Figure 2.
The spike glycoprotein is responsible for the improved replication fitness of Delta variant in primary human epithelial airway cultures
(A) Schemes of Alpha variant, Delta variant, and Delta variant bearing Alpha spike. The spike gene of Delta variant was swapped with Alpha variant, resulting in a chimeric SARS-CoV-2 of “Alpha-spike/Delta-backbone.”
(B) Plaques of Alpha-spike/Delta-backbone chimeric virus. The plaque images were taken on day 2.5 post infection of Vero E6 cells. The average diameter of the plaques are presented in the parentheses.
(C and D) Replication competitions between Delta and Alpha-spike/Delta-backbone (C) and Alpha-spike/Delta-backbone and Alpha (D) on primary HAE cells. Equal PFU amounts of two viruses were mixed and inoculated onto HAE cells at an MOI of 5. After 2 h incubation, the cells were washed thrice with DPBS and maintained for 5 days. Secreted progeny viruses were collected daily by incubating the apical side of the HAE culture with 300 μL DPBS at 37°C for 30 min. Red dots represent individual cell cultures (n = 6) pooled from 2 independent biological repeats; the horizontal lines in each catseye represent the mean; shaded regions represent standard error of the mean; y axes use a log10 scale. Black numbers above each set of values (catseye) indicate the ratios of two viral RNA species. p values were calculated for group coefficient using linear regression model. ∗∗∗p < 0.001.