Single-cell RNA sequencing unveils the clonal and transcriptional landscape of cutaneous T-cell lymphomas

Mycosis fungoides (MF) is the most common subtype of cutaneous T-cell lymphoma, arising from clonal expansion of transformed skin-homing memory T cells. However, the lack of specific markers for malignant lymphocytes prevents distinguishing them from benign T cells in the tumor microenvironment (TME) of MF, thus delaying diagnosis and development of specific treatment, which results in poor clinical outcomes. Here we employed recent advances in single-cell RNA-sequencing to establish the transcriptome of expanded T-cell clones directly in advanced-stage MF skin tumors, allowing the transcriptional profiles of malignant and reactive T lymphocytes to be distinguished. Our analysis identified multiple non-overlapping expanded T-cell clonotypes within individual MF tumors, which included malignant and reactive clones. Heterogeneity among samples was observed not only in the number of expanded clonotypes but also in their TCR specificity and gene expression. While patient-specific tumorigenic pathways were up-regulated by the malignant clones, we also identified a common gene expression that included genes associated with cancer cell metabolism, de novo nucleotide biosynthesis, and invasion. Most non-malignant clones originated from CD8+ T cells and commonly presented an exhausted immune phenotype and activated Th1/Th2 pathways. While non-clonal infiltrating CD4+ and CD8+ lymphocytes from all tumors shared anti-inflammatory and immunosuppressive pathways, patient-specific mechanisms included the TNFR2 signaling cascade, ferroptosis, and cytotoxic pathways by gamma/delta T cells. Thus, scRNAseq reveals new insights in MF pathogenesis by providing an unprecedented report of the transcriptomes of malignant and reactive T cells in the TME of individual patients and offers novel prospective targets for personalized therapy. Overall design: 4 MF samples and 4 normal samples were analyzed for gene expression and TCR expression. 4 SZ samples and 3 healthy PBMC samples were analyzed for gene expression and TCR expression. 2 blood/skin samples from same patient were analyzed.