Department of Pathology and Laboratory Medicine, Emory University, Atlanta, GA 30322, USA.
J Immunol. 2009 Dec 15;183(12):8216-24. doi: 10.4049/jimmunol.0902550.
CD32A, the major phagocytic FcgammaR in humans, exhibits a polymorphism in the ligand binding domain. Individuals homozygous for the R allelic form of CD32A (CD32A(R) allele) are more susceptible to bacterial infections and autoimmune diseases as compared with H allelic CD32A (CD32A(H)) homozygous and CD32A(R/H) heterozygous individuals. To understand the mechanisms behind this differential susceptibility, we have investigated the dynamics of the interaction of these allelic forms of CD32A when they are simultaneously exposed to immune complexes (IC). Binding studies using Ig fusion proteins of CD32A alleles showed that the R allele has significantly lower binding not only to human IgG2, but also to IgG1 and IgG3 subtypes. Competition assays using purified molecules demonstrated that CD32A(H)-Ig outcompetes CD32A(R)-Ig for IC binding when both alleles simultaneously compete for the same ligand. CD32A(H)-Ig blocked the IC binding mediated by both the allelic forms of cell surface CD32A, whereas CD32A(R)-Ig blocked only CD32A(R) and was unable to cross-block IC binding mediated by CD32A(H). Two-dimensional affinity measurements also demonstrated that CD32A(R) has significantly lower affinity toward all three subtypes as compared with CD32A(H). Our data suggest that the lower binding of CD32A(R) not only to IgG2 but also to IgG1 and IgG3 might be responsible for the lack of clearance of IC leading to increased susceptibility to bacterial infections and autoimmune diseases. Our data further suggests that in humans, inflammatory cells from CD32A(R/H) heterozygous individuals may predominantly use the H allele to mediate Ab-coated target cell binding during phagocytosis and Ab-dependent cellular cytotoxicity, resulting in a phenotype similar to CD32A(H) homozygous individuals.
CD32A 是人类主要的吞噬性 FcγR,其配体结合域存在多态性。与 H 等位基因 CD32A(CD32A(H))纯合子和 CD32A(R/H)杂合子相比,CD32A 等位基因纯合子(CD32A(R)等位基因)个体更容易发生细菌感染和自身免疫性疾病。为了了解这种差异易感性背后的机制,我们研究了这些 CD32A 等位基因形式在同时暴露于免疫复合物(IC)时的相互作用动力学。使用 CD32A 等位基因的 Ig 融合蛋白进行的结合研究表明,R 等位基因不仅与人类 IgG2 的结合显著降低,而且与 IgG1 和 IgG3 亚型的结合也显著降低。使用纯化分子进行的竞争测定表明,当两个等位基因同时竞争相同配体时,CD32A(H)-Ig 会优先与 CD32A(R)-Ig 竞争 IC 结合。CD32A(H)-Ig 阻断了两种细胞表面 CD32A 等位基因介导的 IC 结合,而 CD32A(R)-Ig 仅阻断 CD32A(R),并且无法交叉阻断由 CD32A(H)介导的 IC 结合。二维亲和力测量也表明,与 CD32A(H)相比,CD32A(R)对所有三种亚型的亲和力显著降低。我们的数据表明,CD32A(R)不仅与 IgG2 而且与 IgG1 和 IgG3 的结合降低可能是导致 IC 清除减少从而增加细菌感染和自身免疫性疾病易感性的原因。我们的数据进一步表明,在人类中,CD32A(R/H)杂合子的炎症细胞可能主要利用 H 等位基因来介导吞噬和抗体依赖性细胞毒性过程中抗体包被靶细胞的结合,导致表型类似于 CD32A(H)纯合子个体。