Abstract Rationale Pseudomonas aeruginosa (PA) is commonly associated with hospital-acquired (nosocomial) pneumonia. Despite access to new antibiotics, multidrug resistance to PA has doubled in recent years causing an urgent need for alternative strategies to boost host defense. We recently reported that pre-exposure of mice to LPS results in 100% survival from a lethal PA infection that involves training of neutrophils (PMID: 38830855). Our objective in this study was to determine the translational potential of this finding by testing monophosphoryl lipid A (MPLA), a non-toxic derivative of bacterial lipopolysaccharide (LPS) that induces a spectrum of immunomodulatory effects in the lung and has been used as an immunostimulant. Due to its reduced toxicity, MPLA makes a safer alternative for therapeutic and prophylactic vaccines, which can be used to treat various disease conditions. Here, we investigated whether MPLA-induced immune training protects against lethal infection by a highly pathogenic PA isolate and whether MPLA conditions bone marrow (BM) neutrophils, priming them for enhanced host defense after trafficking them to the infection site. Methods MPLA was intravenously administered into 8-10 week-old C57BL/6 mice. 7 days post-MPLA treatment, mice were infected with the reference strain of PA14 at a dose of 1x106 cfu/mouse. Cells in the BM and airspaces were recovered by bronchoalveolar lavage (BAL) after 24 hours of infection, and analyzed by flow cytometry. In another set of experiments, post-MPLA treatment of congenic CD45.1 C57BL/6 mice, enriched-neutrophils were isolated from the BM and also from BAL after recruitment by brief LPS exposure and the cells were adoptively transferred intratracheally to CD45.2 mice to distinguish donor and recipient cells by flow cytometry. Bacterial burden in peripheral blood and BAL fluid was assayed. Results Our data show that exposure to MPLA provides complete protection to mice that otherwise die 8-10 hours after PA infection. Pre-exposure to MPLA resulted in reduced bacterial burden in the lung with markedly attenuated peripheral dissemination. The profiles of bone marrow and BAL neutrophils were distinct in infected mice after MPLA exposure as compared to the control mice with detection of CXCR2+, CXCR4+, and CD101+ neutrophil populations in the BAL that may potentially contribute to host protection. Adoptive transfer of MPLA-trained neutrophils from BM and BAL also protected against PA infection in recipient mice. Conclusions Collectively, our data demonstrate the ability of non-toxic LPS to train neutrophils for effective host protection suggesting a prophylactic approach to prevent ventilator-associated pneumonia in hospitalized patients. This abstract is funded by: This research is supported by NIH grants P01 HL114453 and R21 AI193306 (to P.R.).
Das et al. (Fri,) studied this question.