Immunophage Synergy: Supporting Innate Immunity with Phages to Combat Bacterial Pneumonia

Bacteriophage (phage) therapy offers a promising strategy for treating (lung) infections caused by multi-drug-resistant (MDR) bacteria. However, its effectiveness is influenced by numerous factors, including the host immune response. Neutrophils play a key role in clearing pathogenic bacteria, yet their involvement in phage therapy remains underexplored. Here, we investigated immune interactions in phage therapy for <italic>Pseudomonas aeruginosa</italic> infection, using a murine pneumonia model with selective depletion of alveolar macrophages or neutrophils before infection. In neutrophil-depleted PAO1-infected mice, phage therapy with a two-phage cocktail administered at 2 hours post-infection (hpi) failed to control the infection. Conversely, the same phage cocktail reduced lung bacterial loads by up to 4 logs at 24 hpi in immunocompetent mice, highlighting the critical role of neutrophils. Interestingly, alveolar macrophages did not exhibit a similar synergistic effect with phage therapy in vivo. While in vivo studies suggest neutrophils are critical for clearing (phage-)resistant bacteria, the exact mechanism—whether through direct neutrophil effector function or indirect activation of other immune pathways—remains uncertain. To examine this further, we employ a human airway organoid infection model with blood-derived neutrophils to analyze the interaction between epithelial cell infection, phage-mediated bacterial lysis, and neutrophil activation. Classical assays alongside advanced imaging techniques, including immunofluorescence and electron microscopy, will provide critical insights into the possible immunophage synergy mechanisms between innate immune cells and phages.