Department of Biomedical Sciences, James H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN, United States.
Front Immunol. 2020 Aug 13;11:1812. doi: 10.3389/fimmu.2020.01812. eCollection 2020.
C-reactive protein (CRP), a component of the innate immune system, is an antipneumococcal plasma protein. Human CRP has been shown to protect mice against infection with lethal doses of by decreasing bacteremia. , CRP binds to phosphocholine-containing substances, such as pneumococcal C-polysaccharide, in a Ca-dependent manner. Phosphocholine-complexed human CRP activates the complement system in both human and murine sera. The mechanism of antipneumococcal action of CRP , however, has not been defined yet. In this study, we tested a decades-old hypothesis that the complement-activating property of phosphocholine-complexed CRP contributes to protection of mice against pneumococcal infection. Our approach was to investigate a CRP mutant, incapable of activating murine complement, in mouse protection experiments. We employed site-directed mutagenesis of CRP, guided by its three-dimensional structure, and identified a mutant H38R which, unlike wild-type CRP, did not activate complement in murine serum. Substitution of His with Arg in CRP did not affect the pentameric structure of CRP, did not affect the binding of CRP to pneumococci, and did not decrease the stability of CRP in mouse circulation. Employing a murine model of pneumococcal infection, we found that passively administered H38R CRP failed to protect mice against infection. Infected mice injected with H38R CRP showed no reduction in bacteremia and did not survive longer, as opposed to infected mice treated with wild-type CRP. Thus, the hypothesis that complement activation by phosphocholine-complexed CRP is an antipneumococcal effector function was supported. We can conclude now that complement activation by phosphocholine-complexed CRP is indeed essential for CRP-mediated protection of mice against pneumococcal infection.
C-反应蛋白（CRP）是先天免疫系统的一个组成部分，也是一种抗肺炎球菌的血浆蛋白。研究表明，人类 CRP 可通过降低菌血症来保护小鼠免受致死剂量的肺炎球菌感染。CRP 以 Ca2+依赖性方式与含有磷酸胆碱的物质（如肺炎球菌 C-多糖）结合。磷酸胆碱复合的人 CRP 可在人血清和鼠血清中激活补体系统。然而，CRP 抗肺炎球菌作用的机制尚未确定。在这项研究中，我们检验了一个存在了几十年的假说，即磷酸胆碱复合的 CRP 激活补体的特性有助于保护小鼠免受肺炎球菌感染。我们的方法是在小鼠保护实验中研究一种不能激活鼠补体的 CRP 突变体。我们采用 CRP 的三维结构为指导进行 CRP 的定点突变，并鉴定出一个突变体 H38R，与野生型 CRP 不同，它不能在鼠血清中激活补体。CRP 中的组氨酸突变为精氨酸不会影响 CRP 的五聚体结构，不会影响 CRP 与肺炎球菌的结合，也不会降低 CRP 在鼠循环中的稳定性。在肺炎球菌感染的小鼠模型中，我们发现被动给予 H38R CRP 不能保护小鼠免受感染。与用野生型 CRP 治疗的感染小鼠相比，注射 H38R CRP 的感染小鼠的菌血症没有减少，也没有存活更长时间。因此，磷酸胆碱复合的 CRP 通过补体激活是一种抗肺炎球菌效应功能的假说得到了支持。我们现在可以得出结论，即磷酸胆碱复合的 CRP 通过补体激活对 CRP 介导的小鼠肺炎球菌感染保护确实是必不可少的。
