Uncontrolled C3 activation causes membranoproliferative glomerulonephritis in mice deficient in complement factor H.

The alternative pathway of complement is activated continuously in vivo through the C3 'tick-over' pathway. This pathway is triggered by the hydrolysis of C3, resulting in the formation of C3 convertase. This, in turn, generates C3b, which mediates many of the biological functions of complement. Factor H, the main regulator of this activation, prevents formation and promotes dissociation of the C3 convertase enzyme, and, together with factor I, mediates the proteolytic inactivation of C3b. Factor H deficiency, described in 29 individuals from 12 families and in pigs, allows unhindered activation of fluid-phase C3 and severe depletion of plasma C3 (ref. 11). Membranoproliferative glomerulonephritis (MPGN) occurs in factor H-deficient humans and pigs. Although MPGN has been reported in other conditions in which uncontrolled activation of C3 occurs, the role of C3 dysregulation in the pathogenesis of MPGN is not understood. Here we show that mice deficient in factor H (Cfh(-/-) mice) develop MPGN spontaneously and are hypersensitive to developing renal injury caused by immune complexes. Introducing a second mutation in the gene encoding complement factor B, which prevents C3 turnover in vivo, obviates the phenotype of Cfh(-/-) mice. Thus, uncontrolled C3 activation in vivo is essential for the development of MPGN associated with deficiency of factor H.

Factor I is required for the development of membranoproliferative glomerulonephritis in factor H-deficient mice.

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.

Spontaneous autoimmunity in 129 and C57BL/6 mice-implications for autoimmunity described in gene-targeted mice.

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 x 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.

Genetic dissection of spontaneous autoimmunity driven by 129-derived chromosome 1 Loci when expressed on C57BL/6 mice.

Extensive evidence indicates that genetic predisposition is a central element in susceptibility to systemic lupus erythematosus both in humans and animals. We have previously shown that a congenic line carrying a 129-derived chromosome 1 interval on the C57BL/6 background developed humoral autoimmunity. To further dissect the contribution to autoimmunity of this 129 interval, we have created six subcongenic strains carrying fractions of the original 129 region and analyzed their serological and cellular phenotypes. At 1 year of age the congenic strain carrying a 129 interval between the microsatellites D1Mit15 (87.9 cM) and D1Mit115 (99.7 cM) (B6.129chr1b) had high levels of autoantibodies, while all the other congenic lines were not significantly different from the C57BL/6 controls. The B6.129chr1b strain displayed only mild proliferative glomerulonephritis despite high levels of IgG and C3 deposited in the kidneys. FACS analysis of the spleens revealed that the B6.129chr1b mice had a marked increase in the percentage of activated T cells associated with a significant reduction in the proportion of CD4(+)CD25(high) regulatory T cells. Moreover, this analysis showed a significantly reduced percentage of marginal zone B cells that preceded autoantibody production. Interestingly the 129chr1b-expressing bone marrow-derived macrophages displayed an impaired uptake of apoptotic cells in vitro. Collectively, our data indicate that the 129chr1b segment when recombined on the C57BL/6 genomic background is sufficient to induce loss of tolerance to nuclear Ags. These findings have important implication for the interpretation of the autoimmune phenotype associated with gene-targeted models.

Murine CD93 (C1qRp) contributes to the removal of apoptotic cells in vivo but is not required for C1q-mediated enhancement of phagocytosis.

Human CD93 (known as C1qRp) has been shown to be a phagocytic receptor involved in the in vitro C1q-dependent enhancement of phagocytosis. However, binding of CD93 to C1q and its function remain controversial. In this study, we have generated CD93-deficient mice (CD93(-/-)) to investigate its biological role(s). The CD93(-/-) mice were viable and showed no gross abnormalities in their development. Thioglycolate-elicited peritoneal macrophages deficient in CD93 showed a similar enhancement in complement- and FcgammaR-dependent uptake of RBC to the wild-type macrophages when plated on C1q-coated surfaces suggesting that the lack of this receptor had no effect on these C1q-mediated events. There was no impairment in either complement- or FcgammaR-dependent phagocytic assays in vivo. By contrast, the CD93(-/-) mice had a significant phagocytic defect in the clearance of apoptotic cells in vivo (human Jurkat T cells and murine thymocytes: p=0.0006 and p=0.0079, respectively) compared with strain-matched controls. However, in vitro, the CD93(-/-) macrophages showed similar engulfment of apoptotic cells to wild-type macrophages. Furthermore, no supporting evidence for a role of CD93 as an adhesion molecule was found using intravital microscopy or analyzing peritoneal cell recruitment in response to three different inflammatory stimuli (thioglycolate, zymosan A, and IL-1beta). Thus, our findings indicate that murine CD93 is expressed on the peritoneal macrophage, especially on thioglycolate-elicited cells, but does not appear to play a key role in C1q-mediated enhancement of phagocytosis or in the intercellular adhesion events tested. However, our results suggest that it may contribute to the in vivo clearance of dying cells.