Immunological Characteristics of Triploid Grass Carp (Ctenopharyngodon idella) and evaluation of its phagocytosis in the macrophages of head kidney

Triploid fish are considered a promising candidate for aquaculture; however, their immunological properties remain incompletely characterized. In this study, we investigated the immunological differences between 10-month-old triploid and diploid grass carp. Triploids exhibited a higher head kidney index and obvious macrophage aggregation in the head kidney compared to diploids. Among the immune and antioxidant indices, serum IgM content, head kidney lysozyme and acid phosphatase activities, and catalase and total superoxide dismutase activities, were significantly elevated in the triploids. Transcriptomic analysis identified 610 differentially expressed genes (DEGs) in the head kidney between triploids and diploids. KEGG pathway enrichment analysis revealed that these DEGs were primarily associated with primary immunodeficiency and phagosome pathways. Notably, genes involved in antigen presentation, immunoregulation, phagocytosis, and inflammation were significantly down-regulated in triploids. In contrast, MYD88 , macrophage proliferation genes, and macrophage marker genes were up-regulated. Further investigation into macrophage function demonstrated that triploid macrophages exhibited reduced phagocytic capacity, but maintained respiratory burst activity comparable to that of diploids. This was accompanied by elevated expression of respiratory burst-related and lysosome-associated genes. Collectively, these findings suggest that although triploid grass carp exhibit diminished macrophage phagocytosis and inflammatory responses, compensatory immune regulatory mechanisms may be at play. This study provides a theoretical foundation for the application of triploid grass carp in aquaculture.