Staphlyococcus aureus Phenol-Soluble Modulins Stimulate the Release of Proinflammatory Cytokines from Keratinocytes and Are Required for Induction of Skin Inflammation

Abstract

Staphylococcus aureus is a human commensal that colonizes the skin. While it is normally innocuous, it has strong associations with atopic dermatitis pathogenesis and has become the leading cause of skin and soft tissue infections in the United States. The factors that dictate the role of S. aureus in disease are still being determined. In this work, we utilized primary keratinocyte culture and an epidermal murine colonization model to investigate the role of S. aureus phenol-soluble modulins (PSMs) in proinflammatory cytokine release and inflammation induction. We demonstrated that many species of Staphylococcus are capable of causing release of interleukin 18 (IL-18) from keratinocytes and that S. aureus PSMs are necessary and sufficient to stimulate IL-18 release from keratinocytes independently of caspase 1. Further, after 7 days of epicutaneous exposure to wild-type S. aureus, but not S. aureus ⌬psm, we saw dramatic changes in gross pathology, as well as systemic release of proinflammatory cytokines. This work demonstrates the importance of PSM peptides in S. aureus-mediated inflammatory cytokine release from keratinocytes in vitro and in vivo and further implicates PSMs as important contributors to pathogenesis. Staphylococcus aureus is a human commensal that lives in the nose, skin, and throat in approximately 30% of the human population (1, 2). While S. aureus is usually harmless, it is an opportunistic pathogen that has become a leading cause of nosocomial infections in the United States (3) and can manifest as skin and soft tissue infections, infective endocarditis, osteomyelitis, and sepsis (1). Further, it has a role in exacerbation of atopic dermatitis (AD), a chronic inflammatory disease of the skin that affects up to 20% of children and up to 3% of adults (4, 5). Chronic cutaneous inflammation during AD results in increased production of extracellular matrix components that permit attachment of S. aureus (6). Subsequently, S. aureus promotes AD by stimulating a strong proinflammatory response (7). S. aureus proteins, such as hemolysin ␣ (HLA), staphylococcal protein A (SPA), and lipoteichoic acids (LTA), promote proinflammatory cytokine production from keratinocytes (7-11). However, this stimulation requires the concurrent presence of a surfactant, such as SDS (11).Keratinocytes serve as the first line of defense against cutaneous pathogens and are able to stimulate an immune response by releasing cytokines, defensins, and antimicrobial cationic peptides, such as LL-37, to fight off pathogens (12). One cytokine that is produced by keratinocytes and has a role in promoting AD is interleukin 18 (IL-18) (13,14). IL-18 is a proinflammatory cytokine that is cleaved and activated primarily by caspase 1 (15). Caspase 1 activation occurs following exposure to danger-associated molecular patterns (DAMPs) or pathogen-associated molecular patterns (PAMPs), which stimulate the activation of the inflammasome (16). Caspase 1 activation can also result in the activation of the proinflammatory cytokine IL-1␤ (17)...

Staphylococcus aureus Phenol-Soluble Modulins Impair Interleukin Expression in Bovine Mammary Epithelial Cells

“…Furthermore, cell lysis may also explain the release of IL-8, as the PSM concentrations used in those experiments were in the cytolytic range (12). Similarly, the stimulation of PSM-induced IL-1␤ and IL-18 release from human keratinocytes, highly specialized epithelial cells, was also likely associated with cell lysis (15). Treatment of bovine PS cells with PSM␣3 at 2 g/ml for 8 h resulted in increased interleukin expression in the absence of a cytotoxic effect (Fig.…”

“…Moreover, PSMs are responsible for modulation of cytokine secretion. It was demonstrated that PSMs induce the release of IL-8 from neutrophils (13) and IL-18 from human keratinocytes, likely through the lytic release (15). In contrast, simultaneous treatment of dendritic cells with S. aureus cell lysate and PSM␣ inhibited the secretion of TNF, IL-6, and IL-12, whereas the secretion of the anti-inflammatory cytokine IL-10 was increased (18).…”

“…Recently, it was discovered that virulence of S. aureus, in particular of community-associated methicillin-resistant S. aureus strains, depends on phenol-soluble modulins (PSMs), a family of secreted amphipathic, ␣-helical peptides with a variety of biological functions (12). S. aureus PSMs have potent cytolytic activity against many cell types, including neutrophils, monocytes, erythrocytes, keratinocytes, and osteoblasts (13)(14)(15). PSMs also have proinflammatory activities: they stimulate leukocytes and initiate proinflammatory responses, including neutrophil chemoattraction and activation (12,16).…”

The Role of the Skin Microbiome in Atopic Dermatitis

“…Protein A can induce TNF-α production in keratinocytes in a non-toxic manner [64]. Finally, S. aureus-secreted PSMs are also known to increase inflammation in keratinocytes [69]. Although much is known of the virulence factors discussed above, more are still being discovered.…”

Characterization of the Pathogenicity of Streptococcus intermedius TYG1620 Isolated from a Human Brain Abscess Based on the Complete Genome Sequence with Transcriptome Analysis and Transposon Mutagenesis in a Murine Subcutaneous Abscess Model

“…4F). Recently, staphylococcal PSMs have been implicated in skin and soft tissue infection (SSTI) (28)(29)(30) and have shown significantly higher levels of expression in methicillin-resistant S. aureus (MRSA) strains isolated from SSTIs. These observations imply that the small ORFs detected in the present study might be candidates that contribute to in vivo abscess formation.…”