Tryptophan catabolism reprograms immunity to prevent hemolymph bacterial dysbiosis and support molting in Spätzle-deficient Helicoverpa armigera larvae

BACKGROUND: Molting is a critical yet vulnerable stage in insect development, during which the peritrophic matrix is temporarily disrupted. This transient loss of gut barrier integrity can allow gut bacteria to escape into the hemocoel, where they may proliferate in the nutrient-rich hemolymph, leading to developmental impairments or septicemia. Despite this threat, insects typically complete molting successfully, suggesting the involvement of robust immune mechanisms that control hemolymph bacterial load. RESULTS: This study identifies the Spätzle gene cluster (HaSpz3-6) in the cotton bollworm (Helicoverpa armigera) as essential for limiting hemolymph bacterial proliferation during molting. Knockdown of HaSpz3-6 suppresses antimicrobial peptide expression, leading to Bacillus spp. overgrowth and delayed molting. The ensuing bacterial proliferation triggers tryptophan catabolism, elevating serotonin and N-acetylserotonin (NAS) levels. Serotonin enhances phagocytosis, which cooperates with reactive oxygen species (ROS) to reduce bacterial burden. NAS subsequently mitigates oxidative stress by scavenging excess ROS. CONCLUSIONS: These findings reveal that Spätzle-mediated immunity is crucial for bacterial regulation during molting. When this pathway is impaired, tryptophan catabolism provides a compensatory defense, albeit with developmental trade-offs. Overall, this study underscores the plasticity of insect immune responses and highlights the critical role of Spätzle in safeguarding developmental transitions.