Exogenous factors in the pathogenesis of atopic dermatitis: Irritants and cutaneous infections.

Atopic dermatitis (AD) is a chronic relapsing inflammatory cutaneous disease that is often associated with other atopic symptoms, such as food allergy, allergic rhinitis and asthma, leading to significant morbidity and healthcare costs. The pathogenesis of AD is complicated and multifactorial. Although the aetiology of AD remains incompletely understood, recent studies have provided further insight into AD pathophysiology, demonstrating that the interaction among genetic predisposition, immune dysfunction and environmental provocation factors contributes to its development. However, the increasing prevalence of AD suggests that environmental factors such as irritation and cutaneous infection play a crucial role in triggering and/or aggravating the disease. Of note, AD skin is susceptible to bacterial, fungal and viral infections, and microorganisms may colonize the skin and aggravate AD symptoms. Overall, understanding the mechanisms by which these risk factors affect the cutaneous immunity of patients with AD is of great importance for developing a precision medicine approach for treatment. This review summarizes recent developments in exogenous factors involved in the pathogenesis of AD, with special emphasis on irritants and microbial infections.

Eczema Herpeticum: Clinical and Pathophysiological Aspects.

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease in the world. AD is a complex pathology mainly characterized by an impaired skin barrier, immune response dysfunction, and unbalanced skin microbiota. Moreover, AD patients exhibit an increased risk of developing bacterial and viral infections. One of the most current, and potentially life-threatening, viral infection is caused by herpes simplex virus (HSV), which occurs in about 3% of AD patients under the name of eczema herpeticum (EH). Following a first part dedicated to the clinical features, virological diagnosis, and current treatments of EH, this review will focus on the description of the pathophysiology and, more particularly, the presently known predisposing factors to herpetic complications in AD patients. These factors include those related to impairment of the skin barrier such as deficit in filaggrin and anomalies in tight and adherens junctions. In addition, low production of the antimicrobial peptides cathelicidin LL-37 and human ß-defensins; overexpression of cytokines such as interleukin (IL)-4, IL-13, IL-25, IL-33, and thymic stromal lymphopoietin (TSLP); or downregulation of type I to III interferons as well as defect in functions of immune cells such as dendritic, natural killer, and regulatory T cells have been involved. Otherwise, genetic polymorphisms and AD topical calcineurin inhibitor treatments have been associated with an increased risk of EH. Finally, dysbiosis of skin microbiota characterized in AD patients by Staphylococcus aureus colonization and toxin secretion, such as α-toxin, has been described as promoting HSV replication and could therefore contribute to EH.

Replicated methylation changes associated with eczema herpeticum and allergic response.

BACKGROUND: Although epigenetic mechanisms are important risk factors for allergic disease, few studies have evaluated DNA methylation differences associated with atopic dermatitis (AD), and none has focused on AD with eczema herpeticum (ADEH+). We will determine how methylation varies in AD individuals with/without EH and associated traits. We modeled differences in genome-wide DNA methylation in whole blood cells from 90 ADEH+, 83 ADEH-, and 84 non-atopic, healthy control subjects, replicating in 36 ADEH+, 53 ADEH-, and 55 non-atopic healthy control subjects. We adjusted for cell-type composition in our models and used genome-wide and candidate-gene approaches. RESULTS: We replicated one CpG which was significantly differentially methylated by severity, with suggestive replication at four others showing differential methylation by phenotype or severity. Not adjusting for eosinophil content, we identified 490 significantly differentially methylated CpGs (ADEH+ vs healthy controls, genome-wide). Many of these associated with severity measures, especially eosinophil count (431/490 sites). CONCLUSIONS: We identified a CpG in IL4 associated with serum tIgE levels, supporting a role for Th2 immune mediating mechanisms in AD. Changes in eosinophil level, a measure of disease severity, are associated with methylation changes, providing a potential mechanism for phenotypic changes in immune response-related traits.

Atypical Varicella-Zoster Kaposi Varicelliform Eruption in Sézary Syndrome.

Patients with mycosis fungoides experience considerable morbidity and mortality from secondary bacterial and viral infections. Staphylococcus aureus, ß-hemolytic streptococci, herpes simplex virus, and herpes zoster virus remain the most common infectious pathogens in this group of patients. With depressed cellular immunity and diminished skin barrier as the main precipitating risk factors, immunocompromised patients can often present with an atypical presentation of a common dermatologic condition. The case herein discusses a clinically atypical nonvesicular Kaposi varicelliform eruption secondary to a varicella-zoster virus in a patient with Sézary syndrome. Concurrent polypharmacy in these patients is also a risk factor for development of drug hypersensitivity reactions. However, given their immunocompromised status, first and foremost, a careful inspection should be made of every atypical skin eruption in search of an infectious etiology, and afterward, an appropriate treatment should be promptly initiated.

Ankyrin repeat domain 1 regulates innate immune responses against herpes simplex virus 1: A potential role in eczema herpeticum.

BACKGROUND: Atopic dermatitis (AD) is a common inflammatory skin disease. A subset of patients with AD are susceptible to disseminated herpes simplex virus (HSV) infection, a complication termed eczema herpeticum (ADEH+). The immune mechanisms causing ADEH+ remain elusive. Using RNA sequencing, we recently found that ankyrin repeat domain 1 (ANKRD1) was significantly induced in human PBMCs upon HSV-1 stimulation, and its induction in patients with ADEH+ was significantly reduced compared with that seen in AD patients without a history of eczema herpeticum (ADEH-). OBJECTIVE: We sought to validate ANKRD1 gene expression in nonatopic (NA) subjects, patients with ADEH-, and patients with ADEH+ and to delineate the biological function of ANKRD1 and the signaling pathway or pathways involved. METHODS: Purification of human PBMCs, monocytes, B cells, dendritic cells, T cells, and natural killer cells; RNA extraction and quantitative RT-PCR; small interfering RNA technique; co-immunoprecipitation; and Western blot assays were used. RESULTS: ANKRD1 expression was significantly reduced in PBMCs from patients with ADEH+ after HSV-1 stimulation compared with PBMCs from patients with ADEH-. We found that the induction of ANKRD1 by HSV-1 and multiple pattern recognition receptor agonists are mediated by inflammatory cytokines. Silencing ANKRD1 gene expression in antigen-presenting cells led to increased viral load and reduced IFNB1 and IL29 production. Using co-immunoprecipitation methods, we demonstrated that ANKRD1 formed protein complexes with interferon regulatory factor (IRF) 3 and IRF7, which are important transcription factors regulating signaling transduction of pattern recognition receptors. Overexpression of ANKRD1 enhanced the IRF3-mediated signaling pathways. CONCLUSION: ANKRD1 is involved in IRF3-mediated antiviral innate immune signaling pathways. Its reduced expression in patients with ADEH+ might contribute to the pathogenesis of ADEH+.

IL-1R Type 1-Deficient Mice Demonstrate an Impaired Host Immune Response against Cutaneous Vaccinia Virus Infection.

The IL-1 superfamily of cytokines and receptors has been studied extensively. However, the specific roles of IL-1 elements in host immunity to cutaneous viral infection remain elusive. In this study, we applied vaccinia virus (VACV) by scarification to IL-1R1 knockout mice (IL-1R1-/-) and found that these mice developed markedly larger lesions with higher viral genome copies in skin than did wild-type mice. The phenotype of infected IL-1R1-/- mice was similar to eczema vaccinatum, a severe side effect of VACV vaccination that may develop in humans with atopic dermatitis. Interestingly, the impaired cutaneous response of IL-1R1-/- mice did not reflect a systemic immune deficiency, because immunized IL-1R1-/- mice survived subsequent lethal VACV intranasal challenge, or defects of T cell activation or T cell homing to the site of inoculation. Histologic evaluation revealed that VACV infection and replication after scarification were limited to the epidermal layer of wild-type mice, whereas lack of IL-1R1 permitted extension of VACV infection into dermal layers of the skin. We explored the etiology of this discrepancy and determined that IL-1R1-/- mice contained significantly more macrophages and monocyte-derived dendritic cells in the dermis after VACV scarification. These cells were vulnerable to VACV infection and may augment the transmission of virus to adjacent skin, thus leading to larger skin lesions and satellite lesions in IL-1R1-/- mice. These results suggest new therapeutic strategies for treatment of eczema vaccinatum and inform assessment of risks in patients receiving IL-1 blocking Abs for treatment of chronic inflammatory disorders.

Infectious Complications in Atopic Dermatitis.

Atopic dermatitis is characterized by the interplay of skin barrier defects with the immune system and skin microbiome that causes patients to be at risk for infectious complications. This article reviews the pathogenesis of atopic dermatitis and the mechanisms through which patients are at risk for infection from bacterial, viral, and fungal pathogens. Although these complications may be managed acutely, prevention of secondary infections depends on a multipronged approach in the maintenance of skin integrity, control of flares, and microbial pathogens.

Defective natural killer cell activity in a mouse model of eczema herpeticum.

BACKGROUND: Patients with atopic dermatitis (AD) are susceptible to several viruses, including herpes simplex virus (HSV). Some patients experience 1 or more episodes of a severe skin infection caused by HSV termed eczema herpeticum (EH). There are numerous mouse models of AD, but no established model exists for EH. OBJECTIVE: We sought to establish and characterize a mouse model of EH. METHODS: We infected AD-like skin lesions with HSV1 to induce severe skin lesions in a dermatitis-prone mouse strain of NC/Nga. Gene expression was investigated by using a microarray and quantitative PCR; antibody titers were measured by means of ELISA; and natural killer (NK) cell, cytotoxic T-cell, regulatory T-cell, and follicular helper T-cell populations were evaluated by using flow cytometry. The role of NK cells in HSV1-induced development of severe skin lesions was examined by means of depletion and adoptive transfer. RESULTS: Inoculation of HSV1 induced severe erosive skin lesions in eczematous mice, which had an impaired skin barrier, but milder lesions in small numbers of normal mice. Eczematous mice exhibited lower NK cell activity but similar cytotoxic T-cell activity and humoral immune responses compared with normal mice. The role of NK cells in controlling HSV1-induced skin lesions was demonstrated by experiments depleting or transferring NK cells. CONCLUSION: A murine model of EH with an impaired skin barrier was established in this study. We demonstrated a critical role of defective NK activities in the development of HSV1-induced severe skin lesions in eczematous mice.

Filaggrin deficiency promotes the dissemination of cutaneously inoculated vaccinia virus.

BACKGROUND: Eczema vaccinatum is a life-threatening complication of smallpox vaccination in patients with atopic dermatitis (AD) characterized by dissemination of vaccinia virus (VV) in the skin and internal organs. Mutations in the filaggrin (FLG) gene, the most common genetic risk factor for AD, confer a greater risk for eczema herpeticum in patients with AD, suggesting that it impairs the response to cutaneous viral infections. OBJECTIVE: We sought to determine the effects of FLG deficiency on the response of mice to cutaneous VV inoculation. METHODS: VV was inoculated by means of scarification of unsensitized skin or skin topically sensitized with ovalbumin in FLG-deficient flaky tail (ft/ft) mice or wild-type (WT) control mice. The sizes of primary and satellite skin lesions were measured, and hematoxylin and eosin staining was performed. VV genome copy numbers and cytokine mRNA levels were measured by using quantitative PCR. RESULTS: VV inoculation in unsensitized skin of ft/ft mice, independent of the matted hair mutation, resulted in larger primary lesions, more abundant satellite lesions, heavier viral loads in internal organs, greater epidermal thickness, dermal cellular infiltration, and higher local Il17a, Il4, Il13, and Ifng mRNA levels than in WT control mice. VV inoculation at sites of topical ovalbumin application amplified all of these features in ft/ft mice but had no detectable effect in WT control mice. The number of satellite lesions and the viral loads in internal organs after cutaneous VV inoculation were significantly reduced in both unsensitized and topically sensitized ft/ftxIl17a(-/-) mice. CONCLUSION: FLG deficiency predisposes to eczema vaccinatum. This is mediated primarily through production of IL-17A.

IL-10 suppresses IL-17-mediated dermal inflammation and reduces the systemic burden of Vaccinia virus in a mouse model of eczema vaccinatum.

Individuals with atopic dermatitis (AD) are susceptible to a severe, potentially fatal, systemic infection and inflammatory response following exposure to Vaccinia virus (VV). IL-10 acts both as an inducer of Th2 responses and as a regulator of T cell activation. It has been shown to limit skin inflammation elicited by contact sensitizers. AD exacerbations have been associated with decreased IL-10 function. We used IL-10(-/-) mice to test the role of the cytokine in VV immunity. They exhibited larger primary lesions and increased cutaneous neutrophil infiltration compared to wild-type (WT) counterparts. This was associated with enhanced production of IL-17A, IL-17F and CXCL2. Paradoxically, despite intact adaptive immune responses, tissue viral burdens were increased in IL-10(-/-) mice. These findings suggest that IL-10 is important in limiting skin inflammation induced by VV and that abnormal IL-17-driven neutrophil recruitment at the primary infection site in the skin results in increased systemic viral dissemination.

Pathological role of regulatory T cells in the initiation and maintenance of eczema herpeticum lesions.

It remains unknown why the occurrence of eczema herpeticum (EH) caused by an extensive disseminated cutaneous infection with HSV-1 or HSV-2 is associated with the exacerbation of atopic dermatitis lesions after withdrawal of treatment. Although regulatory T cells (Tregs) limit the magnitude of HSV-specific T cell responses in mice, their role in the induction and resolution of EH has not been defined. We initially investigated the frequencies, phenotype, and function of Tregs in the peripheral blood of atopic dermatitis with EH (ADEH) patients at onset and after clinical resolution, atopic dermatitis patients without EH, and healthy controls. Tregs with the skin-homing phenotype and the activated/induced phenotype were expanded at onset and contracted upon resolution. Treg-suppressive capacity was retained in ADEH patients and, the expanded Tregs suppressed IFN-γ production from HSV-1-specific CD8(+) and CD4(+) T cells. The increased frequency of CD14(dim)CD16(+) proinflammatory monocytes (pMOs) was also observed in the blood and EH skin lesions. Thus, pMOs detected in ADEH patients at onset were characterized by an increased ability to produce IL-10 and a decreased ability to produce proinflammatory cytokines, unlike their normal counterparts. Our coculture study using Tregs and pMOs showed that the pMOs can promote the expansion of inducible Tregs. Tregs were detected frequently in the vicinity of HSV-expressing and varicella zoster virus-expressing CD16(+) monocytes in the EH lesions. Expansions of functional Tregs, together with pMOs, initially required for ameliorating excessive inflammation occurring after withdrawal of topical corticosteroids could, in turn, contribute to the initiation and progression of HSV reactivation, resulting in the onset of EH.

Atopic dermatitis complicated by eczema herpeticum is associated with HLA B7 and reduced interferon-γ-producing CD8+ T cells.

BACKGROUND: The increased susceptibility of patients with atopic dermatitis (AD) to disseminated viral skin infections such as eczema herpeticum (ADEH+) is poorly understood. OBJECTIVES: The primary goal of the current study was to determine whether ADEH+ subjects have identifiable defects in cell-mediated immunity that reduce their ability to control viral infections. MATERIALS AND METHODS: In this study, we evaluated cytokine expression by various subsets of peripheral blood mononuclear cells from ADEH+ (n = 24) compared with AD without a history of viral infections (ADEH-) (n = 20) before and after treatment with herpes simplex virus (HSV). RESULTS: We found that interferon (IFN)-γ expression after HSV treatment was lower in the CD8+ T cells and monocytes from patients with ADEH+ compared with patients who are ADEH- or nonatopic. Given the induction of CD8+ T cells as the result of antigen presentation by human leucocyte antigen (HLA) class I, consistent with the findings described above we also found that the HLA B7 allele was significantly associated with risk of the ADEH+ phenotype (odds ratio = 1·91, P = 0·02, 125 ADEH+ and 161 ADEH- subjects). CONCLUSIONS: These data suggest that defects in viral-induced IFN-γ from CD8+ T cells contribute to the ADEH+ phenotype.

Why is eczema herpeticum unexpectedly rare?

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease of humans, affecting approximately 17% of children. AD patients are especially susceptible to cutaneous bacterial and viral infections, and may develop severe or fatal herpes simplex virus (HSV) infection (eczema herpeticum, EH), requiring intensive antiviral therapy. However, even though a majority of adults show serologic evidence of previous HSV exposure, EH occurs in less than 3% of AD patients. The unexpected rarity of AD patients with EH (ADEH+) suggests that multiple host factors play a role in the clinical expression of this complex phenotype. Recent studies comparing ADEH+ versus ADEH- patients reveal that patients prone to ADEH+ have more severe AD skin disease, biomarkers associated with Th2 helper cell responses (reduced interferon levels, circulating eosinophil counts, increased serum IgE and allergen sensitization) and decreased epidermal expression of filaggrin and antimicrobial peptides. ADEH+ subjects are also more likely to have a history of food allergy or asthma, early onset of AD and a history of other cutaneous infections with Staphylococcus aureus or molluscum contagiosum.

Susceptibility to vaccinia virus infection and spread in mice is determined by age at infection, allergen sensitization and mast cell status.

BACKGROUND: Patients, especially young children, with atopic dermatitis are at an increased risk of developing eczema vaccinatum, a severe reaction to the smallpox vaccine, either through direct vaccination or indirect contact with a person recently vaccinated. METHODS: Using a mouse model of infection, the severity of vaccinia-induced lesions was assessed from their appearance and viral DNA content. The response to vaccinia inoculation was assessed in young and adult mice, allergen-sensitized mice, and in mast cell-deficient mice. RESULTS: Young age, sensitization to an allergen prior to infection, and a mast cell deficit, accomplished by using mast cell-deficient mice, resulted in more severe viral lesions at the site of inoculation, according to lesion appearance and viral DNA content. All three factors combined demonstrated maximal susceptibility, characterized by the severity of primary lesions and the development of secondary (satellite) lesions, as occurs in eczema vaccinatum in humans. Resistance to the appearance of satellite lesions could be restored by adoptive transfer of bone marrow-derived mast cells from either wild-type or cathelicidin-related antimicrobial peptide-deficient mice. Primary lesions were more severe following the latter transfer, indicating that cathelicidin-related antimicrobial peptide does contribute to the protective activity of mast cells against infection. CONCLUSIONS: The combination of young age, allergen sensitization and a mast cell deficit resulted in the most severe lesions, including satellite lesions. Understanding the factors determining the relative resistance/sensitivity to vaccinia virus will aid in the development of strategies for preventing and treating adverse reactions which can occur after smallpox vaccination.

Genetic variants in interferon regulatory factor 2 (IRF2) are associated with atopic dermatitis and eczema herpeticum.

Interferon regulatory factor 2 (IRF2) is a member of a family of transcriptional factors involved in the modulation of IFN-induced immune responses to viral infection. To test whether genetic variants in IRF2 predict risk of atopic dermatitis (AD) and ADEH (atopic dermatitis complicated by eczema herpeticum), we genotyped 78 IRF2 tagging single-nucleotide polymorphisms (SNPs) in both European-American (n = 435) and African-American (n = 339) populations. Significant associations were observed between AD and two SNPs (rs793814, P = 0.007, odds ratio (OR) = 0.52; rs3756094, P = 0.037, OR = 0.66) among European Americans and one SNP (rs3775572, P = 0.016, OR = 0.46) among African Americans. Significant associations were also observed between ADEH and five SNPs (P = 0.049-0.022) among European Americans. The association with ADEH was further strengthened by haplotype analyses, wherein a five-SNP (CAGGA) haplotype showed the strongest association with ADEH (P = 0.0008). Eight IRF2 SNPs were significantly associated with IFN-γ production after herpes simplex virus (HSV) stimulation (P = 0.048-0.0008), including an AD-associated SNP (rs13139310, P = 0.008). Our findings suggest that distinct markers in IRF2 may be associated with AD and ADEH, which may depend upon ethnic ancestry, and genetic variants in IRF2 may contribute to an abnormal immune response to HSV.

Increased susceptibility to cutaneous viral infections in atopic dermatitis: the roles of regulatory T cells and innate immune defects.

Much attention has been focused on the elucidation of mechanisms whereby atopic dermatitis (AD) skin lesions are especially susceptible to certain viral infections, such as herpes simplex virus (HSV). Although one of the most likely hypotheses is that the primary defect is in an impaired epidermal barrier function, alternative hypotheses include an imbalance between antiviral immune responses and regulatory T (T(reg)) cells, and the defects in the innate immune system. Eczema herpeticum (EH) occurs almost exclusively in patients with AD, particularly in those who fail to control skin inflammation. According to our scenario, expansions of T(reg) cells would be initially required for preventing such excessive inflammation resulting from the failure, and the expansions could in turn contribute to HSV reactivation, resulting in the initiation and progression of EH. A selective impairment of Toll-like-receptor-2-mediated proinflammatory cytokine production by monocytes could be the additional mechanism responsible for the increased susceptibility of AD subjects to curtain viral infections. Here we provide several potential explanations for why AD patients are at greater risk for eczema molluscatum.