Mast Cells Limit the Exacerbation of Chronic Allergic Contact Dermatitis in Response to Repeated Allergen Exposure.

Allergic contact dermatitis is a chronic T cell-driven inflammatory skin disease that is caused by repeated exposure to contact allergens. Based on murine studies of acute contact hypersensitivity, mast cells (MCs) are believed to play a role in its pathogenesis. The role of MCs in chronic allergic contact dermatitis has not been investigated, in part because of the lack of murine models for chronic contact hypersensitivity. We developed and used a chronic contact hypersensitivity model in wild-type and MC-deficient mice and assessed skin inflammatory responses to identify and characterize the role of MCs in chronic allergic contact dermatitis. Ear swelling chronic contact hypersensitivity responses increased markedly, up to 4-fold, in MC-deficient KitW-sh/W-sh (Sash) and MCPT5-Cre+iDTR+ mice compared with wild-type mice. Local engraftment with MCs protected Sash mice from exacerbated ear swelling after repeated oxazolone challenge. Chronic contact hypersensitivity skin of Sash mice exhibited elevated levels of IFN-γ, IL-17α, and IL-23, as well as increased accumulation of Ag-specific IFN-γ-producing CD8+ tissue-resident memory T (TRM) cells. The CD8+ T cell mitogen IL-15, which was increased in oxazolone-challenged skin of Sash mice during the accumulation of cutaneous TRM cells, was efficiently degraded by MCs in vitro. MCs protect from the exacerbated allergic skin inflammation induced by repeated allergen challenge, at least in part, via effects on CD8+ TRM cells. MCs may notably influence the course of chronic allergic contact dermatitis. A better understanding of their role and the underlying mechanisms may lead to better approaches for the treatment of this common, disabling, and costly condition.

NOD2 Agonism Counter-Regulates Human Type 2 T Cell Functions in Peripheral Blood Mononuclear Cell Cultures: Implications for Atopic Dermatitis.

Atopic dermatitis (AD) is known as a skin disease; however, T cell immunopathology found in blood is associated with its severity. Skin Staphylococcus aureus (S. aureus) and associated host-pathogen dynamics are important to chronic T helper 2 (Th2)-dominated inflammation in AD, yet they remain poorly understood. This study sought to investigate the effects of S. aureus-derived molecules and skin alarmins on human peripheral blood mononuclear cells, specifically testing Th2-type cells, cytokines, and chemokines known to be associated with AD. We first show that six significantly elevated Th2-related chemokine biomarkers distinguish blood from adult AD patients compared to healthy controls ex vivo; in addition, TARC/CCL17, LDH, and PDGF-AA/AB correlated significantly with disease severity. We then demonstrate that these robust AD-associated biomarkers, as well as associated type 2 T cell functions, are readily reproduced from healthy blood mononuclear cells exposed to the alarmin TSLP and the S. aureus superantigen SEB in a human in vitro model, including IL-13, IL-5, and TARC secretion as well as OX-40-expressing activated memory T cells. We further show that the agonism of nucleotide-binding oligomerization domain-containing protein (NOD)2 inhibits this IL-13 secretion and memory Th2 and Tc2 cell functional activation while inducing significantly increased pSTAT3 and IL-6, both critical for Th17 cell responses. These findings identify NOD2 as a potential regulator of type 2 immune responses in humans and highlight its role as an endogenous inhibitor of pathogenic IL-13 that may open avenues for its therapeutic targeting in AD.

Skin Sodium Accumulates in Psoriasis and Reflects Disease Severity.

Sodium can accumulate in the skin at concentrations exceeding serum levels. A high sodium environment can lead to pathogenic T helper 17 cell expansion. Psoriasis is a chronic inflammatory skin disease in which IL-17‒producing T helper 17 cells play a crucial role. In an observational study, we measured skin sodium content in patients with psoriasis and in age-matched healthy controls by Sodium-23 magnetic resonance imaging. Patients with PASI > 5 showed significantly higher sodium and water content in the skin but not in other tissues than those with lower PASI or healthy controls. Skin sodium concentrations measured by Sodium-23 spectroscopy or by atomic absorption spectrometry in ashed-skin biopsies verified the findings with Sodium-23 magnetic resonance imaging. In vitro T helper 17 cell differentiation of naive CD4+ cells from patients with psoriasis markedly induced IL-17A expression under increased sodium chloride concentrations. The imiquimod-induced psoriasis mouse model replicated the human findings. Extracellular tracer Chromium-51-EDTA measurements in imiquimod- and sham-treated skin showed similar extracellular volumes, rendering excessive water of intracellular origin. Chronic genetic IL-17A‒driven psoriasis mouse models underlined the role of IL-17A in dermal sodium accumulation and inflammation. Our data describe skin sodium as a pathophysiological feature of psoriasis, which could open new avenues for its treatment.

Mast cell-mediated reactions in vivo.

Mast cells are involved in many physiological reactions in which their functions can be very diverse. Models of allergic skin inflammation and systemic anaphylactic reactions in mice are validated methods in which the role of mast cells is well established. In this chapter, we therefore present protocols for passive cutaneous anaphylaxis and contact hypersensitivity, i.e., models which can be used to identify and characterize the role of mast cells as well as mast cell mediators and receptors in allergic IgE-dependent and -independent skin inflammation, and for passive systemic anaphylaxis, a model ideally suited to characterize the systemic effects of mast cell-derived mediators and mast cell receptors.