Identification of an isg15 gene in Nile tilapia (Oreochromis niloticus) and analysis of its role in immune response modulation.

Interferon-stimulated gene 15 (Isg15), a ubiquitin-like protein induced by type I interferon (IFN), plays a pivotal role in antiviral innate immunity; however, its antibacterial functions in teleosts remain less clear. In this study, we identified and characterized an isg15 gene (Onisg15) from Nile tilapia (Oreochromis niloticus) and explored its regulatory role in antibacterial immune responses. Onisg15 encodes a 156-amino-acid protein containing two conserved ubiquitin-like (UBL) domains and a canonical C-terminal LRLRGG conjugation motif, which are characteristic structural features of Isg15 homologs. The UBL domains adopt typical β-grasp folds and α-helical structures. The Onisg15 gene was constitutively expressed in all tested tissues of healthy tilapia, with the highest transcript levels detected in the heart. Infections with Edwardsiella piscicida and Streptococcus agalactiae both significantly induced Onisg15 expression in the head kidney, spleen, liver, and peripheral blood. Recombinant OnIsg15 (rOnIsg15) expressed and purified from Escherichia coli markedly enhanced both the bactericidal activity and respiratory burst capacity of tilapia primary head kidney macrophages. Moreover, OnIsg15 significantly elevated the transcript levels of type I IFN (IFNc, IFNh, IFNd1, IFNd2) and proinflammatory cytokines (IFNγ, IL-6, and IL-8). Taken together, these findings suggest that OnIsg15 likely contributes to host immune defense against bacterial infection by modulating macrophage effector functions and proinflammatory cytokine expression.