Assessing the Persistence and Relevance of Small T-LGL Clones via TRBC1 in a Flowcytometric Lymphoid Screening Tube

Recently, monoclonal antibodies targeting one of the two mutually exclusive constant regions of the TCR β receptor chain (TRBC1 and TRBC2) have been proposed as a surrogate tool for assessing T-cell clonality by flowcytometry. The detection of T-cell clones is typically interpreted in the context of malignancy. However, small T-cell clones of uncertain significance (T-CUS) are frequently identified, posing diagnostic and clinical challenges. Since 2022, TRBC1 was incorporated as a marker for T-cell clonality in our flowcytometric lymphoid screening tube (LST), a tube designed to detect both B- and T-cell aberrancies in the blood of patients with clinical suspicion of hematological malignancy. The objective of the current study was to systematically evaluate the frequency and size of T-cell clones identified at initial diagnosis and to evaluate clonal evolution in follow-up samples.

In this single-centre retrospective study (2022-2024), peripheral blood samples of 3495 unique patients were tested for T-cell clonality using TRBC1. The monotypic T-cell clones of > 50/μL were stratified into three groups: T-CUS1 (50-500 monotypic cells/μL), T-CUS2 (500-2000 monotypic cells/μL), and T-LGL (> 2000 monotypic cells/μL). Follow-up samples were obtained at least 6 months after initial testing (range: 6-30 months) and potential indicators of progression to T-CUS2 or T-LGL were evaluated.

The majority of T-CUS clones (22/36, 61%) were not detected (< 50 monotypic cells/μL) at follow-up, thereby demonstrating the importance of evaluation of the persistance of small T-cell clones. Fifteen percent of patients with a T-CUS1 progressed to T-CUS2, but none to T-LGL (95% CI: 0%-15%). 22% (95% CI: 3%-60%) of T-CUS2 patients evolved to T-LGL, all of which showed aberrant expression of CD57. In the patient group who showed evolution to T-CUS2 or T-LGL, significantly lower hemoglobin levels and neutrophil counts were observed at the time of T-CUS detection. Additionally, in this group, comparable increases in monotypic T-cell clones were observed over time, with a main increment of 90 monotypic cells/μL per month (95% CI: 34%-145%).

The low number of persistence observed in T-CUS at follow-up highlights the need for cautious interpretation of small T-cell clones (T-CUS1). Low hemoglobin and neutrophil counts at diagnosis might indicate higher risk for progression and need for closer follow-up.