Intertriginous mycosis fungoides: A distinct presentation of cutaneous T-cell lymphoma that may be caused by malignant follicular helper T cells

Background: Follicular helper T cells are a subset of helper T cells that facilitate B-cell recruitment and maturation. Rare cases of cutaneous T-cell lymphoma manifesting as de novo tumor lesions in intertriginous skin contain an infiltrate rich in B cells. These cases may represent malignant counterparts of skin-homing follicular helper T cells. Observations: Two men and 1 woman (age range, 35-58 years) were seen with predominantly intertriginous tumor-stage cutaneous T-cell lymphoma lesions characterized by the absence of epidermotropism and the presence of a mixed infiltrate rich in B cells. Two of the patients died of the disease less than 3 years from the initial diagnosis. The surviving patient has aggressive disease and underwent hematopoietic stem cell transplantation. Two of the patients had a prominent CXCL13⁺, Bcl6/CD3⁺, and programmed death protein 1-positive follicular helper T-cell population. Conclusions: The intertriginous tumor variant of cutaneous T-cell lymphoma is heterogeneous but may be associated in some cases with a follicular helper T-cell immunophenotype. These patients may follow an aggressive clinical course. Tumor progression in sanctuary sites on patients receiving phototherapy may manifest as a similar clinical phenotype. Further characterization of the disease process is needed to confirm this observation.