Abstract
Purpose: :
Staphylococcus aureus is a frequent cause of microbial keratitis, linked with non–surgical traumas and contact lens wear. If left unchecked, these infections can result in vision–threatening corneal erosions and ulcers. We previously found that the fibronectin–binding protein mediates attachment of S. aureus to immortalized human corneal epithelial cells (hCEC). Adherent organisms are capable of invading host cells by activating tyrosine kinase signaling pathways and actin polymerization. S. aureus secretes a panoply of proteins which can alter the response of epithelial cells to the presence of attached bacteria. Most of these toxins are modulated by global regulators Agr and Sar. The goal of this study is to determine whether and how quorum–regulated Agr/Sar–dependent toxins derange the response of corneal epithelial cells to the presence of adherent S. aureus, enabling colonization and ensuing disease.
Methods: :
Late–exponential phase cultures of wild–type S. aureus RN6390 and isogenic agr/sar mutant ALC135 were incubated with confluent human corneal and corneal/limbal epithelial cells (hCEC and hCLE) in serum–free defined keratinocyte medium for up to 4 hours. Co–cultures were assessed for epithelial cell viability and cytokine production through ELISA.
Results: :
Cytokines arrays showed that hCEC/hCLE cultured with sublethal inoculums of S. aureus secrete significant quantities of IFNγ, IL6 and IL8. Corneal epithelial cells exposed to wild type S. aureus strain secrete less IFNγ (18%) than cells exposed to an isogenic mutant, defective in toxin production. At 4 hours post–infection, similar trends in cytokine production were observed with IL1α and IL2 albeit at modest levels. In contrast, comparable levels of IL6 and IL8 were measured in wild type and non–toxigenic S. aureus infections.
Conclusions: :
These experiments indicate that cytokine secretion from human corneal epithelium is influenced by the presence of secreted S. aureus factors. Specifically, S. aureus determinants controlled by global regulators Agr/Sar negatively affect the proinflammatory response mediated by IFNγ. Since these regulators control the expression of >30 secreted factors in S. aureus, it is unclear which determinants mediate this effect.
Keywords: Staphylococcus • keratitis • cytokines/chemokines