Molecular mechanisms of adhesive interactions between HEV and lymphocytes that regulate homeostatic lymphocyte homing.

淋巴细胞归巢到淋巴结和派尔集合淋巴结是由循环淋巴细胞和高内皮微静脉（HEV）之间的级联粘附相互作用介导的。HEV-淋巴细胞相互作用的初始阶段主要由淋巴细胞粘附分子L-选择素控制，其识别由HEV表达的唾液粘蛋白核心蛋白展示的硫酸化和唾液酸化O-连接寡糖。在这里，我们研究了戊型肝炎病毒特异性细胞粘附机制，调节稳态淋巴细胞归巢淋巴结和派尔集合淋巴结。通过小鼠HEV基因表达谱分析，我们鉴定了两种新的HEV相关唾液粘蛋白：内粘蛋白和肾粘蛋白。分子克隆和核苷酸序列分析表明，内粘蛋白在其胞外区含有一个粘蛋白样结构域，而nepmucin含有一个粘蛋白样结构域和一个IG结构域。有趣的是，虽然内粘蛋白在多种组织的血管内皮细胞中表达，但神经粘蛋白在外周淋巴结中表达，而不是在派尔集合淋巴结中表达。在淋巴结HEV中，内粘蛋白和髓粘蛋白都被L-选择素反应性糖链修饰，并可以结合可溶性L-选择素。此外，在通过糖修饰酶（包括C2 GnT、FucTVII和LSST）的特定组合进行适当糖基化后，内粘蛋白和肾粘蛋白可以在其粘蛋白结构域中展示L-选择素反应性寡糖，并在体外生理流动条件下支持淋巴细胞滚动。此外，nepmucin的Ig结构域似乎直接支持淋巴细胞结合。这些观察结果共同表明，内粘蛋白和nepmucin代表新的粘蛋白型配体的L-选择素在HEVs，他们发挥重要作用，通过HEVs淋巴细胞跟踪。

Lymphocyte homing to lymph nodes and Peyer's patches is mediated by cascade adhesive interactions between circulating lymphocytes and high endothelial venules (HEVs). The initial phase of the HEV-lymphocyte interaction is mainly governed by the lymphocyte adhesion molecule L-selectin, which recognizes sulfated and sialylated O-linked oligosaccharide displayed by sialomucin core proteins expressed by HEVs. Here we investigated HEV specific cell adhesion mechanisms that regulate homeostatic lymphocyte homing to lymph nodes and Peyer's patches. By gene expression profiling of mouse HEVs, we identified two novel HEV associated sialomucins, endomucin and nepmucin. Molecular cloning and nucleotide sequencing revealed that endomucin contains a mucin-like domain in its extracellular region, whereas nepmucin contains a mucin-like domain and an Ig domain. Interestingly, while endomucin is expressed in vascular endothelial cells in a variety of tissues, nepmucin is expressed in those in peripheral lymph nodes but not in Peyer's patches. In lymph node HEVs, both endomucin and nepmucin are decorated with L-selectin-reactive sugar chains and can bind soluble L-selectin. Furthermore, upon appropriate glycosylation by a specific combination of sugar-modification enzymes, including C2GnT, FucTVII, and LSST, endomucin and nepmucin can display L-selectin-reactive oligosaccharides in their mucin-domain and support lymphocyte rolling under physiological flow conditions in vitro. In addirion, the Ig-domain of nepmucin appears directly support lymphocyte binding. These observations collectively suggest that endomucin and nepmucin represent novel mucin-type ligands for L-selectin in HEVs and that they play important role in lymphocyte tracking via HEVs.