CD200 Expression in Folliculotropic Mycosis Fungoides Emerging During Dupilumab Treatment

Folliculotropic mycosis fungoides (FMF) or mycosis fungoides with folliculotropism (MF‐F) is increasingly recognized in patients initially diagnosed with atopic dermatitis (AD) who subsequently worsen during dupilumab therapy. We report a case of FMF that became clinically apparent after dupilumab treatment. A 47‐year‐old man with a long‐standing diagnosis of AD developed persistent facial plaques and tumors despite treatment with dupilumab. Histopathological examination revealed folliculotropic infiltration of atypical lymphocytes with monoclonal T‐cell receptor gene rearrangement, leading to a diagnosis of FMF. To explore the role of CD200, immunohistochemical staining was performed in the present case, six additional FMF or MF‐F cases (two diagnosed during dupilumab treatment and four without dupilumab treatment history), and four AD cases. Strong CD200 expression was detected in the tumor cells of the present case and in both additional MF cases diagnosed during dupilumab therapy. In one case, CD200 expression was observed exclusively in the facial tumorous lesion but not in an erythematous lesion on the trunk. Among four FMF or MF‐F cases without dupilumab exposure, only one showed CD200 positivity in a tumorous lesion on the face. In contrast, CD200 expression was absent in all AD cases, regardless of dupilumab treatment history. These findings suggest that CD200 expression may be associated with a subset of folliculotropic MF lesions, particularly tumorous lesions and cases that become clinically apparent during dupilumab treatment.