Tunicate Lectins as Acute Phase Reactants in Innate Immunity

早期对无脊椎动物凝集素的研究表明，由于无脊椎动物缺乏由免疫球蛋白、B细胞和T细胞介导的适应性免疫，这些蛋白质将为成虫或卵提供内部保护，使其免受微生物或真菌的感染。现代对动物凝集素的研究在这些基础上得到了极大的扩展，从脊椎动物中发现了直接参与先天免疫功能的凝集素，如内毒素结合分子、调味素和补体激活因子。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，这些凝集素需要在其碳水化合物识别结构域(CRD)中与配体结合，并共享CRD序列基序。最近，另一个主要的凝集素家族Galectin家族的一些成员被提出参与免疫功能。Galectin家族是另一个不需要钙+结合的凝集素家族，具有不同的CRD基序。本实验室以原索动物Clavelina picta为模式生物，将凝集素作为非我识别分子，作为无脊椎动物免疫反应的第一步。原脊椎动物缺乏适应性免疫，因此是研究无脊椎动物先天免疫机制的合适模型，这些机制可能是通过脊椎动物谱系保存下来的。在先前NSF的支持下，这个实验室已经：(A)在微囊藻中鉴定和鉴定了一种独特的可溶性C型凝集素(CPL-III)，它不仅在单个多肽亚单位中携带MBL-和选择素样CRD，而且还结合‘非自身’(环境细菌)和‘自身’(衣膜硫酸多糖)配体，(B)部分鉴定了补体激活的假定分子伙伴丝氨酸蛋白酶MASP和补体成分C3，(C)证明在血细胞表面存在CPL-III样蛋白，以及(D)鉴定了两种不同的Galectin，其中一个在血细胞中有明显的表达。通过阐述CPL-III、MASP、C3和Galectin的结构和功能，并研究它们的协同活性，本研究将建立C型凝集素和Galectins在补体激活和调理中的联系。初步研究结果表明，Picta凝集素是研究天然免疫中两类凝集素(C型凝集素和Galectins)潜在的协同表达和生物学作用的理想模型系统。CPL-III不同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‘自身’和‘非自身’碳水化合物配体的结合，可能有助于深入了解哺乳动物集合素和选择素的进化史。此外，建立CPL-III作为完整功能的凝集素介导的补体激活途径的触发器的作用，将构成最强烈的迹象，表明该途径在进化上先于抗体介导的途径，正如早些时候在后口无脊椎动物中发现C3时提出的那样。这将代表着我们对先天性免疫的理解取得了根本性的进步，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.