Complement System Part I – Molecular Mechanisms of Activation and Regulation

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins and the membrane attack complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly we will discuss the development and benefits of therapies using complement inhibitors

Autoantibodies Against C3b—Functional Consequences and Disease Relevance

The complement component C3 is at the heart of the complement cascade. It is a complex protein, which generates different functional activated fragments (C3a, C3b, iC3b, C3c, C3d). C3b is a constituent of the alternative pathway C3 convertase (C3bBb), binds multiple regulators, and receptors, affecting thus the functioning of the immune system. The activated forms of C3 are a target for autoantibodies. This review focuses on the discovery, disease relevance, and functional consequences of the anti-C3b autoantibodies. They were discovered about 70 years ago and named immunoconglutinins. They were found after infections and considered convalescent factors. At the end of the twentieth century IgG against C3b were found in systemic lupus erythematosus and recently in lupus nephritis, correlating with the disease severity and flare. Cases of C3 glomerulopathy and immune complex glomerulonephritis were also reported. These antibodies recognize epitopes, shared between C3(H2O)/C3b/iC3b/C3c and have overt functional activity. They correlate with low plasmatic C3 levels in patients. In vitro, they increase the activity of the alternative pathway C3 convertase, without being C3 nephritic factors. They perturb the binding of the negative regulators Complement Receptor 1 and Factor H. The clear functional consequences and association with disease severity warrant further studies to establish the link between the anti-C3b autoantibodies and tissue injury. Comparative studies with such antibodies, found in patients with infections, may help to uncover their origin and epitopes specificity. Patients with complement overactivation due to presence of anti-C3b antibodies may benefit from therapeutic targeting of C3

Lésions endothéliales liées à un défaut de contrôle du complément (de la génétique du complément au syndrome hémolytique et urémique)

L identification fréquente de mutations des protéines régulatrices du complément suggère que les lésions endothéliales du syndrome hémolytique et urémique atypique (SHUa) résultent d une activation incontrôlée de la voie alterne du complément. Les mutations, en soi, ne constituent cependant que des facteurs de susceptibilité et les mécanismes menant de l anomalie de régulation du complément au développement de lésions de microangiopathie thrombotique rénale restent mal compris. L objectif de ce projet était donc d'étudier certains mécanismes de l activation du complément à la surface des cellules endothéliales dans le SHUa et les conséquences de cette activation pour l endothélium. Dans ce but, ce travail s est initialement concentré sur la mutation C3R139W, dont nous avons réalisé la caractérisation phénotypique et fonctionnelle et qui a constitué un modèle d étude des conséquences d un complément dérégulé sur l endothélium. Cette mutation a été identifiée de façon sporadique chez 4% des patients de la cohorte française de SHUa. Son étude phénotypique a mis en avant une évolution fonctionnelle rénale souvent sévère mais inhomogène ainsi qu une fréquence non négligeable d événements cardio-vasculaires. Sa caractérisation fonctionnelle a révélé une augmentation de son affinité pour le facteur B, à l origine de la formation d une hyper C3 convertase , échappant également en partie aux systèmes de contrôle (diminution de liaison avec la MCP). L étude de ses conséquences endothéliales a montré, à la surface de cellules pré-activées, une augmentation des produits d activation du complément et de l expression membranaire de facteur tissulaire, faisant le lien avec l acquisition d un phénotype endothélial prothrombotique. Nous rapportons également, sous l effet de sérum porteur de cette mutation, une majoration de la perméabilité et du détachement cellulaire, susceptibles de traduire une souffrance endothéliale. Dans la deuxième partie et en vue de préciser les liens entre anomalie du complément et activation des cellules endothéliales, nous nous sommes intéressés au rôle de l hémolyse, dénominateur commun des SHU. Nous avons ainsi montré que l hème libre activait la voie alterne du complément dans le sérum et à la surface des cellules endothéliales et ce, de façon exacerbé, en cas de dysrégulation sous-jacente du complément. Nous avons identifié plusieurs mécanismes d action par lesquels l hème peut activer le complément : il favorise les interactions C3/C3 et ainsi la formation d une hyper C3/C5 convertase, déclenche une mobilisation des corps de Weibel-Palade à l origine de l expression membranaire de P-selectine, qui est capable d activer la voie alterne du complément et induit une diminution de l expression membranaire des régulateurs MCP et DAF. Par ces travaux, nous avons précisé les liens entre activation du complément et acquisition d un phénotype endothélial prothrombotique dans le SHUa. Nous avons notamment identifié l hémolyse comme un acteur potentiel de l amplification des lésions endothéliales complément-dépendantes. Son contrôle pourrait ainsi constituer une nouvelle voie thérapeutique dans le SHU.Pas de résumé en anglaisPARIS5-Bibliotheque electronique (751069902) / SudocSudocFranceF

Complement System Part II: Role in Immunity

International audienceThe complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target

Complement Factor H Gene Abnormalities in Haemolytic Uraemic Syndrome: From Point Mutations to Hybrid Gene

Noris and Remuzzi discuss a new study showing an association between atypical haemolytic uremic syndrome and a hybrid complement gene,CFH/CFHL1

Phenotypic expansion of DGKE-associated diseases.

Atypical hemolytic uremic syndrome (aHUS) is usually characterized by uncontrolled complement activation. The recent discovery of loss-of-function mutations in DGKE in patients with aHUS and normal complement levels challenged this observation. DGKE, encoding diacylglycerol kinase-ε, has not been implicated in the complement cascade but hypothetically leads to a prothrombotic state. The discovery of this novel mechanism has potential implications for the treatment of infants with aHUS, who are increasingly treated with complement blocking agents. In this study, we used homozygosity mapping and whole-exome sequencing to identify a novel truncating mutation in DGKE (p.K101X) in a consanguineous family with patients affected by thrombotic microangiopathy characterized by significant serum complement activation and consumption of the complement fraction C3. Aggressive plasma infusion therapy controlled systemic symptoms and prevented renal failure, suggesting that this treatment can significantly affect the natural history of this aggressive disease. Our study expands the clinical phenotypes associated with mutations in DGKE and challenges the benefits of complement blockade treatment in such patients. Mechanistic studies of DGKE and aHUS are, therefore, essential to the design of appropriate therapeutic strategies in patients with DGKE mutations

Mutations in Complement Regulatory Proteins Predispose to Preeclampsia: A Genetic Analysis of the PROMISSE Cohort

Jane Salmon and colleagues studied 250 pregnant patients with SLE and/or antiphospholipid antibodies and found an association of risk variants in complement regulatory proteins in patients who developed preeclampsia, as well as in preeclampsia patients lacking autoimmune disease

Both Monoclonal and Polyclonal Immunoglobulin Contingents Mediate Complement Activation in Monoclonal Gammopathy Associated-C3 Glomerulopathy

C3 glomerulopathy (C3G) results from acquired or genetic abnormalities in the complement alternative pathway (AP). C3G with monoclonal immunoglobulin (MIg-C3G) was recently included in the spectrum of “monoclonal gammopathy of renal significance.” However, mechanisms of complement dysregulation in MIg-C3G are not described and the pathogenic effect of the monoclonal immunoglobulin is not understood. The purpose of this study was to investigate the mechanisms of complement dysregulation in a cohort of 41 patients with MIg-C3G. Low C3 level and elevated sC5b-9, both biomarkers of C3 and C5 convertase activation, were present in 44 and 78% of patients, respectively. Rare pathogenic variants were identified in 2/28 (7%) tested patients suggesting that the disease is acquired in a large majority of patients. Anti-complement auto-antibodies were found in 20/41 (49%) patients, including anti-FH (17%), anti-CR1 (27%), anti-FI (5%) auto-antibodies, and C3 Nephritic Factor (7%) and were polyclonal in 77% of patients. Using cofactor assay, the regulation of the AP was altered in presence of purified IgG from 3/9 and 4/7 patients with anti-FH or anti-CR1 antibodies respectively. By using fluid and solid phase AP activation, we showed that total purified IgG of 22/34 (65%) MIg-C3G patients were able to enhance C3 convertase activity. In five documented cases, we showed that the C3 convertase enhancement was mostly due to the monoclonal immunoglobulin, thus paving the way for a new mechanism of complement dysregulation in C3G. All together the results highlight the contribution of both polyclonal and monoclonal Ig in MIg-C3G. They provide direct insights to treatment approaches and opened up a potential way to a personalized therapeutic strategy based on chemotherapy adapted to the B cell clone or immunosuppressive therapy