Role of the Property of C-Reactive Protein to Activate the Classical Pathway of Complement in Protecting Mice from Pneumococcal Infection

Abstract

C-reactive protein (CRP) is not an acute-phase protein in mice, and therefore, mice are widely used to investigate the functions of human CRP. It has been shown that CRP protects mice from pneumococcal infection, and an active complement system is required for full protection. In this study, we assessed the contribution of CRP’s ability of activating the classical pathway of complement in the protection of mice from lethal infection with virulent Streptococcus pneumoniae type 3. We used two CRP mutants, Y175A and K114A. The Y175A CRP does not bind C1q and does not activate complement in human serum. The K114A CRP binds C1q and activates complement more efficiently than wild-type CRP. Passively administered, both CRP mutants and the wild-type CRP protected mice from infection equally. Infected mice injected with wild-type or mutant CRP had reduced bacteremia, resulting in lower mortality and increased longevity compared with mice that did not receive CRP. Thus, the protection of mice was independent of CRP-mediated activation of the classical pathway of complement. To confirm that human CRP does not differentiate between human and mouse complement, we analyzed the binding of human CRP to mouse C1q. Surprisingly, CRP did not react with mouse C1q, although both mutant and wild-type CRP activated mouse C3, indicating species specificity of CRP-C1q interaction. We conclude that the mouse is an unfit animal for exploring CRP-mediated activation of the classical complement pathway, and that the characteristic of CRP to activate the classical complement pathway has no role in protecting mice from infection.

The Classical and Regulatory Functions of C1q in Immunity and Autoimmunity

“…Like IgM, CRP also binds to phosphorylcholine on AC and then recruits C1q and elicits complement activation (27). Binding of CRP to phosphorylcholine on bacteria also elicit C1q-mediated complement activation (28). Compared with direct C1q opsonization, C1q-mediated C3 deposition on AC is likely to lead to more effective AC phagocytosis as a single C1q bound to AC can cause the deposition of many C3 molecules (1).…”

“…Another well-studied C1q-binding molecule is CRP. C1q binds to CRP and similarly activates the complement classical pathway (27,28). The difference between CRP and immune complexes as C1q ligands is that CRP but not immune complexes also binds factor H (71).…”

Purification of recombinant C-reactive protein mutants

“…The significance of the recognition function of CRP in its alternate structural pentameric conformation in host defense is unknown. However, ex vivo and in vivo experiments in animal models of inflammatory diseases indicate that CRP exerts, although limited, anti-pneumococcal, anti-atherosclerotic, anti-arthritic and antiamyloidogenic functions Agrawal et al, 2008Agrawal et al, , 2010Chang et al, 2012;Gang et al, 2012Gang et al, , 2015Jiang et al, 2006;Jones et al, 2011;Kovacs et al, 2007;Nayeri et al, 2010;Ngwa et al, 2016;Ozawa et al, 2016;Simons et al, 2014;Singh et al, 2008;Suresh et al, 2006Suresh et al, , 2007Szalai et al, 1995;Yother et al, 1982). CRP has also been shown to alleviate experimental autoimmune encephalomyelitis (Zhang et al, 2015).…”

“…For 2002; Gang et al, 2012Gang et al, , 2015Suresh et al, 2006Suresh et al, , 2007. Recombinant CRP mutants can be expressed in mammalian cells and, if expressed, can be purified from the cell culture media.…”

A Novel RBP-Jκ-Dependent Switch from C/EBPβ to C/EBPζ at the C/EBP Binding Site on the C-Reactive Protein Promoter

“…C -reactive protein (CRP) 3 is a host-defense protein. The serum concentration of CRP increases in chronic and acute inflammatory and in some noninflammatory states (1)(2)(3)(4).…”