Introduction: During the Coronavirus Disease 2019 (COVID-19) pandemic caused by Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2), 3 distinct phenotypes have emerged in children. The majority of children have mild or no symptoms. Similar to adults, a minority of children can be severely affected with respiratory distress requiring intensive care. Finally, they may develop a phenomenon presumed unique to children termed Multisystem Inflammatory Syndrome in Children (MIS-C). MIS-C is a hyperinflammatory syndrome characterized by fever and organ dysfunction (particularly cardiac) in the setting of recent COVID-19 infection. Reports from the adult literature have invoked thrombotic microangiopathy (TMA) and complement activation as a potential cause for severe manifestations of COVID-19 (Zhang et al. NEJM. 2020; Campbell et al. Circulation 2020). Soluble C5b9 (sC5b-9), the terminal complement complex, has been implicated as a marker of hematopoietic stem cell transplant associated TMA (HSCT-TMA; Jodele et al. Blood 2014). We sought to elucidate the role of terminal complement activation and TMA in the different pediatric disease phenotypes.

Methods: We enrolled children admitted to the Children's Hospital of Philadelphia during the COVID-19 pandemic who had evidence of SARS-CoV-2 infection on reverse transcriptase polymerase chain reaction (RT-PCR) from mucosa, or met clinical criteria for MIS-C. Patients (pts) were classified in to 3 categories: minimal COVID-19 symptoms or incidental finding of SARS-CoV-2 infection, severe COVID-19 requiring ventilatory support, or MIS-C. To investigate the role of TMA in children with COVID-19 we measured sC5b-9 in plasma of pts with the 3 manifestations of SARS-CoV-2, and in healthy controls. sC5b9 was measured in triplicate at two dilutions by ELISA. Proinflammatory cytokines were measured using V-Plex Pro-inflammatory Panel 1 Human Kits and analyzed on a QuickPlex SQ120. P-values were computed using Dunn's multiple comparisons test after Kruskal-Wallis testing. Blood smears were examined by a hematologist and hematopathologist for schistocytes.

Results: 50 pts were enrolled on whom complete sC5b9 data were available: minimal COVID-19 (N=18), severe COVID-19 (N=11), and MIS-C (N=21). Plasma was obtained on healthy controls (N=26). The median sC5b9 level in healthy controls (57 ng/mL) differed significantly (p<0.001 in each case; Figure 1A) from that in pts with minimal disease (392 ng/mL), severe disease (646 ng/mL), and MIS-C (630 ng/mL); differences between MIS-C, minimal, and severe were not statistically significant. Elevations in sC5b9 correlated in a statistically significant manner with the maximum creatinine and blood urea nitrogen (BUN) measured during hospitalization (Figure 1B&C), but not age (p=0.512). sC5b9 did not correlate with lactate dehydrogenase (LDH), nor with the lowest levels of fibrinogen, hemoglobin or platelet counts. Of pts with available data, 19/26 (73.1%) had elevated LDH, 2/31 (6.4%) had hypofibrinogenemia, 35/47 (74.5%) were anemic, and 28/47 (59.6%) were thrombocytopenic.

Pro-inflammatory cytokines were measured. Of particular interest to TMA is the neutrophil chemotactic factor IL-8, because of its role as a marker of endothelial damage (Dvorak et al. Front Pediatr 2019). Levels of IL-8 differed significantly between pts with MIS-C (p=0.0166) or pts with severe COVID-19 (p=0.0079), when compared to minimal COVID-19 pts; but not between pts with MIS-C and severe disease (p = 0.99).

Blood smears were available on 34 patients. Schistocytes were present in 13/15 (87%) patients with MIS-C, 7/8 (87%) patients with severe COVID-19 and 5/11 (45%) patients with minimal COVID-19 (χ2=6.59, p=0.037).

Conclusions: We demonstrate derangements of the final common pathway of complement activation in children with the 3 presentations of SARS-CoV-2. Strikingly, sC5b9s were abnormal even in children with minimal disease or incidental infection. Renal dysfunction correlated with elevations in sC5b9, strengthening the evidence that TMA plays a role in the pathophysiology of SARS-CoV-2 infection. Future work is aimed at further characterizing the role of the complement cascade in the pathogenesis of MIS-C and COVID-19 in children. The long-term complications of endothelial damage and complement activation are unknown and extended follow-up is warranted.


Diorio:Children's Hospital of Philadelphia: Research Funding; University of Pennsylvania: Research Funding. Lambert:22qSociety: Consultancy; RDMD ITP study: Consultancy; Octapharma: Consultancy, Research Funding; Educational Concepts in Medicine: Consultancy; Shionogi: Consultancy; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Argenix: Consultancy; CdLS Foundation: Consultancy; Sysmex: Research Funding; Dova: Consultancy, Membership on an entity's Board of Directors or advisory committees; Principia: Consultancy, Membership on an entity's Board of Directors or advisory committees; ClinGen: Honoraria; Platelet Disorder Support Association (PDSA): Consultancy; AstraZeneca: Research Funding; Bayer: Consultancy; ITP Australia: Consultancy. Henrickson:Horizon Pharma: Other: ad hoc board meeting. Odom John:Burroughs Wellcome: Research Funding; NIAID: Research Funding. Bassiri:CSL Behring: Other: Spouse receives stocks . Behrens:NIH/NIAID: Research Funding. Teachey:Janssen: Consultancy; Amgen: Consultancy; La Roche: Consultancy; Sobi: Consultancy.

Author notes


Asterisk with author names denotes non-ASH members.