Johansen F E, Braathen R, Brandtzaeg P
Laboratory for Immunohistochemistry and Immunopathology (LIIPAT), Institute of Pathology, University of Oslo, Rikshospitalet, N-0027 Oslo, Norway.
Scand J Immunol. 2000 Sep;52(3):240-8. doi: 10.1046/j.1365-3083.2000.00790.x.
The joining (J) chain is a small polypeptide, expressed by mucosal and glandular plasma cells, which regulates polymer formation of immunoglobulin (Ig)A and IgM. J-chain incorporation into polymeric IgA (pIgA, mainly dimers) and pentameric IgM endows these antibodies with several salient features. First, a high valency of antigen-binding sites, which makes them suitable for agglutinating bacteria and viruses; little or no complement-activating potential, which allows them to operate in a noninflammatory fashion; and, most importantly, only J-chain-containing polymers show high affinity for the polymeric Ig receptor (pIgR), also known as transmembrane secretory component (SC). This epithelial glycoprotein mediates active external transfer of pIgA and pentameric IgM to exocrine secretions. Thus, secretory IgA (SIgA) and SIgM, as well as free SC, are generated by endoproteolytic cleavage of the pIgR extracellular domain. The secretory antibodies form the 'first line' of defence against pathogens and noxious substances that favour the mucosae as their portal of entry. The J chain is involved in creating the binding site for pIgR/SC in the Ig polymers, not only by determining the polymeric quaternary structure but apparently also by interacting directly with the receptor protein. Therefore, both the J chain and the pIgR/SC are key proteins in secretory immunity.
连接（J）链是一种小的多肽，由黏膜和腺浆细胞表达，它调节免疫球蛋白（Ig）A和IgM的聚合物形成。J链掺入聚合IgA（pIgA，主要是二聚体）和五聚体IgM赋予这些抗体若干显著特征。首先，具有高化合价的抗原结合位点，这使其适合凝集细菌和病毒；几乎没有或没有补体激活潜能，这使其能够以非炎症方式发挥作用；最重要的是，只有含J链的聚合物对聚合Ig受体（pIgR）表现出高亲和力，pIgR也称为跨膜分泌成分（SC）。这种上皮糖蛋白介导pIgA和五聚体IgM向外分泌液的主动外向转运。因此，分泌型IgA（SIgA）和分泌型IgM以及游离的SC是由pIgR细胞外结构域的内蛋白水解切割产生的。分泌性抗体形成抵御病原体和有害物质的“第一道防线”，这些病原体和有害物质倾向于通过黏膜作为其进入门户。J链不仅通过确定聚合物四级结构，而且显然还通过与受体蛋白直接相互作用，参与在Ig聚合物中创建pIgR/SC的结合位点。因此，J链和pIgR/SC都是分泌免疫中的关键蛋白。
