Immune response of gilthead seabream (Sparus aurata) following experimental infection with Aeromonas hydrophila

The Gram-negative bacteria Aeromonas hydrophila is a heterogeneous organism that causes the disease known as motile aeromonad septicaemia, which is responsible for serious economic loss in seabream culture due to bacterial infections. However, the immune mechanisms involved in this disease in fish are still poorly understood. For the purpose of this study, gilthead seabream (Sparus aurata L.) specimens received a double intraperitoneal injection of bacterial inoculums: a primary infection with 1 × 107 cell ml−1 A. hydrophila, followed by a secondary infection with 1 × 108 cell ml−1 fourteen days later. Changes in cellular innate immune parameters – phagocytosis, respiratory burst activity and peroxidase leucocyte content – were evaluated 24 and 48 h after each injection. Simultaneously, the expression levels of nine immune-relevant genes (TLR, NCCRP-1, HEP, TCR, IgM, MHC-IIα, IL-1β, C3 and CSF-1R) were measured in the head-kidney, spleen, intestine and liver, by using q-PCR. Generally, the results showed a significant decrease in cellular immune responses during the primary infection and a significant enhanced during the second infection, principally in respiratory burst and peroxidase activity, thus indicating a recovery of the immune system against this bacterial pathogen. Finally, transcript levels of immune genes were down-regulated during the first infection, except for the IL-1β gene. In contrast, mRNA expression levels during the re-infection were significantly up-regulated. The results seem to suggest a relatively fast elimination of the bacteria and recovery of fish during the secondary infection.