Sexual dimorphism of antiviral immunity in grass carp (Ctenopharyngodon idella): TLR7 and TLR8b promote resistance to GCRV in males, whereas TLR8a increases susceptibility in females.

Sexual dimorphism of immune responses is widespread in vertebrates. In mammals, Toll-like receptor 7 (TLR7) and TLR8 are tandemly localized on the X chromosome and constitute the key molecular basis for sex-biased immunity. However, most teleosts lack typical sex chromosomes, and due to whole-genome duplication (WGD), the TLR family has expanded and functionally diverged. In grass carp (Ctenopharyngodon idella), CiTLR7 and CiTLR8a are tandemly located on chromosome 9, while CiTLR8b resides on chromosome 10. The two TLR8 paralogs have undergone functional divergence, yet their involvement in sex-dependent immune regulation remains unclear. Using a type II grass carp reovirus (GCRV-II) immersion infection model, we systematically analyzed sex-specific immune responses and the regulatory roles of CiTLR7 and CiTLR8a/b. Females exhibited higher susceptibility, mortality, tissue damage, and viral load. In contrast, males displayed stronger immune defense, with elevated innate immune activity, IgM expression, and activation of antiviral molecules, including AP-1/JunD, NF-κB1, and IFN1, effectively suppressing viral replication. Histopathological analysis and transcript levels of GCRV-VP4/VP56 confirmed these observations. Notably, CiTLR7 and CiTLR8b were highly expressed in males and resistant individuals, enhancing antiviral immunity, whereas CiTLR8a was upregulated in females and susceptible individuals, suggesting a potential pro-viral role. These findings reveal functional differentiation among CiTLR7 and CiTLR8a/b, highlighting their critical involvement in sex-dependent immune regulation. Overall, this study demonstrates the importance of the TLR7/8 signaling pathway in fish immune sexual dimorphism, providing new theoretical insights for understanding vertebrate sex-biased immunity and for disease-resistant breeding in aquaculture.