Primary cutaneous T-cell lymphoma expressing FOXP3: A case report supporting the existence of malignancies of regulatory T cells

Regulatory T (Treg) cells, which represent 5% to 10% of peripheral T cells, regulate the activities of T-cell subsets by performing immunosuppressive functions and thus preventing the development of autoimmune responses. The majority of Treg cells are CD4⁺, CD25⁺, and FOXP3⁺. Recently, it has been demonstrated that the tumor cells in adult T-cell leukemia lymphomas can function as Treg, raising the question of whether any variant of primary cutaneous T-cell lymphoma may also express a regulatory phenotype. We describe an extraordinary case of primary cutaneous T-cell lymphoma clinically characterized by protean cutaneous manifestations and histologically showing a pattern consistent with epidermotropic pleomorphic medium-/large-cell primary cutaneous T-cell lymphoma. The majority of neoplastic cells were CD4⁺ CD25⁺ T cells and strongly expressed FOXP3. With this background, the current case, characterized by an aggressive course requiring polychemotherapy, may support the existence of lymphoproliferative malignancies of Treg cells.