T-cell lymphomas are a heterogeneous group of malignancies involving T lymphocytes and generally characterized by a poor prognosis. Among them, cutaneous T-cell lymphomas involve primarily the skin. Mycosis fungoides and Sézary syndrome are the most frequent cutaneous T-cell lymphomas. The inventors studied the regulatory T phenotype of Sézary cells and showed the expression of CCR8 (CD198) by Sézary cells and other T-cell lymphoma cell lines. CCR8 therefore appears as a useful diagnostic, prognostic and follow-up marker, and as a potential therapeutic target in T-cell lymphomas. Therapeutic depletion of CCR8-expressing cancer cells would eliminate tumor cells and also activate the anti-tumor immunity in T-cell lymphomas.