Mucosal BCG Vaccination and Immune Layering: Route-Dependent Programming of Immunity at the Respiratory Interface

Mycobacterium bovis Bacille Calmette-Guérin (BCG), the only licensed vaccine against tuberculosis, provides inconsistent protection against pulmonary tuberculosis, reflecting an incomplete understanding of how vaccine-induced immunity is organized within tissues. Emerging evidence indicates that the route of vaccination is not merely a technical variable but a critical determinant of immune programming. Whereas parenteral BCG primarily elicits systemic immune responses, mucosal delivery reprograms immunity at the respiratory interface by promoting localized trained innate immunity, tissue-resident memory T (T_RM) cells, and early containment of infection. In this review, we propose an integrated framework of "immune layering," in which protection emerges through the coordinated interactions of epithelial regulation, trained innate immunity, tissue-resident adaptive memory, regulatory homeostasis, and systemic immune support across spatial and temporal scales. Within this framework, trained innate immunity serves as an initial conditioning layer that shapes subsequent adaptive differentiation, whereas epithelial- and microbiota-associated regulatory networks establish the tissue context in which immune responses are initiated, organized, and maintained. Importantly, effective mucosal immunity depends on a dynamically regulated equilibrium rather than maximal immune activation. The dissociation between enhanced early pulmonary immune responses and limited long-term protection underscores the influence of tissue-specific regulatory constraints and environmental context on vaccine efficacy. This framework redefines correlates of protection by identifying the vaccination route and tissue-level immune organization as fundamental determinants of protective immunity, thereby providing a conceptual foundation for the rational development of next-generation mucosal tuberculosis vaccines.