Microalgal delivery of recombinant fish interferon modulates gut microbiota and enhances antiviral immunity in fish

Viral infections remain a challenge to aquaculture, resulting in severe economic losses and threatening fish health worldwide. As a key immunomodulatory and antiviral factor, interferon (IFN) plays a crucial role in regulating immune responses. We constructed a high-level expression strain of recombinant interferon (Rec-IFN) using Synechococcus sp. PCC 7002, which also served as a delivery system, and evaluated its efficacy as a dietary immunostimulant in gibel carp (Carassius gibelio) and zebrafish (Danio rerio). Analysis of the intestinal microbiome indicated that the Rec-IFN diet promoted beneficial bacteria such as Cetobacterium while reducing opportunistic pathogens including Aeromonas and Vibrio in gibel carp. Notably, after Cyprinid herpesvirus-2 (CyHV-2) infection, the Rec-IFN diet enhanced microbial connectivity, helping to preserve gut microbiota function. In zebrafish, the Rec-IFN diet increased species richness and evenness, while reducing opportunistic pathogens such as Vibrio. Transcriptomic analysis revealed specific activation of the Toll-like receptor signaling pathway and a reduction of immune overstimulation following infection in zebrafish. Our findings demonstrate that the Rec-IFN diet significantly enhanced the host IFN response and alleviated virus-induced damage to intestinal and immune organs, reduced viral load, and decreased mortality. This research offers a protective effect by reducing the severity of infections caused by both DNA virus (CyHV-2) and RNA virus (SVCV). It provides new insights into the application of Rec-IFN microalgae as an effective oral immunotherapeutic strategy for reducing losses from viral infections in aquaculture.