PAN4, an AI-designed host-directed immunomodulatory peptide, promotes PGRP-SC1a-associated intestinal immune homeostasis in Drosophila melanogaster.

We identify PAN4 (GAYTFKIRRK), an AI-designed host-directed immunomodulatory peptide, as a modulator of intestinal immune homeostasis in Drosophila melanogaster. PAN4 lacks detectable bactericidal activity against Pseudomonas aeruginosa in vitro but improves host survival following infection, supporting a host-directed mode of action rather than direct pathogen killing. Genetic analyses indicate that PAN4-associated protective phenotypes require PGRP-SC1a function, a negative regulator of the Imd pathway. Consistently, PAN4 attenuated infection-induced suppression of PGRP-SC1a-associated responses and maintained immune-related homeostasis without broadly inducing antimicrobial peptide expression. PAN4 expression was also associated with multiple physiological alterations in the intestine, including changes in mitochondrial signal, luminal acidification, calcium-associated responses in enteroendocrine cells, and microbiota composition. These observations suggest that PAN4 activity is associated with broad intestinal physiological changes, although the causal relationships among these phenotypes remain to be determined. Functional assays further show that PAN4 reduces stress-associated intestinal barrier dysfunction and suppresses abnormal intestinal stem cell overproliferation, linking immune regulation with tissue homeostasis. Together, these findings identify PAN4 as an AI-designed host-directed immunomodulatory peptide that contributes to intestinal immune homeostasis and tissue resilience in Drosophila. Consistent with these observations, oral PAN4 administration in honeybees was associated with a context-dependent protective effect following bacterial challenge, suggesting that PAN4-associated protective effects may extend beyond Drosophila under appropriate delivery conditions.