Comprehensive analysis of the immunological differences in the intestinal barrier of improved grass carp and their parents

Disease-resistant grass carp (DR-GGC) was an improved grass carp developed by backcrossing gynogenetic grass carp (GGC, female) with grass carp (Ctenopharyngodon iIdella, GC, male), which had better disease resistance advantages than GC in aquaculture. However, little was known about the immune mechanism of disease resistance advantages of DR-GGC compared with GC. This study aimed to investigate the differences in molecular mechanisms of disease resistance among DR-GGC, GGC and GC by comparing intestinal histopathology, antioxidant activity in the liver and spleen, tight junction protein gene expression, intestinal microbiota structure, intestinal transcriptome and metabolomics following intestinal inflammation induced by Aeromonas hydrophila. The histopathological results showed that the intestinal villi of DR-GGC, GGC and GC were damaged with shedding and fusion, the intestinal damage of GC being the most severe. Tight junction protein gene expression in the intestine and antioxidant enzyme activity in the liver and spleen were significantly higher in DR-GGC than in GC (P < 0.05), whereas serum creatinine (CRE) and plasma free hemoglobin (FHB) were significantly below that in GC (P < 0.05). Meanwhile, the transcriptome results showed that the differentially expressed genes (DEGs) were significantly enriched in immune response-related pathways in DR-GGC and GGC compared with GC (P < 0.05). The DEGs were also enriched in cellular and molecular processes in DR-GGC and GC compared with GGC (P < 0.05). The metabolomics results revealed lipid and oxidative metabolism-related differential metabolites (DMs) among the three grass carp groups. Furthermore, twenty amino acids and their metabolites were up-regulated in DR-GGC compared with GC, including aspartic acid, arginine, ornithine and others, and cloud significantly reduces intestinal inflammation by regulating the immune and oxidative responses. And the combined analysis of the transcriptome and metabolome revealed that the DEGs and DMs in these pathways were significantly correlated and regulated. Additionally, the intestinal microbiota analysis showed that DR-GGC had an increased abundance of probiotic Fusobacteriota at the phylum level and Cetobacterium at the genus level. Conversely, GC had an increased abundance of pathogenic Bacteroidetes at the phylum level and Aeromonas at the genus level. In conclusion, this study comprehensively confirmed the greater disease resistance of DR-GGC and further elucidated the potential molecular mechanism of better intestinal immune barrier resistance than that of GC, which is of significant value for the development of the GC.