The role of innate immune signaling in the pathogenesis of atopic dermatitis and consequences for treatments

Toll-like receptors in atopic dermatitis: pathogenesis and therapeutic implications

Toll-like receptors (TLR), the key players of the innate immune system, contribute to the pathogenesis of atopic dermatitis (AD) through multiple pathways. TLRs play a crucial role in delaying barrier repair, promoting Th2-mediated dermatitis, shifting the response toward Th1 in the chronic phase, and contributing to the establishment of the itch-scratch cycle, as well as mediating the effects of UV radiation. The dysregulation of proinflammatory and immunomodulatory effects of TLRs can be attributed to their ligand structures, receptor heterodimerization, the relative frequency of each TLR, interactions with other receptors/signalling pathways, cytokine milieu, and genetic polymorphisms. Current AD treatments like vitamin-D analogs, tacrolimus, and cyclosporine partially work through TLR modulation. Direct TLR stimulation using different compounds has shown therapeutic benefits in preclinical studies. However, significant challenges exist, including off-target effects due to ubiquitous TLR expression and complex roles in immune responses. Future directions include CRISPR-based gene editing to understand TLR functions, development of specific TLR modulators for targeted therapy, and machine learning applications to predict drug responses and identify novel ligands. Patient heterogeneity, including the presence or absence of polymorphisms, variations in TLR expression levels, and differences in immune responses, underscores the need for personalized therapeutic approaches.

The immunomodulatory activity of Orthosiphon aristatus against atopic dermatitis: Evidence-based on network pharmacology and molecular simulations

Objectives

To explore the potential activity of Orthosiphon aristatus (OA) against atopic dermatitis (AD).

Methods

Phytocompounds from OA were identified through chromatography analysis, then continued to target identification and functional annotation to explore the potential target of OA. Then, network pharmacology from annotated proteins determined protein targets for OA phytocompounds. Protein with highest rank according to the betweenness and closeness algorithm then continued to molecular docking and validated through molecular dynamics analysis.

Results

Chromatography data analysis revealed thirty-six compounds, predominantly classified as carboxylic acid, fatty acyls, and polyphenols. Upon identifying these compounds, network biology-based target identification revealed their potential bioactivity in modulating inflammation in AD. Tumour Necrosis Factor-alpha (TNF-α) and Prostaglandin G/H synthase 2 (PTGS2) emerged as the most probable targets based on hub centrality in the protein-protein interaction network. Later, molecular docking analyses highlighted sixteen compounds with good inhibitory activity against these two proteins. Notably, molecular dynamics simulation revealed that three compounds out of the previous sixteen potential compounds were more likely to act as the TNF-α and PTGS2 inhibitor as well as their native inhibitor. Those compounds are (1R,9R)-5-Cyclohexyl-11- (propylsulfonyl)-7,11- diazatricyclo[7.3.1.02,7]trideca- 2,4-dien-6-one, also known as ZINC8297940, as the best TNF-α inhibitor along with dl-Leucineamide and Benazol P as the potential inhibitor of PTGS2.

Conclusions

These findings suggest that OA may exert therapeutic effects against AD by controlling inflammation through TNF-α and PTGS2 signalling pathways.

Molecular and cellular mechanisms of itch and pain in atopic dermatitis and implications for novel therapeutics

Atopic dermatitis is a chronic inflammatory skin disease. Patients with atopic dermatitis experience inflammatory lesions associated with intense itch and pain, which lead to sleep disturbance and poor mental health and quality of life. We review the molecular mechanisms underlying itch and pain symptoms in atopic dermatitis and discuss the current clinical development of treatments for moderate-to-severe atopic dermatitis. The molecular pathology of atopic dermatitis includes aberrant immune activation involving significant cross-talk among the skin and immune and neuronal cells. Exogenous and endogenous triggers modulate stimulation of mediators including cytokine/chemokine expression/release by the skin and immune cells, which causes inflammation, skin barrier disruption, activation and growth of sensory neurons, itch and pain. These complex interactions among cell types are mediated primarily by cytokines, but also involve chemokines, neurotransmitters, lipids, proteases, antimicrobial peptides, agonists of ion channels or various G protein-coupled receptors. Patients with atopic dermatitis have a cytokine profile characterised by abnormal levels of interleukins 4, 12, 13, 18, 22, 31 and 33; thymic stromal lymphopoietin; and interferon gamma. Cytokine receptors mainly signal through the Janus kinase/signal transducer and activator of transcription pathway. Among emerging novel therapeutics, several Janus kinase inhibitors are being developed for topical or systemic treatment of moderate-to-severe atopic dermatitis because of their potential to modulate cytokine expression and release. Janus kinase inhibitors lead to changes in gene expression that have favourable effects on local and systemic cytokine release, and probably other mediators, thus successfully modulating molecular mechanisms responsible for itch and pain in atopic dermatitis.

Does tocilizumab have a role in dermatology? A review of clinical applications, its adverse side effects and practical considerations

Tocilizumab is a humanized monoclonal antibody to the interleukin-6 (IL-6) receptor which was first approved for use in refractory rheumatoid arthritis almost a decade ago. Since then, its use has expanded to a number of rheumatological and inflammatory conditions. In dermatology, off-label use of tocilizumab has been reported to be efficacious in morphoea, systemic sclerosis, psoriasis, atopic dermatitis, vitiligo, graft-versus-host disease, pyoderma gangrenosum, Behcet's disease, Schnitzler's syndrome, sarcoidosis, and cutaneous adverse reactions. That being said, the evidence demonstrating tocilizumab's efficacy in dermatology is predominantly low-level case-based evidence, and one must consider the potential for publication bias. In this review we will discuss the reported clinical applications of tocilizumab in dermatology, mechanisms of action, and the range of associated adverse effects (both cutaneous and non-cutaneous) that can occur. Additionally, we will discuss the role of tocilizumab in the management of COVID-19.

Alternate expression of PEPT1 and PEPT2 in epidermal differentiation is required for NOD2 immune responses by bacteria-derived muramyl dipeptide

Peptide transporters 1 and 2 (PEPT1 and PEPT2) are proton-coupled oligopeptide transporter members of the solute carrier 15 family and play a role in the cellular uptake of di/tri-peptides and peptidomimetics. Our previous work showed that PEPT2 is predominantly expressed within undifferentiated keratinocytes. Here we show that PEPT2 expression decreases as keratinocyte differentiation progresses and that PEPT1 alternately is expressed at later stages. Absolute quantification using quantitative polymerase chain reaction revealed that the expression level of PEPT1 is about 17 times greater than that of PEPT2. Immunohistochemical study of human skin provided evidence of PEPT1 in the epidermis. The uptake of glycylsarcosine into keratinocytes was significantly blocked by PEPT inhibitors, including nateglinide and glibenclamide. Moreover, we found that PEPT1 knockdown in differentiated keratinocytes significantly suppressed the influence of a bacterial-derived peptide, muramyl dipeptide (MDP), on the production of proinflammatory cytokine interleukin-8, implying that bacteria-derived oligopeptides can be transported by PEPT1 in advanced differentiated keratinocytes. Taken together, PEPT1 and PEPT2 may concertedly play an important role in MDP-NOD2 signaling in the epidermis, which provides new insight into the mechanisms of skin homeostasis against microbial pathogens.

Staphylococcus aureus-derived extracellular vesicles induce monocyte recruitment by activating human dermal microvascular endothelial cells in vitro

BACKGROUND

Atopic dermatitis (AD) represents the most common inflammatory skin disorder in children showing massive infiltration of immune cells. The colonization of AD-afflicted skin by Staphylococcus aureus and S. aureus-derived extracellular vesicles (SEVs) has been associated with AD pathogenesis; however, the molecular mechanism underlying SEV-mediated inflammatory responses remains unclear.

OBJECTIVE

We investigated how SEVs can mediate inflammatory responses in AD pathogenesis by examining the effect of SEVs on human dermal microvascular endothelia cells (HDMECs).

METHODS

HDMECs were treated with SEVs, and the expression of cell adhesion molecules or cytokines was assessed using RT-qPCR, Western blot or cytokine array analyses. The receptor for SEVs and related signalling molecules in HDMECs were addressed and verified via gene knockdown or inhibitor experiments. The recruitment assay of human THP-1 monocytic cells on HDMECs was performed after SEV treatment in the presence or absence of the verified receptor or signalling molecule.

RESULTS

SEVs, but not other gram-positive bacteria-derived extracellular vesicles, directly activated HDMECs by increasing the expression of cell adhesion molecules (E-selectin, VCAM1 and ICAM1) and that of IL-6, the inflammatory cytokine; consequently, they enhanced the recruitment of THP-1 monocytic cells to HDMECs. The SEV-induced HDMEC activation was dependent on Toll-like receptor 4 and the NF-κB signalling pathway, which was rapidly activated within 1 hour post-treatment and followed by an upregulation of cell adhesion molecules and IL-6 at later time-points. Moreover, SEV-mediated HDMEC responses were more rapid and intense than those induced by the same protein concentrations of S. aureus extracts.

CONCLUSIONS & CLINICAL RELEVANCE

SEVs as proinflammatory factors could mediate immune cell infiltration in AD by efficiently inducing endothelial cell activation and monocyte recruitment, which may provide insights into alleviating the S. aureus-mediated onset or progression of AD and its phenotypes.

Atopic dermatitis

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease, with a lifetime prevalence of up to 20% and substantial effects on quality of life. AD is characterized by intense itch, recurrent eczematous lesions and a fluctuating course. AD has a strong heritability component and is closely related to and commonly co-occurs with other atopic diseases (such as asthma and allergic rhinitis). Several pathophysiological mechanisms contribute to AD aetiology and clinical manifestations. Impairment of epidermal barrier function, for example, owing to deficiency in the structural protein filaggrin, can promote inflammation and T cell infiltration. The immune response in AD is skewed towards T helper 2 cell-mediated pathways and can in turn favour epidermal barrier disruption. Other contributing factors to AD onset include dysbiosis of the skin microbiota (in particular overgrowth of Staphylococcus aureus), systemic immune responses (including immunoglobulin E (IgE)-mediated sensitization) and neuroinflammation, which is involved in itch. Current treatments for AD include topical moisturizers and anti-inflammatory agents (such as corticosteroids, calcineurin inhibitors and cAMP-specific 3',5'-cyclic phosphodiesterase 4 (PDE4) inhibitors), phototherapy and systemic immunosuppressants. Translational research has fostered the development of targeted small molecules and biologic therapies, especially for moderate-to-severe disease.

A Phage Lysin Fused to a Cell-Penetrating Peptide Kills Intracellular Methicillin-Resistant Staphylococcus aureus in Keratinocytes and Has Potential as a Treatment for Skin Infections in Mice

Staphylococcus aureus is the main pathogen that causes skin and skin structure infections and is able to survive and persist in keratinocytes of the epidermis. Since the evolution of multidrug-resistant bacteria, the use of phages and their lysins has presented a promising alternative approach to treatment. In this study, a cell wall hydrolase (also called lysin) derived from Staphylococcus phage JD007 (JDlys) was identified. JDlys showed strong lytic activity against methicillin-resistant Staphylococcus aureus (MRSA) strains from different sources and of different multilocus sequence typing (MLST) types. Furthermore, a fusion protein consisting of a cell-penetrating peptide derived from the trans-activating transcription (Tat) factor fused to JDlys (CPP_Tat-JDlys) was used to kill MRSA bacteria causing intracellular infections. CPP_Tat-JDlys, in which the fusion of CPP_Tat to JDlys had almost no effect on the bacteriolytic activity of JDlys, was able to effectively eliminate intracellular MRSA bacteria and alleviate the inflammatory response and cell damage caused by MRSA. Specifically, CPP_Tat-JDlys was able to combat MRSA-induced murine skin infections and, consequently, expedite the healing of cutaneous abscesses. These data suggest that the novel antimicrobial CPP-JDlys may be a worthwhile candidate as a treatment for skin and skin structure infections caused by MRSA.IMPORTANCES. aureus is the main cause of skin and skin structure infections due to its ability to invade and survive in the epithelial barrier. Due to the overuse of antibiotics in humans and animals, S. aureus has shown a high capacity for acquiring and accumulating mechanisms of resistance to antibiotics. Moreover, most antibiotics are usually limited in their ability to overcome the intracellular persistence of bacteria causing skin and skin structure infections. So, it is critical to seek a novel antimicrobial agent to eradicate intracellular S. aureus In this study, a cell-penetrating peptide fused to lysin (CPP-JDlys) was engineered. Our results show that CPP-JDlys can enter keratinocytes and effectively eliminate intracellular MRSA. Meanwhile, experiments with mice revealed that CPP-JDlys efficiently inhibits the proliferation of MRSA in murine skin and thus shortens the course of wound healing. Our results indicate that the CPP-fused lysin has potential for use for the treatment of skin infections caused by MRSA.

Immune response patterns in non-communicable inflammatory skin diseases

Non-communicable inflammatory skin diseases (ncISD) such as psoriasis or atopic eczema are a major cause of global disease burden. Due to their impact and complexity, ncISD represent a major challenge of modern medicine. Dermatology textbooks describe more than 100 different ncISD based on clinical phenotype and histological architecture. In the last decades, this historical description was complemented by increasing molecular knowledge - and this knowledge is now being translated into specific therapeutics. Combining the enormous advances made in lymphocyte immunology and molecular genetics with clinical and histological phenotyping reveals six immune response patterns of the skin - type I immune cells cause the lichenoid pattern characterized by immune-mediated cell death of keratinocytes; type II immune cells underlie the eczematous pattern with impaired epidermal barrier, infection and eosinophils as well as the bullous pattern with loss of epithelial integrity; Th17 cells and ILC3 mediate the psoriatic pattern characterized by acanthosis, high metabolic activity and neutrophils; dysbalance of regulatory T cells causes either the fibrogenic pattern with rarefication of cells and dermal thickening or the granulomatous pattern defined by formation of granulomas. With more and more specific therapeutic agents approved, classifying ncISD also according to their immune response pattern will become highly relevant. This review defines the six immune response patterns of ncISD and highlights therapeutic strategies targeting key lymphocyte mediators.

Comparative analysis of Lacistema pubescens and dexamethasone on topical treatment of skin inflammation in a chronic disease model and side effects

Objectives

This study aimed to evaluate the chronic topical anti-inflammatory activity of the pharmaceutical formulation ProHLP containing the hexane fraction of Lacistema pubescens (HLP). It was also investigated the possible cutaneous and systemic adverse effects of HLP and ProHLP in mice when compared to dexamethasone.

Methods

The chronic topical anti-inflammatory activity was determined by croton oil multiple application-induced mouse ear oedema model. Histopathological analyses of ear tissue samples sensitized with croton oil were performed. Cutaneous atrophy induced by HLP and topical glucocorticoid treatments and excision skin wounds model to evidenced possible adverse reactions were also determined.

Key findings

ProHLP significantly reduced the mice ear oedema and considerably accelerated the wound-healing process. Also, HLP did not lead cutaneous atrophy and preserved the clinical aspect of the thymus, adrenal and spleen, unlike dexamethasone.

Conclusions

The results suggested that ProHLP is an efficient and safer pharmaceutical formulation to treat chronic inflammatory diseases.

Profile of dupilumab and its potential in the treatment of inadequately controlled moderate-to-severe atopic dermatitis

Atopic dermatitis (AD) is a common inflammatory skin disorder that manifests as eczematous lesions, often associated with allergic rhinitis and asthma. Historically, moderate-to-severe disease has been managed with systemic immunosuppression, such as oral corticosteroids, which result in relapse and limiting side effects. Due to recent advancements in the identification of interleukin (IL)-4 and IL-13 as key mediators in AD, new biological agents have been developed for treatment. Dupilumab is a recently approved monoclonal antibody that targets the alpha subunit of the IL-4 receptor and, thus, downregulates activity of IL-4 and IL-13. This review discusses the profile of dupilumab and its potential for efficacy and safety in treating moderate-to-severe AD by reviewing data from Phase I–III clinical trials. Results suggest that dupilumab shows great therapeutic promise for AD. Further studies investigating extended use of dupilumab and dupilumab in comparison to other agents are needed to establish long-term efficacy and safety.

Skin barrier and dry skin in the mature patient

Dry skin is the most common clinical manifestation of dermatologic diseases, and it presents with itching, redness, and desquamation-signs and clinical manifestations that are not only physically uncomfortable but also affect patients psychologically. The water content in the stratum corneum is largely dependent on the composition and amount of the intercellular lipids, which regulate the loss of water from the skin, and on the levels of hygroscopic substances of the natural moisturizing factors, which are responsible for retention of water in the stratum corneum. Prevention of water loss and penetration of potentially toxic substances and microorganisms into the body are the most important functions of the skin, which acts as a natural frontier between the inner organism and the environment. Skin barrier defects occur in several skin diseases, but the influence of aging on the skin barrier function is largely unknown and conflicting results have been reported. In this review, the structure and function of the barrier in relation to the aging process are discussed.

Evaluation of hyaluronic acid-P40 conjugated cream in a mouse model of dermatitis induced by oxazolone

P40 is a particulate fraction or fragment isolated from Corynebacterium granulosum, which exhibits a wide spectrum of pharmacological functions including antitumor, antibacterial, phagocytic, antiviral and cytokine induction effects. In the present study, the immunomodulatory potential of P40-conjugated with hyaluronic acid was assessed in a mouse model of dermatitis induced by oxazolone. Oxazolone-induced allergic contact dermatitis is a T cell-mediated Th2-like hypersensitivity reaction, which mimics the corresponding reaction in humans. Female cluster of differentiation-1 mice were sensitized on days 0 and 1 by the application of 2% oxazolone onto a shaved back. The disease was induced by re-challenge on day 7 using 15% oxazolone in the inner and outer of the left ears of the mice. Mice were topically treated with hyaluronic acid-P40 conjugate cream or with placebo to the inner and outer surface of the left ear for 7 consecutive days starting from 1 h after the sensitization. A significant reduction in ear thickness and weight and in edema and leukocyte recruitment were observed in the mice treated with hyaluronic-P40 conjugate cream compared with mice treated with the cream base alone (P<0.05). Thus, P40-conjugated with hyaluronic acid may constitute an innovative dermatitis treatment.

Cellular and molecular immunologic mechanisms in patients with atopic dermatitis

Atopic dermatitis (AD) is a complex skin disease frequently associated with other diseases of the atopic diathesis. Recent evidence supports the concept that AD can also recognize other comorbidities, such as chronic inflammatory bowel or cardiovascular diseases. These comorbidities might result from chronic cutaneous inflammation or from a common, yet-to-be-defined immunologic background leading to immune deviations. The activation of immune cells and their migration to the skin play an essential role in the pathogenesis of AD. In patients with AD, an underlying immune deviation might result in higher susceptibility of the skin to environmental factors. There is a high unmet medical need to define immunologic endotypes of AD because it has significant implications on upcoming stratification of the phenotype of AD and the resulting targeted therapies in the development of precision medicine. This review article emphasizes studies on environmental factors affecting AD development and novel biological agents used in the treatment of AD. Best evidence of the clinical efficacy of novel immunologic approaches using biological agents in patients with AD is available for the anti-IL-4 receptor α-chain antibody dupilumab, but a number of studies are currently ongoing with other specific antagonists to immune system players. These targeted molecules can be expressed on or drive the cellular players infiltrating the skin (eg, T lymphocytes, dendritic cells, or eosinophils). Such approaches can have immunomodulatory and thereby beneficial clinical effects on the overall skin condition, as well as on the underlying immune deviation that might play a role in comorbidities. An effect of these immunologic treatments on pruritus and the disturbed microbiome in patients with AD has other potential consequences for treatment.

Keratinocytes as sensors and central players in the immune defense against Staphylococcus aureus in the skin

Healthy human skin provides an effective mechanical as well as immunologic barrier against pathogenic microorganisms with keratinocytes as the main cell type in the epidermis actively participating and orchestrating the innate immune response of the skin. As constituent of the outermost layer encountering potential pathogens they have to sense signals from the environment and must be able to initiate a differential immune response to harmless commensals and harmful pathogens. Staphylococci are among the most abundant colonizers of the skin: Whereas Staphylococcus epidermidis is part of the skin microbiota and ubiquitously colonizes human skin, Staphylococcus aureus is only rarely found on healthy human skin, but frequently colonizes the skin of atopic dermatitis (AD) patients. This review highlights recent advances in understanding how keratinocytes as sessile innate immune cells orchestrate an effective defense against S. aureus in healthy skin and the mechanisms leading to an impaired keratinocyte function in AD patients.

The etiopathogenesis of atopic dermatitis: barrier disruption, immunological derangement, and pruritus

Atopic dermatitis (AD) is a common chronic skin inflammatory disorder characterized by recurrent eczema accompanied by an intractable itch that leads to an impaired quality of life. Extensive recent studies have shed light on the multifaceted pathogenesis of the disease. The complex interplay among skin barrier deficiency, immunological derangement, and pruritus contributes to the development, progression, and chronicity of the disease. Abnormalities in filaggrin, other stratum corneum constituents, and tight junctions induce and/or promote skin inflammation. This inflammation, in turn, can further deteriorate the barrier function by downregulating a myriad of essential barrier-maintaining molecules. Pruritus in AD, which may be due to hyperinnervation of the epidermis, increases pruritogens, and central sensitization compromises the skin integrity and promotes inflammation. There are unmet needs in the treatment of AD. Based on the detailed evidence available to date, certain disease mechanisms can be chosen as treatment targets. Numerous clinical trials of biological agents are currently being conducted and are expected to provide treatments for patients suffering from AD in the future. This review summarizes the etiopathogenesis of the disease and provides a rationale for choosing the novel targeted therapy that will be available in the future.

Chitinase-3-Like Protein 1 (YKL-40) Reflects the Severity of Symptoms in Atopic Dermatitis

Chitinase-3-like protein 1 (YKL-40) is suggested to be associated with type 2 T helper response and atopy. The aim of the study was the evaluation of serum YKL-40 level in atopic dermatitis. The study was performed on 59 patients: 27 males and 32 females, aged from 18 to 64 years. The severity of the disease was assessed by the SCORAD and objective SCORAD indexes. The severity of pruritus was measured by the visual analogue scale. Blood samples were taken to examine serum level of YKL-40, total IgE level, C-reactive protein level, white blood cell count, and neutrophil count. YKL-40 serum levels were significantly higher in patients with atopic dermatitis compared to the controls. There was a positive correlation between YKL-40 concentration and SCORAD, objective SCORAD, and pruritus. This study has shown that YKL-40 serum level is increased in patients with atopic dermatitis and reflects the severity of symptoms.

Skin barrier in atopic dermatitis: beyond filaggrin

Atopic dermatitis is a chronic inflammatory skin disease with a complex pathogenesis, where changes in skin barrier and imbalance of the immune system are relevant factors. The skin forms a mechanic and immune barrier, regulating water loss from the internal to the external environment, and protecting the individual from external aggressions, such as microorganisms, ultraviolet radiation and physical trauma. Main components of the skin barrier are located in the outer layers of the epidermis (such as filaggrin), the proteins that form the tight junction (TJ) and components of the innate immune system. Recent data involving skin barrier reveal new information regarding its structure and its role in the mechanic-immunological defense; atopic dermatitis (AD) is an example of a disease related to dysfunctions associated with this complex.