Metabolic re-programming of keratinocytes in response to contact allergens.

BACKGROUND: Allergic contact dermatitis (ACD) is a common skin disease caused by the recognition of haptens by the immune system. Keratinocytes play an important role in the initiation and facilitation of inflammatory responses in ACD. Immune responses are associated with major changes in metabolism. However, metabolic re-programming is not well studied in ACD; specifically, knowledge of metabolic alterations in structural cells is lacking. METHODS: Metabolic re-programming in ACD was studied using publicly available transcriptome datasets. Primary pooled keratinocytes and a keratinocyte cell line (HaCaT) were stimulated with contact allergens, and inflammatory responses and expression of metabolic markers were measured by qPCR and flow cytometry, respectively. RESULTS: ACD is characterized by metabolic re-programming with a metabolic profile similar to atopic dermatitis. Exposure to contact allergens causes a wide array of metabolic alterations. Stimulation of keratinocytes with contact allergens induced inflammatory responses typical for ACD and was associated with an up-regulation of proteins representative for glucose uptake, fatty acid metabolism, oxidative phosphorylation and to some extent arginine biosynthesis. Changes in these metabolic pathways were also observed when comparing lesional with non-lesional contact dermatitis skin. CONCLUSIONS: ACD is, similarly to other inflammatory skin diseases, characterized by metabolic re-programming. Contact allergen exposure induces expression of a wide array of metabolic pathways, which is at least in part mediated through metabolic re-programming of keratinocytes.

γδ T cells and inflammatory skin diseases.

Approximately 25% of the population suffers from skin diseases. The most common forms of skin diseases are the inflammatory skin diseases such as allergic contact dermatitis, psoriasis, and atopic dermatitis. These diseases are described as T cell-mediated diseases induced by either allergens or autoantigens. Classically, the focus has been on the role of αß T cells, but it is becoming increasingly clear that γδ T cells play a central role in inflammatory skin diseases. In particular, an important role of IL-17A-producing γδ T cells in these inflammatory skin diseases has been shown in various disease models in mice. Interestingly, various epidermal proteins, which appear to be linked to inflammatory conditions in the skin by yet undescribed mechanisms, are expressed by specific subsets of thymic epithelial cells and mutations in these proteins seem to affect γδ T cell development. The focus of this review is how mutations in epidermal proteins affect γδ T cell development and how γδ T cells, and in particular of IL-17A-producing γδ T cells, contribute to inflammatory skin diseases such as allergic contact dermatitis, psoriasis, and atopic dermatitis.

CD4+ T cells inhibit the generation of CD8+ epidermal-resident memory T cells directed against clinically relevant contact allergens.

BACKGROUND: CD8+ epidermal-resident memory T (TRM ) cells play central roles in local flare-up responses to experimental contact allergens by inducing massive influx of neutrophils to the epidermis upon allergen challenge. Whether similar immunopathogenic mechanisms are involved in the responses to clinically relevant contact allergens is unknown. METHODS: The immune response to cinnamal, ρ-phenylenediamine (PPD) and methylisothiazolinone (MI) was studied in a well-established mouse model for allergic contact dermatitis that includes formation of TRM cells by ELISA, flow cytometry, fluorescence microscopy analyses and cell depletion protocols. RESULTS: We show that the formation of CD4+ and CD8+ epidermal TRM cells and the inflammatory response are highly allergen-dependent. However, the magnitude of the flare-up responses correlated with the number of epidermal CD8+ TRM cells, CXCL1/CXCL2 release and recruitment of neutrophils to the epidermis. Finally, depletion of CD4+ T cells strongly enhanced the number of epidermal CD8+ TRM cells, the flare-up response and the epidermal infiltration of neutrophils for all allergens. CONCLUSION: As the first, this study demonstrates that clinically relevant contact allergens have the ability to generate pathogenic, epidermal CD8+ TRM cells that recruit neutrophils following re-exposure to the allergen, but that this normally is counteracted by the simultaneous induction of anti-inflammatory CD4+ T cells.

The junctional adhesion molecule-like protein (JAML) is important for the inflammatory response during contact hypersensitivity.

BACKGROUND: The junctional adhesion molecule-like protein (JAML) plays important roles in wound healing and activation of epidermal γδ T cells in mice. Whether JAML plays a role in contact hypersensitivity (CHS), the animal model of allergic contact dermatitis (ACD), is not known. METHODS: To examine the role of JAML in CHS, we used various mouse models of CHS in JAML knockout (KO) and wild-type (WT) mice. Furthermore, the expression of the JAML ligand coxsackievirus and adenovirus receptor (CXADR) on keratinocytes was accessed in vitro and in vivo. RESULTS: JAML KO mice had a diminished inflammatory response during both the sensitization and elicitation phase of CHS and had reduced numbers of CD8+ and CD4+ T cells in the epidermis. Furthermore, interferon γ (IFNγ), interleukin 1ß (IL-1ß) and CXCL10 production were significantly reduced in JAML KO mice during the elicitation phase. We found that CD8+ T cells express JAML and that JAML is essential for rapid flare-up responses to contact allergens. Finally, we show that keratinocytes up-regulate the JAML ligand CXADR following exposure to contact allergens. CONCLUSION: Our study is the first to show a central role of JAML in CHS and reveals a potential new target for the treatment of ACD in humans.

CD100 boosts the inflammatory response in the challenge phase of allergic contact dermatitis in mice.

BACKGROUND: Allergic contact dermatitis (ACD) is an inflammatory disease with a complex pathophysiology in which epidermal-resident memory CD8+ T (TRM ) cells play a key role. The mechanisms involved in the activation of CD8+ TRM cells during allergic flare-up responses are not understood. METHODS: The expression of CD100 and its ligand Plexin B2 on CD8+ TRM cells and keratinocytes before and after allergen exposure was determined by flow cytometry and RT-qPCR. The role of CD100 in the inflammatory response during the challenge phase of ACD was determined in a model of ACD in CD100 knockout and wild-type mice. RESULTS: We show that CD8+ TRM cells express CD100 during homeostatic conditions and up-regulate it following re-exposure of allergen-experienced skin to the experimental contact allergen 1-fluoro-2,4-dinitrobenzene (DNFB). Furthermore, Plexin B2 is up-regulated on keratinocytes following exposure to some contact allergens. We show that loss of CD100 results in a reduced inflammatory response to DNFB with impaired production of IFNγ, IL-17A, CXCL1, CXCL2, CXCL5, and IL-1ß and decreased recruitment of neutrophils to the epidermis. CONCLUSION: Our study demonstrates that CD100 is expressed on CD8+ TRM cells and is required for full activation of CD8+ TRM cells and the flare-up response of ACD.

CD8+ tissue-resident memory T cells recruit neutrophils that are essential for flare-ups in contact dermatitis.

BACKGROUND: Allergic contact dermatitis (ACD) is classically described as a delayed-type hypersensitivity reaction. However, patients often experience flare-ups characterized by itching erythema, edema, and often vesicles occurring within hours after re-exposure of previously sensitized skin to the specific contact allergen. Recent studies have indicated that skin-resident memory T (TRM ) cells play a central role in ACD. However, the pathogenic role of TRM cells in allergen-induced flare-ups is not known. METHODS: By the use of various mouse models and cell depletion protocols, we investigated the role of epidermal TRM cells in flare-up reactions to the experimental contact allergen 1-fluoro-2,4-dinitrobenzene. The inflammatory response was measured by changes in ear thickness, and the cellular composition in epidermis was determined by flow cytometry and confocal microscopy. Finally, adaptive transfer and inhibitors were used to determine the role of TRM cells, neutrophils, and CXCL1/CXCL2 in the response. RESULTS: We show that CD8+ TRM cells initiate massive infiltration of neutrophils in the epidermis within 12 h after re-exposure to the contact allergen. Depletion of neutrophils before re-exposure to the allergen abrogated the flare-up reactions. Furthermore, we demonstrate that CD8+ TRM cells mediate neutrophil recruitment by inducing CXCL1 and CXCL2 production in the skin, and that blockage of the C-X-C chemokine receptor type 1 and 2 inhibits flare-up reactions and neutrophil infiltration. CONCLUSION: As the first, we show that epidermal CD8+ TRM cells cause ACD flare-ups by rapid recruitment of neutrophils to the epidermis.

The role of interleukin-1ß in the immune response to contact allergens.

Interleukin-1ß (IL-1ß) is an important pro-inflammatory cytokine that has an effect on almost every cell lineage in the body. By blocking IL-1ß and investigating the IL-1ß signaling pathway, several studies have demonstrated a central role of IL-1ß in the response to contact allergens. This review summarizes the current literature regarding the basic immunological mechanisms mediated by IL-1ß in the different phases of allergic contact dermatitis (ACD) and highlights potential IL-1ß-targeted treatment options, which in the future may be relevant in the treatment of patients with ACD. This review is based primarily on studies using various mouse models and human in vitro studies, since clinical studies on the effect of IL-1ß in ACD are lacking.

Induced Human Regulatory T Cells Express the Glucagon-like Peptide-1 Receptor.

The glucagon-like peptide-1 receptor (GLP-1R) plays a key role in metabolism and is an important therapeutic target in diabetes and obesity. Recent studies in experimental animals have shown that certain subsets of T cells express functional GLP-1R, indicating an immune regulatory role of GLP-1. In contrast, less is known about the expression and function of the GLP-1R in human T cells. Here, we provide evidence that activated human T cells express GLP-1R. The expressed GLP-1R was functional, as stimulation with a GLP-1R agonist triggered an increase in intracellular cAMP, which was abrogated by a GLP-1R antagonist. Analysis of CD4+ T cells activated under T helper (Th) 1, Th2, Th17 and regulatory T (Treg) cell differentiation conditions indicated that GLP-1R expression was most pronounced in induced Treg (iTreg) cells. Through multimodal single-cell CITE- and TCR-sequencing, we detected GLP-1R expression in 29-34% of the FoxP3+CD25+CD127- iTreg cells. GLP-1R+ cells showed no difference in their TCR-gene usage nor CDR3 lengths. Finally, we demonstrated the presence of GLP-1R+CD4+ T cells in skin from patients with allergic contact dermatitis. Taken together, the present data demonstrate that T cell activation triggers the expression of functional GLP-1R in human CD4+ T cells. Given the high induction of GLP-1R in human iTreg cells, we hypothesize that GLP-1R+ iTreg cells play a key role in the anti-inflammatory effects ascribed to GLP-1R agonists in humans.

Dendritic Epidermal T Cells in Allergic Contact Dermatitis.

Allergic contact dermatitis (ACD) is a common inflammatory skin disease with a prevalence of approximately 20% in the European population. ACD is caused by contact allergens that are reactive chemicals able to modify non-immunogenic self-proteins to become immunogenic proteins. The most frequent contact allergens are metals, fragrances, and preservatives. ACD clinically manifests as pruritic eczematous lesions, erythema, local papules, and oedema. ACD is a T cell-mediated disease, involving both CD4+ and CD8+ T cells. In addition, γδ T cells appear to play an important role in the immune response to contact allergens. However, it is debated whether γδ T cells act in a pro- or anti-inflammatory manner. A special subset of γδ T cells, named dendritic epidermal T cells (DETC), is found in the epidermis of mice and it plays an important role in immunosurveillance of the skin. DETC are essential in sensing the contact allergen-induced stressed environment. Thus, allergen-induced activation of DETC is partly mediated by numerous allergen-induced stress proteins expressed on the keratinocytes (KC). Several stress proteins, like mouse UL-16-binding protein-like transcript 1 (Mult-1), histocompatibility 60 (H60) and retinoic acid early inducible-1 (Rae-1) α-ε family in mice and major histocompatibility complex (MHC) class I-chain-related A (MICA) in humans, are upregulated on allergen-exposed KC. Allergen-induced stress proteins expressed on the KC are consequently recognized by NKG2D receptor on DETC. This review focuses on the role of γδ T cells in ACD, with DETC in the spotlight, and on the role of stress proteins in contact allergen-induced activation of DETC.

Pathogenic CD8+ Epidermis-Resident Memory T Cells Displace Dendritic Epidermal T Cells in Allergic Dermatitis.

The skin is our interface with the outside world, and consequently it is exposed to a wide range of microbes and allergens. Recent studies have indicated that allergen-specific skin-resident memory T (TRM) cells play a role in allergic contact dermatitis (ACD). However, the composition and dynamics of the epidermal T-cell subsets during ACD are not known. Here we show that exposure of the skin to the experimental contact allergen DNFB results in a displacement of the normally occurring dendritic epidermal T cells (DETC) concomitant with an accumulation of epidermal CD8+CD69+CD103+ TRM cells in mice. By studying knockout mice, we provide evidence that CD8+ T cells are required for the displacement of the DETC and that DETC are not required for recruitment of CD8+ TRM cells to the epidermis following allergen exposure. We demonstrate that the magnitude of the allergic reaction correlates with the number of CD8+ epidermal TRM cells, which again correlates with allergen dose and number of allergen exposures. Finally, in an attempt to elucidate why CD8+ epidermal TRM cells persist in the epidermis, we show that CD8+ epidermal TRM cells have a higher proliferative capability and are bioenergetically more stable, displaying a higher spare respiratory capacity than DETC.