Human complement Factor H and Properdin act as soluble pattern recognition receptors and differentially modulate SARS-CoV-2 Infection

Severe cases of SARS-CoV-2 infection are characterised by an imbalanced immune response, excessive inflammation, and the development of acute respiratory distress syndrome, which can lead to multiorgan failure and death. Several studies have demonstrated dysregulated complement activity as an indicator of immunopathogenesis in the SARS-CoV-2 infection. Notably, the complement alternative pathway has been implicated in driving the excessive inflammation during severe SARS-CoV-2 infection. Reduced levels of factor H (FH), a down-regulator of the alternative pathway, and increased levels of properdin (Factor P/FP), the only known up-regulator of the alternative pathway, have been observed in individuals with severe COVID-19 infection. The present study investigated the complement activation-independent, and a more direct role of FH and FP against SARS-CoV-2 infection. Using direct ELISA, the interactions of FH and FP with the SARS-CoV-2 spike (S) and receptor binding domain (RBD) were assessed. Using S protein expressing lentiviral pseudotypes, the cell binding and luciferase-based virus entry assays were employed to assess the potential modulatory effects of FH, FP, and recombinant thrombospondin repeats 4 and 5 (TSR4+5) on SARS-CoV-2 cell entry. We also evaluated the immunomodulatory functions of FH and FP in the cytokine response triggered by SARS-CoV-2 pseudotypes via RT-qPCR. SARS-CoV-2 S and RBD proteins were found to bind both FH and FP. Treatment of A549 cells expressing human ACE2 and TMPRSS2 with FP or TSR4+5 resulted in increased cell entry and binding of SARS-CoV-2 pseudotypes. In silico studies revealed that FP increases affinity between SARS-CoV-2 and host ACE2. The impact of FP on viral cell entry and binding was reversed by anti-FP antibody treatment in A549-hACE2+TMPRSS2 cells. However, FH treatment reduced the cell entry and binding of SARS-CoV-2 lentiviral pseudotypes. Furthermore, the A549-hACE2+TMPRSS2 cells challenged with SARS-CoV-2 spike, envelope, nucleoprotein, and membrane protein expressing alphaviral pseudotypes pre-treated with FP or TSR4+5, exhibited upregulation of the transcripts of pro-inflammatory cytokines, such as IL-1β, IL-8, IL-6, TNF-α, IFN-α and RANTES (as well as NF-κB). Conversely, FH pre-treatment downregulated the expression of these pro-inflammatory cytokines. Treatment of A549-hACE2+TMPRSS2 cells with FP increased S protein-mediated NF-κB activation, while FH treatment reduced it. These findings suggest that FH may act as an inhibitor of SARS-CoV-2 cell entry and binding, thereby attenuating the infection-associated inflammatory response in a complement activation-independent manner. FP may contribute to viral cell entry, binding, and exacerbating the immune response. That may result in potentially influencing the severity of the infection.