Gut microbiota modulates age-related cognitive heterogeneity through the gut-immune-brain axis in male rats

Aging is associated with heterogeneous cognitive outcomes, with some individuals maintaining cognitive resilience and others developing impairments. The gut microbiota modulates host immunity, inflammation, and brain function. However, its role in age-related cognitive heterogeneity remains unclear. In this study, we stratified aged rats into cognitively unimpaired (AU) and impaired (AI) groups using behavioral tests, including the attentional set-shifting task and Morris water maze. 16S rRNA gene sequencing of fecal samples revealed distinct gut microbial communities between the two groups, with AI rats exhibiting reduced diversity, elevated Firmicutes , and decreased Bacteroidetes . Through cross-group fecal microbiota transplantation, we demonstrated that gut microbiota played a causal role in shaping cognitive outcomes. AU recipients of AI microbiota developed cognitive deficits, whereas AI recipients of AU microbiota revealed improved flexibility and memory. To assess intestinal barrier integrity, serum LBP and I-FABP levels were measured as indirect markers of permeability. AU-derived microbiota decreased serum LBP and I-FABP levels, suggesting improved intestinal barrier function, whereas AI-derived microbiota did not significantly alter these markers. Flow cytometric analysis of peripheral blood mononuclear cells revealed reduced Treg and elevated Th1 and Th17 cells. Moreover, serum cytokine measurements revealed increased pro-inflammatory cytokine release in the AI group. These systemic alterations skewed microglial polarization toward a pro-inflammatory state in hippocampal tissues, thereby enhancing neuroinflammation and impairing cognition. Conversely, AU-derived microbiota restored Treg balance, increased anti-inflammatory cytokine levels, and inhibited microglial pro-inflammatory polarization, thereby protecting cognitive function. Our findings establish that gut microbiota modulate age-related cognitive heterogeneity through the gut-immune-brain axis, highlighting gut microbiota as a potential therapeutic target for age-related cognitive decline.