Factor H (FH) is an abundant serum glycoprotein that regulates the alternative pathway of complement-preventing uncontrolled plasma C3 activation and nonspecific damage to host tissues. Age-related macular degeneration (AMD), atypical hemolytic uremic syndrome (aHUS), and membranoproliferative glomerulonephritis type II (MPGN2) are associated with polymorphisms or mutations in the FH gene (Cfh), suggesting the existence of a genotype–phenotype relationship. Although AMD and MPGN2 share pathological similarities with the accumulation of complement-containing debris within the eye and kidney, respectively, aHUS is characterized by renal endothelial injury. This pathological distinction was reflected in our Cfh association analysis, which demonstrated that although AMD and MPGN2 share a Cfh at-risk haplotype, the haplotype for aHUS was unique. FH-deficient mice have uncontrolled plasma C3 activation and spontaneously develop MPGN2 but not aHUS. We show that these mice, transgenically expressing a mouse FH protein functionally equivalent to aHUS-associated human FH mutants, regulate C3 activation in plasma and spontaneously develop aHUS but not MPGN2. These animals represent the first model of aHUS and provide in vivo evidence that effective plasma C3 regulation and the defective control of complement activation on renal endothelium are the critical events in the molecular pathogenesis of FH-associated aHUS.

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder in which complex genetic factors play an important role. Several strains of gene-targeted mice have been reported to develop SLE, implicating the null genes in the causation of disease. However, hybrid strains between 129 and C57BL/6 mice, widely used in the generation of gene-targeted mice, develop spontaneous autoimmunity. Furthermore, the genetic background markedly influences the autoimmune phenotype of SLE in gene-targeted mice. This suggests an important role in the expression of autoimmunity of as-yet-uncharacterised background genes originating from these parental mouse strains. Using genome-wide linkage analysis, we identified several susceptibility loci, derived from 129 and C57BL/6 mice, mapped in the lupus-prone hybrid (129 × C57BL/6) model. By creating a C57BL/6 congenic strain carrying a 129-derived Chromosome 1 segment, we found that this 129 interval was sufficient to mediate the loss of tolerance to nuclear antigens, which had previously been attributed to a disrupted gene. These results demonstrate important epistatic modifiers of autoimmunity in 129 and C57BL/6 mouse strains, widely used in gene targeting. These background gene influences may account for some, or even all, of the autoimmune traits described in some gene-targeted models of SLE.

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Membranoproliferative glomerulonephritis (MPGN) type II (dense deposit disease) is an inflammatory renal disease characterized by electron-dense deposits and complement C3 on the glomerular basement membrane. There is no effective therapy. We investigated the role of C5 activation in a model of MPGN that develops spontaneously in complement factor H-deficient mice (Cfh ؊/؊ ). At 12 months there was a significant reduction in mortality, glomerular cellularity, neutrophil numbers, and serum creatinine levels in Cfh ؊/؊ mice deficient in C5. Excessive glomerular neutrophil numbers, frequently seen in patients with MPGN during disease flares, were also observed in Cfh ؊/؊ mice after the administration of an antiglomerular basement membrane antibody. This exaggerated injurious phenotype was absent in Cfh ؊/؊ mice deficient in C5 but not in Cfh ؊/؊ mice deficient in C6, indicating a key role for C5 activation in the induction of renal lesions. Importantly, the renal injury was completely reversed in Cfh ؊/؊ mice pretreated with an anti-murine C5 antibody. These results demonstrate an important role for C5 in both spontaneous MPGN and experimentally induced nephritis in factor H-deficient mice and provide preliminary evidence that C5 inhibition therapy might be useful in human MPGN type II.complement ͉ inflammation T he complement system is a key component of innate immunity contributing to host defenses against invading pathogens through multiple mechanisms, which include opsonization, cell lysis, and inflammatory cell recruitment, an action principally mediated through the anaphylatoxin C5a. Complement activation is regulated by a complex group of membrane-bound and fluid-phase proteins (1). Factor H is an abundant serum complement regulatory protein that inhibits the alternative pathway of complement activation. It achieves this through several mechanisms, which include inhibition of the alternative pathway C3 convertase enzyme complex (C3bBb) and acting as a cofactor for the factor I-mediated proteolytic degradation of activated C3 (termed C3b) (2, 3). Its critical importance as a regulator of C3 activation in vivo is illustrated by the complement profile reported in factor H-deficient individuals, where alternative pathway activation proceeds unhindered, resulting in markedly reduced C3 levels (4).Factor H deficiency in humans (5, 6), pigs (7), and mice (8) is associated with membranoproliferative glomerulonephritis (MPGN) type II (dense deposit disease). MPGN is characterized by glomerular capillary wall thickening with increased mesangial matrix and mesangial cells (9). Intramembranous glomerular basement membrane (GBM) deposits together with C3 (10), C5 (11), and C9 (12) staining along the GBM in the absence of Ig characterize type II MPGN (13). Patients typically have low C3 levels while C5 levels remain normal (14). MPGN type II is frequently associated with the presence of C3 nephritic factor (C3NeF), an autoantibody that stabilizes the alternative pathway C3 convertase, preventing its inactivation by factor...

The inflammatory kidney disease membranoproliferative glomerulonephritis type II (MPGN2) is associated with dysregulation of the alternative pathway of complement activation. MPGN2 is characterized by the presence of complement C3 along the glomerular basement membrane (GBM). Spontaneous activation of C3 through the alternative pathway is regulated by 2 plasma proteins, factor H and factor I. Deficiency of either of these regulators results in uncontrolled C3 activation, although the breakdown of activated C3 is dependent on factor I. Deficiency of factor H, but not factor I, is associated with MPGN2 in humans, pigs, and mice. To explain this discordance, mice with single or combined deficiencies of these factors were studied. MPGN2 did not develop in mice with combined factor H and I deficiency or in mice deficient in factor I alone. However, administration of a source of factor I to mice with combined factor H and factor I deficiency triggered both activated C3 fragments in plasma and GBM C3 deposition. Mouse renal transplant studies demonstrated that C3 deposited along the GBM was derived from plasma. Together, these findings provide what we believe to be the first evidence that factor I-mediated generation of activated C3 fragments in the circulation is a critical determinant for the development of MPGN2 associated with factor H deficiency.