Host cellular and immune responses in models of inflammatory skin conditions

Abstract

Skin colonisation of varied communities of commensal microorganisms, such as Staphylococcus aureus (SA), Staphylococcus epidermidis (SE) and Staphylococcus capitis (SC) form the microbiome; a necessity for healthy skin. A number of inflammatory skin conditions, including atopic dermatitis (AD) and dandruff, are strongly associated with microbial dysbiosis of the skin. AD is a common skin disease affecting 30% of UK children, it has a complex and multifactorial aetiology that combines genetic, environmental and immune factors. The skin changes characteristic of AD have been shown to provide a favourable niche for SA colonisation. Indeed, dysbiosis and increased numbers of SA are strongly associated with AD lesions, and correlate with increased disease severity. However, whilst tissue invasion by SA in acute infection is understood to result in severe inflammation, whether SA colonisation has a role in maintaining chronic stable inflamed AD is not well established. This research aims to study interactions between staphylococci and the skin. The goal is to explore the differences in the epidermal response to different species, in an attempt to understand how such differential bacterial regulation may contribute to skin tolerance or inflammation.

Staphylococcal challenge of keratinocyte monolayers, showed similar growth curves for SA and SE, whereas when applied to a more realistic human epidermis in a reconstructed human epidermal (RHE) model, SA proliferation was significantly inhibited in contrast to that seen with SE at both high and low colonisation loads after 24 hours (CFU load 102 p=0.037; 106 p=0.0001). These data strongly suggest species specific regulation of staphylococcal growth, mediated by interaction with the epidermis. This finding was validated in human skin explant models.

Additionally, the different staphylococcal species induced a different outcome on dendritic cell activation by bacteria primed epidermal models. To explore the impact of various staphylococci and inoculum doses, multiplex bead array analysis of the soluble inflammatory mediators in culture media from epidermal models was undertaken and showed that although some differences were noted between SA strains, SA induced keratinocyte production of IL-1α, IL-1β, GM-CSF and TNFα after 24hrs of colonisation, whereas SE was less inflammatory.

To investigate the specific signalling within keratinocytes that might account for species specific responses, transcriptomic analysis was undertaken. Interestingly a strong IL-17/23 pathway signal was induced with SA colonisation, whereas SE induced strong upregulation of the negative regulator of inflammation NF-κB inhibitor A20 (TNFAIP3). Pathway analysis showed that a key regulator of response to SA/SE in keratinocytes was regulated through NF-κB and mediated by the IKK complex regulatory network.

This work provides strong evidence of the highly tuned nature of skin keratinocytes to different bacterial species and identifies the key regulatory mechanisms involved in induction of tolerance or inflammation induced by staphylococci.

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Identifiers

ORCID for Michael Ardern-Jones: orcid.org/0000-0003-1466-2016

ORCID for Myron Christodoulides: orcid.org/0000-0002-9663-4731

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