[Complement system and kidney]. / Complément et rein.

In the last few years, there has been a growing interest in the study of complement, fueleld mainly by the design of complement modulators, especially the C5-blocker eculizumab. The latter has significantly improved the prognosis of some nephropathies, such as the atypical hemolytic uremic syndrome. This breakthrough is a perfect example of fundamental translational research leading to clinical applications for patients. Currently, new molecules are being developed and some of them have already demonstrated clinical efficacy, such as avacopan (C5aR blocker) in ANCA vasculitis. As for kidney transplantation, complement modulators may lead to a new perspective in the treatment of some complications, such as humoral rejection. However, complement modulators carry the side effects, especially the infectious, and high costs.

L'étude du complément bénéficie depuis quelques années d'un regain d'intérêt dû en grande partie à la mise au point de nouveaux modulateurs du complément, notamment le bloqueur du C5, l'éculizumab. Ce dernier a significativement amélioré le pronostic de certaines pathologies rénales, comme le syndrome hémolytique et urémique atypique. Cette avancée est un exemple parfait d'une recherche translationnelle ayant débouché sur des applications cliniques pour les patients. Actuellement, de nouvelles molécules sont en cours de développement et certaines ont déjà démontré une efficacité clinique, comme l'avacopan (bloqueur du C5aR) dans les vasculites à anticorps anticytoplasme des polynucléaires neutrophiles. Du côté de la transplantation rénale, les inhibiteurs du complément pourraient faire évoluer le traitement de certaines complications comme le rejet humoral. Cependant, ces nouvelles thérapies ciblées ont des effets secondaires, notamment infectieux, et un coût important.

Upfront use of eculizumab to treat early acute antibody-mediated rejection after kidney allotransplantation and relevance for xenotransplantation.

Acute antibody-mediated rejection (AMR) early after transplant remains a challenge, both in allotransplantation and in xenotransplantation. We report the case of an early and severe acute AMR episode in a kidney transplant recipient that was successfully treated with upfront eculizumab. A 58-year-old woman had been on dialysis since 2014. She underwent a first kidney transplant in 2018 with primary non-function and received several blood transfusions. Postoperatively, she developed anti-HLA antibodies. One year later, she received a second allograft from a deceased donor. At day 0, there was only one preformed low-level donor-specific antibody (DSA) anti-DQ7. After initial excellent allograft function, serum creatinine increased on days 7-9, and this was associated with oligo-anuria. On day 7, there was an increase in her DSA anti-DQ7 and 4 de novo DSA had developed at high MFI values. Allograft biopsy showed severe active AMR with diffuse C4d deposits in peritubular capillaries. The early acute AMR episode was treated with upfront eculizumab administration (2 doses) with efficient CH50 blockade (< 10% CH50). Rituximab was also administered on day 12, and intravenous immunoglobulin (IVIG) was given over the following days. There was an excellent clinical response to eculizumab administration. Eculizumab administration rapidly reversed the acute AMR episode without the need for plasmapheresis. Rituximab and IVIG were also used as B-cell immunomodulators to decrease DSA. Blocking efficiently the terminal complement pathway may become a useful strategy to treat acute AMR in sensitized recipients of allografts, and possibly in recipients of discordant xenografts.

Platelet-Derived Ectosomes Reduce NK Cell Function.

Platelet (PLT) transfusions are potentially life saving for individuals with low PLT numbers; however, previous work revealed that PLT transfusions are associated with increased infection risk. During storage, PLT intended for transfusion continuously shed ectosomes (Ecto) from their surface, which express immunomodulatory molecules like phosphatidylserine or TGF-ß1. Recently, PLT-Ecto were shown to reduce proinflammatory cytokine release by macrophages and to favor the differentiation of naive T cells toward regulatory T cells. Whether PLT-Ecto modify NK cells remains unclear. We exposed purified NK cells and full PBMCs from healthy donors to PLT-Ecto. We found a reduced expression of several activating surface receptors (NKG2D, NKp30, and DNAM-1) and decreased NK cell function, as measured by CD107a expression and IFN-γ production. Pretreatment of PLT-Ecto with anti-TGF-ß1 neutralizing Ab restored surface receptor expression and NK cell function. We further observed a TGF-ß1-mediated upregulation of miR-183, which, in turn, reduced DAP12, an important protein for stabilization and downstream signaling of several activating NK cell receptors. Again, these effects could antagonized, in part, when PLT-Ecto were preincubated with anti-TGF-ß1 Ab. Erythrocyte Ecto did not affect NK cells. Polymorphonuclear cell Ecto expressed MHC class I and inhibited NK cell function. In addition, they induced the secretion of TGF-ß1 by NK cells, which participated in an auto/paracrine manner in the suppressive activity of polymorphonuclear cell-derived Ecto. In sum, our study showed that PLT-Ecto could inhibit NK cell effector function in a TGF-ß1-dependent manner, suggesting that recipients of PLT transfusions may experience reduced NK cell function.

[Complement in diseases]. / Maladies et complément.

The complement system is part of the innate immunity. It is a multifunctional system including more than 30 plasma and membrane proteins. These are activated by an enzymatic cascade and proteolytic reactions producing activating fragments. Complement is : 1) among the first line of defense towards a pathogen, 2) increasing efficacy of the acquired immunity, 3) responsible for the elimination of immune complexes apoptotic and or necrotic cells (waste removal function) and many other cellular and tissue functions. The aim of this article is to analyze the consequences of complement dysregulation in the development of various diseases.

Le système du complément fait partie de l'immunité innée. C'est un système complexe multifonctionnel comprenant plus de 30 protéines plasmatiques et membranaires s'activant par une cascade de protéolyses enzymatiques générant des fragments d'activation. Il est responsable de : 1) la première réaction vis-à-vis d'un agent pathogène ; 2) l'augmentation de l'immunité acquise et 3) l'élimination des complexes immuns et des cellules mortes par apoptose ou nécrose (fonction poubelle) ainsi que de nombreuses autres fonctions cellulaires et tissulaires. Le but de cet article est d'analyser les conséquences de la dérégulation du complément dans le développement de pathologies.

Blockade of C5 in Severe Acute Postinfectious Glomerulonephritis Associated With Anti-Factor H Autoantibody.

Activation of the complement cascade plays an important role in the pathogenesis of postinfectious glomerulonephritis. We report successful terminal complement pathway blockade using an anti-C5 monoclonal antibody (eculizumab) in an 8-year-old child with severe acute postinfectious glomerulonephritis requiring hemodialysis. The child presented with clinical, serologic, and histopathologic criteria for diffuse crescentic postinfectious glomerulonephritis. Complement measurements showed low C3 and C4 levels, with increased SC5b-9 titers. The presence of a transient anti-factor H autoantibody was also identified. Eculizumab (600mg, 2 doses at a 1-week interval) was administered, with a striking recovery of kidney function. There were no additional hemodialysis sessions needed after the first dose of eculizumab, and glomerular filtration rate measured using inulin clearance at 12 months of follow-up was within the normal range (92mL/min/1.73m2). Prompt terminal complement blockade may have improved the outcome in this case of severe acute postinfectious glomerulonephritis.

Microparticles (ectosomes) shed by stored human platelets downregulate macrophages and modify the development of dendritic cells.

Microparticles (MP) shed by platelets (PLT) during storage have procoagulant activities, but little is known about their properties to modify inflammation or immunity. In this study, we studied the capacity of MP present in PLT concentrates to alter the function of macrophages and dendritic cells (DC). The size of the purified MP was between 100 and 1000 nm, and they expressed phosphatidylserine; surface proteins of PLT (CD61, CD36, CD47), including complement inhibitors (CD55, CD59), but not CD63; and proteins acquired from plasma (C1q, C3 fragments, factor H). These characteristics suggest that the MP shed by PLT are formed by budding from the cell surface, corresponding to ectosomes. The purified PLT ectosomes (PLT-Ect) reduced the release of TNF-α and IL-10 by macrophages activated with LPS or zymosan A. In addition, PLT-Ect induced the immediate release of TGF-ß from macrophages, a release that was not modified by LPS or zymosan A. Macrophages had a reduced TNF-α release even 24 h after their exposure to PLT-Ect, suggesting that PLT-Ect induced a modification of the differentiation of macrophages. Similarly, the conventional 6-d differentiation of monocytes to immature DC by IL-4 and GM-CSF was modified by the presence of PLT-Ect during the first 2 d. Immature DC expressed less HLA-DP DQ DR and CD80 and lost part of their phagocytic activity, and their LPS-induced maturation was downmodulated when exposed to PLT-Ect. These data indicate that PLT-Ect shed by stored PLT have intrinsic properties that modify macrophage and DC differentiation toward less reactive states.

Ectosomes released by polymorphonuclear neutrophils induce a MerTK-dependent anti-inflammatory pathway in macrophages.

At the earliest stage of activation, human polymorphonuclear neutrophils release vesicles derived directly from the cell surface. These vesicles, called ectosomes (PMN-Ect), expose phosphatidylserine in the outer membrane leaflet. They inhibit the inflammatory response of human monocyte-derived macrophages and dendritic cells to zymosan A (ZymA) and LPS and induce TGF-ß1 release, suggesting a reprogramming toward a tolerogenic phenotype. The receptors and signaling pathways involved have not yet been defined. Here, we demonstrate that PMN-Ect interfered with ZymA activation of macrophages via inhibition of NFκB p65 phosphorylation and NFκB translocation. The MerTK (Mer receptor tyrosine kinase) and PI3K/Akt pathways played a key role in this immunomodulatory effect as shown using specific MerTK-blocking antibodies and PI3K inhibitors LY294002 and wortmannin. As a result, PMN-Ect reduced the transcription of many proinflammatory genes in ZymA-activated macrophages. In sum, PMN-Ect interacted with the macrophages by activation of the MerTK pathway responsible for down-modulation of the proinflammatory signals generated by ZymA.