Carbohydrate composition and immunomodulatory activity of different glycoforms of α1-acid glycoprotein
Carbohydrate composition and immunomodulatory activity of different glycoforms of α1-acid glycoprotein
复制标题
α1-酸性糖蛋白不同糖型的碳水化合物组成和免疫调节活性
作者：
S. D. Shiyan;N. Bovin
急性时相蛋白α1-酸性糖蛋白(AGP)是人类血液中的正常成分(0.2-1 mg ml−1)，其糖基化和血液中的浓度在炎症过程中发生变化。本文从糖链结构的角度讨论了AGP的免疫调节特性，并对正常献血者血清(NAGP)、流产患者血清(FAGP)、癌症患者血清(CAGP)和胃癌患者腹水(SAGP)制备的AGP样品进行了分析。除fAGP外，所有样品都有5条复合型N-链，但双、三、四天线链的数量以及终止这些链的糖链结构不同。FAGP含有3条乳糖胺和聚乳糖胺类型的N链和3条其他来源的AGP所没有的O链。用ConA亲和层析柱分离得到的NAGP和sAGP的糖型在分支方面相似，但在末端寡糖方面不同。通过测定不同AGP制剂对PHA刺激的人淋巴细胞增殖反应的影响，以及对刺激细胞产生IL-1、IL-2、IL-6和TNF的影响，来检测不同AGP制剂的免疫调节活性。在增殖试验中，与癌症或胎儿AGP相比，NAGP不那么活跃，但在影响细胞因子产生方面更活跃。一些AGP糖体具有相反的免疫调节作用，为了阐明糖链在AGP生物活性中的作用，开发了一种新的方法。N-连接的低聚糖链连接到可溶的聚丙烯酰胺基质上。这种“伪糖蛋白”在其相对分子质量、四、三、双触角链的相对含量以及单、二、三、四唾液酸寡糖的含量上与AGP相似。对人外周血白细胞亚群的流式细胞术分析表明，AGP与假AGP结合的主要靶点是单核细胞和粒细胞，而不是淋巴细胞。这种结合被含有甘露糖或唾液酸的合成糖偶联物抑制。结合曲线数据表明，存在单核细胞和粒细胞两个群体。它们可能有不同的碳水化合物特性。所有这些研究提供的证据都表明，AGP上的碳水化合物链在调节免疫系统中起重要作用，而不是AGP分子本身。
The acute phase protein, α1-acid glycoprotein (AGP), is a normal constituent of human blood (0.2–1 mg ml−1) and its glycosylation and concentration in the blood change during inflammation. In this review of our recent work, we discuss the immunomodulatory properties of AGP in connection with the structure of its carbohydrate chains.AGP samples prepared from normal donor serum (nAGP), serum obtained during abortion (fAGP), serum of cancer patients (cAGP), and ascitic fluid of patients with stomach cancer (sAGP) were subjected to analysis. All the samples except for fAGP had five N-linked chains of the ‘complex’ type, however, the numbers of bi-, tri-, and tetra-antennary chains, as well as glycan structures terminating these chains, were different. fAGP had three N-linked chains of the lactosamine and polylactosamine type and three O-chains which were not present in AGP isolated from the other sources. The glycoforms of nAGP and sAGP that were isolated using a ConA affinity column were similar in respect to their branching, but differed in their terminal oligosaccharides. sAGP was enriched in units ending in Lex and asialoagalacto (GlcNAc-terminating) forms.Immunomodulatory activity of different AGP preparations was tested in vitro by measuring their effect on the proliferative response of human lymphocytes stimulated by PHA, and by determining their influence on the production of IL-1, IL-2, IL-6, and TNF in the stimulated cells. nAGP was less active compared to cancer or fetal AGP in the proliferation test, but more active in affecting cytokine production. Some AGP glycoforms had opposite immunomodulatory effects.A new approach was developed in order to clarify the role of carbohydrate chains in the biological activity of AGP. A pool of N-linked oligosaccharide chains were attached to a soluble polyacrylamide matrix. This ‘pseudoglycoprotein’ was similar to AGP in its molecular weight; in its relative amounts of tetra-, tri-, and bi-antennary chains; and in the content of mono-, di-, tri-, and tetra-sialylated-oligosaccharides. This pseudo-AGP displayed a similar activity to its parent AGP in the biological tests.Analytical flow cytometry of leukocyte subpopulation from human peripheral blood showed that monocytes and granulocytes but not lymphocytes were the main targets for the binding of AGP and pseudo-AGP. This binding was inhibited by synthetic glycoconjugates containing mannose or sialic acid. The binding curve data suggested that there are two monocyte and granulocyte populations. These may have different carbohydrate specificities.All the evidence provided by these studies indicate that it is the carbohydrate chains on AGP that are important in modulating the immune system and not the AGP molecule itself.