Immune Checkpoint Inhibitor Therapy Alters Microbial Composition of the Spleen in a Melanoma Mouse Model

Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment by restimulating anti-tumor immune cells. However, patient responses remain highly variable, with some individuals experiencing robust tumor regression while others show minimal effects. Emerging research has implicated the microbiome as a key modulator of ICI efficacy but there is limited understanding about how ICI therapy influences microbial communities at distal immune sites such as the spleen. In this study, we investigated the impact of ICI treatment on the microbiome of different locations in a melanoma mouse model. 16S rRNA sequencing revealed changes in microbial composition, marked by an increase in the alpha diversity in the spleen after ICI treatment. Taxonomic analyses showed a shift in dominant phyla, notably a progressive enrichment of Firmicutes and a decline in Proteobacteria, suggesting a restructuring of the splenic microbial landscape. Using core microbiome, indicator species, and differential gene expression analysis, we identified several unique bacterial genera associated with ICI treatment. These included Staphylococcus, Blautia and Lachnospiraceae_NK4A163_group from the Firmicutes phylum, which were strongly associated with post-treatment samples and emerged over the course of treatment. Despite these observed changes in the spleen microbial composition, predicted microbial functional pathways remained largely stable. Overall, our study provides initial insight into how ICI treatment affects the microbiome beyond the gut. These findings highlight the spleen as a site of dynamic microbial modulation during ICI therapy and raise the possibility that specific microbial taxa may contribute to treatment-associated immune responses.