Dissecting Immune Determinants in Lesional Skin of Cutaneous T-Cell Lymphoma During Mogamulizumab Therapy

Background: Mycosis fungoides (MF) and Sézary syndrome (SS) are the predominant cutaneous T-cell lymphomas. Although malignant T cells in MF/SS overexpress CCR4 and respond to the anti-CCR4 antibody mogamulizumab, skin response rates vary. We hypothesized that immune components within the tumor microenvironment contribute to differential outcomes. Methods: Imaging mass cytometry with a 37-antibody panel was used to characterize immune and structural elements in FFPE tissues from sixteen MF/SS patients (6 MF, 10 SS) treated with Mogamulizumab, including seven skin responders and nine non-responders. Single-cell phenotyping and spatial analyses were performed using the Visiopharm® Phenoplex™ platform, with supervision. Results: We identified 68,974 cells pre-treatment and 58,852 cells post-treatment. Malignant CD4+ T cells showed reduced baseline CD27, CD103, CD25, and ICOS expression compared with non-malignant CD4+ cells. Baseline MF lesions were enriched for IL-13+ and CD103+ malignant T cells, whereas SS lesions contained higher proportions of CD27+ and LAG3+ cells. IL 13+ malignant cells decreased after treatment, most prominently in MF. Myeloid profiles differed by disease and response: MF lesions exhibited baseline enrichment of M1-like macrophages (CD86+, HLA-DR+), while SS lesions were predominantly M2-polarized macrophages (CD163+, CD206+). Responders showed increased M1-like macrophages, whereas non-responders displayed reduced M1-features. An increase in DC3-like cells was observed in non-responders following treatment. Conclusions: This single-cell spatial atlas reveals shared and subtype-specific immune features in MF/SS. Th2-skewed malignant T-cell status and myeloid polarization correlate with clinical response, supporting their potential as spatial biomarkers for patient stratification in mogamulizumab therapy.