Clinical Immunology Unit, The Chinese University of Hong Kong, Hong Kong, China.
IgGENE, FoTan, Hong Kong, China.
Front Immunol. 2019 Aug 2;10:1820. doi: 10.3389/fimmu.2019.01820. eCollection 2019.
Antibodies are well-known protein mediators of immunity. IgM is the primordial member and the neglected sibling of the later-evolved and more proficient IgG in regard to their therapeutic and diagnostic use. Serendipitously, however, we found a paradox: While murine IgM antibodies specific for guanosine triphosphate (GTP) were able to recognize native guanylyl antigens found in primate or rat muscle tissues by immunofluorescence assays (which mimicked the auto-antibodies from autoimmune patients to skeletal or smooth muscle), the murine and human IgG counterparts failed. The results were replicated in cell-free direct binding assays using small latex microspheres decorated densely with GTP. The IgG antibodies could bind, however, if GTP was presented more spaciously on larger particles or as a univalent hapten. Accordingly, oligomerization of GTP (30-mer) destroyed the binding of the IgG antibodies but enhanced that of the IgMs in inhibition ELISA. We reason that, contrary to current belief, IgM does not bind in a lock-and-key manner like IgG. We hypothesize that whereas the intact and rigid antigen-binding site of IgG hinders the antibody from docking with antigens that are obstructed, in IgM, the two component polypeptides of the antigen-binding site can dissociate from each other and navigate individually through obstacles like the ancestral single-polypeptide antibodies found in sharks and camelids, both components eventually re-grouping around the antigen. We further speculate that polyreactive IgMs, which enigmatically bind to more than one type of antigen, use the same modus operandi. These findings call for a re-look at the clinical potential of IgM antibodies particularly in specific areas of cancer therapy, tissue pathology and vaccine design, where IgG antibodies have failed due to target inaccessibility.
抗体是众所周知的免疫蛋白介质。在治疗和诊断用途方面，IgM 是原始成员，也是后来进化的更有效的 IgG 的被忽视的兄弟姐妹。然而，我们偶然发现了一个悖论：虽然针对三磷酸鸟苷 (GTP) 的鼠源性 IgM 抗体能够通过免疫荧光分析（模拟来自自身免疫患者的针对骨骼肌或平滑肌的自身抗体）识别原代和灵长类或大鼠肌肉组织中的天然鸟苷基抗原，但鼠源性和人源性 IgG 抗体却不能。在使用小乳胶微球进行的无细胞直接结合测定中，结果得到了复制，这些微球上密集地装饰有 GTP。然而，如果 GTP 在较大的颗粒上或作为单价半抗原呈现更宽松的空间，则 IgG 抗体可以结合。因此，GTP（30 -mer）的寡聚化破坏了 IgG 抗体的结合，但在抑制 ELISA 中增强了 IgM 的结合。我们推断，与当前的观点相反，IgM 不像 IgG 那样以锁钥方式结合。我们假设，尽管 IgG 的完整和刚性抗原结合位点阻碍了抗体与被阻塞的抗原结合，但在 IgM 中，抗原结合位点的两个组成多肽可以彼此解离，并单独通过障碍物导航，就像在鲨鱼和骆驼科中发现的祖先单多肽抗体一样，两个组成部分最终围绕抗原重新组合。我们进一步推测，多反应性 IgM 会神秘地结合一种以上类型的抗原，使用相同的操作模式。这些发现呼吁重新审视 IgM 抗体的临床潜力，特别是在癌症治疗、组织病理学和疫苗设计等特定领域，由于靶标不可及，IgG 抗体在此类领域中已经失败。
