Temporal transcriptome analysis of head kidney revealed protective cellular immunity in flounder (Paralichthys olivaceus) immunized with inactivated Edwardsiella piscicida.

Vaccines are effective in preventing infection by pathogens. The inactivated Edwardsiella piscicida vaccine to elicit immune response after vaccination of flounder (Paralichthys olivaceus) has been shown in previous studies. The molecular mechanism of this protection is yet to be clarified. Transcriptome sequencing was employed to investigate head kidney of flounder on day 7 post-immunization with an inactivated vaccine, as well as on days 1 and 7 following E. piscicida infection, 5 weeks after immunization. Cellular immune responses and histopathology were also studied. The analysis revealed that each library generated an average of 50 million raw reads, with over 99.5 % of them passing quality control, and Q30 values exceeding 92.5 % in every instance. The numbers of differentially expressed genes (DEGs) detected were 1257 on day 7 after immunization, and 832 and 1545 on days 1 and 7 after challenge, respectively. These DEGs were mapped to the GO and KEGG databases, revealing their involvement in immune functions such as antigen presentation, cytokine interactions, and cell differentiation. To explore immune-related DEGs linked to vaccine protection, 34 genes were primarily involved in the Toll-like and NOD-like receptor signaling pathways on day 1 post-challenge. Analysis of protein interactions identified ten hub genes (IL6, IL1B, NFKBIA, VCAM1, etc.) involved in immune activation and the initiation of innate responses. 66 DEGs were notably involved in T cell receptor signaling and Th cell differentiation on day 7 post-challenge. Fifteen hub genes (SRC, IL10, JUN, CD28, etc.) were closely associated with cellular immune response and differentiation. On day 7 post-challenge, the vaccine group exhibited a notable increase in CD4 cells. Flounder had low E. piscicida loads in the head kidney after challenge. Responses of multiple signaling pathways and differentiation of T cells provide protection after pathogen infection. The study enhances the understanding of the vaccine's immune protection mechanism.