Galectins are a structurally conserved family of ß-galactoside-binding lectins characterized by a unique sequence motif in the carbohydrate recognition domain, and of wide taxonomic distribution, from fungi to mammals. Their biological functions, initially described as key to embryogenesis and early development via recognition of endogenous ("self") carbohydrate moieties, are currently understood as also encompassing tissue repair, cancer metastasis, angiogenesis, adipogenesis, and regulation of immune homeostasis. More recently, however, numerous studies have contributed to establish a new paradigm by revealing that galectins can also bind to exogenous ("non-self") glycans on the surface of potentially pathogenic virus, bacteria, and eukaryotic parasites, and function both as pathogen recognition receptors (PRRs) and effector factors in innate immunity. Our studies on a galectin from the kuruma shrimp Marsupenaeus japonicus (MjGal), revealed that it functions as a typical PRR. Expression of MjGal is upregulated by infectious challenge, and can recognize both Gram (+) and Gram (-) bacteria. MjGal also recognizes carbohydrates on the shrimp hemocyte surface, and can cross-link microbial pathogens to the hemocytes, promoting their phagocytosis and clearance from circulation. Therefore, MjGal contributes to the shrimp's immune defense against infectious challenge both as a PRR and effector factor. Our studies on galectins from the bivalve mollusks, however, have shown that although they can function in immune defense as MjGal, protistan parasites take advantage of the recognition roles of the host galectins, for successful attachment and host infection. We identified in the eastern oyster Crassostrea virginica two galectins (CvGal1 and CvGal2) that not only recognize a large variety of bacterial species, but also the protozoan parasite Perkinsus marinus. Like the shrimp MjGal, both oyster galectins function as opsonins, and promote parasite adhesion and phagocytosis. However, P. marinus survives intrahemocytic oxidative killing and proliferates, eventually causing systemic infection and death of the oyster host. In the softshell clam Mya arenaria we identified a galectin (MaGal1) that displays carbohydrate specificity and recognition properties for sympatric Perkinsus species (P. marinus and P. chesapeaki), that are different from CvGal1 and CvGal2. Our results suggest that although galectins from bivalves can function as PRRs, Perkinsus parasites have co-evolved with their hosts to subvert the galectins' immune functions for host infection by acquisition of carbohydrate-based mimicry.

中文翻译：

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半乳糖凝集素介导的免疫识别：在选定的虾和双壳类软体动物物种中具有对比结果的调理作用。

半乳糖凝集素是一个结构保守的 ß-半乳糖苷结合凝集素家族，其特征是在碳水化合物识别域中具有独特的序列基序，并且具有广泛的分类学分布，从真菌到哺乳动物。它们的生物学功能最初被描述为通过识别内源性（“自我”）碳水化合物部分对胚胎发生和早期发育的关键，目前被理解为还包括组织修复、癌症转移、血管生成、脂肪生成和免疫稳态调节。然而，最近，许多研究通过揭示半乳糖凝集素还可以与潜在致病病毒、细菌和真核寄生虫表面的外源性（“非自身”）聚糖结合，为建立新的范式做出了贡献，并在先天免疫中作为病原体识别受体 (PRR) 和效应因子发挥作用。我们对库鲁马虾 Marsupenaeus japonicus (MjGal) 的半乳糖凝集素的研究表明，它具有典型的 PRR 功能。MjGal 的表达受到感染性攻击的上调，并且可以识别革兰 (+) 和革兰 (-) 细菌。MjGal 还识别虾血细胞表面的碳水化合物，并且可以将微生物病原体与血细胞交联，促进它们的吞噬作用和从循环中清除。因此，MjGal 作为 PRR 和效应因子有助于虾的免疫防御抵抗感染挑战。然而，我们对来自双壳类软体动物的半乳糖凝集素的研究表明，虽然它们可以像 MjGal 一样在免疫防御中发挥作用，原生生物寄生虫利用宿主半乳糖凝集素的识别作用，成功附着和宿主感染。我们在东部牡蛎 Crassostrea virginica 中发现了两种半乳糖凝集素（CvGal1 和 CvGal2），它们不仅可以识别多种细菌物种，还可以识别原生动物寄生虫 Perkinsus marinus。与虾 MjGal 一样，两种牡蛎半乳凝素都具有调理素的作用，可促进寄生虫粘附和吞噬作用。然而，P. marinus 在血细胞内氧化杀伤中存活并增殖，最终导致牡蛎宿主的全身感染和死亡。在软壳蛤 Mya arenaria 中，我们确定了一种半乳糖凝集素 (MaGal1)，它对同域 Perkinsus 物种（P. marinus 和 P. chesapeaki）显示碳水化合物特异性和识别特性，与 CvGal1 和 CvGal2 不同。