FMDV VP3 Induces IL-10 Expression in Porcine Macrophages via PI3K Interaction and PI3K/AKT-mTOR Pathway Activation

Foot-and-mouth disease virus (FMDV) infection elicits sustained, high-level interleukin-10 (IL-10) secretion in cattle and pigs, which correlates with lymphopenia and immunosuppression. We previously showed that macrophages are the principal source of IL-10 during FMDV infection in mice, but the viral trigger and host pathways remained unknown. In the present study, we examined whether the FMDV structural protein VP3 regulates IL-10 expression. To this end, a eukaryotic VP3 expression vector was transfected into porcine alveolar macrophages (3D4/21 cells), and IL-10 expression together with related signaling pathways was interrogated by qRT-PCR, ELISA, Western blot, co-immunoprecipitation (Co-IP), confocal microscopy, and luciferase reporter assays. The results showed that VP3 significantly increased IL-10 mRNA and protein levels (p < 0.001) in a time-dependent manner. Mechanistically, VP3 promoted phosphorylation of PI3K, AKT, and mTOR; this effect was abolished by the PI3K inhibitor LY294002, which also abrogated VP3-induced IL-10 secretion (p < 0.05). Furthermore, VP3 upregulated mRNA expression of STAT3, ATF1, and CREB (p < 0.05) and enhanced IL-10 promoter activity. The STAT3 inhibitor Stattic reduced IL-10 secretion by 22% (p < 0.05). Co-IP and confocal microscopy confirmed direct binding of VP3 to PI3K in the cytoplasm. In conclusion, FMDV VP3 induces IL-10 overexpression by directly activating the PI3K/AKT-mTOR signaling pathway, thereby elucidating a key mechanism of FMDV-induced immunosuppression.