Cadmium disrupts the immune antibacterial function of the freshwater crab Sinopotamon henanense via the Toll/proPO-AS...

Cadmium (Cd) is a harmful pollutant commonly found in rivers near mining areas, which can interfere with the immune system of freshwater organisms. Furthermore, its combined exposure with pathogens exhibits synergistic toxic effects. To explore the immunotoxic mechanism of Cd and Aeromonas hydrophila on the freshwater crab Sinopotamon henanense, this study constructed a dual stress model to investigate the regulatory role of prophenoloxidase activation system (proPO-AS) and Toll pathways in Cd-mediated immunotoxicity. The results showed that Cd disrupted downstream nuclear transcriptional regulatory nodes of the Toll pathway by inhibiting the expression and nuclear translocation of ShDorsal and upregulating ShCactus expression. Meanwhile, Cd impaired the proPO-AS by affecting ShPpae function, thereby reducing host's anti-infective capacity. This study proposes for the first time the ShPpae-ShDorsal-ShCactus axis as a functional hub connecting proPO-AS and Toll pathways, enabling a dual protection mechanism of "rapid innate-effector function + sustained transcription-dependent defense". Upon single challenge with 1/4 LD₅₀A. hydrophila for 24 h or exposure to 7.25 mg/L CdCl₂ for 7 days, ShPpae activated melanization after cellular entry, while ShCactus degradation promoted ShDorsal nuclear translocation and induced Shalf6 expression. In contrast, under 29 mg/L CdCl₂ single exposure for 7 days or combined Cd-bacterial challenge, the expression and subcellular redistribution of ShPpae were inhibited, and the abnormal upregulation of ShCactus led to tight binding with ShDorsal and blocked its nuclear entry, thereby suppressing the transcription of AMPs. This work reveals a theoretical basis for elucidating the immune regulatory rules of aquatic organisms under combined heavy metal-pathogen stress and enriching the research on heavy metal immunotoxicity.