Heat-inactivated Bacillus spores attenuate poly I:C-induced lung inflammation and microbiome alterations through modulation of host immune responses.

Uncontrolled cytokine responses are a major driver of acute lung injury and poor outcomes in respiratory viral infections such as COVID-19. Strategies that can safely restrain hyperinflammation while preserving host defense remain limited. Here, we show that pretreatment with heat-inactivated Bacillus spores markedly protects against the viral mimetic polyinosinic-polycytidylic acid (poly I:C)-induced pulmonary inflammation in BALB/c mice. Structural and in vivo imaging system analyses confirmed that heat treatment was successful in eliminating the viability of spores without rupturing or changing their penetration into the airway via instillation. The results indicated that in a poly I:C-induced model of acute lung injury, spore parabiotics reduced pulmonary edema, histopathological damage, and pro-inflammatory cytokine release. Moreover, parabiotic spores suppressed Ace2 and Tlr3 expression in murine macrophages, suggesting regulation of receptors implicated in downstream inflammatory signaling. Parabiotic treatment attenuated lung microbial alteration triggered by viral mimic challenge. Together, these findings identify spore-based parabiotics as a potentially safe immunomodulatory approach capable of limiting excessive inflammatory responses and supporting respiratory function in the context of viral-like lung inflammation.