Prolonged complement activation in mice

The susceptibility of the kidney to alternative pathway activation-A hypothesis

The glomerulus is often the prime target of dysregulated alternative pathway (AP) activation. In particular, AP activation is the key driver of two severe kidney diseases: atypical hemolytic uremic syndrome and C3 glomerulopathy. Both conditions are associated with a variety of predisposing molecular defects in AP regulation, such as genetic variants in complement regulators, autoantibodies targeting AP proteins, or autoantibodies that stabilize the AP convertases (C3- and C5-activating enzymes). It is noteworthy that these are systemic AP defects, yet in both diseases pathologic complement activation primarily affects the kidneys. In particular, AP activation is often limited to the glomerular capillaries. This tropism of AP-mediated inflammation for the glomerulus points to a unique interaction between AP proteins in plasma and this particular anatomic structure. In this review, we discuss the pre-clinical and clinical data linking the molecular causes of aberrant control of the AP with activation in the glomerulus, and the possible causes of this tropism. Based on these data, we propose a model for why the kidney is so uniquely and frequently targeted in patients with AP defects. Finally, we discuss possible strategies for preventing pathologic AP activation in the kidney.

Physiology and pathology of the C3 amplification cycle: A retrospective

The C3 "Tickover" hypothesis, a mechanism whereby the host maintains constant surveillance of potential invading pathogens, targeting them for elimination through amplified C3b generation and C3-dependent effector mechanisms, was proposed by the late Professor Peter Lachmann in 1973. This unique insight came from a combined understanding of the complement system as it was then defined and the nature of the disease process in rare complement deficiencies and complement-driven diseases. In this review, I give a personal perspective of how understanding of "Tickover" has developed in the subsequent 50 years, culminating in the introduction into the clinic of therapeutic agents designed to combat amplification-driven disease.

Translational mini-review series on complement factor H: renal diseases associated with complement factor H: novel insights from humans and animals

Factor H is the major regulatory protein of the alternative pathway of complement activation. Abnormalities in factor H have been associated with renal disease, namely glomerulonephritis with C3 deposition including membranoproliferative glomerulonephritis (MPGN) and the atypical haemolytic uraemic syndrome (aHUS). Furthermore, a common factor H polymorphism has been identified as a risk factor for the development of age-related macular degeneration. These associations suggest that alternative pathway dysregulation is a common feature in the pathogenesis of these conditions. However, with respect to factor H-associated renal disease, it is now clear that distinct molecular defects in the protein underlie the pathogenesis of glomerulonephritis and HUS. In this paper we review the associations between human factor H dysfunction and renal disease and explore how observations in both spontaneous and engineered animal models of factor H dysfunction have contributed to our understanding of the pathogenesis of factor H-related renal disease.

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.

Effects of decomplementation with cobra venom factor on experimental vasculitis

Mercuric chloride (HgCl2) induces autoimmunity in susceptible rat strains, with hyper-IgE, appearance of a number of autoantibodies, and widespread tissue injury, including necrotizing vasculitis in the gut. In the early phase of tissue injury there is granulocyte infiltration; later there is immunoglobulin deposition along basement membranes in vessels. We have analysed the role of complement in this model using cobra venom factor (CVF), which causes decomplementation lasting around 5 days. The characteristic time course when HgCl2 is given over 10 days is that tissue injury and autoantibody levels reach a peak at around day 15 (start of HgCl2 = day 0). We therefore gave CVF either early (day 0), intermediate (day 5) or late (day 10); a fourth group (controls) received HgCl2 but no CVF. At each time point, CVF caused complete decomplementation which lasted for at least 5 days. Serum IgE and autoantibody levels were similar in all four experimental groups. Tissue injury in the 'early' CVF group and in the 'late' CVF group was not significantly different from controls, but in the intermediate group tissue injury was significantly more severe than in controls. These data indicate that the complement system does not play a major role in the induction of autoantibodies by HgCl2, nor in the effector phase of tissue injury. We speculate that the exacerbation of tissue injury by CVF in the group given this agent at an intermediate stage of the model is explained by the presence of products of C3 activation which have proinflammatory effects during the phase of active granulocyte-mediated tissue injury.