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Cutaneous T-cell lymphoma (CTCL) is characterized by malignant T-cells proliferating in a unique tumor microenvironment (TME) dominated by keratinocytes. Skin colonization and infection by Staphylococcus aureus(S. aureus) is a common cause of morbidity and suspected of fueling disease activity. Here we show that expression of HLA-DR, high-affinity receptors for Staphylococcal enterotoxins (SE), by keratinocytes correlates with IFN-γ expression in the TME. Importantly, IFN-γ induces HLA-DR, SE-binding, and SE-presentation by keratinocytes to malignant T-cells from Sézary syndrome (SS) patients, and malignant and non-malignant T-cell lines derived from SS and Mycosis fungoides patients. Likewise, preincubation of keratinocytes with supernatant from patient-derived SE-producing S. aureustriggers proliferation in malignant T-cells and cytokine release (including IL-2), when cultured with non-malignant T-cells. This is inhibited by pre-treatment with engineered bacteriophage S. aureus-specific endolysins. Furthermore, mutations in the HLA-DR binding sites of SE type-A, and siRNA-mediated knockdown of Janus Kinase-3 (JAK3) and IL-2Rγ block induction of malignant T-cell proliferation. In conclusion, we show that, upon exposure to patient-derived S. aureusand SE, keratinocytes stimulate IL-2Rγ/JAK3-dependent proliferation of malignant and non-malignant T-cells in an environment with non-malignant T-cells. These findings suggest that keratinocytes in the TME play a key role in S. aureusmediated disease activity in CTCL.

Cutaneous T-cell lymphoma (CTCL) is a devastating lymphoid malignancy characterized by the accumulation of malignant T cells in the dermis and epidermis. Skin lesions cause serious symptoms that hamper quality of life and are entry sites for bacterial infection, a major cause of morbidity and mortality in advanced diseases. The mechanism driving the pathological processes that compromise the skin barrier remains unknown. Here, we report increased transepidermal water loss and compromised expression of the skin barrier proteins filaggrin and filaggrin-2 in areas adjacent to TOX-positive T cells in CTCL skin lesions. Malignant T cells secrete mediators (including cytokines such as interleukin 13 [IL-13], IL-22, and oncostatin M) that activate STAT3 signaling and downregulate filaggrin and filaggrin-2 expression in human keratinocytes and reconstructed human epithelium. Consequently, the repression of filaggrins can be counteracted by a cocktail of antibodies targeting these cytokines/receptors, small interfering RNA–mediated knockdown of JAK1/STAT3, and JAK1 inhibitors. Notably, we show that treatment with a clinically approved JAK inhibitor, tofacitinib, increases filaggrin expression in lesional skin from patients with mycosis fungoides. Taken together, these findings indicate that malignant T cells secrete cytokines that induce skin barrier defects via a JAK1/STAT3-dependent mechanism. As clinical grade JAK inhibitors largely abrogate the negative effect of malignant T cells on skin barrier proteins, our findings suggest that such inhibitors provide novel treatment options for patients with CTCL with advanced disease and a compromised skin barrier.