Factor H–Related Protein 5 Interacts with Pentraxin 3 and the Extracellular Matrix and Modulates Complement Activation

Abstract

The physiological roles of the factor H (FH)-related proteins are controversial and poorly understood. Based on genetic studies, FH-related protein 5 (CFHR5) is implicated in glomerular diseases, such as atypical hemolytic uremic syndrome, dense deposit disease, and CFHR5 nephropathy. CFHR5 was also identified in glomerular immune deposits at the protein level. For CFHR5, weak complement regulatory activity and competition for C3b binding with the plasma complement inhibitor FH have been reported, but its function remains elusive. In this study, we identify pentraxin 3 (PTX3) as a novel ligand of CFHR5. Binding of native CFHR5 to PTX3 was detected in human plasma and the interaction was characterized using recombinant proteins. The binding of PTX3 to CFHR5 is of ∼2-fold higher affinity compared with that of FH. CFHR5 dose-dependently inhibited FH binding to PTX3 and also to the monomeric, denatured form of the short pentraxin C–reactive protein. Binding of PTX3 to CFHR5 resulted in increased C1q binding. Additionally, CFHR5 bound to extracellular matrix in vitro in a dose-dependent manner and competed with FH for binding. Altogether, CFHR5 reduced FH binding and its cofactor activity on pentraxins and the extracellular matrix, while at the same time allowed for enhanced C1q binding. Furthermore, CFHR5 allowed formation of the alternative pathway C3 convertase and supported complement activation. Thus, CFHR5 may locally enhance complement activation via interference with the complement-inhibiting function of FH, by enhancement of C1q binding, and by activating complement, thereby contributing to glomerular disease.

The murine factor H-related protein FHR-B promotes complement activation

“…Although there are some controversies regarding the roles of human FHR proteins, a main function identified by recent studies is that they compete with FH for certain self and non-self ligands, such as C3b deposited on surfaces, pentraxins, ECM and, in the case of FHR-3, the FH-binding protein of N. meningitidis (20)(21)(22)(23)(24)(25). In addition to this indirect role in the enhancement of complement activation, some FHRs may directly activate complement by serving as a platform for alternative pathway convertase formation and are able to promote complement activation on host ligands and surfaces (23,24,26).…”

“…Most, particularly early, studies assessed the direct complement regulatory roles of FHRs, and some activities in the regulation of C3 or C5 convertases (15)(16)(17)(18), inhibition of the terminal pathway by FHR-1 (19), and synergistic enhancement of the cofactor activity of FH by FHR-3 and FHR-4 (15) were reported. Recent studies, however, highlight a paradigm change, and described deregulation, i.e., competitive inhibition of FH, as a major function of FHR-1, FHR-2, and FHR-5 (20)(21)(22)(23)(24). FHR-3 was described to compete off FH from binding to fHbp of Neisseria meningitidis (25).…”

“…***p < 0.001, one-way ANOVA. active C3bBb convertase (23,24,26). We therefore tested whether FHR-B was also able to support convertase formation.…”

The role of the alternative pathway of complement activation in glomerular diseases

“…While CFHR3 and CFHR4 were revealed to possess a weak FI cofactor activity, CFHR5 bound C3b in vitro and exhibited both FI cofactor and decay acceleration activity. However, most of these AP-regulating activities of CFHRs are weak, and they were observed in in vitro assays, which used non-physiological CFHRs concentrations [23,24,31,32,[34][35][36].…”

“…This process was called complement deregulation, and so far, CFHR1, CFHR2, and CFHR5 have been described to be a competitive antagonist of CFH [37][38][39][40]. Secondly, CFHR5 may deregulate complement indirectly by competing with CFH in binding to other than C3b physiological ligands, including pentraxin PTX3, CRP and extracellular matrix [36]. Also, CFHR4 and CFHR5 have been reported to directly promote C3 convertase formation by binding to C3b, with stronger ability to assemble C3bBb on CFHR4, and with C3bBb-CFHR4 complex being more resistant to CFH cleavage in comparison with C3bBb [36,41,42].…”