Bacterial extracellular vesicles (bEVs) are nano-sized, mostly
spherical, double-membrane structures secreted by bacteria throughout
their life cycle. In addition to their role in many prokaryotic
processes, they can interact with cells of the host immune system,
enabling a potential for medical application. This study investigated
how bEVs from various pathogenic bacteria modulate human macrophage
immune responses and their subsequent ability to protect lung alveolar
epithelial cells against SARS-CoV-2 infection.Primary human blood-derived macrophages were stimulated with bEVs derived from Legionella pneumophila (Lp), Klebsiella pneumoniae (Kp), Escherichia coli (Ec), Salmonella Typhimurium (Sal) and Streptococcus pneumoniae (Sp). bEVs from Kp, Ec and Sal
strongly induced pro-inflammatory cytokines and interferon-stimulated
genes via Toll-like receptor 4-dependent signaling. In contrast, bEVs
from Lp or Sp elicited minimal or no immune response. Conditioned media from Kp-, Ec- or Sal-
bEV-stimulated macrophages further influenced Calu-3 epithelial cells,
leading to the induction of interferon-stimulated genes. Notably,
SARS-CoV-2 propagation tended to be reduced in Calu-3 cells
pre-stimulated with conditioned media from Kp-bEV- and Ec-bEV-treated macrophages, as demonstrated by decreased infectious virus titers in TCID50 assays.These findings provide new insights into macrophage-bEV interactions, demonstrating that gram-negative Kp- and Ec-derived
bEVs induce a potent interferon-dependent antiviral state that can be
propagated to lung epithelial cells in vitro. This study highlights a
macrophage-driven mechanism of innate immune modulation that extends
beyond immune cells to shape antiviral responsiveness in structural
cells, providing a conceptual framework for future immunomodulatory or
vaccine-oriented strategies.