TO THE EDITOR:

Endothelial cell (EC) injury has emerged as a hallmark of infection resulting from severe respiratory coronavirus 2.1-3 Complement activation or dysregulation is another notable feature of COVID-19,4,5 and elevated plasma levels of sC5b-9, a marker of activation of the complement terminal pathway, have been reported in up to 2/3 of COVID-19 patients,6 and correlate with disease severity.4,6

Complement dysregulation is also a well-established pathogenic mechanism of a rare form of renal thrombotic microangiopathy (TMA), the atypical hemolytic uremic syndrome (aHUS), triggered by complement-induced EC damage.7 

To date, renal TMA has been very rarely documented in the COVID-19 setting.8,9 We report on 5 patients with COVID-19-associated renal TMA, among whom 4 tested patients carried complement genetic susceptibility factors for aHUS.

Patients with COVID-19-associated renal TMA were identified using the databases of the French HUS Registry and the Complement reference Centre in Lausanne. COVID-19 infection was diagnosed in all cases, based on a positive polymerase chain reaction test for severe respiratory coronavirus 2 in oropharyngeal swab samples. Renal TMA was defined by at least 3 of the following criteria: (1) thrombocytopenia (platelet count <150 g/L), (2) mechanical hemolytical anemia (hemoglobin <10 g/dL, lactate dehydrogenase serum level >upper limit of normal [ULN], undetectable haptoglobin, presence of schistocytes on blood smear), (3) acute kidney injury (serum creatinine and/or proteinuria/creatininuria >ULN for age or an increase >15% compared with baseline), and (4) features of TMA in kidney biopsy.

Measurement of plasma C3, C4, CH50, factors H (FH) and I (FI), sC5b-9, and CD46 expression on granulocytes and tests for anti-FH antibodies were performed as previously described.10 Screening for variants and complex rearrangements in complement factor H (CFH), complement factor I, membrane-cofactor protein, C3, and factor B genes was performed using next-generation sequencing and multiplex ligation-dependent probe amplification, as previously described.10 All patients gave informed consent for genetic testing.

Five adult patients with COVID-19-associated renal TMA were identified in 5 French and Swiss nephrology, renal transplantation, and internal medicine units (Table 1; supplemental Figure 1, available on the Blood Web site). None had a personal or familial history of aHUS. Three patients were renal transplant recipients and the cause of their end-stage kidney failure was focal segmental glomerulosclerosis, immunoglobulin A nephropathy (n = 1), and nephroangiosclerosis (n = 1). They presented with a COVID-19-associated renal TMA, 3.5 to 24 months after renal transplantation. At TMA onset, they were receiving tacrolimus (n = 3) and everolimus (n = 1) and none had donor-specific antibodies. All patients had mild respiratory symptoms of COVID-19 and only 1 required low-grade oxygen therapy. The interval between COVID-19 diagnosis and TMA diagnosis ranged from 0 to 30 days. Patients 1, 3, and 4 presented with severe hypertension (systolic 163-212 mm Hg; diastolic 92-130 mm Hg). In all cases, renal dysfunction was severe (serum creatinine, 2.3-10 mg/dL) and 3 patients required hemodialysis. Thrombocytopenia was profound with a platelet count ≤50 g/L in 4 patients. Three patients had extrarenal TMA manifestations: neurological symptoms (confusion, central facial palsy) in patient 2 and intestinal involvement (pain, diarrhea) in patients 3 and 4 (documented by intestinal biopsy disclosing capillary thrombi in patient 3). Renal biopsy performed in 3 patients during the acute phase disclosed typical features of TMA (supplemental Figure 2). In patient 3, a biopsy of the kidney transplant performed 3 weeks after TMA resolution showed duplication/wrinkling of the glomerular basement membrane. In all biopsies, no significant immune deposits were detected. All patients had detectable (>20%) ADAMTS13 activity, and polymerase chain reaction for Shiga toxin in stool was negative in tested patients 2 through 4. No patients had evidence of disseminated intravascular coagulation, with normal fibrinogen, prothrombin, and partial thromboplastin time. Tacrolimus trough levels were 20, 9.2, and 5.7 μg/L in patients 2, 4, and 5, respectively.

Table 1.

Characteristics of 5 patients with COVID-19-associated renal TMA

At TMA diagnosis
PtSex, ageNK//RT
(nephropathy/time from RT)
Time from COVID-19 diagnosis to TMASCr (mg/dL)Plt (g/L)Hb (g/dL)Hapto.
(g/L)
LDH
(×ULN)
Puria
(g/L)
Kidney biopsyCOVID-19 treatmentTMA treatmentFollow-upOutcome
M, 66 y NK 0 d 10 (HD) 50 9.2 <0.3 >ULN NA Glomerular and arteriolar thrombi and EC detachment.
Mild ATN. 
— — 3 mo HD 
M, 71 y RT
(NAS/18 mo) 
12 d 2.3 16 6.9 <0.3 1.7 Kidney biopsy performed 3 wk after TMA resolution Glomerulosclerosis. GBM duplication. Oxygen (3 L/min) PE (n = 4)
Eculizumab (day 4; n = 3)
Temporary discontinuation of tacrolimus/everolimus. 
1 mo SCr 1.7 mg/dL (baseline values) 
M, 35y NK 30 d 1.9/7.9 (HD) 11 9.7 <0.3 Oliguria Glomerular and arteriolar thrombi. Mild ATN. — PE (n = 11)
Eculizumab (day 13; n = 1) 
3 mo HD 
F, 26 y RT
(FSGS/3.5 mo) 
0 d 7.2 (HD) 22 7.2 <0.3 >ULN NA — — PE (n = 3)
Eculizumab (day 21; ongoing)
Temporary tacrolimus discontinuation.
Rituximab (n = 2) 
4 mo SCr 4.2 mg/dL
Eculizumab continued. 
F, 38 y RT
(IgAN/24 mo) 
10d 3.2 103 10.4 <0.3 1.6 0.8 Extensive EC detachment from GBM. Mesangiolysis. — Decrease in tacrolimus dosage. 6 mo SCr 1.8 mg/dL (baseline values) 
At TMA diagnosis
PtSex, ageNK//RT
(nephropathy/time from RT)
Time from COVID-19 diagnosis to TMASCr (mg/dL)Plt (g/L)Hb (g/dL)Hapto.
(g/L)
LDH
(×ULN)
Puria
(g/L)
Kidney biopsyCOVID-19 treatmentTMA treatmentFollow-upOutcome
M, 66 y NK 0 d 10 (HD) 50 9.2 <0.3 >ULN NA Glomerular and arteriolar thrombi and EC detachment.
Mild ATN. 
— — 3 mo HD 
M, 71 y RT
(NAS/18 mo) 
12 d 2.3 16 6.9 <0.3 1.7 Kidney biopsy performed 3 wk after TMA resolution Glomerulosclerosis. GBM duplication. Oxygen (3 L/min) PE (n = 4)
Eculizumab (day 4; n = 3)
Temporary discontinuation of tacrolimus/everolimus. 
1 mo SCr 1.7 mg/dL (baseline values) 
M, 35y NK 30 d 1.9/7.9 (HD) 11 9.7 <0.3 Oliguria Glomerular and arteriolar thrombi. Mild ATN. — PE (n = 11)
Eculizumab (day 13; n = 1) 
3 mo HD 
F, 26 y RT
(FSGS/3.5 mo) 
0 d 7.2 (HD) 22 7.2 <0.3 >ULN NA — — PE (n = 3)
Eculizumab (day 21; ongoing)
Temporary tacrolimus discontinuation.
Rituximab (n = 2) 
4 mo SCr 4.2 mg/dL
Eculizumab continued. 
F, 38 y RT
(IgAN/24 mo) 
10d 3.2 103 10.4 <0.3 1.6 0.8 Extensive EC detachment from GBM. Mesangiolysis. — Decrease in tacrolimus dosage. 6 mo SCr 1.8 mg/dL (baseline values) 

ATN, acute tubular necrosis; F, female; FSGS, focal segmental glomerulosclerosis; GBM, glomerular basement membrane; Hapto, haptoglobin; Hb, hemoglobin; HD, hemodialysis; IgAN, immunoglobulin A nephropathy; LDH, lactate dehydrogenase; M, male; NA, not available; NAS, nephroangiosclerosis; NK, native kidneys; PE, plasma exchange; Plt, platelet count; Pt, patient; Puria, proteinuria; RT, renal transplantation; SCr, serum creatinine; ULN, upper limit of normal.

Complement work-up performed at the time of TMA (Table 2) showed increased plasma C3 and C4 levels in all patients (probably as part of the inflammatory response) and mildly increased sC5b-9 plasma level in 1 patient. FH and FI plasma levels were decreased in 1 patient. Anti-FH antibodies were detected in 2 patients. In 3 of the 4 patients for whom DNA samples were available, a pathogenic rare variant was detected: 1 CFH variant known to be associated to decreased plasma FH levels in vivo, 1 complement FI variant associated to a decreased FI synthesis documented in vitro,11 and 1 C3 variant associated to an impaired C3 regulation.12 An additional patient carried, in homozygosity, the haplotype H3 in the CFH gene (tgtgt), an established risk factor for aHUS.7 

Table 2.

Complement work-up performed in 5 patients with COVID 19-associated renal TMA

PtTime from COVID-19 diagnosis (d)Time from TMA diagnosis (d)C3 (mg/L)C4
(mg/L)
sC5b-9
(ng/mL)
FH
(%)
FI
(%)
CD46
(MFI)
Anti-FH Ab (AU)CFHR1-CFHR3 copy numberComplement geneticsComplement gene variant classification
13 840 301 761 70 100 12.3 Negative CFH c.2266T>A p.Ser756Thr Pathogenic (reduced FH plasma level) 
13 1750 332 248 144 147 ND Negative C3 c.463A>C p.Lys155Gln Pathogenic (confers resistance to cofactor activity)12  
 54 42 1040 187 322 109 130 17 Negative    
34 1320 415 303 91 66 10.3 Negative
(sample collected after PE) 
CFI c.1246A>C p.Ile416Leu Pathogenic (reduced FI plasma level ; Not detected in supernatants and around 25% expression of WT in lysates)11  
 108 77 1160 461 1094* 94 86 13.8 Positive (4130)    
 167 136 1050 357 570 112 66 ND Positive (686)    
19 19 1220 454 ND ND ND ND Positive (1554) CFH homozygous tgtgt haplotype aHUS at-risk haplotype 
 42 42 1470 507 345 130 161 ND Positive (1171)    
 122 122 1210 430 ND ND ND ND Positive (516)    
1270 550 136 120 120 ND Negative ND ND ND 
PtTime from COVID-19 diagnosis (d)Time from TMA diagnosis (d)C3 (mg/L)C4
(mg/L)
sC5b-9
(ng/mL)
FH
(%)
FI
(%)
CD46
(MFI)
Anti-FH Ab (AU)CFHR1-CFHR3 copy numberComplement geneticsComplement gene variant classification
13 840 301 761 70 100 12.3 Negative CFH c.2266T>A p.Ser756Thr Pathogenic (reduced FH plasma level) 
13 1750 332 248 144 147 ND Negative C3 c.463A>C p.Lys155Gln Pathogenic (confers resistance to cofactor activity)12  
 54 42 1040 187 322 109 130 17 Negative    
34 1320 415 303 91 66 10.3 Negative
(sample collected after PE) 
CFI c.1246A>C p.Ile416Leu Pathogenic (reduced FI plasma level ; Not detected in supernatants and around 25% expression of WT in lysates)11  
 108 77 1160 461 1094* 94 86 13.8 Positive (4130)    
 167 136 1050 357 570 112 66 ND Positive (686)    
19 19 1220 454 ND ND ND ND Positive (1554) CFH homozygous tgtgt haplotype aHUS at-risk haplotype 
 42 42 1470 507 345 130 161 ND Positive (1171)    
 122 122 1210 430 ND ND ND ND Positive (516)    
1270 550 136 120 120 ND Negative ND ND ND 

Normal range: C3: 615-1250 mg/L; C4: 93-380 mg/L; FH 70%-130%; FI 70%-130%; CD46: 13-19 MFI; sC5b9: <300 ng/mL.

CFH, complement factor H gene; CFHR, complement factor H-related protein; CFI, complement factor I; MFI, mean fluorescence intensity; ND, not determined; PE, plasma exchange.

*

The patient had several complications related to a perirenal hematoma that occurred after kidney biopsy.

Two patients underwent plasma exchanges with fresh frozen plasma, whereas 3 were treated with eculizumab. Patient 4 received 2 infusions of rituximab for anti-FH antibodies. At the last known follow-up (1-6 months), 2 patients remained dialysis-dependent, 2 renal transplant recipients regained their baseline renal function, and a third had a decreased function of the renal transplant.

Data on COVID-19-associated TMA are scarce9,13,14 (supplemental Table 1). We report here the first series of renal TMA in COVID-19 patients, which includes extensive complement work-up. The first remarkable finding is the sharp contrast between mild respiratory symptoms but severe renal (and in 3 patients extrarenal) TMA. Therefore, COVID-19–related morbidity was mainly due to TMA rather than to the pulmonary involvement. Three of the patients in the present series and 2 from previously published cases are renal transplant recipients. Underlying kidney graft (vascular) damage and tacrolimus/everolimus use and very high trough levels (particularly in patient 2), may have contributed to trigger TMA in the renal graft.

The second remarkable finding is the detection in 3 tested patients of complement gene variants associated to an altered regulation of the complement alternative pathway, and thus to an increased risk of complement-mediated EC injury. An additional tested patient had the at-risk haplotype (H3) in the CFH gene, which predisposes to aHUS. Interestingly, the only previously reported patient with COVID-19–associated renal TMA who was tested for complement gene variants, was found to carry a pathogenic rare variant in the C3 gene and an at-risk haplotype in the membrane-cofactor protein gene.9 Thus, to date, all 5 reported patients with COVID-19–associated renal TMA, who underwent genetic testing, carry a constitutional complement dysregulation. These findings require confirmation in larger series. Noteworthy, 2 patients were positive for anti-FH antibodies, but the pathogenic relevance of these autoantibodies is unclear, and their presence may reflect the stimulation of the immune system because of COVID-19, rather than a causal role in the TMA.

The discussion of the role of systemic complement activation has been one of the central issues in the management of COVID-19 patients.6,15-19 Limited preliminary reports suggested a potential benefit of C5 blockade in severe forms of COVID-19,6,15 but 2 prospective studies did not show any clear benefit of C5 or C5a blockade in this setting.20,21 However, the patients presented here had mild pulmonary COVID-19 manifestations and absent or moderate markers of systemic complement activation. These findings suggest that TMA in our patients resulted predominantly from intrarenal complement activation and ensuing EC damage, associated to a genetic dysregulation of the complement alternative pathway in at least 4 patients (ie, a clinical and genetic pattern of renal TMA similar to the one reported in complement-mediated aHUS). These similarities suggest that HUS is not a unique manifestation of COVID-19, but rather that COVID-19 is a newly identified potential infectious trigger for aHUS, in accordance with previous data on complement-mediated aHUS precipitated by viral infections, mostly influenza strains.22 The awareness of a potential association of COVID-19 with complement-mediated aHUS may help improve patients’ management. The use of anti-complement therapies should be considered in this form of HUS associated with COVID-19, to potentially limit kidney damage. However, the limited number of patients in the present series preclude any firm conclusions, and additional data are required.

In summary, COVID-19 is a potential newly identified trigger for complement-mediated aHUS. The identification of such association has important clinical implications and additional data are required.

The authors are grateful to Carole Gengler (Service of Clinical Pathology, Lausanne University Hospital, Lausanne, Switzerland) for her assistance in reviewing the kidney pathological data.

Contribution: C.E.S., V.F.-B., and F.F. reviewed the data and drafted the manuscript; C.E.S., P.V.M., A.S., S.S., and V.F.-B. performed the complement workup; S.R. and L.D. analyzed the kidney biopsies; all the authors were involved in the management of the patients; and all authors read, commented on, and revised the manuscript before approval.

Conflict-of-interest disclosure: G.K. received fees from SOBI and Roche for invited lectures. V.F.-B. has received fees from Alexion Pharmaceuticals, Roche, BioCryps, and Apellis for invited lectures and/or board membership and is the recipient of a research grant from Alexion Pharmaceuticals. F.F. has received consultancy and/or speaker honoraria from Roche, Alexion, Apellis, Achillion, Novartis, and Alnylam. The remaining authors declare no competing financial interests.

Correspondence: Fadi Fakhouri, Service of Nephrology and Hypertension, Department of Medicine, Lausanne University Hospital, Rue du Bugnon, Lausanne 1005, Switzerland; e-mail: fadi.fakhouri@unil.ch.

Original data are available by e-mail request to the corresponding author.

The online version of this article contains a data supplement.

1.
Ackermann
M
,
Verleden
SE
,
Kuehnel
M
, et al
.
Pulmonary vascular endothelialitis, thrombosis, and angiogenesis in Covid-19
.
N Engl J Med.
2020
;
138
(
18
):
120
-
128
.
2.
Varga
Z
,
Flammer
AJ
,
Steiger
P
, et al
.
Endothelial cell infection and endotheliitis in COVID-19
.
Lancet.
2020
;
138
(
18
):
1417
-
1418
.
3.
Guervilly
C
,
Burtey
S
,
Sabatier
F
, et al
.
Circulating endothelial cells as a marker of endothelial injury in severe COVID -19
.
J Infect Dis.
2020
;
138
(
18
):
1789
-
1793
.
4.
Sinkovits
G
,
Mező
B
,
Réti
M
, et al
.
Complement overactivation and consumption predicts in-hospital mortality in SARS-CoV-2 infection
.
Front Immunol.
2021
;
138
:
663187
.
5.
Yu
J
,
Yuan
X
,
Chen
H
,
Chaturvedi
S
,
Braunstein
EM
,
Brodsky
RA.
Direct activation of the alternative complement pathway by SARS-CoV-2 spike proteins is blocked by factor D inhibition
.
Blood.
2020
;
138
(
18
):
2080
-
2089
.
6.
Peffault de Latour
R
,
Bergeron
A
,
Lengline
E
, et al;
Core Group
.
Complement C5 inhibition in patients with COVID-19 - a promising target?
Haematologica.
2020
;
138
(
18
):
2847
-
2850
.
7.
Fakhouri
F
,
Zuber
J
,
Frémeaux-Bacchi
V
,
Loirat
C.
Haemolytic uraemic syndrome
.
Lancet.
2017
;
138
(
18
):
681
-
696
.
8.
Jhaveri
KD
,
Meir
LR
,
Flores Chang
BS
, et al
.
Thrombotic microangiopathy in a patient with COVID-19
.
Kidney Int.
2020
;
138
(
18
):
509
-
512
.
9.
Mat
O
,
Ghisdal
L
,
Massart
A
, et al
.
Kidney thrombotic microangiopathy after COVID-19 associated with C3 gene mutation
.
Kidney Int Rep.
2021
;
138
(
18
):
1732
-
1737
.
10.
Frémeaux-Bacchi
V
,
Sellier-Leclerc
AL
,
Vieira-Martins
P
, et al
.
Complement gene variants and Shiga toxin-producing Escherichia coli-associated hemolytic uremic syndrome: retrospective genetic and clinical study
.
Clin J Am Soc Nephrol.
2019
;
138
(
18
):
364
-
377
.
11.
Bienaime
F
,
Dragon-Durey
MA
,
Regnier
CH
, et al
.
Mutations in components of complement influence the outcome of Factor I-associated atypical hemolytic uremic syndrome
.
Kidney Int.
2010
;
138
(
18
):
339
-
349
.
12.
Seddon
JM
,
Yu
Y
,
Miller
EC
, et al
.
Rare variants in CFI, C3 and C9 are associated with high risk of advanced age-related macular degeneration
.
Nat Genet.
2013
;
138
(
18
):
1366
-
1370
.
13.
Bascuñana
A
,
Mijaylova
A
,
Vega
A
, et al
.
Thrombotic microangiopathy in a kidney transplant patient with COVID-19
.
Kidney Med.
2021
;
138
(
18
):
124
-
127
.
14.
Jespersen Nizamic
T
,
Huang
Y
,
Alnimri
M
,
Cheng
M
,
Chen
LX
,
Jen
KY.
COVID-19 Manifesting as Renal Allograft Dysfunction, Acute Pancreatitis, and Thrombotic Microangiopathy: A Case Report
.
Transplant Proc.
2021
;
138
(
18
):
1211
-
1214
.
15.
Annane
D
,
Heming
N
,
Grimaldi-Bensouda
L
, et al;
Garches COVID 19 Collaborative Group
.
Eculizumab as an emergency treatment for adult patients with severe COVID-19 in the intensive care unit: a proof-of-concept study
.
EClinicalMedicine.
2020
;
138
:
100590
.
16.
Gavriilaki
E
,
Brodsky
RA.
Severe COVID-19 infection and thrombotic microangiopathy: success does not come easily
.
Br J Haematol.
2020
;
138
(
18
):
e227
-
e230
.
17.
Noris
M
,
Benigni
A
,
Remuzzi
G.
The case of complement activation in COVID-19 multiorgan impact
.
Kidney Int.
2020
;
138
(
18
):
314
-
322
.
18.
Gavriilaki
E
,
Asteris
PG
,
Touloumenidou
T
, et al
.
Genetic justification of severe COVID-19 using a rigorous algorithm
.
Clin Immunol.
2021
;
138
:
108726
.
19.
Carvelli
J
,
Demaria
O
,
Vély
F
, et al;
Explore COVID-19 Marseille Immunopole group
.
Association of COVID-19 inflammation with activation of the C5a-C5aR1 axis
.
Nature.
2020
;
138
(
18
):
146
-
150
.
20.
Chouaki Benmansour
N
,
Carvelli
J
,
Vivier
E.
Complement cascade in severe forms of COVID-19: recent advances in therapy
.
Eur J Immunol.
2021
;
138
(
18
):
1652
-
1659
.
21.
Vlaar
APJ
,
de Bruin
S
,
Busch
M
, et al
.
Anti-C5a antibody IFX-1 (vilobelimab) treatment versus best supportive care for patients with severe COVID-19 (PANAMO): an exploratory, open-label, phase 2 randomised controlled trial
.
Lancet Rheumatol.
2020
;
138
(
18
):
e764
-
e773
.
22.
Bitzan
M
,
Zieg
J.
Influenza-associated thrombotic microangiopathies
.
Pediatr Nephrol.
2018
;
138
(
18
):
2009
-
2025
.

Supplemental data