Novel Insights into the Pathophysiology of Atopic Dermatitis

Atopic dermatitis (AD) is a multifactorial, chronic inflammatory skin disease that is caused by a combination of both genetic conditions and environmental factors. Mutations in the gene encoding filaggrin, which is a natural moisturizer, represent the most significant genetic factors that predispose an individual to the development of AD, as they are associated with the breakdown of the skin barrier, which leads to an increased risk of severe, early onset AD, skin infections, allergen sensitization, and food allergies, as well as the development of asthma and allergic rhinitis. In addition, abnormalities in both the adaptive and innate immune responses play a critical role in the pathogenesis of AD. The acute phase of AD is mainly characterized by the overactivation of type 2 helper T cells, while the chronic phase is characterized by the activation of type 1 helper T cells. Furthermore, environmental factors, allergens, the use of soap and detergents, and microbial infections exacerbate the breakdown of the epidermal barrier and contribute to aberrations of the immune system, further aggravating the manifestations of AD. Our understanding of AD pathophysiology will enable the better use of certain topical products, the development of new products that repair the skin barrier, and the design of new approaches for the prevention and treatment of AD.


Introduction
Atopic dermatitis (AD) or eczema is a multifaceted, chronic inflammatory skin disease that begins in early childhood and is associated with complex disorders of genetics, skin barrier function, the immune system, and environmental factors. 1) AD is usually associated with a family history of atopic disease (asthma, allergic rhinitis, or atopic dermatitis) and is thought to be exacerbated by various factors, such as food allergies, microbial infections, irritants (e.g., wool or synthetic clothing, alkaline soaps, or detergents), climatic factors, and emotions. 2) Epidemiological studies have shown that AD profoundly affects the quality of life of patients, and its prevalence is approximately 10% to 30% in children and 2% to 10% in adults worldwide, depending on the population. 3) 4) Patients with AD are at increased risk for skin infections, sleep disruption, school absences, psychosocial morbidities, occupational disabilities, and emotional stress. 3) 4) Despite recent progress in understanding the complex pathogenesis of AD over the past few 480 Health Topics for Tokyoites decades, a clear understanding of the pathophysiology of this disease remains elusive. Although both external stimuli and immune mechanisms contribute to AD pathogenesis, systemic immune mechanisms are regarded as the chief drivers of the disease. In fact, the infiltration of immune cells, such as T cells, dendritic cells, macrophages, mast cells and eosinophils, and the cytokines and chemokines produced by these cells are increased in AD lesions. 5) In this review, we summarize the current understanding of the pathogenesis of AD in the following sections: (1) genetics of AD; (2) skin barrier dysfunction; (3) immune system dysregulation; and (4) exogenous factors, including environmental factors, irritants, and microbial infections. We believe that understanding the pathogenesis of AD will lead to further improvements in the prevention and management of this skin disorder.

Genetics of AD
Twin concordance rates of AD have been used to emphasize the importance of heredity in susceptibility to this disease. In fact, it has been demonstrated that the concordance rate of AD in monozygotic twins was higher than 70%, while it was only 20% in dizygotic twins, indicating a role for genetics in the development of AD. 6) To date, more than 80 possible genes have been associated with AD susceptibility, including those that play a central role in dysfunction of the skin barrier and immune responses. It is noteworthy that numerous genetic syndromes have atopic eczema as a feature. These syndromes include ichthyosis vulgaris (mutation in filaggrin), Netherton syndrome (serine peptidase inhibitor Kazal-type 5, SPINK-5), severe dermatitis, allergies, and metabolic wasting (Desmoglein 1). 7) To date, of all the genes that have been studied and implicated in AD pathogenesis, the gene encoding filaggrin is the most frequently reported risk factor for AD. Filaggrin, which is a natural moisturizer, is a major structural protein in the stratum corneum. Filaggrin is formed and stored as pro-filaggrin polymers, which are cleaved into filaggrin monomers by several proteases, such as CAP1139 and SASPase. 8) Filaggrin degradation products contribute to the regulation of stratum hydration, maintenance of the acidic pH of skin, immunosuppression, antimicrobial defenses, and skin elasticity. 9) Mutations in filaggrin impair skin barrier function and are associated with an increased risk of severe, early onset AD, skin infections, allergen sensitization, food allergies, 10) and the development of asthma and allergic rhinitis. 11) Furthermore, filaggrin-deficient mice display dry scaly skin and exaggerated immune responses. 12) More than 20 mutations in the filaggrin gene have been described to date, and a strong link between the incidence of AD and mutations in the filaggrin gene has been demonstrated. 13) Approximately 20% to 30% of AD patients, depending on the population, harbor filaggrin mutations. 9) Of note, filaggrin mutations are a significant risk factor not only for AD but also for allergic rhinitis, asthma, and food allergies, demonstrating that skin barrier dysfunction is a primary phenomenon in allergic disease and AD. 14) Interestingly, 40% of people with filaggrin null mutations (resulting in the development of ichythyosis vulgaris) and 60% of people who are heterozygous for these mutations do not develop AD, suggesting that mutations in filaggrin are not the only pathogenic mechanism. 11) 15) Several reports have shown that some genes related to immune pathways are associated with increased AD susceptibility. For example, the association of polymorphisms in genes encoding interleukin (IL)-4, IL-13 and their respective receptors, IL-4R and IL-13R, with AD pathogenesis has been reported. 16) 17) In addition, the genes encoding IL-31, IL-33, thymic stromal lymphopoietin (TSLP) and its receptors, signal transducer and activator of transcription (STAT) 6, and Toll-like receptor 2 have been associated with AD. 18)

Skin barrier dysfunction
The dysfunctional skin barrier has long been considered to be the most significant pathological observation in AD. Defects in the filaggrin gene, decreases in ceramides, and overactivation of certain epidermal proteases are the main causes of barrier dysfunction in AD patients. 19) In addition to filaggrin, transglutaminases, keratins, and intercellular proteins are also indispensable for functional barriers, and defects in these proteins may facilitate the penetration of allergens and microbial stimuli into the skin. 20)-22) As cited above, although filaggrin mutations are a risk factor for AD, there are patients without filaggrin mutations who have dysfunctional skin barriers due to inflammation and itching, indicating that barrier dysfunction is a key component in the pathogenesis of AD. Both inflamed skin and defective barriers in AD facilitate the penetration of allergens that further trigger immune responses, leading to immunoglobulin E (IgE)-mediated allergies. Of note, the inflammation and cellular responses in AD downregulate the expression of filaggrin and thus further cause the breakdown of the skin barrier. Type 2 T helper cell (Th2) cytokines, such as IL-4, IL-13, IL-31, and IL-33, inhibit the production of skin barrier-related proteins, such as filaggrin, loricrin, involucrin, and keratins. 18) Kelleher et al. have shown that impairment of the skin barrier, as assessed by transepidermal water loss (TEWL), at birth and at 2 months predates the onset of AD at 1 year. 23) The persistent activation of some epidermal proteases, such as the kallikrein (KLK)-related peptidases KLK5, KLK7, and KLK14, which have critical roles in corneocyte desquamation, has also been associated with AD susceptibility. 24) The activity of these proteases increases when the skin pH increases. The pH of normal skin surfaces is acidic (5.45.9 on the forearm of a healthy, adult white male) and contributes to the barrier function and antibacterial effects of this tissue. 25) The pH value increases in patients with AD and might exacerbate AD skin lesions. 26) 27)

Immune system dysregulation
Aberrations in both adaptive and innate immunity are seen in patients with AD. Although the abnormal immune responses in AD are partially due to barrier dysfunction and allergen penetration, there are also primary immunological factors that drive the pathophysiology of AD. Among the various immune cells (T cells, dendritic cells, macrophages, mast cells and eosinophils) that are involved in AD development, T cells are one of the key players of adaptive immunity. The acute phase of AD is characterized by the overactivation of Th2, Th22, and Th17 cells, while the chronic phase is characterized by the activation of Th1, Th2 and Th22 cells. 28) Th2 cytokines, such as IL-4 and IL-13, play important roles in chemokine production, skin barrier dysfunction, antimicrobial peptide suppression, and allergic inflammation. 29) 30) The T cell activities in AD are driven by specialized skin dendritic cells, such as Langerhans cells and inflammatory dendritic epidermal cells. These cells overexpress a high-affinity receptor for IgE (FcεRI) that captures allergens for presentation to Th2 cells. 31) 32) IL-4 and IL-13 downregulate filaggrin expression by keratinocytes, 30) which supports the observation of reduced filaggrin expression in lesional AD skin compared to nonlesional AD skin. 33) IL-17 also reduces the expression of filaggrin and involucrin, and Th17 activation is observed in the blood and in acute AD skin lesions. 34)-36) It was also shown that IL-17 production is higher in intrinsic AD with normal IgE levels than in extrinsic AD with high total serum IgE levels and IgE molecules that are specific for allergens. 37) The IL-22 levels are also enhanced in AD skin and are associated with dysfunction of the skin barrier and abnormal expression of epidermal markers. 38)-40) Interestingly, IL-31, which is secreted by Th2 cells, mast cells, macrophages, dendritic cells and eosinophils, 41) induces itch in patients with AD, and together with IL-22 and Th2 cytokines, IL-31 stimulates the expression of keratinization markers, leading to epidermal hyperplasia. 37) Furthermore, TSLP is highly expressed in AD skin, and its production is triggered by allergens and irritants. The elevated expression of TSLP in the skin of 2-month-old infants was found to predate the onset of AD at 2 years of age. 42)

Exogenous factors
The literature suggests that the risk of AD is higher in urban areas than in rural areas in populations of the same ethnicity and genetic background, 43) suggesting the importance of environmental factors in AD pathophysiology. Among the many exogenous factors implicated in AD onset include environmental factors, house dust mites, allergens, microbial infections, antibiotics, and irritants. 44)

Environmental factors
It has been shown that ultraviolet light exhibits immunosuppressive properties, 45) and exposure to sun/ultraviolet B results in elevated serum levels of vitamin D. Interestingly, vitamin D deficiency is associated with severe AD in areas of the skin that are not exposed to light, demonstrating a protective role of vitamin D. 46) In addition, low temperatures have also been associated with the exacerbation of AD, 47) although some skin manifestations may worsen during the summer. 48) Further studies are necessary to clarify the role of temperature in AD pathogenesis. Another important environmental factor that is currently under investigation for its association with AD is air pollution. A study conducted by Jedrychowski et al., has shown that infants whose mothers were prenatally exposed to particulate matter (PM 2.5) and who were then postnatally exposed to the same particulate matter and cigarette smoke showed a 2-fold increase in the prevalence of AD during the first year of life. 49) Another study has shown that AD manifestations are aggravated by the presence of high concentrations of particulate matter, toluene, and other volatile organic compounds. 44) The"hygiene hypothesis"has been proposed to link the rapid increase in AD prevalence to the development and"sanitization"of our society. 50) In fact, a study has reported that larger family size and nursery school attendance are associated with a reduced risk of AD, probably due to the frequent exposure to infections. It is believed that improvements in household amenities and cleanliness lead to the lack of stimulation of the immune system by microbes at an early age, resulting in a high risk of AD. 51)

Irritants
The use of soaps and detergents, which work as surfactants, can cause damage to the skin and provoke scaly and dry skin, thereby weakening the skin barrier function by increasing TEWL; this phenomenon is more severe in AD patients than in normal subjects. 52) Surfactants increase TEWL via their ability to solubilize skin lipids. 52) 53) It has been reported that washing the skin with soap can cause an increase in the pH by 3 U for more than 90 minutes. 53) In addition to increasing the skin pH, soaps and detergents also induce the release of pro-inflammatory cytokines from corneocytes and alter the expression of keratinocyte differentiation markers. 53) Interestingly, Deguchi et al. have succeeded in demonstrating that avoiding detergents is effective in relieving the symptoms of AD in Japanese patients. 54) In addition to detergents, studies have demonstrated that AD prevalence is higher in areas with hard water than in areas with soft water, 55) 56) and that the use of emollients from birth markedly reduces the risk of developing AD in children with a family history of AD. 57)

Microbial infections
Patients with AD are prone to microbial infections, primarily Staphylococcus aureus (S. aureus) infections, which are seen in up to 90% of patients and play a crucial role in triggering and aggravating AD. 14) The weakened skin barrier, aberrations in the cutaneous immune system and trauma from scratching all contribute to an increased risk of bacterial infections. Bacterial virulence factors, such as superantigens, α-and δ-toxin, and protein A secreted by S. aureus, cause skin inflammation and contribute to bacterial persistence. 58) Paradoxically, bacteria that cause infections in AD patients are also usually detected on healthy skin; however, due to the increased oils on healthy skin, these bacteria, which are lipophobic, cannot normally penetrate the skin. In contrast, staphylococci and streptococci invade dry and scaly atopic skin. These bacteria stimulate the immune system, resulting in constant, chronic inflammation in AD patients. Other microorganisms that are known to cause skin infections and trigger inflammation in AD include Malassezia species, Candida albicans, human papillomavirus, Molluscum contagiosum virus, Vaccinia virus and herpes simplex virus. 14) 59) Eczema herpeticum and eczema vaccinatum, which are caused by herpes simplex virus and Vaccinia virus, respectively, are serious problems in AD and are related to the severity of AD. 14) 59) Persistent cutaneous infections in patients with AD are due in part to the diminished production of antimicrobial peptides (AMPs), such as human βdefensins and LL-37, in AD patients compared to psoriasis patients. Although the mechanism by which AMPs are reduced in AD skin is not well understood, the overproduction of Th2 cytokines, such as IL-4, IL-10, IL-13, and IL-31, which inhibit AMPs, and the diminished secretion of IL-17, IL-22, and TNF-α, which stimulate the production of AMPs, are thought to be plausible reasons for the reduction in AMPs in AD. 60)

Conclusion
The pathophysiology of AD is likely more complex than previously documented. AD is a multifactorial and heterogeneous skin disease that occurs as a result of the interactions of a variety of factors, such as mutations in epidermal genes (e.g., filaggrin), dysfunction of the skin barrier, dysregulation of the immune system and various environmental factors (Figure-1).
Understanding AD pathogenesis will allow the better use of some topical products, such as emollients, the avoidance of other topical products, such as alkaline soaps and detergents, and the development of new products that repair or maintain the skin barrier. It is now known that the frequent application of moisturizers, such as physiologic lipid mixtures and ceramide-rich lipids, enhances skin hydration, reduces TEWL, and decreases bacterial colonization. Of note, the steady application of emollients reduces the risk of AD development. 57) 61) In conclusion, further understanding the pathophysiology of AD will allow us to design new approaches for the prevention and treatment of AD.

Figure-1 The interdependent mutualistic relationship between factors that cause atopic dermatitis
Atopic dermatitis is a multifactorial and heterogeneous skin disease that occurs as a result of the interactions of a variety of factors, such as skin barrier impairment, dysregulation of the immune system and various environmental factors. AMP: antimicrobial peptide; IgE: Immunoglobulin E; TEWL: transepidermal water loss; Th: helper T cell.