Mechanisms of immune responses in Acanthopagrus latus to Streptococcus agalactiae infection revealed by transcriptomic analysis.

Acanthopagrus latus (yellowfin seabream) is an economically important fish in the southeast coastal sea of China. Its slower growth rate makes it more prone to diseases in the cultivation period, leading to substantial economic losses. Epidemiological investigations have indicated that Streptococcus agalactiae is one of the most common Gram-positive pathogens, which have garnered increasing attention due to its high contagion and lethality rates in A. latus. In this work, an infection model of yellowfin seabream was established with an intraperitoneal injection of S. agalactiae. Clinical sign observations and various analyses, including histological examination, serum biochemical index assessment, immune-related enzyme level measurement, and transcriptome analysis of tissues (liver and intestine) with obvious clinical signs, were conducted for revealing the effects of S. agalactiae infection and immune response mechanisms in yellowfin seabream. The results indicate that evident clinical signs and multi-tissue damages with the notable changes in indices and significant increase in immune-related enzyme levels in the serum occurred in infected fish. RNA sequencing analysis identified 1130 differentially expressed genes (DEGs) in the liver and 1218 DEGs in the intestine, which were involved in multiple immune- and metabolism-related pathways via KEGG enrichment analysis. The transcriptomic results were further corroborated by quantitative real-time RT-PCR (qRT-PCR) tests of some specific immune-related genes. These findings provide new insights into the molecular immune mechanisms in yellowfin seabream following S. agalactiae infection and offer valuable reference data for disease prevention and molecular breeding (i.e., selective breeding through developing molecular markers of key genes).