Treatment of Congenital Thrombotic Thrombocytopenic Purpura With Eculizumab

A compound heterozygous ADAMTS13 mutation causes congenital thrombotic thrombocytopenic purpura: a case report

Congenital thrombotic thrombocytopenic purpura (cTTP) is a thrombotic microangiopathy (TMA) characterized by severe hereditary ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs 13) deficiency caused by ADAMTS13 mutations. This rare autosomal recessive genetic disorder is often misdiagnosed as immune thrombocytopenia (ITP) or hemolytic uremic syndrome (HUS). Here, we report a 21-year-old male cTTP patient with a compound heterozygous ADAMTS13 mutation. The patient was admitted for acute thrombocytopenia, with a 5-year history of chronic thrombocytopenia and 1 month of renal dysfunction. Initially diagnosed with ITP, he was treated with immunosuppressive therapy, including glucocorticoids and intravenous immunoglobulin, which provided temporary relief but failed to prevent recurrent thrombocytopenia. Ultimately, cTTP was confirmed by the low ADAMTS13 0% activity and two heterozygous variants (c.1335del and c.1045C > T) in the ADAMTS13 gene, and the patient received prophylactic fresh-frozen plasma (FFP) infusions every 2-3 weeks regularly. Interestingly, the patient also exhibited elevated sC5b-9 levels during the acute phase, necessitating differentiation from HUS. This report highlights a cTTP caused by a compound heterozygous ADAMTS13 mutation, although its pathogenesis requires further investigation. Given the atypical clinical manifestations of cTTP, it is necessary to conduct ADAMTS13 activity and even genetic testing in patients with recurrent thrombocytopenia and end-organ damage.

Is Endothelial Activation a Critical Event in Thrombotic Thrombocytopenic Purpura?

Thrombotic thrombocytopenic purpura (TTP) is a severe thrombotic microangiopathy. The current pathophysiologic paradigm suggests that the ADAMTS13 deficiency leads to Ultra Large-Von Willebrand Factor multimers accumulation with generation of disseminated microthrombi. Nevertheless, the role of endothelial cells in this pathology remains an issue. In this review, we discuss the various clinical, in vitro and in vivo experimental data that support the important role of the endothelium in this pathology, suggesting that ADAMTS13 deficiency may be a necessary but not sufficient condition to induce TTP. The "second hit" model suggests that in TTP, in addition to ADAMTS13 deficiency, endogenous or exogenous factors induce endothelial activation affecting mainly microvascular cells. This leads to Weibel-Palade bodies degranulation, resulting in UL-VWF accumulation in microcirculation. This endothelial activation seems to be worsened by various amplification loops, such as the complement system, nucleosomes and free heme.

Soluble Membrane Attack Complex: Biochemistry and Immunobiology

The soluble membrane attack complex (sMAC, a.k.a., sC5b-9 or TCC) is generated on activation of complement and contains the complement proteins C5b, C6, C7, C8, C9 together with the regulatory proteins clusterin and/or vitronectin. sMAC is a member of the MACPF/cholesterol-dependent-cytolysin superfamily of pore-forming molecules that insert into lipid bilayers and disrupt cellular integrity and function. sMAC is a unique complement activation macromolecule as it is comprised of several different subunits. To date no complement-mediated function has been identified for sMAC. sMAC is present in blood and other body fluids under homeostatic conditions and there is abundant evidence documenting changes in sMAC levels during infection, autoimmune disease and trauma. Despite decades of scientific interest in sMAC, the mechanisms regulating its formation in healthy individuals and its biological functions in both health and disease remain poorly understood. Here, we review the structural differences between sMAC and its membrane counterpart, MAC, and examine sMAC immunobiology with respect to its presence in body fluids in health and disease. Finally, we discuss the diagnostic potential of sMAC for diagnostic and prognostic applications and potential utility as a companion diagnostic.

Verotoxin Receptor-Based Pathology and Therapies

Verotoxin, VT (aka Shiga toxin,Stx) is produced by enterohemorrhagic E. coli (EHEC) and is the key pathogenic factor in EHEC-induced hemolytic uremic syndrome (eHUS-hemolytic anemia/thrombocytopenia/glomerular infarct) which can follow gastrointestinal EHEC infection, particularly in children. This AB5 subunit toxin family bind target cell globotriaosyl ceramide (Gb₃), a glycosphingolipid (GSL) (aka CD77, pk blood group antigen) of the globoseries of neutral GSLs, initiating lipid raft-dependent plasma membrane Gb₃ clustering, membrane curvature, invagination, scission, endosomal trafficking, and retrograde traffic via the TGN to the Golgi, and ER. In the ER, A/B subunits separate and the A subunit hijacks the ER reverse translocon (dislocon-used to eliminate misfolded proteins-ER associated degradation-ERAD) for cytosolic access. This property has been used to devise toxoid-based therapy to temporarily block ERAD and rescue the mutant phenotype of several genetic protein misfolding diseases. The A subunit avoids cytosolic proteosomal degradation, to block protein synthesis via its RNA glycanase activity. In humans, Gb₃ is primarily expressed in the kidney, particularly in the glomerular endothelial cells. Here, Gb₃ is in lipid rafts (more ordered membrane domains which accumulate GSLs/cholesterol) whereas renal tubular Gb₃ is in the non-raft membrane fraction, explaining the basic pathology of eHUS (glomerular endothelial infarct). Females are more susceptible and this correlates with higher renal Gb₃ expression. HUS can be associated with encephalopathy, more commonly following verotoxin 2 exposure. Gb₃ is expressed in the microvasculature of the brain. All members of the VT family bind Gb₃, but with varying affinity. VT2e (pig edema toxin) binds Gb₄ preferentially. Verotoxin-specific therapeutics based on chemical analogs of Gb₃, though effective in vitro, have failed in vivo. While some analogs are effective in animal models, there are no good rodent models of eHUS since Gb₃ is not expressed in rodent glomeruli. However, the mouse mimics the neurological symptoms more closely and provides an excellent tool to assess therapeutics. In addition to direct cytotoxicity, other factors including VT–induced cytokine release and aberrant complement cascade, are now appreciated as important in eHUS. Based on atypical HUS therapy, treatment of eHUS patients with anticomplement antibodies has proven effective in some cases. A recent switch using stem cells to try to reverse, rather than prevent VT induced pathology may prove a more effective methodology.

Atypical hemolytic uremic syndrome and complement blockade: established and emerging uses of complement inhibition

PURPOSE OF REVIEW

Atypical hemolytic uremic syndrome (aHUS) is a diagnosis that has captured the interest of specialists across multiple fields. The hallmark features of aHUS are microangiopathic hemolysis and thrombocytopenia, which creates a diagnostic dilemma because of the occurrence of these findings in a wide variety of clinical disorders.

RECENT FINDINGS

In most of the instances, aHUS is a diagnosis of exclusion after ruling out causes such as Shigella toxin, acquired or genetic a disintegrin and metalloproteinase thrombospondin motif 13 deficiency (thrombotic thrombocytopenic purpura), and vitamin B12 deficiency. In the purest sense, aHUS is a genetic condition that is activated (or unmasked) by an environmental exposure. However, it is now evident that complement activation is a feature of many diseases. Variants in complement regulatory genes predispose to microangiopathic hemolysis in many rheumatologic, oncologic, and drug-induced vascular, obstetric, peritransplant, and infectious syndromes.

SUMMARY

Many 'hemolysis syndromes' overlap clinically with aHUS, and we review the literature on the treatment of these conditions with complement inhibition. New reports on the treatment of C3 glomerulopathy, Shiga toxin-related classic hemolytic uremic syndrome, and medication-related thrombotic microangiopathy will be reviewed as well.

Synergistic effects of ADAMTS13 deficiency and complement activation in pathogenesis of thrombotic microangiopathy

Severe deficiency of plasma ADAMTS13 activity is the primary cause of thrombotic thrombocytopenic purpura (TTP) whereas overwhelming activation of complement via an alternative pathway results in atypical hemolytic uremic syndrome (aHUS), the prototypes of thrombotic microangiopathy (TMA). However, clinical and pathogenic distinctions between TTP and aHUS are often quite challenging. Clinical reports have suggested that complement activation may play a role in the development of TTP, which is caused by severe deficiency of plasma ADAMTS13 activity. However, the experimental evidence to support this hypothesis is still lacking. Here, we show that mice with either Adamts13 -/- or a heterozygous mutation of complement factor H (cfh) at amino acid residue of 1206 (ie, cfh W/R ) alone remain asymptomatic despite the presence of occasional microvascular thrombi in various organ tissues. However, mice carrying both Adamts13 -/- and cfh W/R exhibit thrombocytopenia, low haptoglobin, increased fragmentation of erythrocytes in peripheral blood smear, increased plasma levels of lactate dehydrogenase activity, blood urea nitrogen, and creatinine, as well as an increased mortality rate, consistent with the development of TMA. Moreover, mice with a homozygous mutation of cfh (ie, cfh R/R ) with or without Adamts13 -/- developed severe TMA. The mortality rate in mice with Adamts13 -/- cfh R/R was significantly higher than that in mice with cfh R/R alone. Histological and immunohistochemical analyses demonstrated the presence of disseminated platelet-rich thrombi in terminal arterioles and capillaries of major organ tissues in these mice that were either euthanized or died. Together, our results support a synergistic effect of severe ADAMTS13 deficiency and complement activation in pathogenesis of TMA in mice.

Where are we with haemolytic uremic syndrome?

Haemolytic uremic syndrome (HUS) is characterised by microangiopathic haemolytic anaemia with acute kidney injury. It is currently classified into two main categories: Shiga-toxin producing E. coli-hemolytic uremic syndrome (STEC-HUS) and atypical haemolytic uremic syndrome (aHUS). Endothelial cell damage is the common pathway in HUS to developing thrombotic microangiopathy. Atypical HUS includes primary, secondary and aHUS due to metabolic diseases. In the majority of aHUS cases, hyperactivity of the alternative complement pathway plays a central role. Therefore, treatment is based on complement inhibitors like eculizumab, a drug that has revolutionised the natural history of the disease. Relapses are frequent after kidney transplant and thus confer a poor prognosis.

Complement activation associated with ADAMTS13 deficiency may contribute to the characteristic glomerular manifestations in Upshaw-Schulman syndrome

INTRODUCTION

Upshaw-Schulman syndrome (USS) is a congenital form of thrombotic thrombocytopenic purpura (TTP) associated with loss-of-function mutations in the ADAMTS13 gene, possibly leading to aberrant complement activation and vascular injury. However, USS is extremely rare, and there have been no systematic studies correlating histopathological severity with local ADAMTS13 expression and complement activation.

MATERIALS AND METHODS

Here, we compared histopathological features, ADAMTS13 immunoreactivity, and immunoreactivity of complement proteins C4d and C5b-9 among renal biopsy tissues from five USS cases, ten acquired TTP cases, and eleven controls.

RESULTS

Pathological analysis revealed chronic glomerular sclerotic changes in the majority of USS cases (4 of 5), with minor glomerular pathology in the remaining case. In two of these four severe cases, more than half of the glomerular segmental sclerosis area was localized in the perihilar region. The average number of ADAMTS13-positive cells per glomerulus was significantly lower in USS cases than controls (p < 0.05). Conversely, C4d staining was significantly more prevalent in the glomerular capillary walls of USS cases than controls (p < 0.05), while C5b-9 staining did not differ significantly among groups.

CONCLUSIONS

These findings suggest that the severity of glomerular injury in USS is associated with deficient ADAMTS13 expression and local complement activation, particularly in vascular regions with higher endothelial shear stress. We suggest that C4d immunostaining provides evidence for complement-mediated glomerular damage in USS.

Acquired thrombotic thrombocytopenic purpura with isolated CFHR3/1 deletion-rapid remission following complement blockade

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is caused by the abundance of uncleaved ultralarge von Willebrand factor multimers (ULvWF) due to acquired (autoantibody-mediated) or congenital vWF protease ADAMTS13 deficiency. Current treatment recommendations include plasma exchange therapy and immunosuppression for the acquired form and (fresh) frozen plasma for congenital TTP.

CASE-DIAGNOSIS/TREATMENT

A previously healthy, 3-year-old boy presented with acute microangiopathic hemolytic anemia, thrombocytopenia, erythrocyturia and mild proteinuria, but normal renal function, and elevated circulating sC5b-9 levels indicating complement activation. He was diagnosed with atypical hemolytic uremic syndrome and treated with a single dose of eculizumab, followed by prompt resolution of all hematological parameters. However, undetectably low plasma ADAMTS13 activity in the pre-treatment sample, associated with inhibitory ADAMTS13 antibodies, subsequently changed the diagnosis to acquired TTP. vWF protease activity normalized within 15 months without further treatment, and the patient remained in long-term clinical and laboratory remission. Extensive laboratory workup revealed a homozygous deletion of CFHR3/1 negative for anti-CFH antibodies, but no mutations of ADAMTS13, (other) alternative pathway of complement regulators or coagulation factors.

CONCLUSIONS

This case, together with a previous report of a boy with congenital TTP (Pecoraro et al. Am J Kidney Dis 66:1067, 2015), strengthens evolving in-vitro and ex-vivo evidence that ULvWF interferes with complement regulation and contributes to the TTP phenotype. Comprehensive, prospective complement studies in patients with TTP may lead to a better pathophysiological understanding and novel treatment approaches for acquired or congenital forms.

Atypical Hemolytic Uremic Syndrome

Atypical hemolytic uremic syndrome is a rare life-threatening disease of unregulated complement activation. Untreated, the prognosis is generally poor; more than one-half of patients die or develop end-stage renal disease within 1 year. Atypical hemolytic uremic syndrome is characterized by thrombotic microangiopathy with evidence of hemolysis, thrombocytopenia, and renal impairment. This systemic disease affects the kidneys, brain, heart, lungs, gastrointestinal tract, pancreas, and skin. Acquired and genetic abnormalities of complement regulation may be identified in approximately 70% of patients. Plasma therapy is generally ineffective. Eculizumab blocks terminal complement activation, prevents complement-mediated organ damage, and is currently recommended as front-line therapy.

Acquired thrombotic thrombocytopenic purpura and atypical hemolytic uremic syndrome successfully treated with eculizumab

Acquired idiopathic thrombotic thrombocytopenic purpura is a life-threatening disease with a mortality of up to 90%, if not promptly recognized and treated. We report a 64-year-old woman with this condition who presented with left-sided weakness and seizure-like activity preceded by headache and easy bruising. She did not achieve optimal response to plasma exchange, corticosteroids, rituximab, and vincristine. We initiated treatment with eculizumab, following which she had durable remission that continued for 30 months after discontinuation of the drug. We later found that our patient has homozygous deletion in two closely related genes, complement factor H-related 1 and complement factor H-related 3.

Eculizumab in secondary atypical haemolytic uraemic syndrome

Background. Complement dysregulation occurs in thrombotic microangiopathies (TMAs) other than primary atypical haemolytic uraemic syndrome (aHUS). A few of these patients have been reported previously to be successfully treated with eculizumab.

Methods. We identified 29 patients with so-called secondary aHUS who had received eculizumab at 11 Spanish nephrology centres. Primary outcome was TMA resolution, defined by a normalization of platelet count (>150 × 10⁹/L) and haemoglobin, disappearance of all the markers of microangiopathic haemolytic anaemia (MAHA), and improvement of renal function, with a ≥25% reduction of serum creatinine from the onset of eculizumab administration.

Results. Twenty-nine patients with secondary aHUS (15 drug-induced, 8 associated with systemic diseases, 2 with postpartum, 2 with cancer-related, 1 associated with acute humoral rejection and 1 with intestinal lymphangiectasia) were included in this study. The reason to initiate eculizumab treatment was worsening of renal function and persistence of TMA despite treatment of the TMA cause and plasmapheresis. All patients showed severe MAHA and renal function impairment (14 requiring dialysis) prior to eculizumab treatment and 11 presented severe extrarenal manifestations. A rapid resolution of the TMA was observed in 20 patients (68%), 15 of them showing a ≥50% serum creatinine reduction at the last follow-up. Comprehensive genetic and molecular studies in 22 patients identified complement pathogenic variants in only 2 patients. With these two exceptions, eculizumab was discontinued, after a median of 8 weeks of treatment, without the occurrence of aHUS relapses.

Conclusion. Short treatment with eculizumab can result in a rapid improvement of patients with secondary aHUS in whom TMA has persisted and renal function worsened despite treatment of the TMA-inducing condition.

Complement C5-inhibiting therapy for the thrombotic microangiopathies: accumulating evidence, but not a panacea

Thrombotic microangiopathy (TMA), characterized by organ injury occurring consequent to severe endothelial damage, can manifest in a diverse range of diseases. In complement-mediated atypical haemolytic uraemic syndrome (aHUS) a primary defect in complement, such as a mutation or autoantibody leading to over activation of the alternative pathway, predisposes to the development of disease, usually following exposure to an environmental trigger. The elucidation of the pathogenesis of aHUS resulted in the successful introduction of the complement inhibitor eculizumab into clinical practice. In other TMAs, although complement activation may be seen, its role in the pathogenesis remains to be confirmed by an interventional trial. Although many case reports in TMAs other than complement-mediated aHUS hint at efficacy, publication bias, concurrent therapies and in some cases the self-limiting nature of disease make broader interpretation difficult. In this article, we will review the evidence for the role of complement inhibition in complement-mediated aHUS and other TMAs.

Successful kidney transplantation in a patient with congenital thrombotic thrombocytopenic purpura (Upshaw-Schulman syndrome)

BACKGROUND

Congenital thrombotic thrombocytopenic purpura (TTP) may not be recognized until organ failure related to the microvascular thrombosis occurs. Kidney failure may be the initial presenting clinical feature. Kidney transplantation has been contraindicated because of the assumption that the continuing microvascular thrombosis will cause inevitable graft failure.

CASE REPORT

We report a 48-year-old nulliparous woman who presented with end-stage kidney disease that was attributed to hypertension. Her past history included a thromboembolic stroke at age 32, for which she was placed on permanent anticoagulation. Immediately after living unrelated-donor kidney transplant, she developed severe hemolysis and acute decline in urine output for which she received red blood cell and platelet transfusions and an infusion of eculizumab (1200 mg). She promptly responded and was discharged on her fifth postoperative day with a serum creatinine level of 1.0 mg/dL. Two weeks later, thrombocytopenia and hemolysis recurred. By this time, undetectable ADAMTS13 activity (<5%) with no demonstrable inhibitor had been reported. She responded rapidly to plasma infusions. Genetic analysis confirmed the diagnosis of congenital TTP, documenting known pathogenic mutations in each of the ADAMTS13 genes. She continued to receive twice-monthly infusions for 4 months. Surveillance kidney biopsies at 6 and 12 months posttransplant demonstrated no evidence of thrombotic microangiopathy or graft rejection. After 2 years of follow-up her creatinine remains stable at 1.0 mg/dL (estimated glomerular filtration rate, 65 mL/min/1.73 m² ).

CONCLUSION

Our experience suggests that kidney transplantation may be an appropriate management for carefully selected patients with congenital TTP who develop end-stage renal disease.

Interaction between Multimeric von Willebrand Factor and Complement: A Fresh Look to the Pathophysiology of Microvascular Thrombosis

von Willebrand factor (VWF), a multimeric protein with a central role in hemostasis, has been shown to interact with complement components. However, results are contrasting and inconclusive. By studying 20 patients with congenital thrombotic thrombocytopenic purpura (cTTP) who cannot cleave VWF multimers because of genetic ADAMTS13 deficiency, we investigated the mechanism through which VWF modulates complement and its pathophysiological implications for human diseases. Using assays of ex vivo serum-induced C3 and C5b-9 deposits on endothelial cells, we documented that in cTTP, complement is activated via the alternative pathway (AP) on the cell surface. This abnormality was corrected by restoring ADAMTS13 activity in cTTP serum, which prevented VWF multimer accumulation on endothelial cells, or by an anti-VWF Ab. In mechanistic studies we found that VWF interacts with C3b through its three type A domains and initiates AP activation, although assembly of active C5 convertase and formation of the terminal complement products C5a and C5b-9 occur only on the VWF-A2 domain. Finally, we documented that in the condition of ADAMTS13 deficiency, VWF-mediated formation of terminal complement products, particularly C5a, alters the endothelial antithrombogenic properties and induces microvascular thrombosis in a perfusion system. Altogether, the results demonstrated that VWF provides a platform for the activation of the AP of complement, which profoundly alters the phenotype of microvascular endothelial cells. These findings link hemostasis-thrombosis with the AP of complement and open new therapeutic perspectives in cTTP and in general in thrombotic and inflammatory disorders associated with endothelium perturbation, VWF release, and complement activation.

HUS and atypical HUS

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy characterized by intravascular hemolysis, thrombocytopenia, and acute kidney failure. HUS is usually categorized as typical, caused by Shiga toxin-producing Escherichia coli (STEC) infection, as atypical HUS (aHUS), usually caused by uncontrolled complement activation, or as secondary HUS with a coexisting disease. In recent years, a general understanding of the pathogenetic mechanisms driving HUS has increased. Typical HUS (ie, STEC-HUS) follows a gastrointestinal infection with STEC, whereas aHUS is associated primarily with mutations or autoantibodies leading to dysregulated complement activation. Among the 30% to 50% of patients with HUS who have no detectable complement defect, some have either impaired diacylglycerol kinase ε (DGKε) activity, cobalamin C deficiency, or plasminogen deficiency. Some have secondary HUS with a coexisting disease or trigger such as autoimmunity, transplantation, cancer, infection, certain cytotoxic drugs, or pregnancy. The common pathogenetic features in STEC-HUS, aHUS, and secondary HUS are simultaneous damage to endothelial cells, intravascular hemolysis, and activation of platelets leading to a procoagulative state, formation of microthrombi, and tissue damage. In this review, the differences and similarities in the pathogenesis of STEC-HUS, aHUS, and secondary HUS are discussed. Common for the pathogenesis seems to be the vicious cycle of complement activation, endothelial cell damage, platelet activation, and thrombosis. This process can be stopped by therapeutic complement inhibition in most patients with aHUS, but usually not those with a DGKε mutation, and some patients with STEC-HUS or secondary HUS. Therefore, understanding the pathogenesis of the different forms of HUS may prove helpful in clinical practice.

Glomerular C4d deposits can mark structural capillary wall remodelling in thrombotic microangiopathy and transplant glomerulopathy: C4d beyond active antibody-mediated injury: a retrospective study

Peritubular capillary C4d (ptc-C4d) usually marks active antibody-mediated rejection, while pseudolinear glomerular capillary C4d (GBM-C4d) is of undetermined diagnostic significance, especially when seen in isolation without concurrent ptc-C4d. We correlated GBM-C4d with structural GBM abnormalities and active antibody-mediated rejection in 319 renal transplant and 35 control native kidney biopsies. In kidney transplants, ptc-C4d was associated with GBM-C4d in 97% by immunofluorescence microscopy (IF) and 61% by immunohistochemistry (IHC; P < 0.001). Transplant glomerulopathy correlated with GBM-C4d (P < 0.001) and presented with isolated GBM-C4d lacking ptc-C4d in 69% by IF and 40% by IHC. Strong isolated GBM-C4d was found post year-1 in repeat biopsies with transplant glomerulopathy. GBM-C4d staining intensity correlated with Banff cg scores (rs = 0.45, P < 0.001). Stepwise exclusion and multivariate logistic regression corrected for active antibody-mediated rejection showed significant correlations between GBM duplication and GBM-C4d (P = 0.001). Native control biopsies with thrombotic microangiopathies demonstrated GBM-C4d in 92% (IF, P < 0.001) and 35% (IHC). In conclusion, pseudolinear GBM-C4d staining can reflect two phenomena: (i) structural GBM changes with duplication in native and transplant kidneys or (ii) active antibody-mediated rejection typically accompanied by ptc-C4d. While ptc-C4d is a dynamic 'etiologic' marker for active antibody-mediated rejection, isolated strong GBM-C4d can highlight architectural glomerular remodelling.

Complement in disease: a defence system turning offensive

Although the complement system is primarily perceived as a host defence system, a more versatile, yet potentially more harmful side of this innate immune pathway as an inflammatory mediator also exists. The activities that define the ability of the complement system to control microbial threats and eliminate cellular debris - such as sensing molecular danger patterns, generating immediate effectors, and extensively coordinating with other defence pathways - can quickly turn complement from a defence system to an aggressor that drives immune and inflammatory diseases. These host-offensive actions become more pronounced with age and are exacerbated by a variety of genetic factors and autoimmune responses. Complement can also be activated inappropriately, for example in response to biomaterials or transplants. A wealth of research over the past two decades has led to an increasingly finely tuned understanding of complement activation, identified tipping points between physiological and pathological behaviour, and revealed avenues for therapeutic intervention. This Review summarizes our current view of the key activating, regulatory, and effector mechanisms of the complement system, highlighting important crosstalk connections, and, with an emphasis on kidney disease and transplantation, discusses the involvement of complement in clinical conditions and promising therapeutic approaches.

The endothelium as the common denominator in malignant hypertension and thrombotic microangiopathy

The endothelium plays a pivotal role in vascular biology. The endothelium is the primary site of injury in thrombotic microangiopathies including malignant hypertension. Endothelial injury in thrombotic microangiopathies is the result of increased shear stress, toxins, and/or dysregulated complement activation. Endothelial injury can lead to microvascular thrombosis resulting in ischemia and organ dysfunction, the clinical hallmarks of thrombotic microangiopathies. Currently, available therapies target the underlying mechanisms that lead to endothelial injury in these conditions. Ongoing investigations aim at identifying drugs that protect the endothelium.