Immunization with Mycobacterium vaccae attenuates age-associated transcriptional remodeling and microglial reactivity in the rat hippocampus

Aging is associated with progressive changes in the neuroimmune environment that increase vulnerability to cognitive decline and neurodegenerative disease. Characteristics of this process like immune cell activation and pro-inflammatory signaling can eventually lead to behavioral changes and cognitive decline. Consequently, there is a need to discover approaches that dampen harmful aspects of neuroinflammation with age. Here, we tested the hypothesis that Mycobacterium vaccae (M. vaccae) immunization, a potential immunomodulatory therapy, will reduce age-related neuroinflammation in the rat hippocampus. Using single-nucleus RNA sequencing (snRNA-seq), we identified widespread age-associated transcriptional remodeling across multiple hippocampal cell types. Gene ontology and KEGG pathway analyses revealed prominent enrichment of pathways related to mitochondrial function, oxidative phosphorylation, and cellular stress responses in aged animals. Using immunohistochemistry, we demonstrated increased CD68 in aged Iba1⁺ microglia, suggesting enhanced phagocytic activity. M. vaccae immunization substantially attenuated many of these age-associated transcriptional and protein-level changes, dampening stress- and immune-related gene expression and reducing microglial phagocytic activity. Our findings highlight the immunomodulatory potential of M. vaccae to attenuate aspects of age-related neuroinflammation and underscore the complex interplay of hippocampal glial cells in shaping the inflammatory milieu during aging.