Live but not heat-killed Staphylococcus epidermidis elicit an anti-inflammatory phenotype in human embryonic Langerhans cells

Background : In atopic dermatitis (AD), the skin is colonized by high levels of Staphylococcus aureus (SA), while levels of the commensal Staphylococcus epidermidis (SE) are reduced. Levels of SA correlate with the severity of the disease, while transplantation of SE into the skin can ameliorate AD. Objectives : It is hypothesized that the underlying mechanism is supported by Langerhans cells (LCs) which sense a shift in the microbiome and then either continue to perpetuate the disease (with SA) or to act as anti-inflammatory agents, ameliorating the inflammation (with SE). However, many different experimental protocols generating very different results have made the literature on this topic complex and hard to understand. Methods : In this study, we specifically tested human, embryonic stem cell-derived LCs and measured their reactions to SA and SE. We directly compared co-cultures exposed to living SA and SE and to heat-killed bacteria. We analyzed the gene expression of the LCs with qPCR and flow cytometry, the secreted cytokines via ELISA, and their downstream effect on T cells. Results : Our data show that exposure to living SA causes LCs to mature; in turn, this activates a T-cell response of cytokine secretions resembling the inflammatory phenotype observed in AD patients. Meanwhile, SE-primed LCs do not activate T cells, but instead act as anti-inflammatory agents by secreting high levels of IL-10. Importantly, this difference can only be observed in living bacteria. LCs react to heat-killed SE as pro-inflammatory agents, as they do to SA (albeit slightly weaker). Conclusions : Using living bacteria is absolutely crucial when performing experiments to reflect the in vivo situation. Our results expand on the work of others and contribute to the ongoing investigation of interactions of the skin and its microbiome. Why was the study undertaken ? Numerous studies have observed that SE not only correlates with less severe AD, but can ameliorate the disease. This study was undertaken to further investigate the immunological mechanisms related to this effect. What does this study add ? Our data shows that SE modifies T-cell reactions via LCs. Importantly, only living SE has this anti-inflammatory effect; heat-killed SE instead has a pro-inflammatory effect similar to that of S. aureus . What are the implications of this study for the understanding of skin physiology and pathology and/or disease management ? The skin microbiome plays a key role in AD, but at the heart of AD pathophysiology are T cells. We present further evidence that the missing link between the microbiome and the T cells is in fact the LCs. List of abbreviations: AD : Atopic dermatitis; APC : Antigen-presenting cell; DC : Dendritic cell; LC : Langerhans cell; mo-LC : Monocyte-derived Langerhans cell; LN : Lymph nodes; PBMCs : Peripheral blood mononuclear cells; SA : Staphylococcus aureus ; SE : Staphylococcus epidermidis .