T-cell clones of uncertain significance are highly prevalent and show close resemblance to T-cell large granular lymphocytic leukemia. Implications for laboratory diagnostics

Emerging Role of T-cell Receptor Constant β Chain-1 (TRBC1) Expression in the Flow Cytometric Diagnosis of T-cell Malignancies

“…In patients without demonstrable T-cell neoplasia, we found a similar bimodal expression of TRBC1 on CD4-positive and CD8-positive T-cell subsets gated based on distinct immunophenotypic features, with the exception of occasional small CD8-positive T-cell subsets with a unimodal TRBC1 expression pattern [33]. Further analysis revealed that these small subsets were truly clonal based on TCR-Vβ-restriction [31] and T-cell gene rearrangement molecular studies [33], consistent with benign immunodominant clonotypes (see T-CUS section below). In our extensive experience using TRBC1 within a single-tube comprehensive T-cell panel, we have found that T-cell neoplasms virtually always have distinct immunophenotypic features that largely separate them from background benign T-cells, but that this separation based on routine gating strategies is often imperfect and results in a very skewed rather than a purely unimodal TRBC1 expression pattern.…”

“…TRBC1 clonality assessment, together with immunophenotypic aberrancy, allows for accurate quantitation of clonal T-cells. Using this methodology, we found that T-CUS has a much lower clonal T-cell count than T-LGLL, although overlap in clonal size has been noticed [31]. The median clonal T-cell count in T-LGLL patients was 2376 cells/µL (range 198.8-29,905.9 cells/µL), which comprised of 66.0% (range 14.5-97%) of total lymphocytes [31].…”

“…A largely unimodal TRBC1-negative or TRBC1-positive staining pattern is consistent with a restricted (monotypic) TCR β chain constant region expression, indicative of clonality. In addition, we have frequently encountered T-cell clones with a unimodal TRBC1-dim expression pattern, not necessarily associated with dim expression of CD3 (as would be expected in the setting of TCR downregulation) [31], and likely due to a unique TRBC1-positive TCR with diminished avidity for the JOVI.1 antibody for reasons yet unknown.…”

“…The integration of clonality assessment by TRBC1 into routine clinical practice has resulted in the frequent identification of small T-cell clones of uncertain significance (T-CUS) in patients with no clinical evidence of T-cell lymphoma [31], most likely representing dominant T immunoclones ( Figure 2). As predicted from prior reports, most T-CUS detected by TRBC1 staining has an immunophenotype reminiscent of CD8-positive T-cell large granular lymphocytes, with a clone size that only occasionally overlaps with that seen in T-cell large granular lymphocytic leukemia (T-LGLL) [44].…”

“…As T-cell clonality assessment by TRBC1 is implemented in more diagnostic flow cytometry laboratories, we anticipate that the concept of T-CUS will be utilized more routinely in order to best interpret the presence and immunophenotype of small T-cell clones. Given the high prevalence of CD8 + /CD4 − T-CUS, an arbitrary threshold of 15% of all lymphocytes for a clonal CD8-positive T-cell population lacking malignancy-specific immunophenotypic abnormalities (e.g., conspicuously dim expression of CD2, CD3, or CD45; or conspicuously increased light scatter) might be a good practice guide to determine when to raise concern for a CD8-positive T-cell lymphoproliferative disorder [31]. In these common variants of T-CUS, loss of CD5 and/or CD7 is frequently encountered and should not by itself raise concern for neoplasia.…”