<i>Klebsiella pneumoniae</i> ghosts as a novel adjuvant drive dendritic cell maturation and antigen-specific TCM expansion for IFN-γ-mediated immune protection

<i>Klebsiella pneumoniae</i> (<i>K. pneumoniae</i>), a common nosocomial pathogen causing severe pulmonary infections, is often complicated by coinfections. Bacterial ghosts, which are empty bacterial cell envelopes, hold significant promise as vaccine adjuvants. This study aims to develop and evaluate a novel combination vaccine platform utilizing <i>K. pneumoniae</i> ghosts (KP ghosts) to explore their intrinsic immunogenic properties as both vaccine and natural adjuvants. In this study, we showed that KP ghosts enhanced maturation and activation of bone marrow-derived dendritic cells, increasing surface markers (CD40, CD80, CD86, and MHC II) and cytokine secretion (IL-1β, TNF-α, and IL-12p70). The KP ghost-based vaccine provided strong immune protection in mice, significantly improving survival rates and reducing bacterial loads in organs after bacterial challenge. Additionally, to assess the adjuvant potential of KP ghosts, C57BL/6 mice were co-immunized with KP ghosts and a model antigen, ovalbumin (OVA). In comparison to OVA alone, the combination of OVA and KP ghosts elicited higher levels of specific IgG antibodies. Furthermore, OVA combined with KP ghosts increased the expression of the early activation marker CD69 on T cells after <i>in vitro</i> antigen stimulation and raised the frequencies of central memory T cells (Tcm) and CD4⁺ IFN-γ⁺ T cells. In conclusion, KP ghosts are effective as both vaccine and adjuvant components, enhancing the innate immune response of dendritic cells and the antigen-specific response of T cells. These findings highlight KP ghosts as a dual-purpose vaccine/adjuvant platform for broader antibacterial vaccine development.