TRBC1 immunohistochemistry distinguishes cutaneous T-cell lymphoma from inflammatory dermatitis: A retrospective analysis of 39 cases
Authors: Sarah E. Nocco, Mark D. Ewalt, Andrea P. Moy, Natasha E. Lewis, Menglei Zhu, Cecilia Lezcano, Klaus Busam, Melissa Pulitzer
Journal: Journal of the American Academy of Dermatology (2023)

To the Editor: In cutaneous T-cell lymphomas/lymphoproliferative disorders (CTLs), the T cells are usually monoclonal, whereas in reactive infiltrates, they are most often polyclonal.1Pulitzer M. Cutaneous T-cell lymphoma.Clin Lab Med. 2017; 37: 527-546https://doi.org/10.1016/j.cll.2017.06.006Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar Analysis of the T-cell receptor (TCR) using multiplex polymerase chain reaction (PCR) is the standard for clonality testing; however, this method is time-consuming, expensive, and technically limited.2Wang H.W. Raffeld M. Molecular assessments of clonality in lymphoid neoplasms.Semin Hematol. 2019; 56: 37-45https://doi.org/10.1053/j.seminhematol.2018.05.008Crossref PubMed Scopus (9) Google Scholar A monoclonal antibody (mAb) against the C1 constant region of the TCRβ chain (TRBC1) has advanced clonality testing in flow cytometric analysis of αβ T-cell lymphomas,3Berg H. Otteson G.E. Corley H. et al.Flow cytometric evaluation of TRBC1 expression in tissue specimens and body fluids is a novel and specific method for assessment of T-cell clonality and diagnosis of T-cell neoplasms.Cytom B. 2021; 100: 361-369https://doi.org/10.1002/cyto.b.21881Crossref PubMed Scopus (23) Google Scholar including CTLs.4Horna P. Shi M. Jevremovic D. Craig F.E. Comfere N.I. Olteanu H. Utility of TRBC1 expression in the diagnosis of peripheral blood involvement by cutaneous T-cell lymphoma.J Invest Dermatol. 2021; 141: 821-829https://doi.org/10.1016/j.jid.2020.09.011Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar During TCR gene rearrangement, 1 of 2 mutually exclusive TCR constant β chains (TRBC1 or TRBC2) is randomly selected. Normal αβ T cells comprise a mixture of TRBC1+ and TRBC1− (presumptively TRBC2+) cells, whereas neoplastic T cell populations arise from a single clone and exclusively express either TRBC1 or TRBC2. Restricted expression of TRBC1 can thus serve as a marker for clonality.5Maciocia P.M. Wawrzyniecka P.A. Philip B. et al.Targeting the T cell receptor B-chain constant region for immunotherapy of T cell malignancies.Nat Med. 2017; 23: 1416-1423https://doi.org/10.1038/nm.4444Crossref PubMed Scopus (168) Google Scholar Here, we report our results using a TRBC1 mAB in immunohistochemistry (IHC) on FFPE specimens from 30 αβCTLs and 9 rashes as a surrogate for molecular testing. We performed a retrospective review of clinicopathologic data from patients whose biopsy samples were assessed for possible CTL in 2022. These cases needed to have warranted IHC and molecular analysis for the diagnosis and were TCRαβ+. Hematoxylin and eosin–stained sections and CD3 IHC were reviewed using light microscopy (blinded to diagnosis) for atypical lymphoid infiltrates. Furthermore, ≥90% of morphologically atypical cells needed to show CD3 for baseline T-cell distribution. TRBC1 (E6Z3S; 1:600; 79485, Cell Signaling Technology) immunostaining was assessed as a percentage of total atypical CD3+ T-cell infiltrates. Cases were assigned a predictive category of monoclonal (PM) or polyclonal (PP) based on the percentage of atypical T cells labeled by TRBC1, with percentages of ≥70% or ≤25% indicating TRBC1-restriction or -predicted monoclonal (Fig 1). We then compared TRBC1 predictions with clonality interpretations by PCR and clinical data. Supplemental material available on Mendeley at (https://data.mendeley.com/datasets/hpj9j2dnzw/1). Twenty-six (68%) samples were PM and 12 (32%) were PP by TRBC1. Overall concordance with PCR was observed in 21 of 26 (81%) PM samples and 9 of 12 (75%) PP samples, with TRBC1 PM samples showing 88% agreement for monoclonality with fragment analysis (21/24 PCR-clonal cases). Agreement improved to 23 of 26 when PCR results were re-evaluated, resulting in a changed interpretation to monoclonal in 2 cases. TRBC1 PM was more sensitive for CTLs (24/29, 83%) versus PCR (22/29, 76%), although the specificity was lower with PM, rendering a positive predictive value of 89% compared with 96% with PCR. Both techniques showed clonality in all high grade CTLs. Neither technique was strong at confirming reactive disease (negative predictive value: 50% for TRBC1 and 53% for PCR). Qualitatively, when TRBC1 was restricted positive, it aided in the diagnosis of strictly intraepidermal CTL. Our findings indicate that the use of a single anti-TRBC1 antibody with an appropriate pan–T-cell marker allows rapid and low-cost assessment of clonality in archival tissues. Problems with visual "gating" by morphology and emphasis on CD3+ αβCTLs limit the generalizability of the results. Inflamed or polymorphous infiltrates are challenging to address with single IHC techniques. The small sample size and few reactive cases warrant caution in attributing statistical significance to comparisons with PCR. Cutoff values for TRBC1 monotypia require more stringent analysis. As with PCR, false positives are possible because of oligoclonal reactive processes. The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article.