Uncontrolled activation of the alternative pathway of complement is thought to be associated with age-related macular degeneration (AMD). The alternative pathway is continuously activated in the fluid phase, and tissue surfaces require continuous complement inhibition to prevent spontaneous autologous tissue injury. Here, we examined the effects of oxidative stress on the ability of immortalized human retinal pigment epithelial cells (ARPE-19) to regulate complement activation on their cell surface. Combined treatment with H 2 O 2 (to induce oxidative stress) and complementsufficient serum was found to disrupt the barrier function of stable ARPE-19 monolayers as determined by transepithelial resistance (TER) measurements. Neither treatment alone had any effect. TER reduction was correlated with increased cell surface deposition of C3, and could be prevented by using C7-depleted serum, an essential component of the terminal complement pathway. Treatment with H 2 O 2 reduced surface expression of the complement inhibitors DAF, CD55, and CD59, and impaired regulation at the cell surface by factor H present within the serum. Combined treatment of the monolayers with H 2 O 2 and serum elicited polarized secretion of vascular epidermal growth factor (VEGF). Both, secretion of VEGF and TER reduction could be attenuated using either an alternative pathway inhibitor or by blocking VEGF receptor-1/2 signaling. Regarded together, these studies demonstrate that oxidative stress reduces regulation of complement on the surface of ARPE-19 cells, increasing complement activation. This sublytic activation results in VEGF release, which mediates disruption of the cell monolayer. These findings link oxidative stress, complement activation, and apical VEGF release, which have all been associated with the pathogenesis of AMD.

Glomerular IgM and C3 deposits frequently accompany idiopathic FSGS and secondary glomerulosclerosis, but it is unknown whether IgM activates complement, possibly contributing to the pathogenesis of these diseases. We hypothesized that IgM natural antibody binds to neoepitopes exposed in the glomerulus after nonimmune insults, triggering activation of the complement system and further injury. We examined the effects of depleting B cells, using three different strategies, on adriamycin-induced glomerulosclerosis. First, we treated wild-type mice with an anti-murine CD20 antibody, which depletes B cells, before disease induction. Second, we evaluated adriamycin-induced glomerulosclerosis in Jh mice, a strain that lacks mature B cells. Third, we locally depleted peritoneal B cells via hypotonic shock before disease induction. All three strategies reduced deposition of IgM in the glomerulus after administration of adriamycin and attenuated the development of albuminuria. Furthermore, we found that glomerular IgM and C3 were detectable in a subset of patients with FSGS; C3 was present as an activation fragment and colocalized with glomerular IgM, suggesting that glomerular IgM may have bound a cognate ligand. Taken together, these results suggest that IgM activates the complement system within the glomerulus in an animal model of glomerulosclerosis. Strategies that reduce IgM natural antibody or that prevent complement activation may slow the progression of glomerulosclerosis.

During complement activation the C3 protein is cleaved, and C3 activation fragments are covalently fixed to tissues. Tissue-bound C3 fragments are a durable biomarker of tissue inflammation, and these fragments have been exploited as addressable binding ligands for targeted therapeutics and diagnostic agents. We have generated cross-reactive murine monoclonal antibodies against human and mouse C3d, the final C3 degradation fragment generated during complement activation. We developed 3 monoclonal antibodies (3d8b, 3d9a, and 3d29) that preferentially bind to the iC3b, C3dg, and C3d fragments in solution, but do not bind to intact C3 or C3b. The same 3 clones also bind to tissue-bound C3 activation fragments when injected systemically. Using mouse models of renal and ocular disease, we confirmed that, following systemic injection, the antibodies accumulated at sites of C3 fragment deposition within the glomerulus, the renal tubulointerstitium, and the posterior pole of the eye. To detect antibodies bound within the eye, we used optical imaging and observed accumulation of the antibodies within retinal lesions in a model of choroidal neovascularization (CNV). Our results demonstrate that imaging methods that use these antibodies may provide a sensitive means of detecting and monitoring complement activation-associated tissue inflammation.

Purpose:To determine the feasibility of T2-weighted magnetic resonance (MR) imaging in the noninvasive quantifi cation of renal infl ammation by using superparamagnetic iron oxide (SPIO) nanoparticles targeted to tissue-bound C3 activation fragments in a mouse model of lupus nephritis. Materials and Methods:All animal procedures were approved by the University of Colorado-Denver animal care and use committee. SPIO nanoparticles were encapsulated by using aminefunctionalized phospholipids. A recombinant protein containing the C3d-binding region of complement receptor type 2 (CR2) was then conjugated to the surface of the SPIO nanoparticle. Five MRL/lpr mice (a model of lupus nephritis) and six C57BL/6 wild-type mice were assessed with T2-weighted MR imaging at baseline and after SPIO injection. The same fi ve MRL/lpr mice and three C57BL/6 mice also underwent MR imaging after injection of CR2-targeted SPIO. A series of T2-weighted pulses with 16 echo times was used to enable precise T2 mapping and calculation of T2 relaxation times in the cortex and outer and inner medulla of the kidneys, as well as in the spleen, muscle, and fat. The effects of treatment and animal genotype on T2 relaxation times were analyzed with repeatedmeasures analysis of variance. Results:At baseline, the T2-weighted signal intensity in the kidneys of MRL/lpr mice was higher than that in the kidneys of wild-type mice. Injection of untargeted SPIO did not alter the T2-weighted signal in the kidneys in either strain of mice. Injection of CR2-targeted SPIO in MRL/lpr mice, however, caused a signifi cant accumulation of targeted iron oxide with a subsequent decrease in T2 relaxation times in the cortex and outer and inner medulla of the kidneys. No changes in T2 relaxation time were observed in the wild-type mice after injection of targeted SPIO. Conclusion:Injection of CR2-conjugated SPIO caused a signifi cant reduction in T2-weighted MR imaging signal and T2 relaxation time in nephritic kidneys.q RSNA, 2010Supplemental material: http://radiology.rsna.org/lookup /suppl

Summary Acute kidney injury is a common and severe clinical problem. Patients who develop acute kidney injury are at increased risk of death in spite of supportive measures such as hemodialysis. Research in recent years has revealed that tissue inflammation is central to the pathogenesis of renal injury, even after non-immune insults such as ischemia/reperfusion and toxins. Examination of clinical samples and pre-clinical models demonstrate that activation of the complement system is a critical cause of acute kidney injury. Furthermore, complement activation within the injured kidney is a proximal trigger of many downstream inflammatory events within the renal parenchyma that exacerbate injury to the kidney. Complement activation may also account for the systemic inflammatory events that contribute to remote organ injury and patient mortality. Complement inhibitory drugs have now entered clinical use and may provide an important new therapeutic approach for patients suffering from or at high risk of developing acute kidney injury.

Ischemia/reperfusion (I/R) triggers a robust inflammatory response within the kidney. Numerous components of the immune system contribute to the resultant renal injury including the complement system. We sought to identify whether natural antibodies bind to the post-ischemic kidney and contribute to complement activation after I/R. We depleted peritoneal B cells in mice by hypotonic shock. Depletion of the peritoneal B cells prevented the deposition of IgM within the glomeruli after renal I/R, and attenuated renal injury after I/R. We found that glomerular IgM activates the classical pathway of complement but does not cause substantial deposition of C3 within the kidney. Furthermore, mice deficient in classical pathway proteins were not protected from injury, indicating that glomerular IgM does not cause injury through activation of the classical pathway. We also subjected mice deficient in all mature B cells (μMT mice) to renal I/R and found that they sustained worse renal injury than wild-type controls. Serum IL-10 levels were lower in the μMT mice. Regarded together, these results indicate that natural antibody produced by peritoneal B cells binds within the glomerulus after renal I/R and contributes to functional renal injury. However, non-peritoneal B cells attenuate renal injury after I/R, possibly through the production of IL-10.

Factor H is a regulator of the alternative pathway of complement. Genetic studies have revealed that patients with factor H mutations are at increased risk for several types of renal disease. Pathogenic activation of the alternative pathway in acquired diseases, such as ischemic acute kidney injury, also suggests that non-mutated factor H also has a limited ability to control the alternative pathway in the kidney. In the current study we examined the ability of circulating factor H to control alternative pathway activation on renal tubular epithelial cells. We found that an absolute deficiency of factor H prevented complement activation on the cells after ischemia/reperfusion, likely because of consumption of alternative pathway proteins in the fluid phase. In contrast, when the fluid phase regulation by factor H was retained but the interaction of factor H with cell surfaces was blocked, complement activation after renal ischemia/reperfusion increased. Finally, a recombinant form of factor H that is specifically targeted to sites of C3 deposition was found to reduce complement activation in the tubulointerstitium after I/R. These studies demonstrate that native factor H provides critical regulation of the alternative pathway in the renal tubulointerstitium after injury, but that this protection is incomplete. An inhibitor that targeted recombinant factor H to the tissue surface prevented alternative pathway activation and attenuated renal injury after I/R.

Defective control of the alternative pathway of complement is an important risk factor for several renal diseases, including atypical hemolytic uremic syndrome. Infections, drugs, pregnancy, and hemodynamic insults can trigger episodes of atypical hemolytic uremic syndrome in susceptible patients. Although the mechanisms linking these clinical events with disease flares are unknown, recent work has revealed that each of these clinical conditions causes cells to release microparticles. We hypothesized that microparticles released from injured endothelial cells promote intrarenal complement activation. Calcineurin inhibitors cause vascular and renal injury and can trigger hemolytic uremic syndrome. Here, we show that endothelial cells exposed to cyclosporine in vitro and in vivo release microparticles that activate the alternative pathway of complement. Cyclosporine-induced microparticles caused injury to bystander endothelial cells and are associated with complement-mediated injury of the kidneys and vasculature in cyclosporine-treated mice. Cyclosporine-induced microparticles did not bind factor H, an alternative pathway regulatory protein present in plasma, explaining their complement-activating phenotype. Finally, we found that in renal transplant patients, the number of endothelial microparticles in plasma increases 2 weeks after starting tacrolimus, and treatment with tacrolimus associated with increased C3 deposition on endothelial microparticles in the plasma of some patients. These results suggest that injury-associated release of endothelial microparticles is an important mechanism by which systemic insults trigger intravascular complement activation and complement-dependent renal diseases.