Characterizing immune responses of grey mullet (Mugil cephalus) to Nocardia seriolae ex vivo and in vivo

Grey mullet (Mugil cephalus) is an economically valuable species but is highly susceptible to Nocardia seriolae, leading to significant mortality. The immune responses of grey mullet to N. seriolae are not well understood, limiting the development of effective prevention strategies. This study aimed to clarify the immune responses of grey mullet to N. seriolae. Immune cells were exposed to N. seriolae ex vivo for 48 h, followed by de novo transcriptome analysis. In vivo, RT-qPCR was used to compare gene expression in N. seriolae-infected fish and controls at 48 h post-infection. Transcriptome analysis identified 65,276 unigenes, with 654 differentially expressed genes associated with immune pathways, notably Th17 differentiation and IL-17 signaling. KEGG, Ingenuity Pathway Analysis, and GSEA further supported these findings, indicating that N. seriolae activates immune responses against intracellular pathogens and may evade immunity through macrophage alternative activation. The in vivo results showed similar immune responses post-infection. These insights could inform strategies to protect grey mullet from N. seriolae infection.