The Diagnostic Utility of TRBC1 Immunohistochemistry in Mature T-Cell Lymphomas.

T-cell clonality assessment constitutes an essential part of the diagnostic evaluation of suspected T-cell neoplasms. Recent advances in flow cytometry-based analysis of TCR β chain constant region 1 (TRBC1) have introduced an accurate method of assessment of T-cell clonality. Its broader applicability is constrained due to the requirement of viable cells. Furthermore, the utility of the TRBC1 antibody in tissue immunohistochemistry (IHC) has not been comprehensively addressed. Herein, we validated an IHC-based approach to assess T-cell clonality using formalin-fixed, paraffin-embedded (FFPE) tissue. Utilizing DeepLIIF image analysis, we quantified TRBC1 positivity among CD3-positive cells in a training cohort comprising 34 cases of alpha/beta T-cell neoplasms and 29 cases of reactive lymphoid tissue as controls. In an independent validation cohort comprising 29 T-cell neoplasms and 20 controls, similar image quantification was conducted by a pathologist uninvolved in the analysis of the training cohort and blinded to the diagnoses. Receiver operating characteristic (ROC) analysis of the training cohort established the optimal cut-off points for monotypic TRBC1 expression-79.0% or higher indicating monotypic positivity and 36.3% or lower denoting negativity. These thresholds demonstrated robust metrics in both the training (sensitivity 88.2%, specificity 93.1%, positive predictive value 93.8%, negative predictive value 87.1%) and the validation cohorts (sensitivity 93.1%, specificity 95.0%, positive predictive value 96.4%, negative predictive value 90.5%). TRBC1 IHC was correlated with flow cytometry in 52 cases, which demonstrated a strong quantitative correlation of TRBC1 positivity (r = 0.78, p < 0.001) and a high categorical agreement (85.9%) in classifying monotypic versus polytypic staining. Discrepancies in categorization were associated with low tumor percentages. Furthermore, multiplex immunofluorescence (MIF) was performed in 15 cases for targeted quantification of TRBC1 expression in CD3-positive, PAX5-negative cells, achieving a concordance of 86.7% with IHC. In summary, TRBC1 IHC offers a reliable and practical complementary method for assessing T-cell clonality.