Interleukin-9 Promotes Liver Fibrosis in Schistosomiasis Japonica by Regulating Retinoic Acid Expression

Objective: To investigate the molecular mechanism by which interleukin-9 (IL-9) mediates liver fibrosis in schistosomiasis japonica through disruption of retinoic acid (RA) homeostasis, and to determine the protective effect of exogenous RA intervention against hepatic fibrosis in schistosomiasis japonica. Methods: Eighteen mice were randomly assigned to three groups: normal control (NC), Schistosoma japonicum (S. japonicum) infection (SJ), and RA intervention (SJ + RA). After 9 weeks of infection, liver fibrosis indicators were measured by qRT-PCR, Western blotting, ELISA, and immunohistochemistry. In cell experiments, hepatic stellate cells (HSC) were stimulated with IL-9, and an HSC–mast cell transwell co-culture system was established; HSC activation markers were examined, and key genes in the RA synthesis and metabolic pathways were identified by RNA-seq and verified by qRT-PCR. In addition, exogenous RA was applied to TGF-β1-activated HSC to investigate the inhibitory effect of RA on HSC activation. Results: Animal experiments showed that S. japonicum infection significantly upregulated the IL-9 mRNA expression in mouse liver tissues, increased the infiltration of IL-9-positive cells in the egg granuloma areas, and markedly reduced the hepatic RA content. Cell experiments showed that IL-9 stimulation alone did not directly induce HSC activation; however, in the HSC–mast cell co-culture system, IL-9 significantly upregulated the expression of HSC activation markers. RNA-seq analysis combined with qRT-PCR verified that, under HSC–mast cell co-culture conditions, the lipid droplet-associated proteins PLIN2 and PLIN3 were downregulated, with a more pronounced decrease in PLIN3, whereas RDH10, the rate-limiting enzyme of RA synthesis, was upregulated, approaching but not reaching statistical significance. Furthermore, in vivo and in vitro rescue experiments confirmed that exogenous RA supplementation effectively inhibited HSC activation and alleviated the liver fibrosis induced by S. japonicum infection. Conclusions: IL-9 suppressed PLIN2/3 expression and impaired RDH10-mediated RA expression via mast cell paracrine signaling. The resulting decline in local hepatic RA concentration disrupted the quiescent state of HSC, ultimately inducing aberrant HSC activation and thereby promoting the progression of liver fibrosis in schistosomiasis japonica.