Background: Allergic contact dermatitis (ACD) is classically described as a delayedtype 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 (T RM ) cells play a central role in ACD.However, the pathogenic role of T RM 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 T RM 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 T RM cells, neutrophils, and CXCL1/CXCL2 in the response. Results:We show that CD8 + T RM 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 + T RM 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 + T RM cells cause ACD flare-ups by rapid recruitment of neutrophils to the epidermis.

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 (T RM) 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 þ CD69 þ CD103 þ T RM cells in mice. By studying knockout mice, we provide evidence that CD8 þ T cells are required for the displacement of the DETC and that DETC are not required for recruitment of CD8 þ T RM cells to the epidermis following allergen exposure. We demonstrate that the magnitude of the allergic reaction correlates with the number of CD8 þ epidermal T RM cells, which again correlates with allergen dose and number of allergen exposures. Finally, in an attempt to elucidate why CD8 þ epidermal T RM cells persist in the epidermis, we show that CD8 þ epidermal T RM cells have a higher proliferative capability and are bioenergetically more stable, displaying a higher spare respiratory capacity than DETC.

γδ and αβ T cells develop in the thymus from common progenitor cells. 1 Unlike αβ T cells, the majority of murine γδ T cells are functionally committed during intra-thymic differentiation. 2-4 This underlines the importance of the thymic microenvironment in the regulation of the peripheral effector functions of γδ T cells. Central to the thymic microenvironment is the thymic epithelial cells (TEC), divided into cortical TEC (cTEC) and medullary TEC (mTEC). 5 Particularly, the mTEC seem to regulate the functional commitment of γδ T cells within the thymus. Recently, it has become clear that TEC, especially mTEC, are more heterogeneous than previously assumed. Interestingly, subsets of mTEC with expression profiles such as tuft cells and keratinocytes (KC) have been identified. 6,7 KC constitute the majority of cells within the outermost layer of the skin, the epidermis. The epidermis acts as our first line of defense against assaults from pathogens, chemicals, and allergens, thereby playing a critical role in providing protection from injury. Although KC are not classified as immune cells, they play important immunological roles in the first line of defense against pathogens and allergens. Thus, KC express pattern recognition receptors such as Toll-like receptors (TLR) 8 and NOD-like receptors (NLR), 9 some constitutively and others following stimulation, that are able to recognize pathogen-associated molecular patterns and damage-associated molecular patterns, which trigger the production and release of cytokines, chemokines, and anti-microbial peptides that in turn activate immune cells in the skin. 10-12 The immune cells found in the epidermis are T cells and a subset of antigen-presenting cells termed Langerhans cells (LC). 10,12,13 In naïve mice, the primary subset of T cells in the epidermis is a special subset of γδ T cells called dendritic epidermal T cells (DETC) due to their dendritic morphology. 14