Detecting T-cell clones of uncertain significance using anti-TRBC1-based flow cytometry

Flow cytometry (FC) incorporating the T-cell receptor constant beta chain-1 (TRBC1) antibody has rapidly gained traction as the preferred method for T-cell clonality assessment. The widespread use of this assay has identified a high prevalence of T-cell clones of uncertain significance (T-CUS) that are detected in the absence of demonstrable malignancy. We conducted a retrospective evaluation of T-CUS clones identified at our institution following the introduction of a TRBC1 antibody based FC assay in 2020. Bona fide T-CUS was identified in 92 samples from 55 patients and comprised 60 distinct T-cell clones, the majority of which were CD8+ and showed immunophenotypic overlap with T-cell large granular lymphocyte leukaemia (T-LGLL). CD4+, CD4+8+ and CD4-8- clones were also captured and described in this cohort. Expression of the NK marker CD16 was uncommon (16.7%), highlighting this as a potentially useful marker for distinguishing T-CUS from T-LGLL. In the follow-up cohort, 92% of 29 tracked clones reduced in size or stayed the same over a median follow-up of 19.6 months and no cases progressed to overt T-cell malignancy. This study demonstrates serial assessment of T-CUS clones and provides the basis for further prospective studies.