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Clq is the subcomponent of the classical complement pathway that can interact with a range of biochemically and structurally diverse self and nonself ligands. The charged residues belonging to the apex of the global domain of Clq(gClq) heterotrimer as well as the side of the ghB are crucial for Clq binding to these ligands.

Complement coordinates inflammation in macrophages: The serum complement system performs well-established functions in innate immune system recognition and destruction of invading pathogens, but complement proteins produced by immune cells can also execute distinct functions intracellularly. In human macrophages, Niyonzima et al. demonstrate that the complement protein C5a signals on mitochondrial membranes through its receptor C5aR1 to control IL-1β production in response to cholesterol crystal (CC) uptake. C5a/C5aR1 signaling altered mitochondrial activity and increased reactive oxygen species production, promoting IL-1β gene expression and processing. In a mouse model of atherosclerosis, in which CC accumulates in arterial walls, deletion of C5aR1 in myeloid cells reduced the severity of cardiovascular disease. These results identify a function for intracellular complement proteins in myeloid cell mitochondrial metabolism and responses to sterile inflammation. While serum-circulating complement destroys invading pathogens, intracellularly active complement, termed the "complosome," functions as a vital orchestrator of cell-metabolic events underlying T cell effector responses. Whether intracellular complement is also nonredundant for the activity of myeloid immune cells is currently unknown. Here, we show that monocytes and macrophages constitutively express complement component (C) 5 and generate autocrine C5a via formation of an intracellular C5 convertase. Cholesterol crystal sensing by macrophages induced C5aR1 signaling on mitochondrial membranes, which shifted ATP production via reverse electron chain flux toward reactive oxygen species generation and anaerobic glycolysis to favor IL-1β production, both at the transcriptional level and processing of pro–IL-1β. Consequently, atherosclerosis-prone mice lacking macrophage-specific C5ar1 had ameliorated cardiovascular disease on a high-cholesterol diet. Conversely, inflammatory gene signatures and IL-1β produced by cells in unstable atherosclerotic plaques of patients were normalized by a specific cell-permeable C5aR1 antagonist. Deficiency of the macrophage cell-autonomous C5 system also protected mice from crystal nephropathy mediated by folic acid. These data demonstrate the unexpected intracellular formation of a C5 convertase and identify C5aR1 as a direct modulator of mitochondrial function and inflammatory output from myeloid cells. Together, these findings suggest that the complosome is a contributor to the biologic processes underlying sterile inflammation and indicate that targeting this system could be beneficial in macrophage-dependent diseases, such as atherosclerosis. [ABSTRACT FROM AUTHOR]

The article provides a retraction letter for the article "Targeting Neural-Restrictive Silencer Factor Sensitizes Tumor Cells to Antibody-Based Cancer Immunotherapy In Vitro via Multiple Mechanisms" by Martin V. Kolev, Marieta M. Ruseva and Rossen M. Donev.

Mutations in the complement regulatory proteins are associated with several different diseases. Although these mutations cause dysregulated alternative pathway activation throughout the body, the kidneys are the most common site of injury. The susceptibility of the kidney to alternative pathway-mediated injury may be due to limited expression of complement regulatory proteins on several tissue surfaces within the kidney. To examine the roles of the complement regulatory proteins factor H and Crry in protecting distinct renal surfaces from alternative pathway mediated injury, we generated mice with targeted deletions of the genes for both proteins. Surprisingly, mice with combined genetic deletions of factor H and Crry developed significantly milder renal injury than mice deficient in only factor H. Deficiency of both factor H and Crry was associated with C3 deposition at multiple locations within the kidney, but glomerular C3 deposition was lower than that in factor H alone deficient mice. Thus, factor H and Crry are critical for regulating complement activation at distinct anatomic sites within the kidney. However, widespread activation of the alternative pathway reduces injury by depleting the pool of C3 available at any 1 location., (Copyright © 2016 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

The complement-mediated renal diseases C3 glomerulopathy (C3G) and atypical hemolytic uremic syndrome (aHUS) strongly associate with inherited and acquired abnormalities in the regulation of the complement alternative pathway (AP). The major negative regulator of the AP is the plasma protein complement factor H (FH). Abnormalities in FH result in uncontrolled activation of C3 through the AP and associate with susceptibility to both C3G and aHUS. Although previously developed FH-deficient animal models have provided important insights into the mechanisms underlying susceptibility to these unique phenotypes, these models do not entirely reproduce the clinical observations. FH is predominantly synthesized in the liver. We generated mice with hepatocyte-specific FH deficiency and showed that these animals have reduced plasma FH levels with secondary reduction in plasma C3. Unlike mice with complete FH deficiency, hepatocyte-specific FH-deficient animals developed neither plasma C5 depletion nor accumulation of C3 along the glomerular basement membrane. In contrast, subtotal FH deficiency associated with mesangial C3 accumulation consistent with C3G. Although there was no evidence of spontaneous thrombotic microangiopathy, the hepatocyte-specific FH-deficient animals developed severe C5-dependent thrombotic microangiopathy after induction of complement activation within the kidney by accelerated serum nephrotoxic nephritis. Taken together, our data indicate that subtotal FH deficiency can give rise to either spontaneous C3G or aHUS after a complement-activating trigger within the kidney and that the latter is C5 dependent., (Copyright © 2016 by the American Society of Nephrology.)

C3 glomerulopathy is a complement-mediated renal disease that is frequently associated with abnormalities in regulation of the complement alternative pathway. Mice with deficiency of factor H (Cfh(-/-)), a negative alternative pathway regulator, are an established experimental model of C3 glomerulopathy in which complement C3 fragments including iC3b accumulate along the glomerular basement membrane. Here we show that deficiency of complement receptor 3 (CR3), the main receptor for iC3b, enhances the severity of spontaneous renal disease in Cfh(-/-) mice. This effect was found to be dependent on CR3 expression on bone marrow-derived cells. CR3 also mediated renal protection outside the setting of factor H deficiency, as shown by the development of enhanced renal injury in CR3-deficient mice during accelerated nephrotoxic nephritis. The iC3b-CR3 interaction downregulated the proinflammatory cytokine response of both murine and human macrophages to lipopolysaccharide stimulation in vitro, suggesting that the protective effect of CR3 on glomerular injury was mediated via modulation of macrophage-derived proinflammatory cytokines. Thus, CR3 has a protective role in glomerulonephritis and suggests that pharmacologic potentiation of the macrophage CR3 interaction with iC3b could be therapeutically beneficial., (Copyright © 2016 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

C3 glomerulopathy refers to renal disorders characterized by abnormal accumulation of C3 within the kidney, commonly along the glomerular basement membrane (GBM). C3 glomerulopathy is associated with complement alternative pathway dysregulation, which includes functional defects in complement regulator factor H (FH). There is no effective treatment for C3 glomerulopathy. We investigated the efficacy of a recombinant mouse protein composed of domains from complement receptor 2 (CR2) and FH (CR2-FH) in two models of C3 glomerulopathy with either preexisting or triggered C3 deposition along the GBM. FH-deficient mice spontaneously develop renal pathology associated with abnormal C3 accumulation along the GBM and secondary plasma C3 deficiency. CR2-FH partially restored plasma C3 levels in FH-deficient mice 2 hours after intravenous injection. CR2-FH specifically targeted glomerular C3 deposits, reduced the linear C3 reactivity assessed with anti-C3 and anti-C3b/iC3b/C3c antibodies, and prevented further spontaneous accumulation of C3 fragments along the GBM. Reduction in glomerular C3d and C9/C5b-9 reactivity was observed after daily administration of CR2-FH for 1 week. In a second mouse model with combined deficiency of FH and complement factor I, CR2-FH prevented de novo C3 deposition along the GBM. These data show that CR2-FH protects the GBM from both spontaneous and triggered C3 deposition in vivo and indicate that this approach should be tested in C3 glomerulopathy., (Copyright © 2016 by the American Society of Nephrology.)

Complement component C3 is the most abundant complement protein in plasma, central to all three complement activation pathways and essential to complement amplification. Thus, it is one of the most extensively studied complement proteins. This chapter describes the purification of C3 from human and mouse plasma using protein precipitation, followed by classical ion exchange chromatography and gel filtration. The biochemical and functional characteristics of the purified C3 are typically assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and hemolysis assays. The hemolysis assay is a standard technique to assess complement activity monitoring the lysis of red blood cells.

Complement factor H (CFH) is a negative regulator of the alternative pathway of complement, and properdin is the sole positive regulator. CFH-deficient mice (CFH(-/-)) develop uncontrolled C3 activation and spontaneous renal disease characterized by accumulation of C3 along the glomerular basement membrane, but the role of properdin in the pathophysiology is unknown. Here, we studied mice deficient in both CFH and properdin (CFH(-/-).P(-/-)). Although CFH(-/-) mice had plasma depleted of both C3 and C5, CFH(-/-).P(-/-) animals exhibited depletion of C3 predominantly, recapitulating the plasma complement profile observed in humans with properdin-independent C3 nephritic factors. Glomerular inflammation, thickening of the capillary wall, and glomerular C3 staining were significantly increased in CFH(-/-).P(-/-) compared with CFH(-/-) mice. We previously reported that exogenous CFH ameliorates C3 staining of the glomerular basement membrane and triggers the appearance of mesangial C3 deposits in CFH(-/-) mice; here, we show that these effects require properdin. In summary, during uncontrolled activation of C3 driven by complete CFH deficiency, properdin influences the intraglomerular localization of C3, suggesting that therapeutic inhibition of properdin would be detrimental in this setting.

Tumor cells escape clearance by complement by abundantly expressing CD59 and other membrane complement regulators. Recently, we designed a peptide derived from the neural-restrictive silencer factor (REST), REST68, which we showed to inhibit expression of CD59 in tumors lacking the full-length REST and proposed a detailed model for regulation of CD59 expression via interplay between REST and nucleolin (NCL) transcription factors. In this paper, we study in detail the mechanisms for sensitization of malignant cells to Ab-based cancer immunotherapy by the REST68 peptide and the implications of the REST/NCL model for the design of treatment resulting in higher tumor susceptibility. REST68 inhibited CD59 expression in malignant cells expressing either truncated or full-length REST, but not in nonmalignant cells. However, activation of protein kinase C (PKC) in nonmalignant cells, a process that contributes to cellular transformation, phosphorylated NCL and enabled suppression of CD59 expression by the REST68. Combined treatment of different tumor types with REST68 and PKC inhibitor synergized to further suppress CD59 expression and reduce resistance to complement lysis. The combined treatment also increased susceptibility of tumors expressing either of the REST isoforms to PBMC-mediated killing, which, at least in part, accounted for the strong promotion of apoptosis by the REST68/PKC inhibitor. These data demonstrate that REST68 sensitizes tumors to Ab-based cancer immunotherapy via multiple mechanisms. Furthermore, the REST/NCL interplay model for regulation of expression of cd59 and other genes involved in cell survival enables the design of treatments for different tumor types to achieve more efficient tumor clearance.

Alzheimer's disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD, however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review emphasizes on the dual key roles of complement system and complement regulators (CRegs) in disease pathology and progression. The particular focus of this review is on currently evolving strategies for design of complement inhibitors that might aid therapy by restoring the fine balance between activated components of complement system, thus improving the cognitive performance of patients. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.