T‐cell receptor‐δ expression and γδ+ T‐cell infiltrates in primary cutaneous γδ T‐cell lymphoma and other cutaneous T‐cell lymphoproliferative disorders

Aims The diagnosis of cutaneous γδ T‐cell lymphoma (GDTCL) requires the identification of γδ chains of the T‐cell receptor (TCR). Our aim in this study was, by using a new monoclonal antibody (mAb) against TCRδ, to evaluate TCRδ expression in formalin‐fixed paraffin‐embedded (FFPE) skin tissue from TCRγ+ cutaneous T‐cell lymphoma (CTCL), and to assess TCRδ expression within a spectrum of other cutaneous lymphoproliferative disorders (CLPDs). Methods and results Twelve cases (10 patients) with TCRγ+ CTCL and 132 additional CLPD cases (127 patients) were examined, including mycosis fungoides (MF) ( n = 60), cutaneous GDTCL ( n = 15), subcutaneous panniculitis‐like T‐cell lymphoma (SPTCL) ( n = 11), and CD30 + lymphoproliferative disorder (LPD) ( n = 24). Clone H‐41 against TCRδ was used on a Leica Bond‐3 automated stainer to label FFPE slides. H‐41 immunostaining was graded as percentage infiltrate: high (50–100%), moderate (10–49%), and low (0–9%). In TCRγ+ tumours, 12 of 12 (100%) patients showed TCRδ expression comparable to TCRγ expression. No (0%) TCRγ+ cases were negative for TCRδ. In all CLPDs, TCRδ expression was as follows: GDTCL, 16 of 20 cases (14 of 15 patients) high, two moderate, and two low; MF, 0 of 60 cases high, nine moderate, and 51 low; CD30 + LPD, one of 24 cases high, two moderate, and 21 low; and SPTCL, 0 of 11 cases (0 of 9 patients) high, two moderate, and two low. Three MF‐like cases and one SPTCL‐like case showed high expression; the remainder showed low expression. Conclusions mAb H‐41 against TCRδ matches TCRγ in immunostaining FFPE tissues from GDTCL, supporting H‐41 as a replacement for mAb γ3.20. TCRδ expression in our study suggests that the true occurrence of γδ+ non‐GDTCL CTCL/CLPD may be lower than suggested by the recent literature.