Vaccination is one of the key strategies for protection against SARS-CoV-2 infection. However, intramuscular administration does not generate sufficient antibody concentrations in the respiratory mucosa to prevent infection. In contrast, booster intranasal vaccination – even without adjuvants – induces a strong protective immune response in the airways. The mechanisms by which an antigen alone establishes mucosal immunity had remained unclear.
Researchers from Harvard University and Yale University (USA) identified mechanisms underlying the efficacy of intranasal booster immunization. The study was conducted in mice using SARS-CoV-2 vaccines.
Booster intranasal vaccination was shown to transform systemic immunity induced by primary immunization into localized immunity in the respiratory tract.
This transformation follows a defined sequence:
- After primary intramuscular vaccination, pathogen-specific memory B cells are generated in lymph nodes.
- During intranasal booster immunization, epithelial T lymphocytes that detect pathogen-derived antigens produce signalling proteins. These signals disseminate systemically, alerting the organism to viral exposure at the epithelial barrier.
- In response to these signals, memory B cells generated during primary immunization rapidly migrate to the respiratory epithelium. Circulating T lymphocytes are recruited as well.
- Upon reaching the respiratory epithelium, B cells differentiate into plasma cells that produce large amounts of IgA antibodies. T lymphocytes simultaneously secrete additional signaling proteins that continue to recruit B and T cells to the site.
- Consequently, high concentrations of IgA antibodies accumulate in both upper and lower respiratory tract epithelium, effectively preventing infection.
The investigators also clarified why intranasal booster vaccination proved effective without an additional adjuvant. Respiratory epithelial T lymphocytes themselves functioned as a natural adjuvant.
Study Details
In the first experimental series, IgA dynamics following booster vaccination were examined. Mice received primary intramuscular immunization, followed by intranasal boosting on day 14. Beginning on day 2 after booster immunization, a rapid increase in IgA levels in the lung mucosa was observed, peaking on day 7. Even two weeks later, IgA concentrations in the respiratory epithelium remained significantly higher than after intramuscular immunization alone:
Image source: https://www.nature.com/articles/s41590-025-02156-0/figures/1
The researchers then compared the effects of different booster routes – intramuscular, intraperitoneal, intratracheal, and intranasal – on immune landscape remodeling. Significant antibody increases were observed only with intranasal and intratracheal administration. This effect was comparable in male and female mice. The findings indicate that direct antigen delivery to the respiratory mucosa is critical for effective booster responses.
In subsequent experiments, immune landscapes were compared between two groups:
- Group 1 received intramuscular immunization only.
- Group 2 received an additional intranasal booster vaccination.
In both groups, primary intramuscular immunization activated pathogen-specific memory B cells in lymph nodes. These cells were detectable in the inguinal lymph nodes by day 7.
Following booster vaccination, the researchers mapped the migration of activated B cells from lymph nodes to the respiratory epithelium. Although not all B cells could be traced, more than 80% of those reaching the epithelium differentiated into plasma cells capable of producing large amounts of IgA. Thus, epithelial IgA-producing cells originated predominantly from memory B cells generated during primary immunization rather than from de novo activation after boosting.
In mice immunized only intramuscularly, neither B cells nor plasma cells were detected in the respiratory epithelium, indicating that memory B cells formed during primary vaccination are essential for mucosal IgA responses.
Further experiments identified the mechanism driving B-cell migration after booster immunization. Booster exposure triggered recruitment of circulating immune cells to the lungs, including T lymphocytes, NK cells, and other innate immune cells. Genomic analysis revealed that by day 2 post-booster, genes associated with inflammatory responses, antigen presentation, and antiviral defense were upregulated.
Levels of T cell–derived signaling proteins were also measured. Production increased significantly on day 2 and declined rapidly by day 4. These signaling molecules were specifically those known to recruit B cells and circulating T lymphocytes to sites of pathogen detection in the respiratory epithelium.
To assess the contribution of T lymphocytes, mice were treated with an agent that depletes epithelial T cells before booster immunization. Under these conditions, IgA production in the respiratory tract was markedly reduced, while systemic antibody levels in serum remained unchanged. These results demonstrate that T lymphocytes act as a natural adjuvant during booster intranasal vaccination.
In the final experiment, the effect of an additional booster dose was evaluated. A second intranasal booster further increased IgA concentrations in both upper and lower respiratory tracts. This finding is particularly relevant because the nasal mucosa serves as the primary entry point for infection and is the first site exposed to viral challenge.
The authors also suggested that, in the absence of booster intranasal vaccination, periodic inhalation of noninfectious environmental viral doses could potentially maintain elevated antibody levels and immune memory in the nasal epithelium.
Conclusion
Vaccination remains a key strategy for protection against SARS-CoV-2. However, intramuscular immunization alone is insufficient to establish protective immunity in the respiratory epithelium. Booster intranasal vaccination reshapes the immune landscape established by primary immunization and significantly increases antibody concentrations in both upper and lower respiratory tract mucosa.
