High-Sensitive TRBC1-Based Flow Cytometric Assessment of T-Cell Clonality in Tαβ-Large Granular Lymphocytic Leukemia

Flow cytometric (FCM) analysis of the constant region 1 of the T-cell receptor β chain (TRBC1) expression for assessing Tαβ-cell clonality has been recently validated. However, its utility for the diagnosis of clonality of T-large granular lymphocytic leukemia (T-LGLL) needs to be confirmed, since more mature Tαβ cells (i.e., T-LGL normal-counterpart) show broader TRBC1⁺/TRBC1^- ratios vs. total Tαβ cells. We compared the distribution and absolute counts of TRBC1⁺ and TRBC1^- Tαβ-LGL in blood containing polyclonal (n = 25) vs. clonal (n = 29) LGL. Overall, polyclonal TRBC1⁺ or TRBC1^- Tαβ-LGL ranged between 0.36 and 571 cells/μL (3.2-91% TRBC1⁺ cells), whereas the clonal LGL cases showed between 51 and 11,678 cells/μL (<0.9% or >96% TRBC1⁺ cells). Among the distinct TCRVβ families, the CD28^- effector-memory and terminal-effector polyclonal Tαβ cells ranged between 0 and 25 TRBC1⁺ or TRBC1^- cells/μL and between 0 and 100% TRBC1⁺ cells, while clonal LGL ranged between 32 and 5515 TRBC1⁺ or TRBC1^- cells/μL, representing <1.6% or >98% TRBC1⁺ cells. Our data support the utility of the TRBC1-FCM assay for detecting T-cell clonality in expansions of Tαβ-LGL suspected of T-LGLL based on altered percentages of TRBC1⁺ Tαβ cells. However, in the absence of lymphocytosis or in the case of TαβCD4-LGL expansion, the detection of increased absolute cell counts by the TRBC1-FCM assay for more accurately defined subpopulations of Tαβ-LGL-expressing individual TCRVβ families, allows the detection of T-cell clonality, even in the absence of phenotypic aberrations.