Abstract
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<0.05 vs. vehicle PBS group). Furthermore, intratracheal administration of recombinant RBM induced TNF, IL‐6 and IL‐1β release in bronchoalveolar lavage fluid at 24 hours post‐instillation (n=13 per group, p<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.