BCG immunisation through intranasal instillation protects mice against Streptococcus pneumoniae infection by enhancing alveolar macrophage activity

Pneumonia remains a leading cause of morbidity and mortality worldwide, especially among young children, the elderly, and immunocompromised individuals. Streptococcus pneumoniae is the most common bacterial cause of community-acquired pneumonia, and the rise of antimicrobial resistance highlights the urgent need for innovative approaches to strengthen host defences. The Bacille Calmette-Guérin (BCG) vaccine, originally developed to prevent tuberculosis, has been shown to provide heterologous protection against unrelated pathogens, mediated by diverse innate and adaptive immune mechanisms. Furthermore, as mucosal delivery has recently been shown to induce stronger local immune responses, intranasal administration of BCG emerges as a promising strategy against S. pneumoniae infection. Here, we investigated the protective effects of BCG immunisation through intranasal instillation in a murine model of S. pneumoniae infection, in comparison with the classical subcutaneous route. Intranasal BCG administration markedly reduced pneumococcal burden and improved mice survival up to three months post-immunisation. This protection was associated with a dampened inflammatory cytokine response while preserving efficient bacterial clearance. Mechanistically, alveolar macrophages were identified as key mediators of protection, as their depletion abolished the beneficial effect of BCG intranasal instillation. In vitro, BCG stimulation enhanced the phagocytic and bactericidal activities of macrophages against S. pneumoniae, which may explain the clearance of the bacteria observed in vivo in immunised mice. In summary, our study demonstrates that BCG immunisation, through intranasal instillation, provides transient heterologous protection against S. pneumoniae by activating alveolar macrophages. These findings highlight intranasal BCG administration as a promising approach to enhance local lung immunity and reduce susceptibility to bacterial pneumonia.