Preclinical investigation of intranasal adjuvanted COVID-19 vaccine

By Tarun Sai LomteMar 30 2022Reviewed by Aimee Molineux

A recent preclinical study published in Vaccines reported systemic and mucosal immunity against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) via intranasal delivery of a vaccine containing recombinant SARS-CoV-2 antigens, nucleocapsid (NC), and spike subunit (S1).

Background

Typically, coronavirus disease 2019 (COVID-19) causes a mild respiratory illness with symptoms including dry cough, fever, and headache. However, highly vulnerable populations experience severe COVID-19 requiring hospitalization, intensive care, and oxygen support.

In the past, extensive vaccination efforts have prevented or curbed viral epidemics and pandemics, and vaccine research has evolved over the past few decades, resulting in novel vaccine candidates. In the case of SARS-CoV-2, many vaccines have been developed on different platforms, such as plasmid deoxyribonucleic acids (DNAs), messenger ribonucleic acids (mRNAs), inactivated recombinant protein, adenoviral vectors, and recombinant viral vector vaccines. Moreover, vaccine design strategies are growing to utilize targets that improve efficiency and safety profile.

The role of spike (S) protein is critical in humoral and cellular immunity and inhibition of viral infection. S protein mediates receptor recognition, host cell attachment, and fusion. Trial studies have observed strong immunogenicity of recombinant S1. The nucleocapsid, involved in the binding and packaging of viral RNA, has been lately identified in inducing humoral responses in COVID-19 patients making it a potential vaccine candidate.

The study

Researchers evaluated the immune responses against SARS-CoV-2 with recombinant S1 and NC proteins of SARS-CoV-2 in the present study. The potential applicability of the vaccine and enhancement of efficacy mediated by the adjuvants were determined in mice.

Female mice were categorized into six experimental and two control groups with five mice each. Mice in the positive control group were inoculated with a non-infectious virus, and negative control mice received saline. Experimental mice received a combination of NC and S1 antigens with or without cationic (N3) or anionic (L3) lipid adjuvants. Adjuvants (L3 or N3) were used at 1.5% w/v and mixed with 0.1 or 1 µg of recombinant S1 and NC proteins. Vaccines were administered in two doses 21 days apart as intranasal sprays.

Blood samples were collected from the animals on days 14 and 42 to extract serum. A microneutralization assay (MNA) was performed using sera obtained from the mice. ELISpot capture assays determined the interleukin (IL)-5 and interferon (IFN)-γ levels.

Results

The researchers observed antibodies against the S1 and NC antigens in all the animals except those controlled negatively. However, the serological responses varied significantly. Serum IgG titers were significantly elevated in mice receiving vaccines with N3 adjuvant and exhibited nearly a 10-fold increase, and increasing the concentration of anionic L3 to 2% resulted in comparable antibody titers. A similar trend was observed only after the second vaccination for IgA titers, nearly 10-fold higher than the first dose.

Microneutralizing antibody titers differed variably across mice receiving different vaccine doses. Mice in groups 5 (1 µg dose + N3 adjuvant) and 6 (0.1 µg + N3 adjuvant) exhibited significantly higher antibody titers. At the same time, those with L3 adjuvants had higher antibody titers than the positive control group. All immunized mice produced IgA antibodies against the SARS-CoV-2 antigens in the respiratory and gastrointestinal tracts.

Consistently, mice receiving vaccines with N3 adjuvants had higher mucosal IgA titers. All vaccinated animals propagated IL-5 secreting spleen cells when stimulated with the recombinant antigens in vitro. The IL-5 secreting cells were higher in mice immunized with N3 adjuvanted antigens. IFN-γ secreting spleen cells were calculated similarly, and higher IFN-γ cells were detected in mice vaccinated with SARS-CoV-2 antigens adjuvanted with N3.

Conclusions

The study reported systemic and mucosal immune responses in mice post-intranasal vaccination at low concentrations of recombinant SARS-CoV-2 antigens (NC and S1) with mucosal adjuvants. The immunogenicity of NC and S1 was confirmed by the significantly high IgA and IgG titers in the mucosa and serum, respectively, consistent with a previous study in mice that reported immunogenicity of S1 and NC combination when administered intramuscularly.

Notably, mice immunized with recombinant antigens in a cationic adjuvant (N3) formulation demonstrated higher and improved humoral and cellular immune responses than mice in other experimental groups. This effect might be because of the destabilizing effect of the N3 cationic lipids on cell membranes, thereby improving antigens' permeability.

Contrastingly, increased concentration of the anionic L3 adjuvants resulted in significantly higher antibodies after the second vaccine dose. Virus neutralizing serum was detected in mice that received N3-adjuvanted vaccine or 2% L3 adjuvant, but significant neutralization was apparent with N3 formulation, indicating that vaccines with N3 adjuvants elicit functionally superior antibodies.