Langerhans cells and cDC1s play redundant roles in mRNA-LNP induced protective anti-influenza and anti-SARS-CoV-2 responses.

Nucleoside modified mRNA combined with Acuitas Therapeutics' lipid nanoparticles (LNP) have been shown to support robust humoral immune responses in many preclinical animal vaccine studies and later in humans with the SARS-CoV-2 vaccination. We recently showed that this platform is highly inflammatory due to the LNPs' ionizable lipid component. The inflammatory property is key to support the development of potent humoral immune responses. However, the mechanism by which this platform drives T follicular helper cells (Tfh) and humoral immune responses remains unknown. Here we show that lack of Langerhans cells or cDC1s neither significantly affected the induction of PR8 HA and SARS-CoV-2 RBD-specific Tfh cells and humoral immune responses, nor susceptibility towards the lethal challenge of influenza and SARS-CoV-2. However, the combined deletion of these two DC subsets led to a significant decrease in the induction of PR8 HA and SARS-CoV-2 RBD-specific Tfh cell and humoral immune responses. Despite these observed defects, the still high antibody titers were sufficient to confer protection towards lethal viral challenges. We further found that IL-6, but not neutrophils, was required to generate Tfh cells and antibody responses. In summary, here we bring evidence that the mRNA-LNP platform can support protective adaptive immune responses in the absence of specific DC subsets through an IL-6 dependent and neutrophil independent mechanism.

The mRNA-LNP platform's lipid nanoparticle component used in preclinical vaccine studies is highly inflammatory.

Vaccines based on mRNA-containing lipid nanoparticles (LNPs) are a promising new platform used by two leading vaccines against coronavirus disease in 2019 (COVID-19). Clinical trials and ongoing vaccinations present with very high protection levels and varying degrees of side effects. However, the nature of the reported side effects remains poorly defined. Here we present evidence that LNPs used in many preclinical studies are highly inflammatory in mice. Intradermal injection of these LNPs led to rapid and robust inflammatory responses, characterized by massive neutrophil infiltration, activation of diverse inflammatory pathways, and production of various inflammatory cytokines and chemokines. The same dose of LNP delivered intranasally led to similar inflammatory responses in the lung and resulted in a high mortality rate. In summary, here we show that the LNPs used for many preclinical studies are highly inflammatory. Thus, their potent adjuvant activity and reported superiority comparing to other adjuvants in supporting the induction of adaptive immune responses likely stem from their inflammatory nature. Furthermore, the preclinical LNPs are similar to the ones used for human vaccines, which could also explain the observed side effects in humans using this platform.