Tunicate Lectins as Acute Phase Reactants in Innate Immunity

从对无脊椎动物凝集素的早期研究中，提出将其命名为“保护素”以表明，由于无脊椎动物缺乏由免疫球蛋白、B和T细胞介导的适应性免疫，因此这些蛋白质将介导成虫或其卵的内部保护以对抗微生物或真菌感染。对动物凝集素的现代研究在这些基础上得到了巨大的扩展，发现来自脊椎动物的凝集素直接参与先天免疫功能，如LPS结合分子，调理素和补体激活因子。已经描述了许多具有免疫相关功能的凝集素，特别是在C型凝集素中，其需要在其碳水化合物识别结构域（CRD）中的Ca++来结合配体，并且共享CRD序列基序。近年来，半乳糖凝集素家族的一些成员，另一个主要的结构类别的凝集素，不需要Ca++的结合，并共享一个不同的CRD motif，已被提出参与免疫功能。原脊索动物缺乏适应性免疫，因此，是合适的模型来检查先天免疫机制的无脊椎动物，可能已经通过脊椎动物谱系保守。在美国国家科学基金会的支持下，该实验室已经：（a）鉴定和表征了C。picta是一种独特的可溶性C型凝集素（CPL-III），不仅在单个多肽亚基中携带MBL和选择素样CRD，而且还结合“非自身”（环境细菌）和“自我”（膜硫酸化聚糖）配体，（B）实现了补体活化的推定分子配偶体、丝氨酸蛋白酶MASP和补体组分C3的部分表征，（c）证明了在血细胞表面上存在CPL-III样蛋白，和（d）表征了两种不同的半乳糖凝集素，其中一种在血细胞中不同地表达。通过解决CPL-III，MASP，C3和半乳糖凝集素的结构和功能方面，并研究它们的协调活性，本研究将建立C型凝集素和半乳糖凝集素之间的联系，补体激活和调理作用。初步结果表明，C. picta是研究两类凝集素（C型凝集素和半乳糖凝集素）在先天免疫中的潜在协同表达和生物学作用的理想模型系统。CPL-III不同的CRD的串联组织，它们与哺乳动物MBL和选择素的同源性，以及它们与“自我”和“非自我”碳水化合物配体的结合，可以提供对哺乳动物聚集素和选择素的进化历史的洞察。此外，建立CPL-III作为完整功能凝集素介导的补体激活途径的“触发物”的作用，将构成该途径在进化中先于抗体介导的途径的最强指示，如早先在后口无脊椎动物中发现C3时所提出的。这将代表我们对先天免疫的理解的根本性进步，这将超越无脊椎动物/脊椎动物的界限。

From the early studies on invertebrate lectins, their designation as 'protectins' was proposed to suggest that, because invertebrates lack adaptive immunity mediated by immunoglobulins, B and T cells, these proteins would mediate internal protection for the adult or its eggs against microbial or fungal infection. The modern era of research on animal lectins has seen a vast expansion on these foundations, to find lectins from vertebrates in direct participation in innate immune functions, as LPS-binding molecules, opsonins, and complement-activating factors. Numerous lectins with immune-related functions have been described, particularly among the C-type, lectins that require Ca++ in their carbohydrate-recognition domain (CRD) for binding to ligand, and share a CRD sequence motif. Recently, some members of the galectin family, another major structural class of lectins which do not require Ca++ for binding and share a different CRD motif, have been proposed to participate in immune functions.This laboratory has focused on lectins as 'non-self' recognition molecules, as the first step of the immune response in invertebrates, using the protochordate Clavelina picta as the model organism. Protochordates lack adaptive immunity, and thus, are suitable models to examine innate immunity mechanisms of invertebrates that may have been conserved through the vertebrate lineages. With prior NSF support this laboratory has: (a) identified and characterized in C. picta a unique soluble C-type lectin (CPL-III) that not only carries both MBL- and selectin-like CRDs, in a single polypeptide subunit, but also binds both 'non-self' (environmental bacteria) and 'self' (tunic sulfated glycan) ligands, (b) achieved partial characterization of putative molecular partners for complement activation, the serine protease MASP, and complement component C3, (c) demonstrated the presence of a CPL-III-like protein on the surface of the hemocytes, and (d) characterized two distinct galectins, one of which is distinctly expressed in hemocytes. By addressing the structure and functional aspects of CPL-III, MASP, C3, and galectin, and investigating their coordinated activity, this research will establish a link between C-type lectins and galectins in complement activation and opsonization. Preliminary results make C. picta an ideal model system to examine the potential coordinate expression and biological roles of the two lectin classes (C-type lectins and galectins) in innate immunity. The tandem organization of CPL-III distinct CRDs, their homologies to mammalian MBL and selectins, and their binding to 'self' and 'non-self' carbohydrate ligands, may provide insight into the evolutionary history of mammalian collectins and selectins. Furthermore, to establish the role of CPL-III as the 'trigger' of an integrally functional lectin-mediated complement activation pathway, would constitute the strongest indication that this pathway preceded the antibody-mediated pathway in evolution, as proposed earlier upon the discovery of C3 in deuterostome invertebrates. This would represent a fundamental advance in our understanding of innate immunity, which would transcend the invertebrates/vertebrates boundary.