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

Simple Summary: TRBC1 expression analysis by flow cytometry (FCM) has been recently proved to be a useful, simple and fast approach to assessing T alpha beta-cell clonality. The aim of this study was to validate the utility of this assay specifically for the diagnosis of T-cell clonality of T-large granular lymphocytic leukemias (T-LGLL), as more mature polyclonal T alpha beta large granular lymphocytes (T alpha beta-LGL) show broader TRBC1(+)/TRBC1(-) ratios vs. total T alpha beta cells. Our results showed that a TRBC1-FCM assay is also a fast and easy method for detecting T-cell clonality in T-LGLL based on altered (increased or decreased) percentages of TRBC1(+) T alpha beta cells of LGL expansions (i.e., with lymphocytosis) suspected of T-LGLL, whereas in the absence of lymphocytosis (or in T alpha beta CD4-LGLL), the detection of increased absolute cell-counts of more precisely defined subpopulations of T-LGL expressing individual TCRV beta families is required.& nbsp;Flow cytometric (FCM) analysis of the constant region 1 of the T-cell receptor beta chain (TRBC1) expression for assessing T alpha beta-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 T alpha beta cells (i.e., T-LGL normal-counterpart) show broader TRBC1(+)/TRBC1(-) ratios vs. total T alpha beta cells. We compared the distribution and absolute counts of TRBC1(+) and TRBC1(-) T alpha beta-LGL in blood containing polyclonal (n = 25) vs. clonal (n = 29) LGL. Overall, polyclonal TRBC1(+) or TRBC1(-) T alpha beta-LGL ranged between 0.36 and 571 cells/mu L (3.2-91% TRBC1(+) cells), whereas the clonal LGL cases showed between 51 and 11,678 cells/mu L (96% TRBC1(+) cells). Among the distinct TCRV beta families, the CD28(-) effector-memory and terminal-effector polyclonal T alpha beta cells ranged between 0 and 25 TRBC1(+) or TRBC1(-) cells/mu L and between 0 and 100% TRBC1(+) cells, while clonal LGL ranged between 32 and 5515 TRBC1(+) or TRBC1(-) cells/mu L, representing 98% TRBC1(+) cells. Our data support the utility of the TRBC1-FCM assay for detecting T-cell clonality in expansions of T alpha beta-LGL suspected of T-LGLL based on altered percentages of TRBC1(+) T alpha beta cells. However, in the absence of lymphocytosis or in the case of T alpha beta CD4-LGL expansion, the detection of increased absolute cell counts by the TRBC1-FCM assay for more accurately defined subpopulations of T alpha beta-LGL-expressing individual TCRV beta families, allows the detection of T-cell clonality, even in the absence of phenotypic aberrations.