Day Eric S, Cachero Teresa G, Qian Fang, Sun Yaping, Wen Dingyi, Pelletier Marc, Hsu Yen-Ming, Whitty Adrian
Department of Drug Discovery, Biogen Idec, Inc., 14 Cambridge Center, Cambridge, Massachusetts 02142, USA.
Biochemistry. 2005 Feb 15;44(6):1919-31. doi: 10.1021/bi048227k.
BAFF (B cell activating factor of the TNF family, also known as BlyS and TALL-1), a TNF family cytokine critical for the development and function of B cells, has been reported to bind to three receptors, BCMA (B cell maturation protein), TACI (transmembrane activator and CAML [calcium-modulator and cyclophilin ligand] interactor), and BAFFR (BAFF receptor), but with widely conflicting values for the affinity and selectivity of binding. BCMA and TACI additionally bind APRIL (a proliferation-inducing ligand), the TNF family ligand most homologous to BAFF. Using soluble, monomeric forms of the receptors, we demonstrate that BAFFR binds BAFF with K(D) approximately 16 nM, while BCMA binds with K(D) approximately 1.6 microM, indicating a approximately 100-fold selectivity for binding to BAFFR over BCMA. APRIL shows the opposite selectivity, binding to BCMA with K(D) approximately 16 nM while showing no detectable affinity for BAFFR (K(D) > 3 microM). The binding of BAFF or APRIL to these receptors is highly sensitive to assay-dependent avidity effects, likely explaining the widely ranging affinity values reported in the literature. Binding of BAFF to BCMA-Fc, a bivalent fusion protein consisting of the extracellular domain of BCMA fused to the hinge and CH1 and CH2 domains of human IgG1, in solution or coated onto an ELISA plate gave apparent binding affinities of approximately 0.63 and approximately 0.15 nM, respectively, compared to values of K(D(app)) <or= 30 and approximately 100 pM for the corresponding BAFFR/IgG1 fusion protein, BAFFR-Fc. The high selectivity of BAFF for BAFFR versus BCMA is thus partly obscured in these multivalent assays. The intrinsically high selectivity inferred from the measurements with monomeric receptor correlates well with in vivo data from knockout mice, providing a possible explanation for the observations that interruption of the BAFFR gene in the A/WySnJ mouse produces a phenotype similar to the BAFF knockout mouse, while the BCMA knockout mouse has no discernible B cell phenotype.
BAFF（肿瘤坏死因子家族的B细胞激活因子，也称为BlyS和TALL-1）是一种对B细胞的发育和功能至关重要的肿瘤坏死因子家族细胞因子，据报道它可与三种受体结合，即BCMA（B细胞成熟蛋白）、TACI（跨膜激活剂和CAML[钙调节蛋白和亲环素配体]相互作用分子）和BAFFR（BAFF受体），但关于结合亲和力和选择性的值差异很大。BCMA和TACI还可结合APRIL（增殖诱导配体），它是与BAFF同源性最高的肿瘤坏死因子家族配体。使用受体的可溶性单体形式，我们证明BAFFR与BAFF的结合解离常数（K(D)）约为16 nM,而BCMA与BAFF的结合解离常数约为1.6 μM，这表明BAFF与BAFFR结合的选择性比与BCMA高约100倍。APRIL表现出相反的选择性，它与BCMA的结合解离常数约为16 nM，而对BAFFR无明显亲和力（K(D)>3 μM）。BAFF或APRIL与这些受体的结合对检测依赖性亲和力效应高度敏感，这可能解释了文献中报道的广泛的亲和力值差异。BAFF与BCMA-Fc（一种由BCMA的胞外结构域与人IgG1的铰链区、CH1和CH2结构域融合而成的二价融合蛋白）在溶液中或包被在ELISA板上的结合，其表观结合亲和力分别约为0.63和0.15 nM，而相应的BAFFR/IgG1融合蛋白BAFFR-Fc的K(D(app))≤30和约100 pM。因此，在这些多价检测中，BAFF对BAFFR相对于BCMA的高选择性部分被掩盖了。从单体受体测量中推断出的内在高选择性与敲除小鼠的体内数据相关性良好，这为以下观察结果提供了一种可能的解释：A/WySnJ小鼠中BAFFR基因的中断产生的表型与BAFF敲除小鼠相似，而BCMA敲除小鼠没有明显的B细胞表型。
