Claire L. Harris

Gain-of-function mutations in complement factor B are associated with atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is an important cause of acute renal failure in children. Mutations in one or more genes encoding complement-regulatory proteins have been reported in approximately one-third of nondiarrheal, atypical HUS (aHUS) patients, suggesting a defect in the protection of cell surfaces against complement activation in susceptible individuals. Here, we identified a subgroup of aHUS patients showing persistent activation of the complement alternative pathway and found within this subgroup two families with mutations in the gene encoding factor B (BF), a zymogen that carries the catalytic site of the complement alternative pathway convertase (C3bBb). Functional analyses demonstrated that F286L and K323E aHUS-associated BF mutations are gain-of-function mutations that result in enhanced formation of the C3bBb convertase or increased resistance to inactivation by complement regulators. These data expand our understanding of the genetic factors conferring predisposition to aHUS, demonstrate the critical role of the alternative complement pathway in the pathogenesis of aHUS, and provide support for the use of complement-inhibition therapies to prevent or reduce tissue damage caused by dysregulated complement activation.

C3 glomerulopathy: consensus report

C3 glomerulopathy is a recently introduced pathological entity whose original definition was glomerular pathology characterized by C3 accumulation with absent or scanty immunoglobulin deposition. In August 2012, an invited group of experts (comprising the authors of this document) in renal pathology, nephrology, complement biology, and complement therapeutics met to discuss C3 glomerulopathy in the first C3 Glomerulopathy Meeting. The objectives were to reach a consensus on: the definition of C3 glomerulopathy, appropriate complement investigations that should be performed in these patients, and how complement therapeutics should be explored in the condition. This meeting report represents the current consensus view of the group.

Treatment of B-RAF mutant human tumor cells with a MEK inhibitor requires Bim and is enhanced by a BH3 mimetic

B-RAF is frequently mutated in solid tumors, resulting in activation of the MEK/ERK signaling pathway and ultimately tumor cell growth and survival. MEK inhibition in these cells results in cell cycle arrest and cytostasis. Here, we have shown that MEK inhibition also triggers limited apoptosis of human tumor cell lines with B-RAF mutations and that this effect was dependent on upregulation and dephosphorylation of the proapoptotic, Bcl-2 homology 3-only (BH3-only) Bcl-2 family member Bim. However, upregulation of Bim was insufficient for extensive apoptosis and was countered by overexpression of Bcl-2. To overcome apoptotic resistance, we treated the B-RAF mutant cells both with MEK inhibitors and with the BH3 mimetic ABT-737, resulting in profound synergism and extensive tumor cell death. This treatment was successful because of both efficient antagonism of the prosurvival Bcl-2 family member Mcl-1 by Bim and inhibition of Bcl-2 and Bcl-x(L) by ABT-737. Critically, addition of ABT-737 converted the predominantly cytostatic effect of MEK inhibition to a cytotoxic effect, causing long-term tumor regression in mice xenografted with human tumor cell lines. Thus, the therapeutic efficacy of MEK inhibition requires concurrent unleashing of apoptosis by a BH3 mimetic and represents a potent combination treatment for tumors harboring B-RAF mutations.

Human C3 mutation reveals a mechanism of dense deposit disease pathogenesis and provides insights into complement activation and regulation

Dense deposit disease (DDD) is a severe renal disease characterized by accumulation of electron-dense material in the mesangium and glomerular basement membrane. Previously, DDD has been associated with deficiency of factor H (fH), a plasma regulator of the alternative pathway (AP) of complement activation, and studies in animal models have linked pathogenesis to the massive complement factor 3 (C3) activation caused by this deficiency. Here, we identified a unique DDD pedigree that associates disease with a mutation in the C3 gene. Mutant C(3923ΔDG), which lacks 2 amino acids, could not be cleaved to C3b by the AP C3-convertase and was therefore the predominant circulating C3 protein in the patients. However, upon activation to C3b by proteases, or to C3(H₂O) by spontaneous thioester hydrolysis, C(3923ΔDG) generated an active AP C3-convertase that was regulated normally by decay accelerating factor (DAF) but was resistant to decay by fH. Moreover, activated C(3b923ΔDG) and C3(H₂O)(923ΔDG) were resistant to proteolysis by factor I (fI) in the presence of fH, but were efficiently inactivated in the presence of membrane cofactor protein (MCP). These characteristics cause a fluid phase-restricted AP dysregulation in the patients that continuously activated and consumed C3 produced by the normal C3 allele. These findings expose structural requirements in C3 that are critical for recognition of the substrate C3 by the AP C3-convertase and for the regulatory activities of fH, DAF, and MCP, all of which have implications for therapeutic developments.

Complement therapeutics; history and current progress

Complement (C) performs vital roles in immune surveillance, from killing of bacteria to generation of an optimal antibody response. However, the mediators responsible for this protective role can inappropriately target self tissues and cause pathology in many inflammatory diseases, in ischaemia-reperfusion injuries and also as a result of therapeutic intervention, such as in cardiopulmonary bypass. Here we review the history of anti-complement therapeutics and describe the plethora of reagents that have evolved to treat complement-mediated pathologies. These agents range from small compounds, including natural products isolated from plants and synthetic peptides designed to target and inhibit the complement cascade, to large, intricately engineered biological reagents. Recombinant, humanised antibody fragments which inhibit at specific points in the complement cascade have been generated and used successfully in man. Other reagents, mimicking the action of the natural complement regulatory proteins present on the surface of self cells, have also been developed and extensively tested. We discuss the pros and cons of these different reagents and describe recent advances in the field, such as specific targeting of drugs to sites of inflammation, which have opened the door to the use of anti-complement therapy in both acute and chronic inflammatory conditions.

Common polymorphisms in C3, factor B, and factor H collaborate to determine systemic complement activity and disease risk

Common polymorphisms in complement alternative pathway (AP) proteins C3 (C3R102G), factor B (fBR32Q), and factor H (fHV62I) are associated with age-related macular degeneration (AMD) and other pathologies. Our published work showed that fBR32Q influences C3 convertase formation, whereas fHV62I affects factor I cofactor activity. Here we show how C3R102G (C3S/F) influences AP activity. In hemolysis assays, C3102G activated AP more efficiently (EC50 C3102G: 157 nM; C3102R: 191 nM; P < 0.0001). fB binding kinetics and convertase stability were identical, but native and recombinant fH bound more strongly to C3b102R (KD C3b102R: 1.0 μM; C3b102G: 1.4 μM; P < 0.0001). Accelerated decay was unaltered, but fH cofactor activity was reduced for C3b102G, favoring AP amplification. Combining disease “risk” variants (C3102G, fB32R, and fH62V) in add-back assays yielded sixfold higher hemolytic activity compared with “protective” variants (C3102R, fB32Q, and fH62I; P < 0.0001). These data introduce the concept of a functional complotype (combination of polymorphisms) defining complement activity in an individual, thereby influencing susceptibility to AP-driven disease.

Dimerization of complement factor H-related proteins modulates complement activation in vivo

The complement system is a key component regulation influences susceptibility to age-related macular degeneration, meningitis, and kidney disease. Variation includes genomic rearrangements within the complement factor H-related (CFHR) locus. Elucidating the mechanism underlying these associations has been hindered by the lack of understanding of the biological role of CFHR proteins. Here we present unique structural data demonstrating that three of the CFHR proteins contain a shared dimerization motif and that this hitherto unrecognized structural property enables formation of both homodimers and heterodimers. Dimerization confers avidity for tissue-bound complement fragments and enables these proteins to efficiently compete with the physiological complement inhibitor, complement factor H (CFH), for ligand binding. Our data demonstrate that these CFHR proteins function as competitive antagonists of CFH to modulate complement activation in vivo and explain why variation in the CFHRs predisposes to disease.

Antigen‐presenting cell exosomes are protected from complement‐mediated lysis by expression of CD55 and CD59

Exosomes are secreted nanometer‐sized vesicles derived from antigen‐presenting cells, which have attracted recent interest as they likely play important roles in immune regulation, and their use as cell‐free tools for immunotherapy has been proposed. Liposomes used clinically as transport vehicles can activate the complement system, leading to their rapid degradation and significant inflammatory toxicity. The use of isolated exosomes in therapy, therefore, may also elicit complement activation, reducing their potential efficacy. We have examined the expression and functional roles of the membrane regulators of complement (CD46, CD55 and CD59) on antigen‐presenting cell‐derived exosomes. Exosomes express the glycosylphosphatidylinositol (GPI)‐anchored regulators CD55 and CD59,but not the transmembrane protein CD46. Antibody blocking of CD55 in the presence of sensitizing antibody (w6/32) and human serum resulted in increased C3b deposition and significantly increased exosome lysis. Blockade of CD59 also resulted in significant lysis, while blocking both CD55 and CD59 increased lysis still further. We conclude that exosomes express GPI‐anchored complement regulators in order to permit their survival in the extracellular environment.

Complement, a target for therapy in inflammatory and degenerative diseases

The complement system is a key innate immune defence against infection and an important driver of inflammation; however, these very properties can also cause harm. Inappropriate or uncontrolled activation of complement can cause local and/or systemic inflammation, tissue damage and disease. Complement provides numerous options for drug development as it is a proteolytic cascade that involves nine specific proteases, unique multimolecular activation and lytic complexes, an arsenal of natural inhibitors, and numerous receptors that bind to activation fragments. Drug design is facilitated by the increasingly detailed structural understanding of the molecules involved in the complement system. Only two anti-complement drugs are currently on the market, but many more are being developed for diseases that include infectious, inflammatory, degenerative, traumatic and neoplastic disorders. In this Review, we describe the history, current landscape and future directions for anti-complement therapies.

PIG-M transfers the first mannose to glycosylphosphatidylinositol on the lumenal side of the ER.

Glycosylphosphatidylinositol (GPI) acts as a membrane anchor of many cell surface proteins. Its structure and biosynthetic pathway are generally conserved among eukaryotic organisms, with a number of differences. In particular, mammalian and protozoan mannosyltransferases needed for addition of the first mannose (GPI‐MT‐I) have different substrate specificities and are targets of species‐ specific inhibitors of GPI biosynthesis. GPI‐MT‐I, however, has not been molecularly characterized. Characterization of GPI‐MT‐I would also help to clarify the topology of GPI biosynthesis. Here, we report a human cell line defective in GPI‐MT‐I and the gene responsible, PIG‐M. PIG‐M encodes a new type of mannosyltransferase of 423 amino acids, bearing multiple transmembrane domains. PIG‐M has a functionally important DXD motif, a characteristic of many glycosyltransferases, within a domain facing the lumen of the endoplasmic reticulum (ER), indicating that transfer of the first mannose to GPI occurs on the lumenal side of the ER membrane.