EDITORIAL www.jasn.org Unexpected Role for uncontrolled C3 activation results in accumulation of C3 activation products in the kidney was obtained through the Properdin in Complement C3 investigation of mice with gene-targeted fH mutations. fH- Glomerulopathies deficient mice develop uncontrolled AP activation and a sec- ondary deficiency of C3. Elegant studies from Pickering et al.6 demonstrate that these mice develop marked accumu- Mohamed R. Daha lation of C3 along the glomerular basement membrane Leiden University Medical Center, and University Medical Center, (GBM) and glomerular inflammation resembling membra- Groningen, The Netherlands noproliferative GN. A similar pathology has also been de- 7 J Am Soc Nephrol 24: ccc–ccc, 2013. scribed in a breed of pigs with spontaneous fH deficiency. doi: 10.1681/ASN.2012111110 Renal accumulation of C3 in the murine model is de- pendent on AP activation, because mice with a combined deficiency of fH together with a deficiency in fB do not de- The complement system is a major component of innate im- velop renal injury or hypercatabolism of C3.8 In addition, de- munity and plays a key role in our defense against foreign position of C3 along the GBM is dependent on factor I (fI), an pathogens.1 In normal steady state conditions, the comple- enzyme that cleaves C3b to inactive C3b (iC3b), rendering it ment system has no significant biologic activity. There are incapable of binding fB and initiating further AP and terminal three main pathways to generate biologic activity by initiation pathway activation. Studies in mice with a combined defi- of complement activation, including the classic pathway ciency of fH and fI suggest that it is iC3b that deposits along (CP), the lectin pathway (LP), and the alternative pathway the GBM during uncontrolled activation of the AP. (AP). Whereas antigen-antibody complexes mainly activate Studies from the 1970s demonstrate that fP stabilizes the the CP, the LP is initiated by pattern recognition of exposed C3bBb convertase and counteracts the function of fH.2 More complex carbohydrate moieties. The AP is constitutively ac- recent studies show that next to its stabilizing function, fP tive at a low grade due to the spontaneous tick-over of C3. may also serve as an initiator of AP activation on certain Once C3 is activated into C3b and C3a, the C3b fragment is surfaces like bacteria and apoptotic and necrotic cells.9 De- able to covalently bind to foreign and host cell surfaces and ficiencies of properdin are associated with severe meningo- serve as focus for amplification of complement activation coccal and neisserial infections, implicating the crucial role by the interaction with factor B (fB) and factor D (fD). The of fP in defense against foreign pathogens. C3bBb convertase that is generated is labile and requires One of the main questions in C3 glomerulopathies and stabilization by properdin (P) to extend its half-life for ad- aHUS is how to treat these diseases that share hypercatabo- ditional C3 activation.2 lism of C3 through the AP, and how to downregulate C3 AP complement activation is regulated by several mem- activation. Because fP is the only positive regulator of C3bBb brane and fluid phase moieties such as factor H (fH), which function, the thought is that attenuation of C3bBb function enhances decay dissociation of fB from the convertase re- by elimination of fP would be a good choice. sulting in loss of C3 activating potential, both in the fluid In this issue of JASN, two leading research groups report on phase and on cell surfaces. fH is a glycoprotein composed their studies on the effects of fP deletion in gene-targeted of 20 consensus repeats (SCRs) that interact with surface mice.10,11 Using comparable murine models, both Ruseva deposited C3b and polyanionic molecules on host cells and et al.10 and Lesher et al.11 come to the same unexpected find- thereby determines its affinity for tissues.3,4 N-terminal 1–4 ings, namely that fP deletion or depletion with mAbs, in con- SCRs are essential for regulation of complement activity, trast to what one would have expected, results in more severe 2/2 whereas SCRs 19–20 are required for tissue interaction. renal pathology in fH mice. The study by Ruseva et al.10 2/2 Mutations in fH in murine models and humans associate reports that fH mice have complete depletion of circulating 2/2 2/2 with severe kidney disease including C3 GN, such as dense C3 and C5 and that the fH ;fP mice have a predominant deposit disease (DDD) and atypical hemolytic uremia depletion of C3. This profile is in agreement with the comple- (aHUS).5,6 Initial insight into the mechanism through which ment profile seen in patients with fP-independent C3 nephritic factors (C3NeFs).12 Unexpectedly, however, glomerular in- flammation, capillary wall thickening, and glomerular C3 2/2 2/2 Published online ahead of print. Publication date available at www.jasn.org. staining were significantly increased in fH ;fP mice com- 2/2 Correspondence: Dr. Mohamed R. Daha, Leiden University Medical Center, pared with fH mice. Earlier on, these investigators had PO Box 9600, 2300 RC Leiden, Netherlands. Email: [email protected] already shown that administration of exogenous fH amelio- 2/2 Copyright © 2013 by the American Society of Nephrology rates GBM deposition of C3 in fH mice and triggers the J Am Soc Nephrol 24: ccc–ccc,2013 ISSN : 1046-6673/2401-ccc 1 EDITORIAL www.jasn.org appearance of mesangial C3 deposits. These studies demonstrate development of agents that control fluid phase AP activation in that fP influences the intraglomerular localization of C3. diseases with hypercatabolism of C3 using analogs with com- The study by Lesher et al.11 describes the generation of mice parable function of fH.15 with a deletion of SCRs 19–20 of fH, a region of fH involved m/m in carbohydrate interaction on cell surfaces. These fH mice, 2/2 6 in contrast to the fH of Pickering et al., have small amounts DISCLOSURES of truncated fH and significantly higher levels of C3 than the 2 2 2/2 m/m None. fH / mice. Like fH mice, the fH mice also exhibit prom- inent C3 and C9 deposition in the kidney with very weak IgG deposition. It is not clear why these IgG deposits are found but REFERENCES the investigators reason that this is not due to an antibody re- – 1. Walport MJ: Complement. First of two parts. NEnglJMed344: 1058– sponse against factor H. At 2 3 months of age, these mice de- 1066, 2001 velop mild glomerular hypercellularity and some evidence of an 2. Fearon DT, Austen KF: Properdin: Binding to C3b and stabilization of m/m inflammatory infiltrate. By 10–12 months, fH mice develop the C3b-dependent C3 convertase. JExpMed142: 856–863, 1975 signs of GN, characterized by increased glomerular hypercellu- 3. Ferreira VP, Herbert AP, Hocking HG, Barlow PN, Pangburn MK: Critical larity, inflammatory infiltrate, and thickening of the GBM to- role of the C-terminal domains of factor H in regulating complement activation at cell surfaces. JImmunol177: 6308–6316, 2006 gether with heavy proteinuria. 4. Józsi M, Oppermann M, Lambris JD, Zipfel PF: The C-terminus of com- To test the hypothesis that amelioration of fP would po- plement factor H is essential for host cell protection. Mol Immunol 44: m/m tentially prevent the occurrence of pathology in the fH 2697–2706, 2007 m/m 2/2 mice, Lesher et al.11 proceeded to generate a fH ;P 5. Pickering MC, Cook HT: Translational mini-review series on comple- mouse model. Like the findings of Ruseva et al.,10 these inves- ment factor H: Renal diseases associated with complement factor H: 11 Novel insights from humans and animals. Clin Exp Immunol 151: 210– tigators also found that deletion of fP did not result in pre- 230, 2008 vention of renal disease but in aggravation of disease activity. 6. Pickering MC, Cook HT, Warren J, Bygrave AE, Moss J, Walport MJ, The investigators concluded that additional factors that affect Botto M: Uncontrolled C3 activation causes membranoproliferative the AP complement activation pathway critically determine glomerulonephritis in mice deficientincomplementfactorH.Nat – the nature and severity of the kidney pathology. Genet 31: 424 428, 2002 7. Høgåsen K, Jansen JH, Mollnes TE, Hovdenes J, Harboe M: Hereditary It is important to mention that depletion of fP with a mAb fi porcine membranoproliferative glomerulonephritis type II is caused by speci c for mouse P also shows an enhanced renal disease in factor H deficiency. JClinInvest95: 1054–1061, 1995 m/m fH mice. On the other hand one should be aware of the 8. 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