Medical Protein Chemistry, Department of Laboratory Medicine, Lund University, S-205 02 Malmö, Sweden;
J Immunol. 2012 Oct 1;189(7):3575-84. doi: 10.4049/jimmunol.1102934. Epub 2012 Aug 27.
Streptococcus pneumoniae (pneumococcus) is a pathogen that causes severe local and life-threatening invasive diseases, which are associated with high mortality rates. Pneumococci have evolved several strategies to evade the host immune system, including complement to disseminate and to survive in various host niches. Thus, pneumococci bind complement inhibitors such as C4b-binding protein (C4BP) and factor H via pneumococcal surface protein C, thereby inhibiting the classical and alternative complement pathways. In this study, we identified the pneumococcal glycolytic enzyme enolase, a nonclassical cell surface and plasminogen-binding protein, as an additional pneumococcal C4BP-binding protein. Furthermore, we demonstrated that human, but not mouse, C4BP bound pneumococci. Recombinant enolase bound in a dose-dependent manner C4BP purified from plasma, and the interaction was reduced by increasing ionic strength. Enolase recruited C4BP and plasminogen, but not factor H, from human serum. Moreover, C4BP and plasminogen bound to different domains of enolase as they did not compete for the interaction with enolase. In direct binding assays with recombinant C4BP mutants lacking individual domains, two binding sites for enolase were identified on the complement control protein (CCP) domain 1/CCP2 and CCP8 of the C4BP α-chains. C4BP bound to the enolase retained its cofactor activity as determined by C4b degradation. Furthermore, in the presence of exogenously added enolase, an increased C4BP binding to and subsequently decreased C3b deposition on pneumococci was observed. Taken together, pneumococci specifically interact with human C4BP via enolase, which represents an additional mechanism of human complement control by this versatile pathogen.
肺炎链球菌（肺炎球菌）是一种病原体，可引起严重的局部和危及生命的侵袭性疾病，死亡率很高。肺炎球菌已经进化出几种策略来逃避宿主免疫系统，包括补体扩散和在各种宿主小生境中存活。因此，肺炎球菌通过肺炎球菌表面蛋白 C结合补体抑制剂，如 C4 结合蛋白（C4BP）和因子 H，从而抑制经典和替代补体途径。在这项研究中，我们确定了肺炎球菌糖酵解酶烯醇酶，一种非经典的细胞表面和纤溶酶原结合蛋白，作为另一种肺炎球菌 C4BP 结合蛋白。此外，我们证明了人 C4BP 而不是鼠 C4BP 结合肺炎球菌。重组烯醇酶以剂量依赖性方式结合从血浆中纯化的 C4BP，并且通过增加离子强度来减少相互作用。烯醇酶从人血清中募集 C4BP 和纤溶酶原，但不募集因子 H。此外，C4BP 和纤溶酶原结合到烯醇酶的不同结构域，因为它们不竞争与烯醇酶的相互作用。在与缺乏单个结构域的重组 C4BP 突变体的直接结合测定中，在 C4BP α 链的补体控制蛋白（CCP）结构域 1/CCP2 和 CCP8 上鉴定出两个烯醇酶结合位点。C4BP 结合到烯醇酶上保留了其作为 C4b 降解的辅因子的活性。此外，在添加外源烯醇酶的情况下，观察到 C4BP 结合增加和随后肺炎球菌上 C3b 沉积减少。总之，肺炎球菌通过烯醇酶特异性地与人 C4BP 相互作用，这代表了这种多功能病原体对人补体控制的另一种机制。
