The interplay between keratinocytes and immune cells, especially T cells, plays an important role in the pathogenesis of chronic inflammatory skin diseases. During psoriasis, keratinocytes attract T cells by releasing chemokines, while skin-infiltrating selfreactive T cells secrete proinflammatory cytokines, e.g., IFNγ and IL-17A, that cause epidermal hyperplasia. Similarly, in chronic graftversus-host disease, allogenic IFNγ-producing Th1/Tc1 and IL-17-producing Th17/Tc17 cells are recruited by keratinocyte-derived chemokines and accumulate in the skin. However, whether keratinocytes act as nonprofessional antigen-presenting cells to directly activate naive human T cells in the epidermis remains unknown. Here, we demonstrate that under proinflammatory conditions, primary human keratinocytes indeed activate naive human T cells. This activation required cell contact and costimulatory signaling via CD58/CD2 and CD54/LFA-1. Naive T cells costimulated by keratinocytes selectively differentiated into Th1 and Th17 cells. In particular, keratinocyte-initiated Th1 differentiation was dependent on costimulation through CD58/CD2. The latter molecule initiated STAT1 signaling and IFNγ production in T cells. Costimulation of T cells by keratinocytes resulting in Th1 and Th17 differentiation represents a new explanation for the local enrichment of Th1 and Th17 cells in the skin of patients with a chronic inflammatory skin disease. Consequently, local interference with T cell-keratinocyte interactions may represent a novel strategy for the treatment of Th1 and Th17 cell-driven skin diseases.In addition, in chronic graft-versus-host disease (GVHD), a major complication of allogenic stem cell transplantation, the KCmediated secretion of chemokines (CXCL9 and CXCL10) leads to the recruitment of alloreactive T cells into the skin. 14 These allogenic T cells predominantly belong to the IFNγ-producing Th1/ Tc1 and IL-17-producing Th17/Tc17 subpopulations and cause cutaneous manifestations, e.g., follicular erythema. [15][16][17] Although the pivotal role of KCs in non-contact-mediated communication during chronic skin inflammation is quite well understood, the direct interaction between KCs and T cells remains elusive. In particular, the potential of KCs to act as nonprofessional APCs, enabling them to costimulate T cells directly in the skin, is still debated.T cells require two distinct signals for activation and clonal expansion. The first signal is transmitted by the antigen-specific T cell receptor (TCR) on T cells, following recognition of antigenic peptides loaded on MHC class I or class II molecules expressed by APCs. The first signal secures the antigen specificity of the immune reaction. The second signal is transmitted through costimulatory receptors, dictating the progression to T cell activation. Between

Cofilin is an essential actin remodeling protein promoting depolymerization and severing of actin filaments. To address the relevance of cofilin for the development and function of T cells in vivo, we generated knock-in mice in which T-cell–specific nonfunctional (nf) cofilin was expressed instead of wild-type (WT) cofilin. Nf cofilin mice lacked peripheral αβ T cells and showed a severe thymus atrophy. This was caused by an early developmental arrest of thymocytes at the double negative (DN) stage. Importantly, even though DN thymocytes expressed the TCRβ chain intracellularly, they completely lacked TCRβ surface expression. In contrast, nf cofilin mice possessed normal numbers of γδ T cells. Their functionality was confirmed in the γδ T-cell–driven, imiquimod (IMQ)-induced, psoriasis-like murine model. Overall, this study not only highlights the importance of cofilin for early αβ T-cell development but also shows for the first time that an actin-binding protein is differentially involved in αβ versus γδ T-cell development.