Alpha-toxin-elicited CX ₃ CL1 release in <i>Staphylococcus aureus</i> pneumonia impairs bactericidal function of human monocytes

<i>Staphylococcus aureus</i> is an important human pathogen causing severe invasive infections. Pathogenesis is attributed to a wide array of virulence factors, including several potent exotoxins such as the pore-forming α-toxin. In this study, we found that patients with <i>S. aureus</i> respiratory tract infections had elevated CX₃CL1 levels in airway fluid and plasma. Using human-organotypic lung models, we observed that stimulation of lung epithelium with α-toxin induces an intensified CX₃CL1 expression apically in the epithelium as well as the release of CX₃CL1. Blocking α-toxin or ADAM10 activity in organotypic lung using an α-toxin-blocking antibody or a specific ADAM10 inhibitor confirmed their role in modulating CX₃CL1 cleavage and release. Analyses of CD14⁺ human monocytes in combination with a CX₃CR1 inhibitor revealed that α-toxin-mediated CX₃CL1 release induces CX₃CL1-dependent chemotaxis. In line with these data, lung tissue from patients with <i>S. aureus</i> respiratory tract infection showed elevated CX₃CL1 and CD14 staining as compared with tissue from patients with non-infectious lung diseases. Functional studies of monocytes showed that CX₃CL1 released by lung models resulted in upregulated CD83 and downregulated CD86, as well as impaired killing of phagocytosed <i>S. aureus</i>. Furthermore, stimulation of monocytes with soluble CX₃CL1 hampered their reactive-oxygen and nitric-oxide production. Taken together our data show that <i>S. aureus</i> triggers the release of lung epithelial CX₃CL1, and we identify an immunomodulatory effect of α-toxin involving its cytotoxic and ADAM10-interacting properties, inducing CX₃CL1 release leading to impaired monocyte effector function.IMPORTANCEExotoxins are essential virulence factors for the pathobiont <i>S. aureus</i> and contribute toward severe invasive infections such as pneumonia. <i>S. aureus</i> α-toxin is a pore-forming exotoxin that causes host cell lysis and severe lung pathology. We found that α-toxin drives the release of membrane-bound chemokine CX₃CL1 by involving ADAM10-mediated proteolytic activity. Furthermore, the release of CX₃CL1 modulated immune responses locally, as demonstrated by enhanced monocyte migration and reduced capacity of monocytes to kill ingested bacteria. CX₃CL1-induced reduction in bacterial killing coincided with impaired production of reactive oxygen and nitric oxide species. This reveals a novel mechanism in the pathogenesis of <i>S. aureus</i> lung infections involving α-toxin-induced release of CX₃CL1, leading to impaired bacterial killing by monocytes.