Early ovariectomy reveals the germline encoding of the “natural” mammalian anti-A-reactive IgM reflecting developmental malignancy.

The germline encoding of a non-immune immunoglobulin M (IgM) molecule in mammals was experimentally documented for the first time as a result of the specifically timed ovariectomy of C57BL/10 mice. The target of this innate antibody involves a trans-species developmental antigen that signifies malignancy when expressed in any non-developmental tissue. Although ovariectomy and castration result in uncontrolled and/or enhanced humoral and cellular immunity involving increased weights of the spleen and thymus with pronounced B and T cell production, the development of mercaptoethanol-sensitive, complement-binding, non-immune anti-A reactivity in murine plasma was not enhanced after an ovariectomy that was performed in C57BL/10 mice prior to the onset of puberty. This non-immune murine anti-A, which is complementary to the GalNAc glycan of syngeneic ovarian glycolipids and distinct from the cross-reactive adaptive anti-A antibody, was strongly downregulated or did not appear in plasma of animals ovariectomized at the age of 20 days. In mouse and man, this molecule most likely gets its complementary footprint from the first, genetically as-yet-undefined trans-species <i>O</i>-linked glycosylation of proteins and after depletion of the volatilely expressed “immature” <i>O</i>-GalNAc transferase activities and release from the transiently appearing glycopeptides, provides the A-reactive glycosidic sites. Consequently, these volatilely expressed non-somatic <i>O</i>-GalNAc-glycosylations of proteins, involving serine/threonine residues, are either identical or metabolically related with those of the mucin-type “aberrant” monosaccharide GalNAcα1-<i>O</i>-Ser/Thr-R or Tn antigen and explain the anti-Tn cross-reactivity of anti-A specific immunoglobulins and the pronounced occurrence of anti-Tn in the plasma of the human histo (blood) group O, in which the A-allelic, phenotype-specific GalNAc glycosylation of plasma proteins does not occur and not affect the anti-Tn levels.

本研究通过对C57BL/10小鼠实施精准时序的卵巢切除术，首次实验证实了哺乳动物中非免疫性免疫球蛋白M（IgM）分子的种系编码。该天然抗体的靶标为一类跨物种发育抗原，此类抗原在非发育组织中表达时可提示恶性肿瘤发生。尽管卵巢切除术与阉割术可引发体液及细胞免疫的失控或增强，具体表现为脾脏与胸腺重量增加，且B细胞、T细胞生成显著增多，但在青春期前接受卵巢切除术的C57BL/10小鼠血浆中，巯基乙醇敏感、补体结合型非免疫性抗A反应并未得到增强。这种非免疫性小鼠抗A抗体可与同基因卵巢糖脂的N-乙酰半乳糖胺（GalNAc）聚糖互补，且有别于交叉反应性适应性抗A抗体；在出生后20日龄接受卵巢切除术的动物血浆中，该抗体被显著下调甚至无法检出。在小鼠与人类体内，该分子的互补结合位点大概率源自首个在遗传层面尚未明确的蛋白质O-连接N-乙酰半乳糖胺（O-GalNAc）糖基化：在瞬时表达的“未成熟”O-GalNAc转移酶活性被耗竭，且从瞬时出现的糖肽中释放后，便可形成抗A反应性糖苷位点。综上，这些发生于丝氨酸/苏氨酸残基的瞬时表达非体细胞O-GalNAc糖基化，要么与黏蛋白型“异常”单糖GalNAcα1-O-Ser/Thr-R（即Tn抗原）完全一致，要么存在代谢相关性；这一机制既解释了抗A特异性免疫球蛋白的抗Tn交叉反应性，也阐明了人类组织（血液）O型个体血浆中抗Tn抗体的高发现象——此类个体不存在A等位基因相关的表型特异性血浆蛋白GalNAc糖基化，且该糖基化也不会对抗Tn抗体水平产生影响。