Fig. 1

Cationic nanocarriers are potent adjuvants for recombinant S-RBD protein vaccine of SARS-CoV-2. a, b Cationic nanocarriers enhanced RBD-induced humoral immune response. NIH mice (n = 6/group) were immunized intranasally (a) and intramuscularly (b) with RBD+ nanocarriers (5 μg RBD/mouse). On day 14 after the first dose, immune sera were collected and titers of the total IgM were examined by ELISA. On day 35, total IgG levels and IgG isotypes were also determined. p value of IgG isotypes was compared to PBS group. c–j Sera were collected on day 35 for functional characterization. Blockade of RBD binding to cell surface ACE2 receptor by immune sera from intranasally (c, d) and intramuscularly (e) immunized mice was measured by flow cytometry (FCM). Neutralization of EGFP-expressing SARS-CoV-2 pseudovirus infection to 293T/ACE2 cells by immune sera from intranasally (f, g) and intramuscularly (i) immunized mice was measured with fluorescent microscopy and FCM. Scale bar, 100 μm. c and f were performed at 1:270 dilution. Neutralization of luciferase-expressing SARS-CoV-2 pseudovirus infection in 293T/ACE2 cells by immune sera from intranasally (h) and intramuscularly (j) immunized mice. n = 3. k, l Cationic nanocarriers + RBD vaccine induced cellular immunity. NIH mice were immunized as aforementioned. We isolated lymphocytes from inguinal lymph nodes and further analyzed the proportion of activated CD69⁺CD8⁺ and CD69⁺CD4⁺ T cells (k) and effector memory (CD44⁺CD62L⁻) T cells in CD4⁺ and CD8⁺ T cell populations (l) by FCM. n = 6. p was compared to PBS group. m After being separated and cultured for 7 days, DCs were cultured in medium with DEX or DEX+ nanocarriers. Then the uptake of DEX by DCs was analyzed by FCM. n = 3. p value was compared to DEX group. n DC were stimulated by nanocarriers for overnight. Maturation surface markers (CD86, CD40) were analyzed by FCM. o Inflammatory cytokines (IL-6, IL-1β, and TNF-α) in the supernatants were detected by ELISA. n = 4. p was compared with untreated group. All the data were presented as mean ± SEM. NS: not significant