Genetic variation in innate immune gene expression influences mortality after traumatic brain injury in Drosophila

Traumatic brain injury (TBI) is a leading cause of disability and death, with outcome severity varying widely even among individuals with comparable injuries. A major challenge is to identify pathways that underlie this variation and could be targeted to improve therapies. Innate immune pathways are candidates because they are rapidly activated after TBI and contribute to neurodegenerative disorders. Using a Drosophila melanogaster TBI model, we examined how genetic background, age, and diet modify effects of evolutionarily conserved Toll and Immune deficiency (Imd) pathways on injury outcomes. These pathways signal through nuclear factor-kappa B (NF-κB) transcription factors Dorsal-related immunity factor (Dif) and Relish (Rel) to activate antimicrobial peptide (AMP) gene expression. We found that genetic diversity among lines from the Drosophila Genetic Reference Panel (DGRP) contributed to variation in AMP expression before and after TBI, with additional effects of age and diet. AMP expression tended to be correlated positively with early mortality following TBI in young flies, but negatively in older flies, suggesting an age-dependent shift in AMP effects from detrimental to protective. Furthermore, heterozygous mutations in Dif or Rel lowered AMP expression in a diet-dependent manner and led to correspondingly reduced early mortality after TBI. These findings show that genetic, biological, and environmental factors influence innate immune pathways, which in turn determine TBI outcomes. Innate immune gene expression before injury emerges as a potential prognostic indicator, pointing to potential new therapeutic strategies.