Staphylococcus aureus stimulates neutrophil targeting chemokine expression in keratinocytes through an autocrine IL-1alpha signaling loop

IL-1R1 signaling facilitates Munro's microabscess formation in psoriasiform imiquimod-induced skin inflammation

Munro's microabscesses contain polymorphonuclear leukocytes and form specifically in the epidermis of psoriasis patients. The mechanism whereby the neutrophils are recruited into the epidermis is poorly understood. Using a combination of human and mouse primary keratinocyte cell cultures and the imiquimod-induced psoriasis-like mouse model of skin inflammation, we explored the role of IL-1 signaling in microabscess formation. In vitro imiquimod stimulated production of IL-1α and neutrophil recruiting chemokines. Imiquimod-activated chemokine expression was dependent upon adenosine signaling and independent of IL-1α and IL-1 receptor type 1 (IL-1R1); nevertheless, IL-1α could enhance chemokine expression initiated by imiquimod. Topical application of imiquimod in vivo led to epidermal microabscess formation, acanthosis, and increased IL-1α and chemokine expression in the skin of wild-type mice. However, in IL-1R1-deficient mice these responses were either absent or dramatically reduced. These results demonstrate that IL-1α and IL-1R1 signaling is essential for microabscess formation, neutrophil recruiting chemokine expression, and acanthosis in psoriasis-like skin inflammation induced by imiquimod.

Activation of epidermal toll-like receptor 2 enhances tight junction function: implications for atopic dermatitis and skin barrier repair

Atopic dermatitis (AD) is characterized by epidermal tight junction (TJ) defects and a propensity for Staphylococcus aureus (S. aureus) skin infections. S. aureus is sensed by many pattern recognition receptors including toll-like receptor (TLR) 2. We hypothesized that an effective innate immune response will include skin barrier repair and that this response is impaired in AD subjects. S. aureus-derived peptidoglycan (PGN) and synthetic TLR2 agonists enhanced TJ barrier and increased expression of TJ proteins, CLDN1, CLDN23, occludin and ZO-1 in primary human keratinocytes. A TLR2 agonist enhanced skin barrier recovery in human epidermis wounded by tape-stripping. Tlr2^−/− mice had a delayed and incomplete barrier recovery following tape-stripping. AD subjects had reduced epidermal TLR2 expression as compared to nonatopic (NA) subjects, which inversely correlated (r= 0.654, P= 0.0004) with transepidermal water loss (TEWL). These observations indicate that TLR2 activation enhances skin barrier in murine and human skin and is an important part of a wound repair response. Reduced epidermal TLR2 expression observed in AD patients may play a role in their incompetent skin barrier.

Interleukin-8 content in the stratum corneum as an indicator of the severity of inflammation in the lesions of atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is an inflammatory skin disease characterized by both acute and chronic eczema. Various markers are used to clinically evaluate the severity of AD. In order to identify a marker of local severity of AD, we measured IL-8, IL-18, vascular endothelial growth factor (VEGF), and transforming growth factor-α (TGF-α) levels in the stratum corneum (scIL-8, scIL-18, scVEGF and scTGF-α) and evaluated the correlation between the levels of these cytokines and the clinical severity scores of localized skin lesions.

METHODS

Stratum corneum samples were collected from the skin lesions of 50 patients with AD using the tape-stripping technique, and the scIL-8, scIL-18, scVEGF and scTGF-α levels were evaluated using the ELISA method. The trans-epidermal water loss and skin water content of the lesions were also measured prior to tape stripping.

RESULTS

The levels of scIL-8, scIL-18, scVEGF and scTGF-α were significantly higher in patients with AD than in healthy controls. Additionally, the levels of scIL-8, scIL-18 and scVEGF significantly correlated with the severity of AD.

CONCLUSIONS

Among these cytokines, scIL-8 showed the highest correlation with the severity scores of lesions in AD as well as other parameters. Our results also suggest that measuring cytokines in the stratum corneum by using ELISA combined with tape stripping is a convenient method to evaluate the severity of skin lesions in AD.

Epithelial-mesenchymal interaction during UVB-induced up-regulation of neutral endopeptidase

We recently reported that overexpression of the elastase NEP (neutral endopeptidase) by fibroblasts plays a pivotal role in the mechanism of UVB-induced skin wrinkling by degrading dermal elastic fibres. Since UVB does not penetrate to the dermis, we hypothesized that factors secreted by UVB-exposed keratinocytes in the epidermis trigger fibroblasts in the dermis to increase their expression of NEP which then degrades the elastic fibres. In the present study, we characterized the epithelial-mesenchymal interaction between keratinocytes and fibroblasts which leads to increased expression of NEP. Human fibroblasts co-cultured with UVB-exposed human keratinocytes in cell inserts significantly increased their expression of NEP at the transcriptional, translational and enzymatic levels. Neutralizing antibodies to IL (interleukin)-1α or GM-CSF (granulocyte/macrophage colony-stimulating factor) significantly abolished the increased expression of NEP at the enzymatic levels in human fibroblasts co-cultured with UVB-exposed human keratinocytes, whereas neutralizing antibodies to IL-6, IL-8 or TNFα (tumour necrosis factor α) had no such effect. The addition of IL-1α or GM-CSF, but not TNFα, IL-6 or IL-8, at concentrations ranging from 1 to 10 nM, significantly stimulated the expression of NEP in human fibroblasts at the transcriptional and translational levels. These findings suggest that IL-1α and GM-CSF are intrinsic cytokines secreted by UVB-exposed keratinocytes that stimulate expression of NEP by fibroblasts.

The double-stranded RNA analogue polyinosinic-polycytidylic acid induces keratinocyte pyroptosis and release of IL-36γ

IL-36 is the common name for the three IL-1 family members IL-36α, IL-36β, and IL-36γ, formerly known as IL-1F6, IL-1F8, and IL-1F9, respectively. IL-36 appears to have pro-inflammatory activities; however, the physiological function of these cytokines remains unknown. Expression of IL-36 by keratinocytes implies its possible involvement in innate immune responses in the skin. We observed that, of the three IL-36 isoforms, human keratinocytes express high levels of IL-36γ. IL-36γ mRNA expression was dramatically induced by the Toll-like receptor ligands polyinosinic-polycytidylic acid (poly(I:C)) and flagellin. Surprisingly, the IL-36γ protein was released by cells treated with poly(I:C), but remained intracellular in cells treated with flagellin only. poly(I:C), but not flagellin, induced cell death and caspase-3/7 activation. Inhibition of caspase-3/7 and caspase-1 blocked extracellular release of IL-36γ from poly(I:C)-treated cells. Furthermore, caspase-1 inhibition prevented poly(I:C)-induced caspase-3/7 activation. Interestingly, transcription of the gene IL36G was dependent on caspase-1, but not caspase-3/7, activation. This demonstrates that the pathways leading to IL36G transcription and caspase-3/7 activation branch after caspase-1. This divergence of the pathways allows the cells to enter a state of de novo protein synthesis before committing to pyroptosis. Overall, our observations suggest that IL-36γ may be an alarmin that signals the cause, e.g., viral infection, of cell death.

Host-pathogen interactions between the skin and Staphylococcus aureus

Staphylococcus aureus is responsible for the vast majority of bacterial skin infections in humans. The propensity for S. aureus to infect skin involves a balance between cutaneous immune defense mechanisms and virulence factors of the pathogen. The tissue architecture of the skin is different from other epithelia especially since it possesses a corneal layer, which is an important barrier that protects against the pathogenic microorganisms in the environment. The skin surface, epidermis, and dermis all contribute to host defense against S. aureus. Conversely, S. aureus utilizes various mechanisms to evade these host defenses to promote colonization and infection of the skin. This review will focus on host-pathogen interactions at the skin interface during the pathogenesis of S. aureus colonization and infection.

Innate and adaptive immune responses against Staphylococcus aureus skin infections

Staphylococcus aureus is an important human pathogen that is responsible for the vast majority of bacterial skin and soft tissue infections in humans. S. aureus can also become more invasive and cause life-threatening infections such as bacteremia, pneumonia, abscesses of various organs, meningitis, osteomyelitis, endocarditis, and sepsis. These infections represent a major public health threat due to the enormous numbers of these infections and the widespread emergence of methicillin-resistant S. aureus (MRSA) strains. MSRA is endemic in hospitals worldwide and is rapidly spreading throughout the normal human population in the community. The increasing frequency of MRSA infections has complicated treatment as these strains are more virulent and are increasingly becoming resistant to multiple different classes of antibiotics. The important role of the immune response against S. aureus infections cannot be overemphasized as humans with certain genetic and acquired immunodeficiency disorders are at an increased risk for infection. Understanding the cutaneous immune responses against S. aureus is essential as most of these infections occur or originate from a site of infection or colonization of the skin and mucosa. This review will summarize the innate immune responses against S. aureus skin infections, including antimicrobial peptides that have direct antimicrobial activity against S. aureus as well as pattern recognition receptors and proinflammatory cytokines that promote neutrophil abscess formation in the skin, which is required for bacterial clearance. Finally, we will discuss the recent discoveries involving IL-17-mediated responses, which provide a key link between cutaneous innate and adaptive immune responses against S. aureus skin infections.

Immunity against Staphylococcus aureus cutaneous infections

Complications arising from cutaneous and soft tissue infections with Staphylococcus aureus are a major clinical problem owing to the high incidence of these infections and the widespread emergence of antibiotic-resistant bacterial strains. If prophylactic vaccines or immunotherapy for certain patient populations are to be developed as an alternative to antibiotics, it will be essential to better understand the immune mechanisms that provide protection against S. aureus skin infections. Recent discoveries have identified a key role for interleukin-1 (IL-1)- and IL-17-mediated immune responses in promoting neutrophil recruitment to the site of infection in the skin, a process that is required for host defence and bacterial clearance. This Review describes these new insights and discusses their potential impact on immune-based therapies and vaccination strategies.

Noninvasive in vivo imaging to evaluate immune responses and antimicrobial therapy against Staphylococcus aureus and USA300 MRSA skin infections

Staphylococcus aureus skin infections represent a significant public health threat because of the emergence of antibiotic-resistant strains such as methicillin-resistant S. aureus (MRSA). As greater understanding of protective immune responses and more effective antimicrobial therapies are needed, a S. aureus skin wound infection model was developed in which full-thickness scalpel cuts on the backs of mice were infected with a bioluminescent S. aureus (methicillin sensitive) or USA300 community-acquired MRSA strain and in vivo imaging was used to noninvasively monitor the bacterial burden. In addition, the infection-induced inflammatory response was quantified using in vivo fluorescence imaging of LysEGFP mice. Using this model, we found that both IL-1α and IL-1β contributed to host defense during a wound infection, whereas IL-1β was more critical during an intradermal S. aureus infection. Furthermore, treatment of a USA300 MRSA skin infection with retapamulin ointment resulted in up to 85-fold reduction in bacterial burden and a 53% decrease in infection-induced inflammation. In contrast, mupirocin ointment had minimal clinical activity against this USA300 strain, resulting in only a 2-fold reduction in bacterial burden. Taken together, this S. aureus wound infection model provides a valuable preclinical screening method to investigate cutaneous immune responses and the efficacy of topical antimicrobial therapies.