To the editor: Microvascular injury, including thrombotic microangiopathy, has been widely reported as a hallmark pathological feature of organ injury in the setting of COVID-191. Accumulating data suggest that complement activation is implicated in the pathogenesis of COVID-19, including endothelial cell damage2, 3, 4, 5. Some of these features are characteristic of atypical hemolytic uremic syndrome (aHUS), a prototypic disease of complement-mediated endothelial cell injury. We report the first case of aHUS relapse triggered by COVID-19. The patient, a 28-year-old white female, was diagnosed with aHUS at the age of three. Genetic analysis revealed the presence of a heterozygous pathogenic variant (R59Stop) in the membrane-cofactor protein (MCP)-encoding gene. Subsequently, she presented three aHUS relapses, at the age of 5, 20 and 27 years. The second relapse required dialysis before hematological and renal remission was obtained following 4 plasma exchanges and treatment with eculizumab for 12 months. The third relapse, triggered by an extra-uterine pregnancy, was treated with 3 months of eculizumab. Kidney biopsy performed at that time revealed moderate interstitial (20%) and glomerular (20 %) fibrosis. The patient had chronic kidney disease stage 3B (glomerular filtration rate of 42 ml/min/1/73m2, estimated using the MDRD formula) and hypertension requiring a treatment combining a calcium-channel blocker and an angiotensin-converting-enzyme inhibitor. In September 2020, she presented with fever, dysphagia, and headache. Clinical examination showed moderate fever, an erythematous throat, infra-centimetric cervical adenopathies and hypertension (150/105 mmHg). She had no lung involvement with a normal chest X-ray. Laboratory tests showed mechanical hemolytic anemia (hemoglobin 8.4 g/dl, lactate dehydrogenase level at 1.5 x upper limit of normal and undetectable haptoglobin), mild thrombocytopenia (platelet count, 106 G/L) acute kidney injury (serum creatinine 2.6 mg/dl vs 1.7 mg/dl at baseline) and significant proteinuria (protein-to-creatinine ratio of 0.21 g/mmol vs 0.05 at baseline). aHUS relapse was diagnosed, and the patient was admitted in our nephrology department. In the context of the re-emergence of the SARS-CoV-2 pandemic in our region, COVID-19 was suspected and confirmed by a positive PCR test in a nasopharyngeal swab. Additional workup showed a decreased C3 serum level (0.65 g/L; normal range 0.9-1.8) and a normal C4 level. Inflammatory markers were normal or moderately increased (C-reactive protein <0.4 mg/dL (<10), ferritin 392 μg/L (13 to 150), D-dimer 512 ng/mL (< 500), IL6 1.8 pg/mL (< 7)). Serum creatinine peaked at 2.9 mg/dl, and platelet count decreased to 88 G/L. Treatment with the C5 blocker eculizumab were immediately restarted, combined with penicillin prophylaxis and anticoagulation due to the increased risk of thrombosis in the setting of COVID-196. Seven days after the start of eculizumab, hematological parameters (platelet count 191 G/L) and renal function (serum creatinine 2.2 mg/dl) improved, and the patient was discharged from hospital. One month, after diagnosis, D-dimer level was normal (261 ng/ml), haptoglobin remained undetectable and mild decrease in C3 plasma level persisted (0.65 g/L). Renal function improved, but serum creatinine (2.0 mg/dl) had not returned to baseline values. It is well established that aHUS relapse may be precipitated by infections, including viral pathogens such as influenza or H1N1 virus 7 , 8 . This case is an illustration that the COVID-19 is to be added to the list of the potential triggers of aHUS relapse. In this setting, the deleterious effect of the coronavirus 19 may arise from (1) a direct toxic effect on endothelial cells, as suggested by autopsies studies 1 and/or (2) a complement activation with ultimately complement-mediated endothelial damage, most particularly in patients with a constitutional defect in complement regulation, as in the patient presented herein. Indeed, it has recently been shown that, in vitro, SARS-CoV-2 activates the complement alternative pathway via its spike surface protein 9 . Similarly, markers of complement activation, including soluble C5b-9, are increased in a significant proportion of COVID-19 patients and correlate to the severity and prognosis of the disease prognosis 4 , 5 . Furthermore, C3 deficiency protects against the development of SARS-Cov infection in mice. 10 . Finally, complement activation may also contribute to the hypercoagulable state in COVID-19 patients 3 . Our patient had a clinically mild form of COVID-19 and no marked systemic inflammation. Nevertheless, virus-driven complement activation did occur and was most probably over amplified in the absence of a tight control of the complement alternative pathway, leading to the development of thrombotic microangiopathy. However, this is to date the only reported case of aHUS relapse triggered by COVID-19, despite the worldwide spread of COVID-19 epidemics. Nevertheless, our observation underlines the need for a close monitoring of aHUS patients who discontinued eculizumab in the setting of COVID-19. It also a further indication that complement blockade should not be discontinued in aHUS during infectious episodes, COVID-19 not being an exception. Uncited reference 6..
