An innovative nasal vaccine strategy to combat COVID

By Neha MathurOct 31 2022Reviewed by Danielle Ellis, B.Sc.

In a recent study published in the journal Science, researchers at Yale University developed a novel coronavirus disease 2019 (COVID-19) vaccination strategy termed 'prime and spike' (P&S) that leveraged existing systemic immunity triggered by parenteral vaccination (prime) to boost immunity at the respiratory mucosa.

Image Credit: Sergey Chips / Shutterstock

Background

The respiratory mucosa is the primary site of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in humans; however, at this site, parenteral vaccination regimens cannot induce adequate protective immunity. Relying on intramuscular (IM) administration, these vaccines have been shown to induce high levels of circulating antibodies, memory B cells, and circulating effector cluster of differentiation (CD)4⁺ and CD8⁺ T cells in preclinical and clinical models. However, they fail to induce tissue-resident memory B (B_RM) cells and T (T_RM) cells and mucosal immunoglobulin G (IgG) and dimeric IgA.

Recent preclinical assessments of vaccines delivered intranasally (IN) induced adequate mucosal immunogenicity at respiratory mucosa. They also conferred immune protection and reduced viral shedding in mice, hamsters, and nonhuman primates. Further, they induced cross-reactive immunity against sarbecoviruses.

About the study

In the present study, researchers hypothesized systemic priming with messenger ribonucleic acid (mRNA)-lipid nanoparticle (LNP) followed by IN boosting conferred adequate protective immunity, unlike parenteral vaccination.

So they vaccinated K18-hACE2 mice with mRNA-based BNT162b2 vaccine IM - the prime dose. After 14 days, they intranasally (IN) administered an unadjuvanted SARS-CoV-2 spike - the booster dose. The team used divergent unadjuvanted intranasal SARS-CoV-2 spike boosters or an immunosilent polyplex encapsulating spike mRNA. They euthanized these mice at days 21 or 28 to assess for mucosal humoral immunity.

First, the team assessed anti-SARS-CoV-2 spike IgG and IgA in nasal turbinates, bronchoalveolar lavage fluid (BALF), and serum. IM prime nor IN spike alone helped mice develop mucosal anti-SARS-CoV-2 antibodies. However, mice that received P&S developed high levels of anti-SARS-CoV-2 IgA and IgG in the nasal wash and BALF.

Further, the researchers compared mucosal CD8+ T cell and antibody responses following P&S under varying conditions. For instance, they tested the efficacy of two-week versus four-week boosting intervals. Likewise, they tested how 25-μl P&S vaccine formulations worked compared to 50-μl intranasal inoculations. Finally, the team evaluated humoral and cellular mucosal immune responses on days 91 and 140 in mice who received IM mRNA-LNP and were boosted with IN spike three months later.

Results

The B_RMcells in the lungs serve as an important local immune effector in protecting against SARS-CoV-2. P&S led to increased antigen-specific B cells within lung tissue. It also increased class-switched antibody-secreting cells (ASC) and class-switched BRM cells in lung tissue expressing IgA or IgG. Thus, P&S elicited local B cell responses in the lung. Furthermore, P&S expanded the lung parenchyma and airway CD8+ TRM and CD4+ TRM cells.

The results showed that the robustness of P&S was modifiable across multiple experimental variables, yet, it did not affect overall immune responses. Most importantly, even delayed IN P&S boosting (up to three months after priming) elicited durable mucosal humoral and cellular immune responses.

Intranasal SARS-CoV-2 spike boosting protects against COVID-19-like disease.(A) Experimental schema: K18-hACE2 mice were IM primed with 0.05 μg of mRNA-LNP and IN boosted with 1 μg of spike IN 14 days post-IM Prime. Six weeks post boost, mice were challenged with 6×10⁴ PFU SCV2 (2019n-CoV/USA_WA1/2020). The first cohort was used to evaluate weight loss and survival up to 14 days post-infection (DPI). The second cohort was used to collect lung and nasal turbinate tissues 2 DPI for viral titer measurement. The third cohort was used to collect lung tissues 5 DPI for histological assessment. (B to D) Weight loss and survival of naïve, IM Prime, or P&S mice from 1 to 14 DPI. (E to F) Measurement of infectious virus titer in lung and nasal turbinate tissues at 2 DPI by plaque assay. (G) Pathology score of lung sections at 5 DPI by hematoxylin and eosin (H&E) staining. (H) Representative H&E staining results from uninfected IM Prime, or P&S mice. Scale bar: 250 μm. Sections are representative of multiple sections from at least five mice per group. (I) Experimental schema: K18-hACE2 mice were IM primed with 0.05 μg of mRNA-LNP and IN boosted with 10 μg of mRNA encapsulated by PACE (IN PACE-Spike) 14 days post-IM Prime. Six weeks post boost, mice were challenged with 6×10⁴ PFU SCV2 (2019n-CoV/USA_WA1/2020). Weight loss and survival were monitored up to 14 DPI. (J to L) Weight loss and survival of naïve, IM Prime, or Prime and PACE-Spike K18-hACE2 mice from 1 to 14 DPI. Mean ± s.e.m.; Statistical significance was calculated by [(D) and (L)] log-rank Mantel-Cox test, [(E) and (F)] one-way ANOVA followed by Tukey’s correction, or (G) Student’s t test; *P≤0.05, **P ≤ 0.01, ***P ≤ 0.001, ****P ≤ 0.0001. Individual data points are represented and pooled from two independent experiments.

Conclusions

FluMist is the only approved respiratory mucosal vaccine that relies on a live attenuated influenza virus. It is only approved for young people and is contraindicated in people with pre-existing respiratory conditions. Several recombinant subunit vaccines are administered IN but require co-formulation with adjuvants to enhance immunogenicity. However, administering such vaccines to the respiratory tract in humans has proven difficult without adjuvants. Also, IN adjuvanted inactivated influenza vaccine has led to Bell's palsy in some cases, likely due to adjuvant toxicity mediating neuronal inflammation. As SARS-CoV-2 continues to evolve and become more immune evasive and transmissible, boosting that induces mucosal immunity is urgently warranted.

The current study described the preclinical development of an alternative vaccine strategy, P&S. P&S utilized diverse unadjuvanted spike subunit protein(s) to elicit strong protective mucosal immunity following mRNA-LNP parenteral vaccination. Since P&S leveraged pre-existing immunity rather than inhibited it, unadjuvanted IN P&S boosting proved more successful in individuals who had received multiple previous vaccine doses. In addition, P&S could also trigger mucosal immunity to other sarbecoviruses, such as SARS-CoV-1. Further, it could be broadly applicable as a booster against new SARS-CoV-2 VOCs in a previously vaccinated individual or as a de novo primary immunization strategy against other emerging respiratory pathogens.