Subcutaneous BCG vaccination protects against streptococcal pneumonia via regulating innate immune responses in the lung

Abstract

Bacillus Calmette-Guerin (BCG) still remains the only licensed vaccine for TB and has been shown to provide nonspecific protection against unrelated pathogens. This has been attributed to the ability of BCG to modulate the innate immune system, known as trained innate immunity (TII). TII is associated with innate immune cells being in a hyper-responsive state leading to enhanced host defense against heterologous infections. Both epidemiological evidence and prospective studies demonstrate cutaneous BCG vaccine-induced TII provides enhanced innate protection against heterologous pathogens. Regardless of the extensive amounts of progress made thus far, the effect of cutaneous BCG vaccination against heterologous respiratory bacterial infections and the underlying mechanisms remain unknown. Here we show for the first time that s.c BCG vaccine-induced TII provides enhanced heterologous innate protection against pulmonary S. pneumoniae infection. We further demonstrate that this enhanced innate protection is mediated by accelerated neutrophilia in the lung and is independent of centrally trained circulating monocytes. New insight from this study will help design novel effective vaccination strategies against unrelated respiratory bacterial pathogens.