Intranasal administration of a recombinant RBD vaccine induced protective immunity against SARS-CoV-2 in mouse

Abstract

The emergence of the global Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) pandemic underscores the importance of the rapid development of a non-invasive vaccine that can be easily administered. A vaccine administered by nasal delivery is endowed with such characteristics against respiratory viruses. In this study, we generated a recombinant SARS-CoV-2 receptor-binding domain (RBD)-based subunit vaccine. Mice were immunized via intranasal inoculation, microneedle-intradermal injection, or intramuscular injection, after which the RBD-specific immune responses were compared. Results showed that when administrated intranasally, the vaccine elicited a robust systemic humoral immunity with high titers of IgG antibodies and neutralizing antibodies as well as a significant mucosal immunity. Besides, antigen-specific T cell responses were also analyzed. These results indicated that the non-invasive intranasal administration should be explored for the future SARS-CoV-2 vaccine design.

Keywords: Intranasal administration; Mucosal immunity; Neutralizing antibody; SARS-CoV-2.

Fig. 1

Characterization of recombinant RBD and immunization schedule of mice. (A) Analysis of purified RBD. Rabbit anti-RBD polyclonal antibody was used for RBD detection. Left panel: SDS-PAGE of purified RBD. Right panel: Western blot of purified RBD. (B) Schematic of the vaccination schedule of mice. They were immunized three times at intervals of 3 weeks through intranasal immunization (i.n.), intradermal immunization (i.d.), or intramuscular immunization (i.m.). Blood samples were collected before each boost immunization and before sacrifice. Vaginal washes were collected for 3 consecutive days before sacrifice. Samples of spleens, nasal washes, bronchoalveolar lavage fluids, and intestines were collected at the time of sacrifice.

Fig. 2

SARS-CoV-2 RBD-specific IgG antibody response in mice after each immunization, as measured by ELISA. i.n.: intranasal immunization, i.d.: intradermal immunization, i.m.: intramuscular immunization. Results are expressed as the mean value ± standard error of the mean (SEM) of seven mice in each group. **0.001 < P < 0.01.

Fig. 3

SARS-CoV-2 RBD-specific IgG1 and IgG2a titers in mice 3 weeks after the last immunization. Results are expressed as the mean value ± standard error of the mean (SEM) of six mice in each group. ***0.0001 < P < 0.001; ****P < 0.0001.

Fig. 4

SARS-CoV-2 RBD-specific secretory IgA titers in (A) nasal washes, (B) bronchoalveolar lavage fluids, (C) vaginal washes, and (D) intestines of mice 3 weeks after the last immunization. Results are expressed as the mean value ± standard error of the mean (SEM) of six mice in each group.

Fig. 5

ELISpot detection of IFN-γ producing cells (A, C, and E) and IL-4 producing cells (B, D, and F) in immunized mice. i.n.: intranasal immunization, i.d.: intradermal immunization, i.m.: intramuscular immunization. Results are expressed as spot-forming cells (SFCs) per 10⁶ spleen cells.

Fig. 6

Cell–cell fusion inhibition assay. (A) Principle of S protein-mediated cell–cell fusion assay. (B) Serum samples in each group were tested in duplicate. Data are presented as relative light units (RLUs) %.