Phloretin Inhibits Post-Entry CCoV Replication and Attenuates TNF-α/NF-κB-Mediated Inflammation

Canine coronavirus (CCoV) triggers severe gastroenteritis in canine hosts, and no specific antiviral drug is currently available for clinical treatment. Phloretin, a naturally occurring dihydrochalcone isolated from apples, possesses extensive anti-inflammatory properties, whereas its potential against CCoV infection has not yet been characterized. The anti-CCoV efficacy of phloretin was assessed in the CRFK cell line via CPE-based IC50 measurement, time-of-addition test and quantitative PCR assays. Transcriptome profiling, network pharmacology, molecular docking, elisa and Western blot (p-p65, IκB-α) were further performed to dissect its underlying anti-inflammatory molecular mechanism. Phloretin suppressed CCoV proliferation with an IC50 value of 63.33 µM and mainly functioned at the post-entry replication phase. This was evidenced by a 2.32 log10 copies/µL decrease in viral load and a 2.48 log10 TCID50/0.1 mL reduction in viral titer. Negligible direct virucidal activity and mild anti-adsorption activity (0.43 log10 TCID50/0.1 mL reduction) were detected. Transcriptomic profiling of CCoV-infected cells revealed remarkable activation of the TNF-α/NF-κB signaling pathway upon viral challenge. Based on this observation, network pharmacology was employed to predict the anti-inflammatory targets of phloretin, identifying TNF, IL-6, and JAK2 as hub targets. Molecular docking further predicted stable binding affinities between phloretin and essential intracellular signaling mediators, including JAK2, p38 MAPK, and p65. Consistently, experimental validation demonstrated that phloretin markedly decreased the transcription and protein production of pro-inflammatory mediators (IL-1β, IL-6, IL-8, IL-18, TNF-α), lowered p-p65 abundance by 25.14%, and restored IκB-α expression to significantly above the control baseline.