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SARS-CoV-2 spike protein induces monocyte apoptosis and interleukin-8 production

Background: In the setting of SARS-CoV-2 infection and COVID-19 illness, a subset of symptomatic patients has been reported to experience severe leukopenia. Viral proteins have been described to have the capacity to induce cell death in peripheral blood cells in infections such as HIV. Given the expression of the cognate receptor, ACE-2, on the surface of Peripheral blood mononuclear cells (PBMCs), we hypothesized that SARS-CoV-2 may induce leukopenia via spike protein ligand-receptor interaction. Methods: PBMCs were isolated from the fresh blood of normal donors and were treated with 1ug/ml of recombinant spike protein, and analyzed for cell death via the Incucyte Live/Cell imaging system. To measure subset specific cell death, PBMCs treated with recombinant spike protein for 48 hours were analyzed by flow cytometry for the expression of cell specific surface receptors and concomitant active caspase 3 expression. Culture supernatant was analyzed by multiplex cytokine analysis to evaluate the presence of pro-inflammatory cytokines. Similar assays were carried out in the presence of a spike- binding domain-antagonistic antibody in order to determine the specific role of spike- ACE2 interaction in causing cell death. Finally, cells from COVID positive patients were analyzed to determine if similar results were observable in-vivo. Results: The treatment of PBMCs with recombinant SARS-CoV-2 spike resulted in significant cell death over time in 2 out of three donors tested (p<0.05) by IncuCyte live imaging analysis. When analyzed for subset specific cell death, a significant increase in cell death (p<0.01), as measured by Caspase 3, was observed in CD14+CD3- cells, correlating with the monocyte population. Supernatants from these cultures demonstrated markedly increased IL-8 production (p=0.0536). Cultures carried out in the presence of a spike antagonistic antibody abrogated the effects of spike protein, indicating a direct relationship between spike-ACE2 interaction and cell death in this sub-population. Similar flow cytometric analysis from 5 febrile patients with COVID-19 demonstrated significantly increased monocyte apoptosis (p<0.05), compared to CD3+ lymphocytes from the same donors;whereas significantly increased monocyte apoptosis was not observed in 5 afebrile COVID-19 patients. Conclusion: These results indicate that SARS-CoV-2 spike protein may induce apoptosis specifically in Monocytes, in an ACE2 dependent manner, in some but not all patients.