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.

C3 nephritic factors are autoantibodies that prolong the half-life or prevent regulation of the alternative pathway C3 convertase, resulting in uncontrolled complement activation. They are strongly associated with renal disease but their role in pathogenesis remains controversial. Here we optimized and compared a panel of assays to identify and interrogate nephritic factor activities. Of 101 patients with histologic or clinically evident disease, 48 were positive in some or all assays. In the presence of properdin, binding of autoantibody was detected in 39 samples and convertase stabilization was detected in 36. Forty-two of 48 nephritic factors tested prevented convertase decay by factor H, and most of these by decay accelerating factor (28) and complement receptor 1 (34). Representative properdin-independent nephritic factors had no effect on C5 cleavage and terminal pathway activity, while properdin-dependent nephritic factors enhanced activity. Biacore analysis of four purified IgG samples confirmed resistance to decay and showed that properdin-independent nephritic factors increased convertase half-life over 50-fold, whereas properdin-dependent nephritic factors increased the half-life 10- to 20-fold and also increased activity of the C3 convertase up to 10-fold. Thus, our study provides a rational approach to detect and characterize nephritic factors in patients.

Envenomation by the spider Loxosceles can result in dermonecrosis and severe ulceration. Our aim was to investigate the role of the complement system and of the endogenous metalloproteinases in the initiation of the pathology of dermonecrosis. Histological analysis of skin of rabbits injected with Loxosceles intermedia venom and purified or recombinant sphingomyelinases showed a large influx of neutrophils, concomitant with dissociation of the collagenous fibers in the dermis. Decomplementation, using cobra venom factor, largely prevented the influx of neutrophils, while influx of neutrophils was also reduced in genetically C6-deficient rabbits, suggesting roles for both C5a and the membrane attack complex in the induction of dermonecrosis. However, C-depletion and C6 deficiency did not prevent the haemorrhage and the collagen injury. Zymography analysis of skin extracts showed the induction of expression of the endogenous gelatinase MMP-9 in the skin of envenomated animals. Rabbit neutrophils contained high levels of MMP-9, expression of which was further increased after incubation with venom, suggesting that these cells may be a source of the MMP-9 found in the skin of envenomated animals. Furthermore, skin fibroblasts also secreted MMP-9 and MMP-2 upon incubation with venom, suggesting that locally produced MMPs can also contribute to proteolytic tissue destruction.

Gene variants in the alternative pathway of the complement system strongly associate with atypical hemolytic uremic syndrome (aHUS), presumably by predisposing to increased complement activation within the kidney. Complement factor H (CFH) is the major regulator of complement activation through the alternative pathway. FH⌬16 -20 animals demonstrated marked hypersensitivity to experimentally triggered renal injury, animals with concomitant C5 deficiency did not. These data demonstrate a critical role for C5 activation in both spontaneous aHUS and experimentally triggered renal injury in animals with defective complement factor H function. This study provides a rationale to investigate therapeutic inhibition of C5 in human aHUS.