Defensive response of European sea bass (Dicentrarchus labrax) against Listonella anguillarum or Photobacterium damselae subsp. piscicida experimental infection

Sea bass were experimentally infected with Listonella anguillarum or Photobacterium damse-lae subsp. piscicida (Phdp). At 24 and 72 h post-infection, the expression analysis ofimmune-relevant genes (IL-1, IL-6, IL-8, Hepcidin), the transcriptional level and detec-tion of HSP70, and the quantification of serum iron were investigated in association withthe histological analysis and the bacterial recognition in tissues by immunohistochemistry.At 15 days post-infection, the specific antibody response was detected in surviving fish,as well as the transcriptional levels of TcR and BcR sequences. Both experimental infec-tions were characterized by a similar acute response, whereas different histological andimmunohistochemistry evidences were observed. In particular, the early reaction appearedsuitable for the clearance of L. anguillarum, thus limiting the histological lesions, the bac-terial dissemination and the further development of acquired immunity in surviving fish.On the contrary, the innate response appeared not enough to resolve the Phdp infection,which was characterized by tissue damage, bacterial widespread and substantial detec-tion of specific humoral immunity in surviving fish, also associated to lymphocytes clonalexpansion. Besides the opportunistic conditions involved in fish vibriosis and pasteurel-losis, the comparison between these experimental infection models seems to suggest thatthe rate of development of the acquired immunity is strictly linked to the activation of thehost innate response combined to the degree of bacterial virulence