Dynamic immune responses of yellowfin seabream (Acanthopagrus latus) following recurrent challenge with Amyloodinium ocellatum.

Amyloodinium ocellatum is a devastating parasite that affects mariculture worldwide. Currently, no effective vaccine is available against this pathogen. This study investigated the dynamic immune responses of Acanthopagrus latus to recurrent infections with A. ocellatum. Following recurrent sub-lethal challenges, a significant reduction in parasite abundance was observed by the third infection, indicating the development of a protective immune response in the fish. To elucidate the underlying immune mechanisms, the A. latus IgM heavy chain (AlIgM) was cloned and recombinantly expressed for the generation of a specific polyclonal antibody. The AlIgM ORF consists of 1341 bp, encoding a protein of 447 amino acids. Despite the observed development of resistance, serum from challenged fish failed to induce a significant increase in the antibody-mediated immobilization of dinospores. Furthermore, measurements of total and parasite-specific IgM levels by indirect enzyme-linked immunosorbent (ELISA) revealed that these levels either remained unchanged or even decreased in repeatedly infected fish. Real-time fluorescence quantitative PCR (qRT-PCR) analysis revealed a significant upregulation of IgT expression in the gills during the fifth infection, suggesting a potential role for this immunoglobulin in mucosal immunity. These results indicate that resistance to A. ocellatum in A. latus is not mediated by a systemic antibody response but is likely conferred by local mucosal immune mechanisms, particularly in the gills.