Systemic BCG Vaccination in “Dirty Mice” induces Protective Trained Immunity against TB

Abstract After a century of discovery of Bacille Calmette-Guerin (BCG), our understanding of its protective mechanism(s) against TB or other pathogens is very limited. We have recently demonstrated that systemic BCG vaccination (intravenously, iv) in specific pathogen-free (SPF) mice imprints hematopoietic stem cells (HSCs) to generate macrophages with a unique protective program against pulmonary M. tuberculosis (Mtb) infection. While this proof of concept study was an important step to determine how we can harness the power of innate (trained) immunity in vaccination against TB, the translation of this novel approach to humans is still unknown. As SPF lab mice incompletely recapitulate the human immune system, additional pre-clinical models are necessary to evaluate the translational potential of systemic BCG vaccination. Thus, in the current study, we hypothesize that systemic BCG vaccination will generate protective HSC-mediated trained immunity against TB in microbe-exposed “dirty” mice whose immune landscape more closely replicates adult human traits. To test this hypothesis, we have established a bedding transfer model from pet shop mice to SPF C57BL/6 mice. Age- and sex-matched dirty or SPF mice were then iv or subcutaneously vaccinated with BCG (1×106 CFU) or PBS (control). At 4 weeks post vaccination, we generated bone marrow derived macrophages and infected them with virulent Mtb (H37Rv; MOI 1). Interestingly, the protective signature of trained immunity was completely intact in BCG-iv vaccinated dirty mice. Collectively, our results indicate that the protective imprinting of systemic BCG vaccination for generation of trained immunity is robust and independent of previous exposure of hosts to other microbes or pathogens.