Complement component 3 d (C3d) is the final cleavage product of the complement component C3 and serves as a crucial role in link innate and adaptive immunity, and increase B-cell sensitivity to an antigen by 1000–10000 fold. The crystal structure of human C3d revealed there are two distinct surfaces, a convex surface containing the thioester-constituting residues that mediate covalent binding to the target antigen, and a concave surface with an acidic pocket responsible for interaction with CR2. In this study, we cloned and sequenced cDNA fragment encoding C3d region from 15 wild bird species. Then, the C3d sequences from wild birds, chicken and mammals were aligned to construct phylogenetic trees. Phylogenetic tree displayed two main branches, indicating mammals and birds, but the bird C3d branch was divided into two main parts, with five wild birds (Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus) clustering much closer to mammals. In addition, the C3d proteins of Ardeola bacchus, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus contained a Glu163 residue at the position at which Lys163 was found in other birds. However, Glu163 have the same charge polarity as Asp163, which is the key amino acid residue comprising the acidic pocket combined with CR2 found at this position in mammals, and Zoothera also possessed Asp163 at this position. Structure modeling analyses also verified that the C3ds of these five wild bird species exhibited the amino acid sequence and structure comprising the typical acidic pocket found in mammals that is required for combination with B cell surface receptors, which contribute electrostatic forces to interact with CR2. Our investigations indicate that some bird C3ds may already have the ability to bind with CR2 by electrostatic force, like mammals. As Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus have more typical C3d concave acid pockets and thus a stronger ability to bind CR2, we speculate that these five wild birds may have a solider immunity against pathogens. Our phylogenetic and structural analyses of bird C3ds provide insights on the evolutionary divergence in the function of immune factors of avian and mammalian.

补体成分 3 d (C3d) 是补体成分 C3 的最终裂解产物，在连接先天免疫和适应性免疫中起关键作用，并将 B 细胞对抗原的敏感性提高 1000-10000 倍。人类 C3d 的晶体结构显示有两个不同的表面，一个包含硫酯构成残基的凸面，该残基介导与靶抗原的共价结合，而一个凹面具有一个酸性口袋，负责与 CR2 相互作用。在这项研究中，我们从 15 种野生鸟类中克隆并测序了编码 C3d 区域的 cDNA 片段。然后，将来自野生鸟类、鸡和哺乳动物的 C3d 序列进行比对以构建系统发育树。系统发育树显示两个主要分支，表示哺乳动物和鸟类，但鸟类 C3d 分支分为两个主要部分，Ardeola bacchus、Zoothera、Bubo、Crossoptilon mantchuricum和Caprimulgus europaeus ) 与哺乳动物更接近。此外，Ardeola bacchus、Bubo、Crossoptilon mantchuricum和Caprimulgus europaeus的 C3d 蛋白在其他鸟类中发现 Lys163 的位置含有一个 Glu163 残基。然而，Glu163 与 Asp163 具有相同的电荷极性，Asp163 是关键氨基酸残基，包括在哺乳动物和Zoothera的该位置发现的与 CR2 结合的酸性口袋在这个位置也拥有 Asp163。结构建模分析还证实，这五种野生鸟类的 C3ds 表现出的氨基酸序列和结构包含哺乳动物中发现的典型酸性口袋，这是与 B 细胞表面受体结合所必需的，B 细胞表面受体有助于静电力与 CR2 相互作用。我们的调查表明，一些鸟类 C3ds 可能已经具有通过静电力与 CR2 结合的能力，例如哺乳动物。作为Ardeola bacchus、Zoothera、Bubo、Crosoptilon mantchuricum 和 Caprimulgus europaeus具有更典型的 C3d 凹酸袋，因此结合 CR2 的能力更强，我们推测这五只野鸟可能对病原体具有更强的免疫力。我们对鸟类 C3ds 的系统发育和结构分析为鸟类和哺乳动物免疫因子功能的进化差异提供了见解。