Multi-organ complement deposition in COVID-19 patients

Increased levels of circulating complement activation products have been reported in COVID-19 patients, but only limited information is available on complement involvement at tissue level. The mechanisms and pathways of local complement activation remain unclear.

We performed immunofluorescence analyses of autopsy specimens of lungs, kidney and liver from nine COVID-19 patients who died of acute respiratory failure. Snap-frozen samples embedded in OCT were stained with antibodies against complement components and activation products, IgG and spike protein of SARS-CoV-2.

Lung deposits of C1q, C4, C3 and C5b-9 were localized in the capillaries of the interalveolar septa and on alveolar cells. IgG displayed a similar even distribution, suggesting classical pathway activation. The spike protein is a potential target of IgG, but its uneven distribution suggests that other viral and tissue molecules may be targeted by IgG. Factor B deposits were also seen in COVID-19 lungs and are consistent with activation of the alternative pathway, whereas MBL and MASP-2 were hardly detectable. Analysis of kidney and liver specimens mirrored findings observed in the lung. Complement deposits were seen on tubules and vessels of the kidney with only mild C5b-9 staining in glomeruli, and on hepatic artery and portal vein of the liver.

Complement deposits in different organs of deceased COVID-19 patients caused by activation of the classical and alternative pathways support the multi-organ nature of the disease.

Grants from the Italian Ministry of Health (COVID-2020-12371808) to PLM and National Institutes of Health HL150146 to NP are gratefully acknowledged.