BAFF selectively enhances the survival of plasmablasts generated from human memory B cells.

The generation of Ig-secreting cells (ISCs) from memory B cells requires interactions between antigen-specific (Ag-specific) B cells, T cells, and dendritic cells. This process must be strictly regulated to ensure sufficient humoral immunity while avoiding production of pathogenic autoantibodies. BAFF, a member of the TNF family, is a key regulator of B cell homeostasis. BAFF exerts its effect by binding to three receptors - transmembrane activator of and CAML interactor (TACI), B cell maturation antigen (BCMA), and BAFF receptor (BAFF-R). To elucidate the contribution of BAFF to the differentiation of B cells into ISCs, we tracked the fate of human memory B cells stimulated with BAFF or CD40L. BAFF and CD40L significantly increased the overall number of surviving B cells. This was achieved via distinct mechanisms. CD40L induced proliferation of nondifferentiated blasts, while BAFF prevented apoptosis of ISCs without enhancing proliferation. The altered responsiveness of activated memory B cells to CD40L and BAFF correlated with changes in surface phenotype such that expression of CD40 and BAFF-R were reduced on ISCs while BCMA was induced. These results suggest BAFF may enhance humoral immunity in vivo by promoting survival of ISCs via a BCMA-dependent mechanism. These findings have wide-ranging implications for the treatment of human immunodeficiencies as well as autoimmune diseases.

EC144 is a potent inhibitor of the heat shock protein 90.

Alkyne 40, 5-(2-amino-4-chloro-7-((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)-7H-pyrrolo[2,3-d]pyrimidin-5-yl)-2-methylpent-4-yn-2-ol (EC144), is a second generation inhibitor of heat shock protein 90 (Hsp90) and is substantially more potent in vitro and in vivo than the first generation inhibitor 14 (BIIB021) that completed phase II clinical trials. Alkyne 40 is more potent than 14 in an Hsp90α binding assay (IC(50) = 1.1 vs 5.1 nM) as well as in its ability to degrade Her-2 in MCF-7 cells (EC(50) = 14 vs 38 nM). In a mouse model of gastric tumors (N87), 40 stops tumor growth at 5 mg/kg and causes partial tumor regressions at 10 mg/kg (po, qd × 5). Under the same conditions, 14 stops tumor growth only at 120 mg/kg, and does not induce partial regressions. Thus, alkyne 40 is approximately 20-fold more efficacious than 14 in mice.

Paucity of clinical disease despite serological autoimmunity and kidney pathology in lupus-prone New Zealand mixed 2328 mice deficient in BAFF.

Constitutive overexpression of B cell-activating factor belonging to the TNF family (BAFF) promotes development of systemic lupus erythematosus (SLE), and treatment of SLE mice with BAFF antagonists ameliorates disease. To determine whether SLE can develop de novo in BAFF-deficient hosts, BAFF-deficient New Zealand Mixed (NZM) 2328 (NZM.Baff(-/-)) mice were generated. In NZM.Baff(-/-) mice, spleen B cells (including CD5(+) B1a and CD5(-) B1b B cells), germinal centers, Ig-secreting cells, and T cells were reduced in comparison to NZM.Baff(+/+) mice. Serum total Ig and autoantibody levels were reduced at 4-6 mo but approached wild-type levels with increasing age, indicating that autoreactive B cells can survive and secrete autoantibodies despite the complete absence of BAFF. At least some of these autoantibodies are nephrophilic in that glomerular deposition of total IgG and IgG1 (but not of IgG2a, IgG2b, or C3) was substantial in NZM.Baff(-/-) mice by 12-13 mo of age. Despite proliferative glomerulonephritis, highlighted by widespread glomerular hyaline thrombi, being common among NZM.Baff(-/-) mice by 6-7 mo of age, severe proteinuria and mortality were greatly attenuated. These results demonstrate that the lifelong absence of BAFF does not protect NZM 2328 mice from serological autoimmunity and renal pathology. Nevertheless, the character of the renal pathology is altered, and the mice are largely spared from clinically overt disease (severe proteinuria and premature death). These observations may have profound ramifications for the use of BAFF antagonists in human SLE and related diseases.

BAFF overexpression and accelerated glomerular disease in mice with an incomplete genetic predisposition to systemic lupus erythematosus.

OBJECTIVE: To determine whether overexpression of BAFF can accelerate the development of systemic lupus erythematosus-associated end-organ disease in hosts with an underlying autoimmune diathesis. METHODS: We introduced a BAFF transgene (Tg) into autoimmune-prone B6.Sle1 and B6.Nba2 mice and evaluated these mice for serologic autoimmunity and renal pathology. RESULTS: B6.Sle1.BAFF and B6.Nba2.BAFF mice, but not non-Tg littermates, frequently developed severe glomerular pathology by 3 months of age. Age-matched B6.BAFF mice, despite renal Ig deposits and increases in B cells and Ig production similar to those in B6.Sle1.BAFF and B6.Nba2.BAFF mice, did not develop glomerular pathology. In B6.Sle1.BAFF and B6.Nba2.BAFF mice, severity of glomerular disease did not obligately correlate with circulating levels of IgG anti-chromatin and/or anti-double-stranded DNA antibodies or with amounts of these autoantibodies deposited in the kidneys. Even in mice with severe glomerular disease, renal tubulointerstitial infiltrates were very limited, and increased proteinuria was not detected. CONCLUSION: BAFF-driven effects on glomerular pathology may be mediated, at least in part, by autoantibodies with specificities other than chromatin and/or by autoantibody-independent means. There is an uncoupling of BAFF-driven precocious glomerular pathology from concomitant development of clinically apparent renal disease, strongly suggesting that BAFF overexpression works in concert with other factors to promote overt renal disease.

Cutting edge: BAFF regulates CD21/35 and CD23 expression independent of its B cell survival function.

Herein we demonstrate that B cell-activating factor of the TNF family (BAFF), a B cell survival factor, also regulates CD21/35 and CD23 expression. BAFF blockade in wild-type mice down-modulates CD21/35 and CD23 on B cells while survival remains intact, and BAFF exposure causes elevated CD21/35 and CD23 expression. Similar down-modulation is observed in bcl-2-transgenic mice treated with a BAFF inhibitor. This is the first evidence that BAFF has a function independent of B cell survival. Reports using CD21/35 and CD23 expression to assess splenic B cell subsets in BAFF-null mice concluded a lack of B cells beyond the immature stage. Since CD21/35 and CD23 are inadequate for delineating B cell subpopulations in BAFF-null mice, we used expression of BAFF-R and several B cell markers to identify more mature splenic B cells in these mice. These data broaden our understanding of BAFF function and correct the view that BAFF-null mice lack mature B cells.

Comparison of soluble decoy IgG fusion proteins of BAFF-R and BCMA as antagonists for BAFF.

BAFF is considered a therapeutic target because dysregulated production of BAFF can induce systemic lupus erythematosus-like phenotype in mice, and elevated levels of BAFF are associated with disease severity in systemic lupus erythematosus and rheumatoid arthritis patients. Fc fusion decoy receptors, BCMA-Fc and BAFF-R-Fc, are therapeutic candidates for blocking BAFF. While studying their interactions with BAFF, we found that BAFF-R-Fc is more effective than BCMA-Fc for blocking BAFF binding to its receptors. We also found that a trimeric BAFF can bind more than one BAFF-R-Fc but only one BCMA-Fc. Moreover, we show that, in contrast to monovalent BAFF-R-Fc, monovalent BCMA does not form stable complexes with BAFF. Differences in their interaction with BAFF predict BAFF-R-Fc would be a better inhibitor. Indeed, we show BAFF-R-Fc is 10-fold more efficacious than BCMA-Fc for blocking BAFF-induced B cell proliferation in vitro and for blocking BAFF-mediated survival of mouse splenic B lymphocytes in vivo.