Introduction: Dual TRBC1–CD3 immunohistochemistry (IHC) provides in-situ visualization of T-cell clonality and shows good concordance with molecular assays. Discordance, however, may reflect biologic or technical factors that alter or diminish antigen expression, creating staining patterns that appear polytypic even when the population is clonal.
Objective: To describe a case of early mycosis fungoides demonstrating apparent discordance between TRBC1–CD3 IHC and T-cell-receptor (TCR) gene rearrangement, and to explore factors that may influence this finding.
Methods: A 34-year-old man with a four-year history of chronic dermatitis-like lesions developed new papules on the forearms and a vesicular plaque on the left foot. Punch biopsies from both sites underwent histopathology, IHC (CD3, CD4, CD5, CD7, dual TRBC1–CD3), and TCR γ/β PCR testing.
Results: Both lesions showed epidermotropic atypical CD4⁺ lymphocytes with angulated nuclei, perinuclear halos, and complete loss of CD5 and CD7 within the epidermis, supporting an aberrant T-cell population. Dual TRBC1–CD3 IHC demonstrated relatively weak TRBC1 and weak CD3 staining, making it difficult to interpret whether there was a monotypic expression pattern within the T cells. These findings were reproducible on a second round of staining using sequential cuts of CD5, CD7, and TRBC1–CD3 from both sites. By contrast, TCR γ/β PCR revealed identical clonal rearrangements in both sites. The patient had negative Sézary flow cytometry and PET/CT, consistent with stage IA (T1N0M0B0).
Conclusions: This case illustrates how apparent discordance between TRBC1–CD3 IHC and TCR PCR can occur when antigen expression is attenuated or variably preserved, whether from biologic modulation of the TCR/CD3 complex or technical factors. Integrating molecular and immunohistochemical data helps distinguish true polytypy from reduced antigen visibility.
