Histopathology, immunoenzyme activity and transcriptome analysis of immune response in silver pomfret infected by cryptokaryon (Cryptorchidism irritant).

Cryptorchidism irritant (CI) infection is a major problem in the culturing process of silver pomfret (Pampus argenteus), which can result in rapid and massive death. However, there is limited information available on the immune response of silver pomfret infected by CI. To address this gap, we sampled naturally infected fish and observed milky white translucent oval CI trophozoites on the gills, body surface, and fin rays. Histological analysis showed that CI infection led to vacuolation of epithelial cells and a decrease in blood cells in the gills. We also performed transcriptome profiling of the gill, kidney complex, and spleen, generating 399,616,194 clean reads that assembled into 101,228 unigenes, which were annotated based on public databases. We detected 14,369 differentially expressed genes, and selected several key immune-related genes for further validation using RT-qPCR. The Graft-versus-host pathway and Allograft rejection pathway were enriched in the gills, leading to inflammation and ulceration. CI infection activated the immune system, increasing levels of interleukin-1 beta and MHC class II antigen, and also activated innate and acquired immune genes in silver pomfret. Furthermore, we measured the activities of five immune-related enzymes (SOD, AKP, CAT, CSH and ACP), which all increased to varying degrees after CI infection. Our findings enhance our understanding of the immune response of fish to parasitic infection and may contribute to the development of strategies to prevent high mortality in CI-stimulated fish in aquaculture.