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Regulatory T‐cell phenotype in association with large cell transformation of mycosis fungoides

Introduction: Tumor cells of primary cutaneous T‐cell lymphomas are able to adopt a regulatory T‐cell phenotype in vitro. The significance of this finding in vivo is matter of debate. Methods: We stained five cases with transformed mycosis fungoides (MF) with an antibody against FOXP3, which is a sensitive and specific marker for the regulatory T‐cell phenotype. Results: Transformed T cells in four of five patients with MF stained positive for FOXP3. One patient who showed no CD30 expression of large transformed T cells was also negative for FOXP3. Comparison of plaques and tumors in one patient showed that FOXP3 and CD30 expression was exclusively observed in large transformed tumor cells whereas malignant T cells without large cell transformation were negative. Conclusion: Transformation of MF to high grade lymphoma may be associated with the adoption of a regulatory T‐cell phenotype. FOXP3 expression may contribute to aggressive behavior of MF after large cell transformation via immune escape mechanism. The significance of this observation is limited by the low case number in this study.