Publication date: Oct 28, 2024

The microbiota plays a crucial role in inducing immune responses. Our previous studies have shown that symbiotic bacterial sensing by the nucleotide-binding oligomerization-domain-containing protein 2 (Nod2) receptor is involved in the mucosal adjuvanticity of cholera toxin. However, cholera toxin’s potential toxicity limits human use. Here, screening other less toxic adjuvants showed that toll-like receptor (TLR) 4 and 7 agonists synergized with the microbiota in inducing adaptive immune responses upon nasal immunization. Particularly, Imiquimod, a TLR7 agonist, exhibited synergistic effects with bacterial component MDP, a Nod2 ligand, in inducing immune responses, such as IL-12p40 and IL-6 productions in bone marrow-derived dendritic cells (BMDCs) and follicular helper T (T) cell differentiation and high-affinity antibody production in immunized mice. The Imiquimod-MDP combination notably elicited immune protection against influenza and SARS-CoV-2 infections. Furthermore, we isolated some bacteria from the nasal cavity of healthy donors, and their Nod2-stimulating activities were measured using a reporter cell line. Staphylococcus aureus, with notable Nod2-stimulating activity, showed higher synergy with Imiquimod than Staphylococcus epidermidis, while the synergistic effects by Imiquimod-bacteria combination disappeared in Nod2-knockout mice. Moreover, the pretreatment with S. aureus enhanced the protective effect of Imiquimod-mediated vaccination against influenza virus compared to S. epidermidis. These results imply that the Imiquimod-MDP and the Imiquimod-bacteria combinations could be novel and promising complex adjuvants in developing intranasal vaccines.

Semantics

Original Article