Johansen F E, Braathen R, Brandtzaeg P
Laboratory for Immunohistochemistry and Immunopathology, Institute of Pathology, University of Oslo, Rikshospitalet, Oslo, Norway.
J Immunol. 2001 Nov 1;167(9):5185-92. doi: 10.4049/jimmunol.167.9.5185.
Local production of secretory (S)IgA provides adaptive immunologic protection of mucosal surfaces, but SIgA is also protective when administered passively, such as in breast milk. Therefore, SIgA is a potential candidate for therapeutic administration, but its complex structure with four different polypeptide chains produced by two distinct cell types complicates recombinant production. The J chain is critical in the structure of SIgA because it is required for efficient polymerization of IgA and for the affinity of such polymers to the secretory component (SC)/polymeric (p)IgR. To better understand the role of the J chain in SIgA production, we have generated various mutant forms of the human J chain and analyzed the function of these mutants when coexpressed with IgA. We found that the C terminus of the J chain was not required for the formation of IgA polymers, but was essential for the binding of pIgA to SC. Likewise, we found that two of the intrachain disulfide bridges (Cys(13):Cys(101) and Cys(109):Cys(134)) were also required for the binding of pIgA to SC but, interestingly, not for IgA polymerization. Conversely, the last intrachain disulfide bridge (Cys(72):Cys(92)) was not essential for either of these two J chain functions. Finally, we demonstrated that the presence of only Cys(15) or Cys(69) was sufficient to support polymerization of IgA, but that these polymers were mostly noncovalently stabilized. Nevertheless, these polymers bound free SC with nearly the same affinity as pIgA containing wild-type J chain, but were transcytosed by pIgR-expressing polarized epithelial cells at a reduced efficiency.
分泌型（S）IgA的局部产生为黏膜表面提供适应性免疫保护，但被动给予时，如母乳中的SIgA也具有保护作用。因此，SIgA是治疗性给药的潜在候选物，但其由两种不同细胞类型产生的具有四条不同多肽链的复杂结构使重组生产变得复杂。J链在SIgA的结构中至关重要，因为它是IgA高效聚合以及此类聚合物与分泌成分（SC）/多聚体（p）IgR亲和力所必需的。为了更好地理解J链在SIgA产生中的作用，我们构建了人J链的各种突变形式，并分析了这些突变体与IgA共表达时的功能。我们发现，J链的C末端对于IgA聚合物的形成不是必需的，但对于pIgA与SC的结合至关重要。同样，我们发现链内的两个二硫键（Cys(13):Cys(101)和Cys(109):Cys(134)）对于pIgA与SC的结合也是必需的，但有趣的是，对于IgA聚合并非必需。相反，最后一个链内二硫键（Cys(72):Cys(92)）对于这两种J链功能都不是必需的。最后，我们证明仅存在Cys(15)或Cys(69)就足以支持IgA的聚合，但这些聚合物大多是非共价稳定的。尽管如此，这些聚合物与游离SC的结合亲和力与含有野生型J链的pIgA几乎相同，但在表达pIgR的极化上皮细胞中跨细胞转运的效率较低。
