The study on the vaccine development using Staphylococcus aureus derived extracellular vesicles

Abstract

S. aureus is an important pathogenic bacterium that causes various infectious diseases in the soft tissue, bloodstream, and lung. Especially, staphylococcal pneumonia is one of the most serious diseases threatening human health. Moreover, with the emergence of the antibiotic-resistant bacteria, the problem with S. aureus infectious disease is becoming increasingly serious. Therefore, a need for an effective preventive therapy against S. aureus infection is urgent. Extracellular vesicles (EVs) derived from bacteria are lipid bi-layered nanovesicles with size ranging from 20~200 nm. Both Gram-negative bacteria, including Escherichia coli, pseudomonas aeruginosa, Acinetobacter baumanii, produce EVs. And also Gram-positive bacterium S. aureus has the ability to produce EVs. EVs derived from S. aureus contain many pro-inflammatory molecules, like enterotoxin (SEQ), IgG-binding protein (Sbi), and Hemolysin. This suggests that EVs derived from S. aureus (SEVs) could induce innate and adaptive immune responses. Previously, studies using SEVs were reported on a murine model of atopic dermatitis. The disease model showed that the skin application of SEVs induces both antibody and T cell responses. Together, SEVs can work as an effective vaccine agent for preventive therapy against S. aureus-induced infection. Here, I aimed to discover a vaccine candidate to prevent S. aureus infection using SEVs. To test this, SEVs were prepared using S. aureus which isolated from clinical samples, and the immunogenicity and efficacy of the candidate were evaluated in vitro and in vivo experiments. As a result, SEVs enhanced the expression of co-stimulatory molecules and the production of immune-modulating cytokines in mouse bone marrow (BM)-derived dendritic cells (DCs). In addition, when compared with sham (PBS)-immunized mice, IgG antibody and T helper cell responses were enhanced in SEV-immunized mice. Moreover, SEV immunization conferred protection against lethality induced by airway challenge with the lethal dose of S. aureus and also pneumonia induced by the sub-lethal dose of S. aureus. The protective effects after airway challenge with S. aureus were also found after the adoptive transfer of splenic T cells, isolated from SEV-immunized mice, to naive mice. However, the protective effects were not found after the adoptive transfer of serum, isolated from SEV-immunized mice, to naive mice. Moreover, the protective effect of SEV immunization was reversed in interferon-gamma (IFN-?) knock-out (KO) mice, but not in IL-17 KO mice. Taken together, these results indicated that active immunization with SEVs confers effective protection against lethality and pneumonia induced by S. aureus infection. The protective effect of SEV vaccination was mediated mainly by IFN-?-producing T (Th1) cells. Finally, I evaluated the efficacy of SEV vaccine candidate for the protection against S. aureus-induced skin infection and inflammation. Active immunization with SEVs exerts protective effect against pathology, such as epidermal thickening, abscess formation, and dermal inflammation, induced by the subcutaneous application of S. aureus. The protective effect of SEV immunization was reversed by treatment with IFN-?- or IL-17-neutralizing antibodies, but not by IL-4-neutralizing antibody. Together, these findings suggest that SEV vaccination has a protective effect against S. aureus-induced skin infection mainly via both Th1 and Th17 cell responses.