Bacillus Calmette–Guérin (BCG) Vaccination and the Immune–Brain Axis: Implications for Neuroprotection and Neurodegenerative Disease

The Bacillus Calmette–Guérin (BCG) vaccine, originally developed for tuberculosis (TB) prevention, has recently attracted attention due to its broader immunomodulatory properties. In addition to its role in TB control, BCG induces trained immunity, a process involving epigenetic and metabolic reprogramming of innate immune cells that leads to altered systemic inflammatory responses. Increasing evidence suggests that these long-term immune adaptations may influence the central nervous system by modulating microglial activation and neuroinflammatory pathways implicated in neurodegenerative diseases. In parallel, chronic infections such as TB are associated with persistent systemic inflammation and immune dysregulation, which may contribute to microglial priming and increased vulnerability to neurodegeneration. This narrative review, based on a targeted literature search of PubMed, Scopus, Web of Science, Embase, and relevant preprint servers, synthesizes current evidence on the relationships between BCG vaccination, trained immunity, and neuroimmune interactions. We focus on studies addressing systemic immune reprogramming, microglial responses, and neuroinflammatory mechanisms relevant to neurodegenerative disorders. The available data suggest that BCG-induced immune modulation may exert context-dependent effects on the brain, with potential neuroprotective implications under certain conditions. However, the evidence remains heterogeneous and largely observational, and causality cannot yet be established. Further mechanistic and prospective studies are required to clarify whether BCG-induced trained immunity can modify the risk or progression of age-related neurodegenerative diseases.