Pathology of Cutaneous T Cell Lymphoma: A Narrative Review

Cutaneous T-cell lymphoma (CTCL) comprises a heterogeneous group of extranodal non-Hodgkin lymphomas. With the publication of the fifth edition of the World Health Organization Classification of Hematolymphoid Tumors, the diagnostic framework for CTCL has shifted from primarily morphologic phenotypes toward an emphasis on molecular drivers. Current research suggests that malignant clones may arise from somatic mutations at the hematopoietic stem cell stage and may follow a continuous hematogenous dissemination model with bidirectional trafficking between the skin and systemic circulation. At the molecular level, genomic instability, often associated with somatic copy-number variations, may promote activation of the janus kinase-signal transducer and activator of transcription (JAK/STAT) signaling pathway through gene-dosage effects. In parallel, chromatin remodeling linked to EZH2 overexpression and reduced special SATB1 expression may support a Th2-polarized program. This phenotype may contribute to epidermal barrier impairment via cytokines such as Interleukins-4 (IL-4) and IL-13, potentially creating conditions permissive for Staphylococcus aureus colonization. Microbial superantigens and exotoxins may further contribute to tumor progression and therapeutic resistance by reinforcing JAK/STAT signaling, particularly STAT3, and reducing CD8+ T-cell–mediated immune surveillance. In the dermis, reprogramming of cancer-associated fibroblasts and polarization of macrophages toward an M2 phenotype may collectively contribute to an immunosuppressive niche. Emerging biomarkers, including CD74, and acquired resistance mechanisms after anti-C-C chemokine receptor 4 therapy further extend the translational relevance of recent pathologic findings. Overall, CTCL evolution appears to be a systemic process shaped by interactions between tumor-intrinsic genetic alterations and the skin microenvironment.