Utility of CD123 immunohistochemistry in differentiating lupus erythematosus from cutaneous T cell lymphoma Stephanie J T Chen, 1,2 Julie Y Tse, 3 Paul W Harms, 1,4 Alexandra C Hristov 1,4 & May P Chan 1,4 1 Department of Pathology, University of Michigan, Ann Arbor, MI, 2 Department of Pathology, University of Iowa, Iowa City, IA, 3 Department of Pathology, Tufts Medical Center, Boston, MA, and 4 Department of Dermatology, University of Michigan, Ann Arbor, MI, USA Date of submission 17 November 2018 Accepted for publication 30 December 2018 Published online Article Accepted 31 December 2018 Chen S J T, Tse J Y, Harms P W, Hristov A C & Chan M P (2019) Histopathology 74, 908–916. https://doi.org/10.1111/his.13817 Utility of CD123 immunohistochemistry in differentiating lupus erythematosus from cutaneous T cell lymphoma Aims: Histopathological overlap between lupus erythe- matosus and certain types of cutaneous T cell lymphoma (CTCL) is well documented. CD123 + plasmacytoid den- dritic cells (PDCs) are typically increased in lupus erythe- matosus, 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. Percent- age, distribution and clustering of CD123 + cells were compared between CLE and MF and between LEP and SPTCL using v 2 and two-tailed t-tests. A higher per- centage 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 percent- age of CD123 + cells than SPTCL (P = 0.01), more fre- quently 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 fib- rinoid/grungy necrosis did not significantly differ between LEP and SPTCL. Dermal mucin also failed to distinguish between groups. Conclusions: CD123 immunostaining is helpful in dif- ferentiating CLE from MF and LEP from SPTCL, but should be interpreted in conjunction with clinico- pathological features and other ancillary studies to ensure accurate diagnosis. Keywords: CD123, cutaneous T cell lymphoma, lupus erythematosus, mycosis fungoides, plasmacytoid dendritic cells, subcutaneous panniculitis-like T cell lymphoma Introduction Histopathological overlap between cutaneous/subcu- taneous lupus erythematosus and cutaneous T cell lymphoma (CTCL) is well recognised, and may present a diagnostic challenge. Although mycosis fungoides (MF) is readily diagnosed when fully devel- oped, classic features such as Pautrier microabscesses are often lacking in early cases. 1–3 Such cases may mimic various inflammatory dermatoses such as eczematous dermatitis, connective tissue disease, pig- mented purpuric dermatosis and lichen sclerosus. 4–6 Of these, cutaneous lupus erythematosus (CLE) may enter the differential diagnosis of MF when interface Address for correspondence: M P Chan MD, University of Michigan, NCRC Building 35, 2800 Plymouth Road, Ann Arbor, MI 48109, USA. e-mail: mpchan@med.umich.edu © 2018 John Wiley & Sons Ltd. Histopathology 2019, 74, 908–916. DOI: 10.1111/his.13817