Utility of CD123 immunohistochemistry in differentiating lupus erythematosus from cutaneous T cell lymphoma

Aims: Histopathological overlap between lupus erythematosus and certain types of cutaneous T cell lymphoma (CTCL) is well documented. CD123⁺ plasmacytoid dendritic cells (PDCs) are typically increased in lupus erythematosus, but have not been well studied in CTCL. We aimed to compare CD123 immunostaining and histopathological features in these conditions.

Methods and results: Skin biopsies of cutaneous lupus erythematosus (CLE, n = 18), lupus erythematosus panniculitis (LEP, n = 17), mycosis fungoides (MF, n = 25) and subcutaneous panniculitis-like T cell lymphoma (SPTCL, n = 9) were retrospectively reviewed and immunostained with CD123. Percentage, distribution and clustering of CD123⁺ cells were compared between CLE and MF and between LEP and SPTCL using χ² and two-tailed t-tests. A higher percentage of CD123⁺ cells was observed in CLE than MF (P < 0.01), more frequently comprising ≥20% of the entire infiltrate (P < 0.01) and forming clusters (P < 0.01). Similarly, LEP showed a higher percentage of CD123⁺ cells than SPTCL (P = 0.01), more frequently comprising ≥20% of the infiltrate (P = 0.04) and forming clusters (P = 0.01). Basal vacuolar change or dyskeratosis was observed in all CLE cases and in 48% cases of MF cases (P = 0.05). Plasma cells were readily identified in 76% cases of LEP but in none of the SPTCL cases (P = 0.01). Adipocyte rimming by lymphocytes, hyaline fat necrosis and fibrinoid/grungy necrosis did not significantly differ between LEP and SPTCL. Dermal mucin also failed to distinguish between groups.

Conclusions: CD123 immunostaining is helpful in differentiating CLE from MF and LEP from SPTCL, but should be interpreted in conjunction with clinicopathological features and other ancillary studies to ensure accurate diagnosis.