Age-associated changes in the gut microbiome impact efficacy of tumor immunomodulatory treatments.

In-situ vaccination (ISV) utilizing nanoparticles (NPs) and therapeutic devices like focused ultrasound (FUS) can trigger immune-mediated killing of both treated and untreated cancer cells. However, the impact of confounding factors such as aging and gut microbiota composition on therapeutic outcomes remains poorly understood. In this study, we sequentially treated young mice (∼8 weeks) and old mice (>18 months) with bilateral melanoma using FUS and calreticulin nanoparticles (CRT-NP) to enhance immunogenic cell death. The combination of CRT-NP and FUS (CFUS) demonstrated greater efficacy in inducing regression of treated and untreated tumors in young mice compared to old mice. The diminished effectiveness in older mice was associated with significant differences in gut microbiome composition, characterized by alterations in bacterial species and splenic immune cells. Specifically, young mice exposed to CFUS exhibited higher abundance of Bacteroidetes and Verrucomicrobia, which was not observed in the aged cohorts. Turicibacter, Anaerotruncus, and Ruminiclostridium demonstrated negative correlations with CD8+ T cells but positive correlations with CD4+ T cells and MDSC cells in both age groups. Taxon set enrichment analysis revealed 58 significantly enriched host gene targets in the young cluster compared to only 11 in the aged cluster. These findings highlight the relationship between ISV treatment efficacy and gut microbiome composition, suggesting that interventions such as diet modification, probiotics, or fecal microbiota transplantation may hold potential as therapeutic strategies to enhance immune responses against solid tumors.