Single-cell RNA sequencing comparison of CD4⁺, CD8⁺ and T-cell receptor γδ⁺ cutaneous T-cell lymphomas reveals subset-specific molecular phenotypes

Background Malignant clones of primary cutaneous T-cell lymphomas (CTCL) can show a CD4⁺, CD8⁺ or T-cell receptor (TCR)-γδ⁺ phenotype, but their individual impact on tumour biology and skin lesion formation remains ill defined. Objectives To perform a comprehensive molecular characterization of CD4⁺ vs. CD8⁺ and TCR-γδ⁺ CTCL lesions. Methods We performed single-cell RNA sequencing (scRNAseq) of 18 CTCL skin biopsies to compare classic CD4⁺ advanced-stage mycosis fungoides (MF) with TCR-γ/δ⁺ MF and primary cutaneous CD8⁺ aggressive epidermotropic cytotoxic T-cell lymphoma (Berti lymphoma). Results Malignant clones of TCR-γ/δ⁺ MF and Bertilymphoma showed similar clustering patterns distinct from CD4⁺ MF, along with increased expression of cytotoxic markers such as NKG7, CTSW, GZMA and GZMM. Only advanced-stage CD4⁺ MF clones expressed central memory T-cell markers (SELL, CCR7, LEF1), alongside B1/B2 blood involvement, whereas TCR-γδ⁺ MF and Berti lymphoma harboured a more tissue-resident phenotype (CD69, CXCR4, NR4A1) without detectable cells in the blood. CD4⁺ MF and TCR-γδ⁺ MF skin lesions harboured strong type 2 immune activation across myeloid cells, while Berti lymphoma was more skewed toward type 1 immune responses. Both CD4⁺ MF and TCR-γδ⁺ MF lesions showed upregulation of keratinocyte hyperactivation markers such as S100A genes and KRT16. This increase was entirely absent in Berti lymphoma, possibly reflecting an aberrant keratinocyte response to invading tumour cells, which could contribute to the formation of the typical ulceronecrotic lesions within this entity. Conclusions Our scRNAseq profiling study reveals specific molecular patterns associated with distinct CTCL subtypes.