The Utility of TRBC1 Immunohistochemistry in the Evaluation of T-Cell Lymphomas.

T-cell receptor β chain constant region 1 (TRBC1) is a useful marker for the detection of clonal T-cell populations and has been used in flow cytometry, with limited data on its utility as an immunohistochemical marker. We evaluated TRBC1 by immunohistochemistry (IHC) in 42 CD3-positive, T-cell receptor alpha beta-positive T-cell lymphomas and compared their expression pattern to a control cohort of 35 reactive lymph nodes and tonsils. TRBC1 restriction was evident in 37/42 (88%) lymphomas but indeterminate in 5 cases, most of which were follicular helper T-cell lymphomas, angioimmunoblastic type/nodal T follicular helper cell lymphomas, angioimmunoblastic type (AITL). 34/35 (97%) control cases had ∼50% as much TRBC1 staining as CD3; however, one tonsil, while not TRBC1 restricted, had much less staining than CD3, which was confirmed by TRBC1 flow cytometry. TRBC1 IHC was helpful in demonstrating clonality in most T-cell lymphomas, including all cases lacking flow cytometric studies, and in 2 cases without definite molecular evidence of clonality. In differentiating T-cell lymphoma from reactive lymphoid tissue, it had a sensitivity of 88% and specificity of 97%. Thus, TRBC1 IHC is a useful, rapid, and inexpensive tool in the diagnosis of T-cell lymphomas.