Upregulation of CD55 complement regulator in distinct PBMC subpopulations of COVID-19 patients is associated with suppression of interferon responses.

Complement activation has been verified in COVID-19 patients by both increased serum levels of complement factors C3a and C5b-9 and increased complement deposition at the tissue levels. Complement regulatory proteins (CRPs) CD55, CD46, CD59 and CR1 act to control complement overactivation and eliminate complement deposition and cell lysis. The aim of the study was to investigate the expression of CRPs in COVID-19 in order to identify potential dysregulated expression patterns of CRPs and address whether these may contribute to disease pathogenesis. Single cell RNA-sequencing (scRNA-seq) analysis performed on isolated PBMCs revealed an increase of CD55 expression in severe and critical COVID-19 patients compared to healthy controls. This increase was also detected upon integrated subclustering analysis of the monocyte, T cell and B cell populations. Flow cytometric analysis verified the distinct pattern of upregulated CD55 expression in monocyte and T cell sub populations of severe COVID-19 patients. This upregulation was associated with decreased expression of interferon stimulated genes (ISGs) in patients with severe COVID-19 suggesting a potential suppressor effect of CD55 on interferon responses. The present study identifies a COVID-19 specific CD55 expression pattern in PBMC subpopulations that coincides with reduced interferon responses thus indicating that the complement regulator CD55 may contribute to COVID-19 pathogenesis.