Cyclo(Phe-Pro) (cFP) is a secondary metabolite produced by certain bacteria and fungi. Although recent studies highlight the role of Cyclo(Phe-Pro) in cell-to-cell communication by bacteria, its role in the context of the host immune response is poorly understood. In this study, we investigated the role of Cyclo(Phe-Pro) produced by the human pathogen Vibrio vulnificus in the modulation of innate immune responses toward the pathogen. METHODS AND RESULTS: Cyclo(Phe-Pro) suppressed the production of proinflammatory cytokines, nitric oxide, and reactive oxygen species in a lipopolysaccharide (LPS)-stimulated monocyte/macrophage cell line and in bone marrow-derived macrophages. Specifically, Cyclo(Phe-Pro) inhibited inhibitory κB (IκB) kinase (IKK) phosphorylation, IκBα degradation, and nuclear factor κB (NF-κB) translocation to the cell nucleus, indicating that Cyclo(Phe-Pro) affects the NF-κB pathway. We searched for genes that are responsible for Cyclo(Phe-Pro) production in V. vulnificus and identified VVMO6_03017 as a causative gene. A deletion of VVMO6_03017 diminished Cyclo(Phe-Pro) production and decreased virulence in subcutaneously inoculated mice. CONCLUSIONS: In summary, Cyclo(Phe-Pro) produced by V. vulnificus actively suppresses the innate immune responses of the host, thereby facilitating its survival and propagation in the host environment. |