Expression of T-follicular helper markers in sequential biopsies of progressive mycosis fungoides and other primary cutaneous T-cell lymphomas.

T-follicular helper (Tfh) lymphocytes represent the neoplastic cells of angioimmunoblastic T-cell lymphoma and have been observed also in several cutaneous T-cell lymphomas (CTCLs) and extracutaneous T-cell lymphomas, including peripheral T-cell lymphoma, not otherwise specified, mycosis fungoides (MF), cutaneous CD4 small/medium T-cell lymphoma (CD4SMTCL), and Sezary syndrome. We studied a large number of different types of primary CTCL for expression of Tfh markers, including 36 biopsies from 21 patients with MF (with sequential biopsies from patch stage and tumor stage of 15 patients), 13 patients with CD4SMTCL, 9 with lymphomatoid papulosis, 11 with cutaneous anaplastic large cell lymphoma (cALCL), 2 with cutaneous γ/δ T-cell lymphoma, 8 with subcutaneous panniculitis-like T-cell lymphoma, 3 with cutaneous aggressive epidermotropic CD8 cytotoxic T-cell lymphoma, 6 with cutaneous peripheral T-cell lymphoma, not otherwise specified, and 1 with Sezary syndrome. Expression of at least 3 of 5 markers (PD-1, CXCL-13, ICOS, Bcl-6, and CD10) in >10% of tumor cells was observed in 33 biopsies (MF = 20; CD4SMTCL = 11; 1 each cutaneous anaplastic large cell lymphoma and subcutaneous panniculitis-like T-cell lymphoma, respectively). Our study shows that a Tfh phenotype is very common in MF and CD4SMTCL but can be observed rarely also in other types of CTCL. Expression of Tfh markers should not be used for classification of any entity of CTCL and may only integrate other immunohistochemical stainings for a more accurate characterization of these disorders. Precise distinction of Tfh-positive CTCLs from secondary skin manifestations of angioimmunoblastic T-cell lymphoma cannot rest on demonstration of a Tfh phenotype alone and should be achieved by a synthesis of clinical, histological, and phenotypic features.