Fig. 3: SARS-CoV-2 titration, histopathology and immunohistochemistry to detect viral antigen of K18-hACE2 geriatric mice tissues.

a–c Determination of infectious viral particles (IVP) in lung, nasal turbinate (NT) and brain samples from SARS-CoV-2 inoculated K18-hACE2 geriatric mice after immunisation. Supernatants from the homogenized lungs (a), nasal turbinates (b) and brain (c) were evaluated for the presence of infectious virus by titration in Vero E6 cells. Viral titres are expressed as the TCID₅₀ per mL of tissue lysate. The dotted lines represent the limit of quantification (LOQ) of the Reed-Muench method. Single data points, mean and standard deviation are shown for each group. Differences between groups were analysed using the Peto&Peto left-censored samples test. P values < 0.05 were considered significant: *. d–i Histopathological assessment and detection of SARS-CoV-2 nucleoprotein (NP) by IHC in tissue samples from SARS-CoV-2 inoculated K18-hACE2 geriatric mice. The lesion severity and extent in lung (d), nasal turbinate (e), and brain (f) samples were expressed using a semi-quantitative score: 0 = none, 1 = mild, 2 = moderate, 3 = severe. The presence and amount of viral antigen in lung (g), nasal turbinates (h), and brain (i) samples were assessed by immunohistochemistry (IHC) using a semi-quantitative score: 0 = absence of viral antigen; 1 = low amount, multifocal localisation; 2 = moderate amount, multifocal localisation; 3 = high amount, diffuse localization. Differences between groups were analysed using an asymptotic generalized Pearson chi-squared test; multiple comparisons were corrected by false discovery rate (FDR). P values < 0.05 were considered significant; *p value < 0.05; **p value < 0.005. NT : nasal turbinate.