Background

Tuberculosis (TB) is the infectious disease with the highest number of deaths worldwide based on a single pathogen. The causative agent, Mycobacterium tuberculosis (Mtb), penetrates deep into the alveoli via respiratory droplets and causes latent infection. Currently, the only available vaccine is Bacillus Calmette-Guérin (BCG), developed over 100 years ago. While effective for severe tuberculosis in newborns, it has the drawback of showing a wide variation in adult pulmonary tuberculosis prevention rates ranging from 0% to 80% depending on the population.

With one-quarter of the global population in a latent TB state, there is an urgent need for immune technologies to prevent adult pulmonary infection. mRNA and Lipid Nanoparticle (LNP) technologies, pillars of infectious disease response, have begun to be applied to TB vaccine research. However, conventional intramuscular mRNA-LNP methods, while effective for systemic immune induction, have the limitation of failing to establish defensive cells in the respiratory mucosa, the gateway for pathogen invasion.

Key Findings

A research team led by Professor Inne Lentacker at Ghent University (Belgium) designed a heterologous prime-pull immunotherapy combining mRNA-LNP loaded with the glycolipid adjuvant alpha-Galactosylceramide (αGC) with intranasal administration. The strategy involves encapsulating mRNA encoding the key TB antigens ESAT-6 and Ag85B into LNPs formulated with αGC to stimulate invariant Natural Killer T (iNKT) cells. The αGC-containing LNPs coordinated immune response pathways by activating iNKT cells in a controlled manner without excessive inflammation.

In the biological assay, αGC-containing mRNA-LNPs promoted the proliferation of CD4+ T cells with an activated memory phenotype (CD44+CD69+CD62L-) in the spleen. The modulation of helper T cell (Th) responses was also notable. By maintaining Th1 responses while inducing Th2 and Th17 responses, a multifaceted defense network was formed. In the CB6F1/J mouse model reflecting genetic diversity, ESAT-6 and Ag85B-specific CD8+ cytotoxic T cell responses were selectively expressed. This result, showing that a response deficient in a single strain was induced depending on the genetic background, provides evidence that host genetic polymorphism must be treated as an essential variable during vaccine evaluation.

The peak of the respiratory defense barrier was achieved using a prime-pull technique that combined systemic administration with local mucosal stimulation. This method first induces systemic immunity via intramuscular injection (prime) and then administers antigens to the airway mucosa to recruit immune cells to the lung tissue (pull). Applying this protocol led to a surge in the frequency of CD4+ T cells with a tissue-resident memory T cell (TRM) phenotype that reside in lung tissue and repel early invading pathogens. The αGC formulation in LNPs demonstrated a synergistic effect in enhancing T cell infiltration and resident engraftment within lung tissue.

Significance and Outlook

The success of respiratory infectious disease vaccines depends on how densely a cellular immune network is established in the lung mucosa. This study demonstrates that, moving beyond methods focused solely on antigen expression rates, mucosal resident memory immunity can be actively constructed through the optimization of lipid adjuvants and administration routes. Can this platform technology expand beyond TB to respond to various respiratory pathogens? The design of integrating iNKT-stimulating ligands into nanoparticle formulations holds the potential to broaden the horizons of next-generation vaccine development.

The practical hurdles to be overcome for clinical entry are also clear. It is essential to establish optimal doses and intervals to prevent iNKT cell anergy that may occur with repeated αGC administration. Furthermore, infection challenge tests must be conducted to verify whether the TRM-inducing efficacy observed in small animal models translates to equivalent protective immunity in primates or in actual human lung environments. Securing the physicochemical stability and mucosal absorption safety of inhalation and nasal formulations is also a key factor for commercialization.