mRNA-LNP vaccines targeting conserved virulence factors elicit protective immunity against lethal Legionella pneumophila infection

Abstract

Legionnaires’ disease (LD), caused by the intracellular pathogen Legionella pneumophila , imposes a substantial global health burden, yet no vaccine has been clinically approved. Here, we develop mRNA-LNP vaccines encoding conserved virulence factors Pal, PilE, and FlaA. In BALB/c mice, immunization expands T follicular helper and germinal center B cells, elicits high-titer IgG, and induces potent IFN-γ⁺ Th1-type responses, with no observed systemic toxicity. In a lethal challenge model (2 × 10 8 CFU in susceptible A/J mice), monovalent FlaA-LNP confers superior protection with 100% survival and sterilizing immunity, whereas PilE-LNP is less protective. Following sublethal challenge, spatiotemporal profiling in A/J mice reveals systemic CD8⁺ effector responses. At the pulmonary mucosa, more protective vaccine formulations (Pal-, FlaA-, and MIX-LNP) expand CD4⁺ and CD8⁺ memory T cells; yet within the tissue-resident memory T (TRM) subset, the increase is selectively restricted to the CD8⁺. This CD8⁺ TRM enrichment, together with increased alveolar macrophages and mucosal sIgA responses, correlates with protective efficacy and defines the local immune landscape. In the bone marrow, plasma cell fate is antigen-dependent: FlaA is associated with acute short-lived plasma cell bursts, whereas PilE is required for long-lived plasma cell seeding. Correlation analyses further link the Tfh–GCB axis and local lung microenvironment (including memory T cells, alveolar macrophages, and mucosal sIgA) to protective efficacy. Collectively, these findings demonstrate that mRNA-LNP vaccines orchestrate a multilayered immune defense against L. pneumophila , coupling acute humoral recall, mucosal CD8⁺ TRM establishment, and antigen-dependent plasma cell fate, providing a translatable mechanistic framework for next-generation vaccines. It schematically summarizes antigen selection from Legionella pneumophila, mRNA-LNP vaccine construction, immunization and immune responses, lethal intratracheal challenge protection, and the mechanistic immune landscape in the lung, spleen, and bone marrow