Alpha-toxin-elicited CXCL1 release in pneumonia impairs bactericidal function of human monocytes

Staphylococcus aureus 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 S. aureus respiratory tract infections had elevated CX$_{3}$CL1 levels in airway fluid and plasma. Using human-organotypic lung models, we observed that stimulation of lung epithelium with α-toxin induces an intensified CX$_{3}$CL1 expression apically in the epithelium as well as the release of CX$_{3}$CL1. 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 CX$_{3}$CL1 cleavage and release. Analyses of CD14$^{+}$ human monocytes in combination with a CX$_{3}$CR1 inhibitor revealed that α-toxin-mediated CX$_{3}$CL1 release induces CX$_{3}$CL1-dependent chemotaxis. In line with these data, lung tissue from patients with S. aureus respiratory tract infection showed elevated CX$_{3}$CL1 and CD14 staining as compared with tissue from patients with non-infectious lung diseases. Functional studies of monocytes showed that CX$_{3}$CL1 released by lung models resulted in upregulated CD83 and downregulated CD86, as well as impaired killing of phagocytosed S. aureus. Furthermore, stimulation of monocytes with soluble CX$_{3}$CL1 hampered their reactive-oxygen and nitric-oxide production. Taken together our data show that S. aureus triggers the release of lung epithelial CX$_{3}$CL1, and we identify an immunomodulatory effect of α-toxin involving its cytotoxic and ADAM10-interacting properties, inducing CX$_{3}$CL1 release leading to impaired monocyte effector function. IMPORTANCE: Exotoxins are essential virulence factors for the pathobiont S. aureus and contribute toward severe invasive infections such as pneumonia. S. aureus 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 CX$_{3}$CL1 by involving ADAM10-mediated proteolytic activity. Furthermore, the release of CX$_{3}$CL1 modulated immune responses locally, as demonstrated by enhanced monocyte migration and reduced capacity of monocytes to kill ingested bacteria. CX$_{3}$CL1-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 S. aureus lung infections involving α-toxin-induced release of CX$_{3}$CL1, leading to impaired bacterial killing by monocytes.