatocytes where the asialoglycoprotein receptor is localized, this nonparenchymal cell lectin may have preference for O-linked glycoproteins.

BAFF, APRIL and their receptors: structure, function and signaling.

BAFF, APRIL and their receptors play important immunological roles, especially in the B cell arm of the immune system. A number of splice isoforms have been described for both ligands and receptors in this subfamily, some of which are conserved between mouse and human, while others are species-specific. Structural and mutational analyses have revealed key determinants of receptor-ligand specificity. BAFF-R has a strong selectivity for BAFF; BCMA has a higher affinity for APRIL than for BAFF, while TACI binds both ligands equally well. The molecular signaling events downstream of BAFF-R, BCMA and TACI are still incompletely characterized. Survival appears to be mediated by upregulation of Bcl-2 family members through NF-kappaB activation, degradation of the pro-apototic Bim protein, and control of subcellular localization of PCKdelta. Very little is known about other signaling events associated with receptor engagement by BAFF and APRIL that lead for example to B cell activation or to CD40L-independent Ig switch.

Synthetic anti-BR3 antibodies that mimic BAFF binding and target both human and murine B cells.

BR3, which is expressed on all mature B cells, is a specific receptor for the B-cell survival and maturation factor BAFF (B-cell-activating factor belonging to the tumor necrosis factor [TNF] family). In order to investigate the consequences of targeting BR3 in murine models and to assess the potential of BR3 antibodies as human therapeutics, synthetic antibody phage libraries were employed to identify BAFF-blocking antibodies cross-reactive to murine and human BR3, which share 52% identity in their extracellular domains. We found an antibody, CB1, which exhibits muM affinity for murine BR3 and very weak affinity for the human receptor. CB3s, an affinity-matured variant of CB1, has sub-nM affinity for BR3 from both species. Alanine scanning and crystallographic structural analysis of the CB3s/BR3 complex reveal that CB3s mimics BAFF by interacting with a similar region of the BR3 surface. Despite this similarity in binding epitopes, CB1 variants antagonize BAFF-dependent human B-cell proliferation in vitro and are effective at reducing murine B-cell populations in vivo, showing significant promise as therapeutics for human B-cell-mediated diseases.