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During severe inflammatory and infectious diseases, various mediators modulate the equilibrium of vascular tone, inflammation, coagulation and thrombosis. This Review describes the interactive roles of the renin–angiotensin system, the complement system, and the closely linked kallikrein–kinin and contact systems in cell biological functions such as vascular tone and leakage, inflammation, chemotaxis, thrombosis and cell proliferation. Specific attention is given to the role of these systems in systemic inflammation in the vasculature and tissues during hereditary angioedema, cardiovascular and renal glomerular disease, vasculitides and COVID-19. Moreover, we discuss the therapeutic implications of these complex interactions, given that modulation of one system may affect the other systems, with beneficial or deleterious consequences., The renin–angiotensin, complement and kallikrein–kinin systems comprise a multitude of mediators that modulate physiological responses during inflammatory and infectious diseases. This Review investigates the complex interactions between these systems and how these are dysregulated in various conditions, including cardiovascular diseases and COVID-19, as well as their therapeutic implications.

Certain kidney diseases are associated with complement activation although a renal triggering factor has not been identified. Here we demonstrated that renin, a kidney-specific enzyme, cleaves C3 into C3b and C3a, in a manner identical to the C3 convertase. Cleavage was specifically blocked by the renin inhibitor aliskiren. Renin-mediated C3 cleavage and its inhibition by aliskiren also occurred in serum. Generation of C3 cleavage products was demonstrated by immunoblotting, detecting the cleavage product C3b, by N-terminal sequencing of the cleavage product, and by ELISA for C3a release. Functional assays showed mast cell chemotaxis towards the cleavage product C3a and release of factor Ba when the cleavage product C3b was combined with factor B and factor D. The renin-mediated C3 cleavage product bound to factor B. In the presence of aliskiren this did not occur, and less C3 deposited on renin-producing cells. The effect of aliskiren was studied in three patients with dense deposit disease and this demonstrated decreased systemic and renal complement activation (increased C3, decreased C3a and C5a, decreased renal C3 and C5b-9 deposition and/or decreased glomerular basement membrane thickness) over a follow-up period of four to seven years. Thus, renin can trigger complement activation, an effect inhibited by aliskiren. Since renin concentrations are higher in renal tissue than systemically, this may explain the renal propensity of complement-mediated disease in the presence of complement mutations or auto-antibodies.

Background & purpose Infants on chronic peritoneal dialysis (PD) have an increased risk of developing neurological morbidities; however, the underlying biological mechanisms are poorly understood. In this clinical study, we investigated whether PD-mediated impairment of nitric oxide (NO) bioavailability and signaling, in patients with persistently low systolic blood pressure (SBP), can explain the occurrence of cerebral ischemia. Methods & results Repeated blood pressure measurements, serial neuroimaging studies, and investigations of systemic nitrate and nitrite levels, as well as NO signaling, were performed in ten pediatric patients on PD. We consistently observed the loss of both inorganic nitrate (-17 ± 3%, P < 0.05) and nitrite (-34 ± 4%, P < 0.05) during PD, which may result in impairment of the nitrate-nitrite-NO pathway. Indeed, PD was associated with significant reduction of cyclic guanosine monophosphate levels (-59.4 ± 15%, P < 0.05). This reduction in NO signaling was partly prevented by using a commercially available PD solution supplemented with l-arginine. Although PD compromised nitrate-nitrite-NO signaling in all cases, only infants with persistently low SBP developed ischemic cerebral complications. Conclusions Our data suggests that PD impairs NO homeostasis and predisposes infants with persistently low SBP to cerebral ischemia. These findings improve current understanding of the pathogenesis of infantile cerebral ischemia induced by PD and may lead to the new treatment strategies to reduce neurological morbidities. (Less)

The complement system is activated in the vasculature during thrombotic and inflammatory conditions. Activation may be associated with chronic inflammation on the endothelial surface leading to complement deposition. Complement mutations allow uninhibited complement activation to occur on platelets, neutrophils, monocytes, and aggregates thereof, as well as on red blood cells and endothelial cells. Furthermore, complement activation on the cells leads to the shedding of cell derived-microvesicles that may express complement and tissue factor thus promoting inflammation and thrombosis. Complement deposition on red blood cells triggers hemolysis and the release of red blood cell-derived microvesicles that are prothrombotic. Microvesicles are small membrane vesicles ranging from 0.1 to 1 μm, shed by cells during activation, injury and/or apoptosis that express components of the parent cell. Microvesicles are released during inflammatory and vascular conditions. The repertoire of inflammatory markers on endothelial cell-derived microvesicles shed during inflammation is large and includes complement. These circulating microvesicles may reflect the ongoing inflammatory process but may also contribute to its propagation. This overview will describe complement activation on blood and endothelial cells and the release of microvesicles from these cells during hemolytic uremic syndrome, thrombotic thrombocytopenic purpura and vasculitis, clinical conditions associated with enhanced thrombosis and inflammation.

Shiga toxin (Stx) is the main virulence factor of enterohemorrhagic Escherichia coli, which are non-invasive strains that can lead to hemolytic uremic syndrome (HUS), associated with renal failure and death. Although bacteremia does not occur, bacterial virulence factors gain access to the circulation and are thereafter presumed to cause target organ damage. Stx was previously shown to circulate bound to blood cells but the mechanism by which it would potentially transfer to target organ cells has not been elucidated. Here we show that blood cell-derived microvesicles, shed during HUS, contain Stx and are found within patient renal cortical cells. The finding was reproduced in mice infected with Stx-producing Escherichia coli exhibiting Stx-containing blood cell-derived microvesicles in the circulation that reached the kidney where they were transferred into glomerular and peritubular capillary endothelial cells and further through their basement membranes followed by podocytes and tubular epithelial cells, respectively. In vitro studies demonstrated that blood cell-derived microvesicles containing Stx undergo endocytosis in glomerular endothelial cells leading to cell death secondary to inhibited protein synthesis. This study demonstrates a novel virulence mechanism whereby bacterial toxin is transferred within host blood cell-derived microvesicles in which it may evade the host immune system., Author Summary Shiga toxin-producing enterohemorrhagic Escherichia coli are non-invasive bacteria that, after ingestion, cause disease by systemic release of toxins and other virulence factors. These infections cause high morbidity, including hemolytic uremic syndrome with severe anemia, low platelet counts, renal failure, and mortality. The most common clinical isolate is E. coli O157:H7. In 2011 an E. coli O104:H4 strain caused a large outbreak in Europe with high mortality. After Shiga toxin damages intestinal cells it comes in contact with blood cells and thus gains access to the circulation. In this study we have shown that the toxin is released into circulating host blood cell-derived microvesicles, in which it retains its toxicity but evades the host immune response. Our results suggest that these microvesicles can enter target organ cells in the kidney and transfer toxin into these cells as well as between cells. Such a mechanism of virulence has not been previously described in bacterial infection.

The complement system is activated in the vasculature during thrombotic and inflammatory conditions. Activation may be associated with chronic inflammation on the endothelial surface leading to complement deposition. Complement mutations allow uninhibited complement activation to occur on platelets, neutrophils, monocytes, and aggregates thereof, as well as on red blood cells and endothelial cells. Furthermore, complement activation on the cells leads to the shedding of cell derived-microvesicles that may express complement and tissue factor thus promoting inflammation and thrombosis. Complement deposition on red blood cells triggers hemolysis and the release of red blood cell-derived microvesicles that are prothrombotic. Microvesicles are small membrane vesicles ranging from 0.1 to 1 μm, shed by cells during activation, injury and/or apoptosis that express components of the parent cell. Microvesicles are released during inflammatory and vascular conditions. The repertoire of inflammatory markers on endothelial cell-derived microvesicles shed during inflammation is large and includes complement. These circulating microvesicles may reflect the ongoing inflammatory process but may also contribute to its propagation. This overview will describe complement activation on blood and endothelial cells and the release of microvesicles from these cells during hemolytic uremic syndrome, thrombotic thrombocytopenic purpura and vasculitis, clinical conditions associated with enhanced thrombosis and inflammation.

Immunoglobulin A (IgA) nephropathy is a chronic glomerulonephritis with excessive glomerular deposition of IgA1, C3 and C5b-9, which may lead to renal failure. We describe the clinical course of an adolescent with rapidly progressive disease leading to renal failure in spite of immunosuppressive treatment. Due to refractory disease the patient was treated with eculizumab (anti-C5) for 3 months in an attempt to rescue renal function. Treatment led to clinical improvement with stabilization of the glomerular filtration rate and reduced proteinuria. Discontinuation of treatment led to a rapid deterioration of renal function. This was followed by a single dose of eculizumab, which again reduced creatinine levels temporarily. Early initiation of eculizumab therapy in patients with progressive IgA nephropathy may have a beneficial effect by blocking complement-mediated renal inflammation.

We describe a case of inferior vena cava thrombosis (IVC) leading to bilateral renal vein thrombosis and renal failure in a neonate, which was successfully treated by thrombolysis. A male neonate, born at term by vaginal delivery (Apgar score 9–10–10) and weighing 4210 g at birth after a normal pregnancy, presented at 9 days of age due to failure to thrive and gross haematuria. At admission the child weighed 4200 g and appeared to be dehydrated. He was anuric with a serum creatinine of 222 μmol/L (reference 14–37 μmol/L), severe metabolic acidosis and respiratory difficulties. He had a palpable abdominal mass on the left side of the abdomen. Ultrasound examination showed enlarged hyperechogenic kidneys, especially on the left side. The renal veins and IVC could not be visualised. Magnetic resonance (MR) angiography revealed thrombosis of the IVC from the bifurcation up to the hepatic veins (Fig. 1a, b) and extending into both renal veins. There was no evidence of adrenal haemorrhage. Treatment with warfarin (Waran; Nycomed, Zurich, Switzerland) was initiated (Fig. 1c), and systemic lowmolecular weight heparin (LMWH) (dalteparin sodium, Fragmin; Pfizer, New York, NY). The dose was adjusted by following levels of anti-Factor Xa geared at 0.5–1.0 kIE/L. Because the ultrasound did not show any change in the size of the thrombus, warfarin was discontinued, and local fibrinolysis was initiated [1, 2]. An angio-catheter was inserted into the occluding thrombus in the IVC, and continuous infusion of recombinant tissue plasminogen activator (rt-PA) (Actilyse; Boehringer, Ingelheim, Germany) was administered at 0.1 mg/kg/hour. The rt-PA treatment was monitored by an analysis of plasma fibrinogen levels. Systemic LMWH was continued in order to prevent thromboembolism, and fresh frozen plasma was infused daily. Cranial ultrasound was performed daily due to the risk of intracranial haemorrhage. Three days after the patient had been admitted, systemic LMWH was switched to unfractionated heparin (LEO Pharma, Ballerup, Denmark) infusion because the level of anti-FXa was too low. The dose of unfractionated heparin was monitored by activated clotting time targeted at 180–200 s. Local rt-PA treatment was discontinued due to excessive bleeding from the peritoneal dialysis catheter insertion site. Pediatr Nephrol (2009) 24:2069–2071 DOI 10.1007/s00467-009-1172-3

Background. Atypical haemolytic uraemic syndrome (aHUS) is associated with dysfunction of the alternative pathway of complement. Disease activity subsides as renal failure progresses but recurs upon renal transplantation, indicating that viable renal tissue contributes to disease activity. We present evidence of cerebrovascular occlusive disease indicating that vascular injury may occur in the absence of kidneys. Methods. A currently 12-year-old girl developed renal failure at the age of 20 months. She underwent bilateral nephrectomy and renal transplantation but lost the transplant due to recurrences. She was on haemodialysis for 7 years. At 10 years of age she developed a transient ischaemic attack. Imaging, genetic investigation and mutation characterization were performed. Results. Imaging demonstrated occlusion and stenosis of the carotid arteries. Two complement mutations, a novel mutation in factor B and a previously described mutation in factor I, and the H3-factor H haplotype, were identified. The factor B mutation, L433S, did not induce excessive complement activation in vitro. Measurement of C3 degradation products indicated ongoing complement activation. In spite of the patient being anephric, treatment was initiated with eculizumab, a humanized anti-C5 antibody that blocks terminal complement activation. She underwent a successful kidney transplant 9 months later and has not developed a recurrence or progression of vascular stenosis 1 year later. Conclusions. The course of disease in this patient with aHUS suggests that complement-mediated vascular injury may occur in the total absence of renal tissue and overt recurrences. To our knowledge, this is the first description of eculizumab treatment in an anephric aHUS patient.

ADAMTS13 mRNA, which encodes the von Willebrand factor-cleaving protease, has been detected in a variety of tissues, including the kidney. The aim of our study was to characterize tubular expression and bioactivity of ADAMTS13. ADAMTS13 mRNA was detected in cultured primary human renal tubular epithelial cells (HRTEC) and in A498 cells, a human renal carcinoma cell line, by real-time PCR. Protein was detected using immunofluorescence and immunoblotting. Immunoblots demonstrated that the protein was secreted. The protease was proteolytically active in both cell lysates and cleaved the FRETS–VWF73 substrate. ADAMTS13 was demonstrated in situ in the renal cortex by immunohistochemistry. Protease was detected in both the proximal and distal renal tubules in normal renal tissue (n = 3) as well as in patients with tubular disorders (n = 3). Immunoblotting revealed that ADAMTS13 was present in the urine of patients with tubulopathy (n = 5) but not in normal urine. ADAMTS13 in urine had a molecular size similar to that in plasma, which indicates that the protease originates in the tubuli because such large proteins do not normally pass the glomerular filter. In conclusion, human renal tubular epithelial cells synthesize biologically active ADAMTS13 which may, after release from tubuli, regulate hemostasis in the local microenvironment.

Enterohemorrhagic Escherichia coli (EHEC) infection leads to marked intestinal injury. Sigmoid colon obtained from two children during EHEC infection exhibited abundant TUNEL-positive cells. To define which bacterial virulence factors contribute to intestinal injury the presence of Shiga toxin-2 (Stx2), intimin and the type III secretion system were correlated with symptoms and intestinal damage. C3H/HeN mice were inoculated with Stx2-producing (86-24) and non-producing (87-23) E. coli O157:H7 strains and 86-24 mutants lacking eae, encoding intimin (strain UMD619) or escN regulating the expression of type III secretion effectors (strain CVD451). Severe symptoms developed in mice inoculated with 86-24 and 87-23. Few mice inoculated with the mutant strains developed severe symptoms. Strain 86-24 exhibited higher fecal bacterial counts, followed by 87-23, whereas strains UMD619 and CVD451 showed minimal fecal counts. More TUNEL-positive cells were found in proximal and distal colons of mice inoculated with strain 86-24 compared with strains 87-23 and CVD451 (p

Enterohemorrhagic Escherichia coli produce an attaching and effacing lesion upon adhering to the intestinal epithelium. Bacterial factors involved in this histopathology include the intimin adhesin and E. coli secreted proteins (Esps) A and B. In this study we investigated the serum antibody responses to recombinant E. coli O157:H7 intimin, EspA, and EspB by immunoblotting. Canadian patients with O157:H7 infection (n=10), Swedish patients with O157:H7 (n=21), non-O157 (n=18), or infection from which the serotype was not available (n=3), and asymptomatic household members (n=25) were studied and compared with Canadian (n=20) and Swedish controls (n=52). In Canadian patients, IgG antibodies to intimin, EspA, and EspB were analyzed, in Swedish patients and their household members IgA, IgG, and IgM antibodies to EspA and EspB were studied. Patients and household members mounted an antibody response to the antigens. Significantly more patients developed an acute response to EspB compared with controls (P

BACKGROUND: Aggregates formed between leukocytes and platelets in the circulation lead to release of tissue factor (TF)-bearing microparticles contributing to a prothrombotic state. As enterohemorrhagic Escherichia coli (EHEC) may cause hemolytic uremic syndrome (HUS), in which microthrombi cause tissue damage, this study investigated whether the interaction between blood cells and EHEC virulence factors Shiga toxin (Stx) and lipopolysaccharide (LPS) led to release of TF. METHODOLOGY/PRINCIPAL FINDINGS: The interaction between Stx or LPS and blood cells induced platelet-leukocyte aggregate formation and tissue factor (TF) release, as detected by flow cytometry in whole blood. O157LPS was more potent than other LPS serotypes. Aggregates formed mainly between monocytes and platelets and less so between neutrophils and platelets. Stimulated blood cells in complex expressed activation markers, and microparticles were released. Microparticles originated mainly from platelets and monocytes and expressed TF. TF-expressing microparticles, and functional TF in plasma, increased when blood cells were simultaneously exposed to the EHEC virulence factors and high shear stress. Stx and LPS in combination had a more pronounced effect on platelet-monocyte aggregate formation, and TF expression on these aggregates, than each virulence factor alone. Whole blood and plasma from HUS patients (n = 4) were analyzed. All patients had an increase in leukocyte-platelet aggregates, mainly between monocytes and platelets, on which TF was expressed during the acute phase of disease. Patients also exhibited an increase in microparticles, mainly originating from platelets and monocytes, bearing surface-bound TF, and functional TF was detected in their plasma. Blood cell aggregates, microparticles, and TF decreased upon recovery. CONCLUSIONS/SIGNIFICANCE: By triggering TF release in the circulation, Stx and LPS can induce a prothrombotic state contributing to the pathogenesis of HUS.