The complement system is important for cellular integrity and tissue homeostasis. Complement activation mediates the removal of microorganisms and the clearance of modified self cells, such as apoptotic cells. Complement regulators control the spontaneously activated complement cascade and any disturbances in this delicate balance can result in damage to tissues and in autoimmune disease. Therefore, insights into the mechanisms of complement regulation are crucial for understanding disease pathology and for enabling the development of diagnostic tools and therapies for complement-associated diseases.

Oxidative stress and enhanced lipid peroxidation are linked to many chronic inflammatory diseases, including age-related macular degeneration (AMD). AMD is the leading cause of blindness in Western societies, but its aetiology remains largely unknown. Malondialdehyde (MDA) is a common lipid peroxidation product that accumulates in many pathophysiological processes, including AMD. Here we identify complement factor H (CFH) as a major MDA-binding protein that can block both the uptake of MDA-modified proteins by macrophages and MDA-induced proinflammatory effects in vivo in mice. The CFH polymorphism H402, which is strongly associated with AMD, markedly reduces the ability of CFH to bind MDA, indicating a causal link to disease aetiology. Our findings provide important mechanistic insights into innate immune responses to oxidative stress, which may be exploited in the prevention of and therapy for AMD and other chronic inflammatory diseases.

Atypical hemolytic uremic syndrome (aHUS) is a severe renal disease that is associated with defective complement regulation caused by multiple factors. We previously described the deficiency of factor H-related proteins CFHR1 and CFHR3 as predisposing factor for aHUS.

Homozygous deletion of a 84-kb genomic fragment in human chromosome 1 that encompasses the CFHR1 and CFHR3 genes represents a risk factor for hemolytic uremic syndrome (HUS) but has a protective effect in age-related macular degeneration (AMD). Here we identify CFHR1 as a novel inhibitor of the complement pathway that blocks C5 convertase activity and interferes with C5b surface deposition and MAC formation. This activity is distinct from complement factor H, and apparently factor H and CFHR1 control complement activation in a sequential manner. As both proteins bind to the same or similar sites at the cellular surfaces, the gain of CFHR1 activity presumably is at the expense of CFH-mediated function (inhibition of the C3 convertase). In HUS, the absence of CFHR1 may result in reduced inhibition of terminal complex formation and in reduced protection of endothelial cells upon complement attack. These findings provide new insights into complement regulation on the cell surface and biosurfaces and likely define the role of CFHR1 in human diseases. IntroductionThe complement system is important for host innate and adaptive immunity and mounts a protective immune response to invading microbes. 1 The alternative complement pathway is spontaneously activated, and generates C3 convertases (C3bBb) that cleave the central component C3 to the anaphylactic peptide C3a and C3b. 2,3 C3b attached to a foreign surface binds factor B and generates the C3 convertase (C3bBb), which enhances further complement activation resulting in opsonization and phagocytosis of particles. Binding of an additional C3b molecule to the C3 convertase forms the C5 convertase (C3bBbC3b) of the alternative pathway. This convertase cleaves C5 and generates the potent chemoattractant C5a as well as C5b, which initiates the terminal complement pathway assembly. 4 C5b immediately undergoes conformational changes and binds C6 and C7 in a nonenzymatic manner. The assembled C5b67 complex is released from the convertase and attaches to lipid bilayers. Upon binding of C8 and C9, the lytic membrane attack complex (MAC) is formed. 3,5 Once activated, this powerful defense system is tightly controlled on host cell surfaces by both membrane-anchored and surface-attached soluble regulators. Proper and coordinated function of these regulators is essential for tissue integrity. Single gene mutations predispose to severe renal and retinal diseases, that is, hemolytic uremic syndrome (HUS; OMIM no. 235400), membranoproliferative glomerulonephritis type II (MPGN II; OMIM no. 609814), or age-related macular degeneration (AMD; OMIM no. 603075). 6,7 HUS is caused by occlusion of arterioles and capillaries in the kidney, due to endothelial cell and platelet damage. 8 MPGN II is a rare renal disease, with formation of dense deposits at the glomerular basement membrane and thickening of the peripheral capillary walls. 9 Similarly, the retinal disease AMD, which causes visual impairment of elderly people, is caused by deposits (drusen) that form on the Bruch membrane and le...

The three genospecies Borrelia burgdorferi, Borrelia garinii, and Borrelia afzelii, all causative agents of Lyme disease, differ in their susceptibilities to human complement-mediated lysis. We recently reported that serum resistance of borrelias correlates largely with their ability to bind the human complement regulators FHL-1/ reconectin and factor H.