Molecular Studies and an ex vivo Complement Assay on Endothelium Highlight the Genetic Complexity of Atypical Hemolytic Uremic Syndrome: The Case of a Pedigree With a Null CD46 Variant

Atypical hemolytic uremic syndrome (aHUS) is an ultra-rare disease characterized by microangiopathic hemolysis, thrombocytopenia, and renal impairment and is associated with dysregulation of the alternative complement pathway on the microvascular endothelium. Outcomes have improved greatly with pharmacologic complement C5 blockade. Abnormalities in complement genes (CFH, CD46, CFI, CFB, C3, and THBD), CFH–CFHR genomic rearrangements, and anti-FH antibodies have been reported in 40–60% of cases. The penetrance of aHUS is incomplete in carriers of complement gene abnormalities; and multiple hits, including the CFH–H3 and CD46_GGAAC risk haplotypes and the CFHR1^*B risk allele, as well as environmental factors, contribute to disease development. Here, we investigated the determinants of penetrance of aHUS associated with CD46 genetic abnormalities. We studied 485 aHUS patients and found CD46 rare variants (RVs) in about 10%. The c.286+2T>G RV was the most prevalent (13/485) and was associated with <30% penetrance. We conducted an in-depth study of a large pedigree including a proband who is heterozygous for the c.286+2T>G RV who experienced a severe form of aHUS and developed end-stage renal failure. The father and paternal uncle with the same variant in homozygosity and six heterozygous relatives are unaffected. Flow cytometry analysis showed about 50% reduction of CD46 expression on blood mononuclear cells from the heterozygous proband and over 90% reduction in cells from the proband's unaffected homozygous father and aunt. Further genetic studies did not reveal RVs in known aHUS-associated genes or common genetic modifiers that segregated with the disease. Importantly, a specific ex vivo test showed excessive complement deposition on endothelial cells exposed to sera from the proband, and also from his mother and maternal uncle, who do not carry the c.286+2T>G RV, indicating that they share a circulating defect that results in complement dysregulation on the endothelium. These results highlight the complexity of the genetics of aHUS and indicate that CD46 deficiency may not be enough to induce aHUS. We hypothesize that the proband inherited from his mother a genetic abnormality in a complement circulating factor that has not been identified yet, which synergized with the CD46 RV in predisposing him to the aHUS phenotype.

The case of complement activation in COVID-19 multiorgan impact

The novel coronavirus disease COVID-19 originates in the lungs, but it may extend to other organs, causing, in severe cases, multiorgan damage, including cardiac injury and acute kidney injury. In severe cases, the presence of kidney injury is associated with increased risk of death, highlighting the relevance of this organ as a target of SARS-CoV-2 infection. COVID-19-associated tissue injury is not primarily mediated by viral infection, but rather is a result of the inflammatory host immune response, which drives hypercytokinemia and aggressive inflammation that affect lung parenchymal cells, diminishing oxygen uptake, but also endothelial cells, resulting in endotheliitis and thrombotic events and intravascular coagulation. The complement system represents the first response of the host immune system to SARS-CoV-2 infection, but there is growing evidence that unrestrained activation of complement induced by the virus in the lungs and other organs plays a major role in acute and chronic inflammation, endothelial cell dysfunction, thrombus formation, and intravascular coagulation, and ultimately contributes to multiple organ failure and death. In this review, we discuss the relative role of the different complement activation products in the pathogenesis of COVID-19-associated tissue inflammation and thrombosis and propose the hypothesis that blockade of the terminal complement pathway may represent a potential therapeutic option for the prevention and treatment of lung and multiorgan damage.

Terminal complement effectors in atypical hemolytic uremic syndrome: C5a, C5b-9, or a bit of both?

The role of the terminal complement pathway as the cause of atypical hemolytic uremic syndrome (aHUS) is widely recognized, but the relative contribution of the effectors C5a/C5aR1 and C5b-9 to disease pathogenesis has not been defined. Using FH^R/R mice carrying a factor H mutation that causes cell surface complement alternative pathway dysregulation, Ueda documented that in FH^R/R mice, C5b-9 causes renal thrombotic microangiopathy (TMA) whereas C5a/C5aR drives macrovascular thrombosis. This commentary addresses the implications and limitations of this study.

Transplantation-Induced Ischemia-Reperfusion Injury Modulates Antigen Presentation by Donor Renal CD11c+F4/80+ Macrophages through IL-1R8 Regulation

BACKGROUND

In donor kidneys subjected to ischemia-reperfusion injury during kidney transplant, phagocytes coexpressing the F4/80 and CD11c molecules mediate proinflammatory responses and trigger adaptive immunity in transplantation through antigen presentation. After injury, however, resident renal macrophages coexpressing these surface markers acquire a proreparative phenotype, which is pivotal in controlling inflammation and fibrosis. No data are currently available regarding the effects of transplant-induced ischemia-reperfusion injury on the ability of donor-derived resident renal macrophages to act as professional antigen-presenting cells.

METHODS

We evaluated the phenotype and function of intragraft CD11c⁺F4/80⁺ renal macrophages after cold ischemia. We also assessed the modifications of donor renal macrophages after reversible ischemia-reperfusion injury in a mouse model of congeneic renal transplantation. To investigate the role played by IL-1R8, we conducted in vitro and in vivo studies comparing cells and grafts from wild-type and IL-R8-deficient donors.

RESULTS

Cold ischemia and reversible ischemia-reperfusion injury dampened antigen presentation by renal macrophages, skewed their polarization toward the M₂ phenotype, and increased surface expression of IL-1R8, diminishing activation mediated by toll-like receptor 4. Ischemic IL-1R8-deficient donor renal macrophages acquired an M₁ phenotype, effectively induced IFNγ and IL-17 responses, and failed to orchestrate tissue repair, resulting in severe graft fibrosis and aberrant humoral immune responses.

CONCLUSIONS

IL-1R8 is a key regulator of donor renal macrophage functions after ischemia-reperfusion injury, crucial to guiding the phenotype and antigen-presenting role of these cells. It may therefore represent an intriguing pathway to explore with respect to modulating responses against autoantigens and alloantigens after kidney transplant.

P772 Percutaneous transcatheter patent foramen ovale closure: does size really matter?

Introduction

Patent foramen ovale (PFO) has been associated with cryptogenic stroke. Percutaneous transcatheter PFO closure has been in development for the last years. In recent studies, it has shown a reduction of recurrent ischemic stroke events. Several manufactures of PFO device closure have developed guidelines for selection of device size based on anatomic measurements obtained through echocardiography. A study showed that according to the guidelines few patients had adequate tissue rim to allow device implantation, and yet almost all the patients had an effective closure.

Purpose and methods

The aim of this study was to evaluate the characteristics of PFO with transesophageal echocardiogram (TEE) (size, distance between foramen ovale-superior vena cava (FO-SVC) and distance between FO-aortic annulus (FO-AoAn)) and to determinate the correlation between the size of PFO device implanted and the theoretical device size according to the guidelines.

We performed a retrospective study among patients who underwent a PFO closure procedure between 2006 and 2018.

Results

42 patients were included, 55% male, mean age 56 years and mean BSA 1,83 m². The PFO mean diameter and length were 3 and 13 mm respectively. The majority of PFO were complex: 91% of patients had defects larger than 8mm and 36% had atrial septum aneurism. Characteristic of PFO were measured (Table). In older patients the distance between PFO-SVC and PFO-AoAn was larger and the septum secundum was thicker (p 0.02). The right-to-left shunt was moderated or severe in most cases (75%). The PFO closure device was successfully implanted in all patients. The majority of them received an Amplatzer PFO device (45%) followed by an Hyperion PFO device. The size of the device was 25mm in half of the patients followed by 30mm device. According to the guidelines, in 67% of the cases the wrong size was chosen and the mean size of the device should had been 30mm (SD ±5,2). TEE examination during follow up showed no residual or mild residual shunt in 80% of the patients. Residual shunt was more prevalent with larger devices (p 0.008)

Conclusions

These results showed that the size of PFO closure device implanted in our population differed from the size recommended by guidelines. Smaller devices have been used. According to our results, larger devices are associated with more residual shunt, which suggests that a different approach may be necessary to establish new protocols for PFO device closure

Table Minimal Maximal Mean SD SVC (mm) 10 30 17.55 ±3,7 AoAn (mm) 9 23 14.1 ±3,5 Minimal and maximal distance between PFO-SVC and AoAn

Autotaxin Inhibitor Protects from Chronic Allograft Injury in Rat Kidney Allotransplantation

BACKGROUND

Tissue fibrosis is the final common phase of chronic allograft injury, the leading cause of late graft loss in kidney transplantation. Preclinical evidence points to the involvement of lysophosphatidic acid (LPA), a bioactive phospholipid, in the development of renal fibrosis.

OBJECTIVES

We assessed whether treatment with an orally available inhibitor of autotaxin (ATXi), the main LPA-producing enzyme, could slow the progression of chronic allograft injury in a fully major histocompatibility complex-mismatched rat kidney transplant model and compared its effects with those of the angiotensin-converting enzyme inhibitor lisinopril.

METHODS

Kidney allograft recipients were given ciclosporin for the first 15 postoperative days to prevent early acute rejection. Thereafter, they received either no treatment or ATXi or lisinopril and were followed for 180 days after transplantation.

RESULTS

Renal LPA levels were increased in allograft rats, providing the rationale for using ATXi in this model. Chronic treatment with ATXi or lisinopril limited progressive proteinuria and ameliorated tubulointerstitial fibrosis compared with allograft rats, although the effects were more robust under ATX inhibition. The administration of ATXi, but not lisinopril, attenuated systemic hypertension, reduced intragraft T cell infiltration, and eventually improved renal graft survival.

CONCLUSIONS

In summary, ATXi had protective effects on indices of chronic allograft injury and could be of therapeutic add-on value in the kidney transplant setting. Notably, an ATX inhibitor is currently being investigated in 2 large phase 3 studies in idiopathic pulmonary fibrosis, underscoring the clinical relevance of our findings.

Impact of a Complement Factor H Gene Variant on Renal Dysfunction, Cardiovascular Events, and Response to ACE Inhibitor Therapy in Type 2 Diabetes

Complement activation has been increasingly implicated in the pathogenesis of type 2 diabetes and its chronic complications. It is unknown whether complement factor H (CFH) genetic variants, which have been previously associated with complement-mediated organ damage likely due to inefficient complement modulation, influence the risk of renal and cardiovascular events and response to therapy with angiotensin-converting enzyme inhibitors (ACEi) in type 2 diabetic patients. Here, we have analyzed the c.2808G>T, (p.Glu936Asp) CFH polymorphism, which tags the H3 CFH haplotype associated to low plasma factor H levels and predisposing to atypical hemolytic uremic syndrome, in 1,158 type 2 diabetics prospectively followed in the Bergamo nephrologic complications of type 2 diabetes randomized, controlled clinical trial (BENEDICT) that evaluated the effect of the ACEi trandolapril on new onset microalbuminuria. At multivariable Cox analysis, the p.Glu936Asp polymorphism (Asp/Asp homozygotes, recessive model) was associated with increased risk of microalbuminuria [adjusted hazard ratio (HR) 3.25 (95% CI 1.46–7.24), P = 0.0038] and cardiovascular events [adjusted HR 2.68 (95% CI 1.23–5.87), P = 0.013]. The p.Glu936Asp genotype significantly interacted with ACEi therapy in predicting microalbuminuria. ACEi therapy was not nephroprotective in Asp/Asp homozygotes [adjusted HR 1.54 (0.18–13.07), P = 0.691 vs. non-ACEi-treated Asp/Asp patients], whereas it significantly reduced microalbuminuria events in Glu/Asp or Glu/Glu patients [adjusted HR 0.38 (0.24–0.60), P < 0.0001 vs. non-ACEi-treated Glu/Asp or Glu/Glu patients]. Among ACEi-treated patients, the risk of developing cardiovascular events was higher in Asp/Asp homozygotes than in Glu/Asp or Glu/Glu patients [adjusted HR 3.26 (1.29–8.28), P = 0.013]. Our results indicate that type 2 diabetic patients Asp/Asp homozygotes in the p.Glu936Asp CFH polymorphism are at increased risk of microalbuminuria and cardiovascular complications and may be less likely to benefit from ACEi therapy. Further studies are required to confirm our findings.

Rare Functional Variants in Complement Genes and Anti-FH Autoantibodies-Associated aHUS

Atypical hemolytic uremic syndrome (aHUS) is a rare disease characterized by microangiopathic hemolytic anemia, thrombocytopenia and renal failure. It is caused by genetic or acquired defects of the complement alternative pathway. Factor H autoantibodies (anti-FHs) have been reported in 10% of aHUS patients and are associated with the deficiency of factor H-related 1 (FHR1). However, FHR1 deficiency is not enough to cause aHUS, since it is also present in about 5% of Caucasian healthy subjects. In this study we evaluated the prevalence of genetic variants in CFH, CD46, CFI, CFB, C3, and THBD in aHUS patients with anti-FHs, using healthy subjects with FHR1 deficiency, here defined “supercontrols,” as a reference group. “Supercontrols” are more informative than general population because they share at least one risk factor (FHR1 deficiency) with aHUS patients. We analyzed anti-FHs in 305 patients and 30 were positive. The large majority were children (median age: 7.7 [IQR, 6.6–9.9] years) and 83% lacked FHR1 (n = 25, cases) due to the homozygous CFHR3-CFHR1 deletion (n = 20), or the compound heterozygous CFHR3-CFHR1 and CFHR1-CFHR4 deletions (n = 4), or the heterozygous CFHR3-CFHR1 deletion combined with a frameshift mutation in CFHR1 that generates a premature stop codon (n = 1). Of the 960 healthy adult subjects 48 had the FHR1 deficiency (“supercontrols”). Rare likely pathogenetic variants in CFH, THBD, and C3 were found in 24% of cases (n = 6) compared to 2.1% of the “supercontrols” (P-value = 0.005). We also found that the CFH H3 and the CD46_GGAAC haplotypes are not associated with anti-FHs aHUS, whereas these haplotypes are enriched in aHUS patients without anti-FHs, which highlights the differences in the genetic basis of the two forms of the disease. Finally, we confirm that common infections are environmental factors that contribute to the development of anti-FHs aHUS in genetically predisposed individuals, which fits with the sharp peak of incidence during scholar-age. Further studies are needed to fully elucidate the complex genetic and environmental factors underlying anti-FHs aHUS and to establish whether the combination of anti-FHs with likely pathogenetic variants or other risk factors influences disease outcome and response to therapies.

C5 Convertase Blockade in Membranoproliferative Glomerulonephritis: A Single-Arm Clinical Trial

RATIONALE & OBJECTIVE

Primary membranoproliferative glomerulonephritis (MPGN) is a rare glomerulopathy characterized by complement dysregulation. MPGN progresses rapidly to kidney failure when it is associated with nephrotic syndrome. We assessed the effects of C5 convertase blockade in patients with MPGN and terminal complement activation.

STUDY DESIGN

Prospective off-on-off-on open-label clinical trial.

SETTING & PARTICIPANTS

Consenting patients with immune complex-mediated MPGN (n=6) or C3 glomerulonephritis (n=4) with sC5b-9 (serum complement membrane attack complex) plasma levels>1,000ng/mL and 24-hour proteinuria with protein excretion>3.5g identified from the Italian Registry of MPGN and followed up at the Istituto di Ricerche Farmacologiche Mario Negri IRCCS (Bergamo, Italy) between March 4, 2014, and January 7, 2015.

INTERVENTION

Anti-C5 monoclonal antibody eculizumab administered during 2 sequential 48-week treatment periods separated by one 12-week washout period.

OUTCOMES

Primary outcome was change in 24-hour proteinuria (median of 3 consecutive measurements) at 24 and 48 weeks.

RESULTS

Median proteinuria decreased from protein excretion of 6.03 (interquartile range [IQR], 4.8-12.4) g/d at baseline to 3.74 (IQR, 3.2-4.4) g/d at 24 weeks (P=0.01) and to 5.06 (IQR, 3.1-5.8) g/d (P=0.006) at 48 weeks of treatment, recovered toward baseline during the washout period, and did not significantly decrease thereafter. Hypoalbuminemia, dyslipidemia, and glomerular sieving function improved during the first treatment period. 3 patients achieved partial remission of nephrotic syndrome and all had undetectable C3 nephritic factors before treatment. Mean measured glomerular filtration rate was 69.7±35.2 versus 87.4±55.1 and 75.8±42.7 versus 76.6±44.1mL/min/1.73m² at the start versus the end of the first and second treatment periods, respectively, among all 10 study participants. Unlike C3, sC5b-9 plasma levels normalized during both treatment periods and recovered toward baseline during the washout in all patients.

LIMITATIONS

Single-arm design, small sample size.

CONCLUSIONS

Eculizumab blunted terminal complement activation in all patients with immune complex-mediated MPGN or C3 glomerulonephritis and nephrotic syndrome, but persistently reduced proteinuria in just a subgroup.

TRIAL REGISTRATION

Registered in the EU Clinical Trials Register with study no. 2013-003826-10.

Hemolytic Uremic Syndrome in an Infant with Primary Hyperoxaluria Type II: An Unreported Clinical Association

A 6-month-old boy presented with acute renal failure, thrombocytopenia, and severe non-immune hemolytic anemia. Infection by Shiga-like toxin-producing Escherichia coli and other causes of microangiopathic hemolysis were ruled out, leading to a diagnosis of atypical hemolytic uremic syndrome (aHUS). Neither pathogenic variants in HUS-associated genes nor anti-factor H antibodies were identified. Copy number variation analysis uncovered 4 copies of complement factor H related genes, CFHR1-CFHR4, conceivably leading to higher than normal levels of the corresponding proteins. However, this abnormality was also found in the healthy relatives, neither explaining the disease nor the excessive complement deposition on endothelial cells detected by an ex-vivo test. Whole-exome sequencing revealed a pathogenic homozygous variant in GRHPR encoding the glyoxylate and hydroxypyruvate reductase. Recessive GRHPR mutations cause primary hyperoxaluria type 2 (PH2). The presence of renal calculi in the patient and elevated oxalate levels in the urine were consistent with the genetic diagnosis of PH2. We hypothesize that, in this patient, hyperoxaluria caused by the GRHPR genetic defect triggered endothelial perturbation and complement activation, which was amplified by impaired factor H regulatory activity due to the increased -CFHR1-CFHR4 copy numbers, resulting in aHUS.

An Ex Vivo Test of Complement Activation on Endothelium for Individualized Eculizumab Therapy in Hemolytic Uremic Syndrome

RATIONALE & OBJECTIVE

Although primary atypical hemolytic uremic syndrome (aHUS) is associated with abnormalities in complement genes and antibodies to complement factor H, the role of complement in secondary aHUS remains debatable. We evaluated the usefulness of an ex vivo test to: (1) detect complement activation within the endothelium in primary and secondary aHUS, (2) differentiate active disease from remission, (3) monitor the effectiveness of eculizumab therapy, and (4) identify relapses during eculizumab dosage tapering and after discontinuation of treatment.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

121 patients with primary aHUS and 28 with secondary aHUS. Serum samples were collected during acute episodes, following remission, and during eculizumab treatment and were assessed using a serum-induced ex vivo C5b-9 endothelial deposition test.

RESULTS

Serum-induced C5b-9 deposition on cultured microvascular endothelium was quantified by calculating the endothelial area covered by C5b-9 staining; values were expressed as percentage of C5b-9 deposits induced by a serum pool from healthy controls. Testing with adenosine diphosphate-activated endothelium demonstrated elevated C5b-9 deposits for all untreated patients with aHUS independent of disease activity, while testing with unstimulated endothelium demonstrated deposits only in active disease. Similar findings were observed in secondary aHUS. Serum-induced C5b-9 deposits on activated and unstimulated endothelium normalized during eculizumab treatment. 96% (22/23) of patients receiving eculizumab at extended 3- or 4-week dosing intervals demonstrated normal C5b-9 deposits on activated endothelium, despite most patients having CH50Eq (serum complement activity) > 20 UEq/mL, indicating that adequate complement control was achieved even with incomplete blockade of circulating C5. During eculizumab dosage tapering or after treatment discontinuation, all patients experiencing relapses versus only 6% (1/17) of those in stable remission had elevated C5b-9 deposits on unstimulated endothelium.

LIMITATIONS

The C5b-9 endothelial deposition test can be performed in only specialized laboratories. Findings on eculizumab dosage tapering need to be confirmed with longitudinal monitoring of C5b-9 deposition.

CONCLUSIONS

The C5b-9 endothelial deposition assay may represent an advance in our ability to monitor aHUS activity and individualize therapy.

C3 glomerulopathy - understanding a rare complement-driven renal disease

The C3 glomerulopathies are a group of rare kidney diseases characterized by complement dysregulation occurring in the fluid phase and in the glomerular microenvironment, which results in prominent complement C3 deposition in kidney biopsy samples. The two major subgroups of C3 glomerulopathy - dense deposit disease (DDD) and C3 glomerulonephritis (C3GN) - have overlapping clinical and pathological features suggestive of a disease continuum. Dysregulation of the complement alternative pathway is fundamental to the manifestations of C3 glomerulopathy, although terminal pathway dysregulation is also common. Disease is driven by acquired factors in most patients - namely, autoantibodies that target the C3 or C5 convertases. These autoantibodies drive complement dysregulation by increasing the half-life of these vital but normally short-lived enzymes. Genetic variation in complement-related genes is a less frequent cause. No disease-specific treatments are available, although immunosuppressive agents and terminal complement pathway blockers are helpful in some patients. Unfortunately, no treatment is universally effective or curative. In aggregate, the limited data on renal transplantation point to a high risk of disease recurrence (both DDD and C3GN) in allograft recipients. Clinical trials are underway to test the efficacy of several first-generation drugs that target the alternative complement pathway.

Unraveling the Molecular Mechanisms Underlying Complement Dysregulation by Nephritic Factors in C3G and IC-MPGN

Membranoproliferative glomerulonephritis (MPGN) was recently classified as C3 glomerulopathies (C3G), and immune-complex (IC) mediated MPGN. Dysregulation of the complement alternative pathway, driven by acquired and/or genetic defects, plays a pathogenetic role in C3G. However, alternative pathway abnormalities were also found in IC-MPGN. The most common acquired drivers are the C3 nephritic factors (C3NeFs), heterogeneous autoantibodies that stabilize the C3 convertase, C3bBb. C3NeFs are traditionally detected by hemolytic assays based on sheep erythrocyte lysis, which however do not provide a direct molecular estimation of C3bBb formation and decay. We set up a microplate/western blot assay that specifically detects and quantifies C3bBb, and its precursor, the C3 proconvertase C3bB, to investigate the complex mechanistic effects of C3NeFs from patients with primary IC-MPGN (n = 13) and C3G (n = 13). In the absence of properdin, 9/26 patients had C3NeF IgGs stabilizing C3bBb against spontaneous and FH-accelerated decay. In the presence of properdin the IgGs of all but one patient had C3bBb-stabilizing activity. Properdin-independent C3NeFs were identified mostly in DDD patients, while properdin-dependent C3NeFs associated with either C3GN or IC-MPGN and with higher incidence of nephrotic syndrome. When we grouped patients based on our recent cluster analysis, patients in cluster 3, with highly electron-dense intramembranous deposits, low C3, and mostly normal sC5b-9 levels, had a higher prevalence of properdin-independent C3NeFs than patients in clusters 1 and 2. Conversely, about 70% of cluster 1 and 2 patients, with subendothelial, subepithelial, and mesangial deposits, low C3 levels and high sC5b-9 levels, had properdin-dependent C3NeFs. The flexibility of the assay allowed us to get deep insights into C3NeF mechanisms of action, showing that: (1) most C3NeFs bind strongly and irreversibly to C3 convertase; (2) C3NeFs and FH recognize different epitopes in C3 convertase; (3) C3NeFs bind rapidly to C3 convertase and antagonize the decay accelerating activity of FH on newly formed complexes; (4) C3NeFs do not affect formation and stability of the C3 proconvertase. Thus, our study provides a molecular approach to detecting and characterizing C3NeFs. The results highlight different mechanisms of complement dysregulation resulting in different complement profiles and patterns of glomerular injury, and this may have therapeutic implications.

Statistical Validation of Rare Complement Variants Provides Insights into the Molecular Basis of Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy

Atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G) are associated with dysregulation and overactivation of the complement alternative pathway. Typically, gene analysis for aHUS and C3G is undertaken in small patient numbers, yet it is unclear which genes most frequently predispose to aHUS or C3G. Accordingly, we performed a six-center analysis of 610 rare genetic variants in 13 mostly complement genes (CFH, CFI, CD46, C3, CFB, CFHR1, CFHR3, CFHR4, CFHR5, CFP, PLG, DGKE, and THBD) from >3500 patients with aHUS and C3G. We report 371 novel rare variants (RVs) for aHUS and 82 for C3G. Our new interactive Database of Complement Gene Variants was used to extract allele frequency data for these 13 genes using the Exome Aggregation Consortium server as the reference genome. For aHUS, significantly more protein-altering rare variation was found in five genes CFH, CFI, CD46, C3, and DGKE than in the Exome Aggregation Consortium (allele frequency < 0.01%), thus correlating these with aHUS. For C3G, an association was only found for RVs in C3 and the N-terminal C3b-binding or C-terminal nonsurface-associated regions of CFH In conclusion, the RV analyses showed nonrandom distributions over the affected proteins, and different distributions were observed between aHUS and C3G that clarify their phenotypes.

Factor H Competitor Generated by Gene Conversion Events Associates with Atypical Hemolytic Uremic Syndrome

Atypical hemolytic uremic syndrome (aHUS), a rare form of thrombotic microangiopathy caused by complement pathogenic variants, mainly affects the kidney microvasculature. A retrospective genetic analysis in our aHUS cohort (n=513) using multiple ligation probe amplification uncovered nine unrelated patients carrying a genetic abnormality in the complement factor H related 1 gene (CFHR1) that originates by recurrent gene conversion events between the CFH and CFHR1 genes. The novel CFHR1 mutants encode an FHR-1 protein with two amino acid substitutions, L290S and A296V, converting the FHR-1 C terminus into that of factor H (FH). Next-generation massive-parallel DNA sequencing (NGS) analysis did not detect these genetic abnormalities. In addition to the CFHR1 mutant, six patients carried the previously uncharacterized CFH-411T variant. In functional analyses, the mutant FHR-1 protein strongly competed the binding of FH to cell surfaces, impairing complement regulation, whereas the CFH-411T polymorphism lacked functional consequences. Carriers of the CFHR1 mutation presented with severe aHUS during adulthood; 57% of affected women in this cohort presented during the postpartum period. Analyses in patients and unaffected carriers showed that FH plasma levels determined by the nonmutated chromosome modulate disease penetrance. Crucially, in the activated endothelial (HMEC-1) cell assay, reduced FH plasma levels produced by the nonmutated chromosome correlated inversely with impairment of complement regulation, measured as C5b-9 deposition. Our data advance understanding of the genetic complexities underlying aHUS, illustrate the importance of performing functional analysis, and support the use of complementary assays to disclose genetic abnormalities not revealed by current NGS analysis.

In-vitro and in-vivo consequences of mutations in the von Willebrand factor cleaving protease ADAMTS13 in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disease characterized by microvascular thrombosis, often associated with deficiency of the von Willebrand factor (VWF) cleaving protease ADAMTS13.We investigated the spectrum of ADAMTS13 gene mutations in patients with TTP and congenital ADAMTS13 deficiency to establish the consequences on ADAMTS13 processing and activity. We describe five missense (V88M, G1239V, R1060W, R1123C and R1219W), 1 nonsense (W1016Stop) and 1 insertion (82_83insT) mutations. In two patients no mutation was identified despite undetectable protease activity. Expression in HEK293 mammalian cells (V88M, G1239V, R1123C and R1219W) documented that three missense mutants were not secreted, whereas theV88M was secreted at low levels and with reduced activity. We also provide evidence that impaired secretion of ADAMTS13 mutants observed in vitro translates into severely reduced ADAMTS13 antigen levels in patients in vivo. To evaluate whether the small amounts of mutant protease present in the circulation of patients had VWF cleaving activity, WT and mutant rADAMTS13 were stably expressed in Drosophila S2 cells under the influence of the Drosophila BiP protein signal sequence, which allows protein secretion. Drosophila expression system showed a 40–60% protease activity in the mutants. Several single nucleotide polymorphisms (SNPs) within exons and intron boundaries were found in patients, suggesting that the interplay of SNPs could at least in part account for ADAMTS13 functional abnormalities in patients without mutations. In conclusion, defective secretion and impaired activity of the mutants concur to determine an almost complete deficiency of ADAMTS13 activity in patients with a homozygous or two heterozygous ADAMTS13 mutations.

Human mesenchymal stromal cells transplanted into mice stimulate renal tubular cells and enhance mitochondrial function

Mesenchymal stromal cells (MSCs) are renoprotective and drive regeneration following injury, although cellular targets of such an effect are still ill-defined. Here, we show that human umbilical cord (UC)-MSCs transplanted into mice stimulate tubular cells to regain mitochondrial mass and function, associated with enhanced microtubule-rich projections that appear to mediate mitochondrial trafficking to create a reparative dialogue among adjacent tubular cells. Treatment with UC-MSCs in mice with cisplatin-induced acute kidney injury (AKI) regulates mitochondrial biogenesis in proximal tubuli by enhancing PGC1α expression, NAD⁺ biosynthesis and Sirtuin 3 (SIRT3) activity, thus fostering antioxidant defenses and ATP production. The functional role of SIRT3 in tubular recovery is highlighted by data that in SIRT3-deficient mice with AKI, UC-MSC treatment fails to induce renoprotection. These data document a previously unrecognized mechanism through which UC-MSCs facilitate renal repair, so as to induce global metabolic reprogramming of damaged tubular cells to sustain energy supply.

Mesenchymal stromal cells drive renal regeneration following injury. Here, the authors show that human mesenchymal stromal cells, when transplanted into mice with acute kidney injury, stimulate renal tubular cell growth and enhance mitochondrial function via SIRT3.

Cluster Analysis Identifies Distinct Pathogenetic Patterns in C3 Glomerulopathies/Immune Complex-Mediated Membranoproliferative GN

Membranoproliferative GN (MPGN) was recently reclassified as alternative pathway complement-mediated C3 glomerulopathy (C3G) and immune complex-mediated membranoproliferative GN (IC-MPGN). However, genetic and acquired alternative pathway abnormalities are also observed in IC-MPGN. Here, we explored the presence of distinct disease entities characterized by specific pathophysiologic mechanisms. We performed unsupervised hierarchical clustering, a data-driven statistical approach, on histologic, genetic, and clinical data and data regarding serum/plasma complement parameters from 173 patients with C3G/IC-MPGN. This approach divided patients into four clusters, indicating the existence of four different pathogenetic patterns. Specifically, this analysis separated patients with fluid-phase complement activation (clusters 1-3) who had low serum C3 levels and a high prevalence of genetic and acquired alternative pathway abnormalities from patients with solid-phase complement activation (cluster 4) who had normal or mildly altered serum C3, late disease onset, and poor renal survival. In patients with fluid-phase complement activation, those in clusters 1 and 2 had massive activation of the alternative pathway, including activation of the terminal pathway, and the highest prevalence of subendothelial deposits, but those in cluster 2 had additional activation of the classic pathway and the highest prevalence of nephrotic syndrome at disease onset. Patients in cluster 3 had prevalent activation of C3 convertase and highly electron-dense intramembranous deposits. In addition, we provide a simple algorithm to assign patients with C3G/IC-MPGN to specific clusters. These distinct clusters may facilitate clarification of disease etiology, improve risk assessment for ESRD, and pave the way for personalized treatment.

Extracellular vesicles derived from T regulatory cells suppress T cell proliferation and prolong allograft survival

We have previously shown that rat allogeneic DC, made immature by adenoviral gene transfer of the dominant negative form of IKK2, gave rise in-vitro to a unique population of CD4⁺CD25⁻ regulatory T cells (dnIKK2-Treg). These cells inhibited Tcell response in-vitro, without needing cell-to-cell contact, and induced kidney allograft survival prolongation in-vivo. Deep insight into the mechanisms behind dnIKK2-Treg-induced suppression of Tcell proliferation remained elusive. Here we document that dnIKK2-Treg release extracellular vesicles (EV) riched in exosomes, fully accounting for the cell-contact independent immunosuppressive activity of parent cells. DnIKK2-Treg-EV contain a unique molecular cargo of specific miRNAs and iNOS, which, once delivered into target cells, blocked cell cycle progression and induced apoptosis. DnIKK2-Treg-EV-exposed T cells were in turn converted into regulatory cells. Notably, when administered in-vivo, dnIKK2-Treg-EV prolonged kidney allograft survival. DnIKK2-Treg-derived EV could be a tool for manipulating the immune system and for discovering novel potential immunosuppressive molecules in the context of allotransplantation.