Edwardsiella virulence protein P modulates immune response and cell death in Japanese medaka, Oryzias latipes

• Δ evpP infection induces early upregulation of inflammatory genes in fish. • Deletion of EvpP accelerates cell death in macrophages and neutrophils. • EvpP transiently suppresses host cell death, enabling temporal immune evasion. Edwardsiellosis, caused by Edwardsiella piscicida , leads to major losses in global aquaculture, and understanding its infection mechanisms is critical for effective disease control. The type VI secretion system (T6SS) effector Edwardsiella virulence protein P (EvpP) is a key virulence factor that inhibits inflammasome assembly and suppresses pyroptosis in mice; however, its role in fish pyroptosis responses remains unclear. In this study, EvpP function was explored in Japanese medaka ( Oryzias latipes ) using an EvpP deletion strain (Δ evpP ) and a wild-type (WT) strain. Fluorescence imaging and quantitative real-time PCR (qPCR) showed that early intestinal colonization occurred in both groups, whereas bacterial loads in the kidney, liver, and posterior intestine were significantly lower in the Δ evpP -infected group at later time points (3 days post-infection, dpi). Immune-related gene ( il1b and mcp1b ) expression was significantly upregulated during Δ evpP infection. Flow cytometry with propidium iodide (PI) staining indicated increased cell death in the Δ evpP -infected group at 1 dpi, whereas the WT-infected group showed greater levels at 3 dpi. At 1 dpi, kidney macrophages and neutrophils showed significant between-group differences in cell death. These temporal differences indicate that EvpP delays early host cell death, consistent with the suppression of inflammasome-associated responses during the initial stage of infection. These findings were achieved using a Japanese medaka teleost model relevant to marine fish for the first time and suggest that mechanisms related to the inflammasome-associated responses observed in mice are also likely present in fish.