Circulating miRNAs involved in the immune response of the European seabass (Dicentrarchus labrax)

Understanding the immune response in fish through transcriptomic and microRNA (miRNA) profiling may un- lock critical insights into disease resistance mechanisms. The objective of the present study was to examine the immune modulation of the European seabass (Dicentrarchus labrax) following bacterial infection and vaccination. Therefore, sequencing of circulating miRNA isolated from blood serum and 3′UTR transcriptome sequencing of head kidney was conducted. In the infected fish 19 miRNAs were found to be differentially expressed. This included two novel miRNAs exhibiting high levels in the infected fish. Regarding circulating miRNAs following vaccination, three specific miRNAs have been identified that demonstrated a substantial increase in expression. Two of them, miR-216b and miR-30a-5p, have been documented to possess the capacity to delay the progression of viral infections. 3′UTR sequencing analysis of the infected fish revealed no significant enrichment of down- regulated transcripts. However, there was a significant enrichment of up-regulated transcripts related to ribo- some biogenesis and protein processing. In vaccinated fish up-regulated transcripts did not demonstrate sub- stantial enrichment. Down-regulated genes on the other hand were involved in cytoskeleton organization and apoptosis, indicating that cellular disruption might be a potential hindrance to effective immunity. Overall, these results provide first insights into the progression and regulation of host immune responses to pathogen infection and vaccination. Moreover, the detection of in total 13 differential expressed circulating miRNAs, including regulators of critical innate immunity-related genes such as Toll-like receptor 18, suggests a potential for circulating miRNAs to play a significant role in the post-transcriptional control of fish immune defenses.