Expression of helper T cell master regulators in inflammatory dermatoses and primary cutaneous T-cell lymphomas: Diagnostic implications

Background Mycosis fungoides (MF) is a neoplasm of skin-homing CD4⁺ helper T (T_H) lymphocytes with dysregulation of T_H1 and T_H2 immunity. Diagnosis of MF is challenging, as there is significant morphologic overlap with other dermatologic entities. Objective We investigated diagnostic utility of T_H1- and T_H2-specific markers, T-bet, and GATA-3, respectively, in MF and its reactive and neoplastic mimics.

Methods Immunohistochemical staining for CD3/T-bet and CD3/GATA-3 was performed on inflammatory dermatoses (n = 56), MF (n = 37), Sezary syndrome (SS; n = 8), and cutaneous anaplastic large cell lymphoma (C-ALCL; n = 14).

Results Inflammatory dermatoses showed epidermal T cells predominantly expressing GATA-3, except psoriasis, which exhibited a mixed GATA-3/T-bet staining. In contrast, neoplastic T cells in patch stage MF showed markedly increased T-bet positivity with minimal GATA-3 expression. Plaque stage MF had a mixed T-bet/GATA-3 phenotype, whereas tumor stage MF and SS exhibited diffuse GATA-3 expression. C-ALCL lacked significant staining for both markers.

Limitations Sample size was relatively small.

Conclusions A predominance of T-bet⁺ T cells in the epidermis support patch stage MF over dermatitis. A predominance of GATA-3⁺ T cells in the dermis support CD30⁺ MF with large cell transformation over C-ALCL. These stains do not allow distinction between dermatitis and cutaneous infiltrates of SS.