Low-frequency ultrasound alleviates pulmonary inflammation induced by Klebsiella pneumoniae in mice by inhibiting the TNFR1/NF-κB pathway

Background: Therapeutic ultrasound has been found to promote tissue healing and reduce inflammation in noninfectious diseases, but its efficacy in infectious inflammation remains unclear. Here, we employ the mice pneumonia model to explore the anti-inflammatory effects of low-frequency ultrasound (LFU) and elucidate its potential molecular mechanisms. Methods: Pneumonia in mice was induced by intratracheal instillation of 100 mu L of a 4.5 x 108 CFU/mL Klebsiella pneumoniae (Kp) bacterial suspension. A single LFU treatment (29.36 kHz, 270 mW/cm2, 10 min) was applied to the chest of mice at 6 or 48 h after infection. Biological samples were collected for gene, protein, and cellular experiments. Results: LFU demonstrated good anti-inflammatory effects in mice during the recovery phase of Kp infection (48 h after infection). Although LFU alone had no bactericidal effects, it slightly reduced the pathological score of lung injury and significantly decreased the infiltration of CD45+ leukocytes. Additionally, the protein levels of TNF-alpha, GM-CSF and COX-2 in the bronchoalveolar lavage fluid were significantly reduced. Bulk RNA-sequencing results showed that the TNF receptor (TNFR)/NF-kappa B pathway was up-regulated after Kp infection, which was suppressed after LFU treatment. Western blot and immunofluorescence revealed LFU significantly reduced the protein levels of TNFR1, p-p65, and nuclear p65. The anti-inflammatory effect of LFU was comparable to a 20 mg/kg NF-kappa B inhibitor and superior to a 15 mg/kg TNFR antagonist. Conclusion: LFU exerts anti-inflammatory effects by inhibiting the TNFR1/NF-kappa B pathway during the recovery period of Kp infection, reducing inflammatory transcription and thereby decreasing the release of inflammatory factors.