The pandemic COVID-19 disease with severe acute respiratory syndrome (SARS) is mediated by the coronavirus SARS-CoV-2. The highly conserved surface spike glycoprotein of the SARS-coronavirus is critical for viral entry into cells, a central step in pathogenesis. To further understand and characterize viral spike proteins in host cell immune responses, we generated two species of spike proteins: a recombinant receptor binding domain (RBD) and a truncated part of RBD, the receptor binding motif (RBM). In cell cultures with murine macrophage-like RAW 264.7 cells or with thioglycollate-elicited peritoneal primary mouse macrophages, recombinant RBD and RBM dose-dependently induced the release of TNF and HMGB1, pro-inflammatory molecules observed in increased systemic levels in COVID-19 patients (Chen R et al, Heliyon 6, 2020, e05672). Intratracheal administration of RBD (100 µg/mouse) or RBM (100 µg/mouse) also induced HMGB1 release in bronchoalveolar lung (BAL) fluid at 24 hours post-instillation (HMGB1 levels = 71 ± 30 ng/mL in PBS group vs. 253 ± 19* ng/mL in RBD group and 284 ± 42* ng/mL in RBM group;n=3-4 mice/group;*: p&lt;0.05 vs. vehicle PBS group). Furthermore, intratracheal administration of recombinant RBM induced TNF, IL-6 and IL-1? release in bronchoalveolar lavage fluid at 24 hours post-instillation (n=13 per group, p&lt;0.05 vs.PBS group). Taken together, our studies reveal that recombinant RBD and RBM induce pro-inflammatory cytokine release in vivo and in cultured murine macrophages.
Induction of Pro-Inflammatory Cytokines TNF, IL-6, and HMGB1 by SARS-CoV-2 Spike Proteins in Mice and in Murine Macrophage Cultures