Outer membrane vesicles attenuate Klebsiella pneumoniae infection injury by affecting macrophage polarisation and helper T cell differentiation

Klebsiella pneumoniae (KP) is a zoonotic bacterium that exhibits conditional pathogenicity, which can result in severe liver abscesses, pulmonary damage, and potentially fatal outcomes. Research has demonstrated that the outer membrane vesicles (OMVs) released by it can provide significant protection to infected animals and may serve as a promising candidate antigen for the development of a novel vaccine. Nevertheless, the specific mechanisms through which OMVs mitigate the detrimental effects of KP infection by promoting the polarization pathways of macrophages and T helper cells remain poorly understood. In this study, we first verified the protective properties of Klebsiella pneumoniae outer membrane vesicles (KP_OMVs) in KP-infected mice and explored the protective mechanisms by analysing transcriptome data. Then we constructed a model of in vitro macrophage polarisation, an in vivo model for Th differentiation, and a KP infection model in KP_OMVs-immunized mice. The polarisation markers were validated by qRT-PCR, ELISA, Western blot, and IHC. The results showed that KP_OMVs were able to provide specific protection for mice with a maximum protection rate of 80%. Besides, the results of transcriptome analysis suggested that the protective mechanism might be related to Th cells and macrophage polarisation. Mice immunized with KP_OMVs were able to achieve rapid bacterial clearance after KP infection through an M1/Th1 immune response. Subsequently, tissue repair was accomplished through Th2/M2 immune response in the late stage of KP infection to avoid causing inflammatory damage. This study provides a theoretical basis for the practical application of the KP_OMVs vaccine.