Naturally-derived bacterial nano-particles engage diverse innate receptors, driving the activation of dendritic cells and leading to the establishment of potent adaptive immune responses.

Abstract

Abstract Most non-living vaccines in clinical use are thought to mediate protection primarily via antibody responses; however, many intracellular bacterial infections require cell-mediated immunity (CMI) for protection. This renders traditional strategies insufficient and drives a need for new approaches to elicit CMI. Outer-membrane vesicles (OMVs) derived from Gram-negative bacteria are an effective vaccine platform with precedence for safe use in humans. Unlike most subunit antigens and synthetic nano-particles, OMVs contain endogenous immunostimulatory ligands and deliver antigens in their native orientation. We hypothesized that OMVs could activate antigen presenting cells through the engagement of various innate receptors and drive CMI through the three key activation steps: antigen-presentation, co-stimulation and cytokine production. Following OMV uptake in vitro and in vivo, dendritic cells (DCs) increased surface MHC class I and II (MHCcII), CD80, and CD86 expression. OMV-stimulated DCs produced significantly more of the key T cell polarizing cytokines IL-1β, IL-6, IL-18 and IL-12p70 compared to DCs treated with heat-killed (HK) bacteria. TLR-4 contributed to OMV-mediated expression of MHCcII, CD80, and IL-1β, IL-6, IL-18 and IL-12p70, while the NLRP3 inflammasome was required for OMV-induced production of IL-1β and IL-18. Following vaccination of mice, Th1- and Th17-type T cell responses were observed after ex vivo restimulation with OMVs and HK bacteria. In addition, OMV vaccination produced functional CD8+ T cells capable of killing bacteria-infected cells. Collectively these results support the utility of the OMV platform as a non-living vaccine that can lead to protective CMI against bacterial pathogens.