T cells are key players in autoimmune diseases by supporting the production of autoantibodies. However, their contribution to the effector phase of antibody-mediated autoimmune dermatoses, i.e., tissue injury and inflammation of the skin, has not been investigated. In this paper, we demonstrate that T cells amplify the development of autoantibody-induced tissue injury in a prototypical, organspecific autoimmune disease, namely epidermolysis bullosa acquisita ( T cells are essential regulators of host defense and exhibit direct cytotoxic as well as regulatory properties. The presence or absence of pro-inflammatory compared with regulatory T cell subsets affects the development and outcome of inflammatory reactions. Misbalance of T cell populations leads to autoimmune disorders, including systemic lupus erythematosus (SLE), different autoimmune bullous dermatoses (AIBDs) and rheumatoid arthritis (RA) [1][2][3] . In these diseases, the contribution of T cells to antibody production and maintenance of the autoimmune response has clearly been demonstrated 4,5 . In recent decades, the understanding of autoantibody-induced tissue injury has greatly improved. However, the role of T cells during the effector phase of autoimmune skin blistering diseases, i.e., tissue injury and inflammation in the targeted organs, is not completely understood. In this study, we investigated the role of T cells during this phase, using a mouse model of epidermolysis bullosa acquisita (EBA), a prototypical organ-specific autoimmune disease 6,7 . EBA is caused by autoantibodies directed against type VII collagen (COL7), an integral component of anchoring fibrils 8 . Animal models, employing antibody transfer into mice 9,10 , have added to a greater understanding of the mechanisms leading to blistering in EBA 9,11,12 . Based on the current understanding of EBA pathogenesis, the effector phase of EBA is predominantly driven by neutrophils -their depletion leading to a complete absence of experimental EBA 13. With regard to T cell involvement during this phase, in vivo and in vitro data have been contradictory. In vivo data indicated a T cell-independent process: Transfer of total IgG isolated from rabbits that had been immunized with COL7 into T cell-deficient mice induced subepidermal blistering 9 . However, in that study, no wild-type control for evaluation of the extent of blistering was included. In other

Pemphigoid diseases refer to a group of severe autoimmune skin blistering diseases characterized by subepidermal blistering and loss of dermal-epidermal adhesion induced by autoantibody and immune cell infiltrate at the dermal-epidermal junction and upper dermis. Here, we explore the role of the immune cell-secreted serine protease, granzyme B, in pemphigoid disease pathogenesis using three independent murine models. In all models, granzyme B knockout or topical pharmacological inhibition significantly reduces total blistering area compared to controls. In vivo and in vitro studies show that granzyme B contributes to blistering by degrading key anchoring proteins in the dermal-epidermal junction that are necessary for dermal-epidermal adhesion. Further, granzyme B mediates IL-8/macrophage inflammatory protein-2 secretion, lesional neutrophil infiltration, and lesional neutrophil elastase activity. Clinically, granzyme B is elevated and abundant in human pemphigoid disease blister fluids and lesional skin. Collectively, granzyme B is a potential therapeutic target in pemphigoid diseases.

Background and Purpose: Pemphigus is caused by autoantibodies against desmoglein (Dsg) 1, Dsg3, and/or non-Dsg antigens. Pemphigus vulgaris (PV) is the most common manifestation of pemphigus, with painful erosions on mucous membranes. In most cases, blistering also occurs on the skin, leading to areas of extensive denudation. Despite improvements in pemphigus treatment, time to achieve remission is long, severe adverse events are frequent and 20% of patients do not respond adequately. Current clinical developments focus exclusively on modulating B cell function or autoantibody half-life. However, topical modulation of PV autoantibody-induced blistering is an attractive target because it could promptly relieve symptoms. Experimental Approach: To address this issue, we performed an unbiased screening in a complex biological system using 141 low MW inhibitors from a chemical library. Specifically, we evaluated PV IgG-induced Dsg3 internalization in HaCaT keratinocytes. Validation of the 20 identified compounds was performed using keratinocyte fragmentation assays, as well as a human skin organ culture (HSOC) model. key Results: Overall, this approach led to the identification of four molecules involved in PV IgG-induced skin pathology: MEK1, TrkA, PI3Kα, and VEGFR2. Conclusion and Implications: This unbiased screening revealed novel mechanisms by which PV autoantibodies induce blistering in keratinocytes and identified new treatment targets for this severe and potentially life-threatening skin disease.

Regulatory T cells (Tregs) are well known for their modulatory functions in adaptive immunity. Through regulation of T cell functions, Tregs have also been demonstrated to indirectly curb myeloid cell-driven inflammation. However, direct effects of Tregs on myeloid cell functions are insufficiently characterized, especially in the context of myeloid cell-mediated diseases, such as pemphigoid diseases (PDs). PDs are caused by autoantibodies targeting structural proteins of the skin. Autoantibody binding triggers myeloid cell activation through specific activation of Fc gamma receptors, leading to skin inflammation and subepidermal blistering. Here, we used mouse models to address the potential contribution of Tregs to PD pathogenesis in vivo. Depletion of Tregs induced excessive inflammation and blistering both clinically and histologically in two different PD mouse models. Of note, in the skin of Treg-depleted mice with PD, we detected increased expression of different cytokines, including Th2-specific IL-4, IL-10, and IL-13 as well as pro-inflammatory Th1 cytokine IFN-γ and the T cell chemoattractant CXCL-9. We next aimed to determine whether Tregs alter the migratory behavior of myeloid cells, dampen immune complex (IC)-induced myeloid cell activation, or both. In vitro experiments demonstrated that co-incubation of IC-activated myeloid cells with Tregs had no impact on the release of reactive oxygen species (ROS) but downregulated β2 integrin expression. Hence, Tregs mitigate PD by altering the migratory capabilities of myeloid cells rather than their release of ROS. Modulating cytokine expression by administering an excess of IL-10 or blocking IFN-γ may be used in clinical translation of these findings.

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