Distinguishing T-cell Large Granular Lymphocytic Leukemia from Reactive Conditions: Laboratory Tools and Challenges in Their Use

Single-Antibody Evaluation of T-Cell Receptor β Constant Chain Monotypia by Flow Cytometry Facilitates the Diagnosis of T-Cell Large Granular Lymphocytic Leukemia

OBJECTIVES

The diagnosis of T-cell large granular lymphocytic leukemia (T-LGLL) is challenging because of overlapping immunophenotypic features with reactive T cells and limitations of T-cell clonality assays. We studied whether adding an antibody against T-cell receptor β constant region 1 (TRBC1) to a comprehensive flow cytometry panel could facilitate the diagnosis of T-LGLL.

METHODS

We added TRBC1 antibody to the standard T-cell and natural killer (NK) cell panel to assess T-cell clonality in 56 T-LGLLs and 34 reactive lymphocytoses. In addition, 20 chronic lymphoproliferative disorder of NK cells (CLPD-NKs) and 10 reactive NK-cell lymphocytoses were analyzed.

RESULTS

Clonal T cells were detected in all available T-LGLLs by monotypic TRBC1 expression and clonal/equivocal T-cell receptor gene rearrangement (TCGR) studies, compared with only 27% of T-LGLLs by killer-cell immunoglobulin-like receptor (KIR) restriction. Overall, 85% of T-LGLLs had a blood tumor burden greater than 500 cells/µL. Thirty-four reactive cases showed polytypic TRBC1 expression, except for 5 that revealed small T-cell clones of uncertain significance. All CLPD-NKs showed expected clonal KIR expression and negative TRBC1 expression.

CONCLUSIONS

Addition of TRBC1 antibody to the routine flow cytometry assay could replace the TCGR molecular study and KIR flow cytometric analysis to assess clonality, simplifying the diagnosis of T-LGLL.

Flow Cytometry Applications in the Diagnosis of T/NK-Cell Lymphoproliferative Disorders

This article provides an overview of the role of flow cytometry in the diagnosis, prognosis, and follow-up of T and NK-cell lymphoproliferative disorders. For each category, we will briefly discuss the immunophenotypic features of normal T and NK cells, and address technical issues in flow cytometry, the approach to diagnosis in various contexts, pitfalls in interpretation, and its use in follow-up and post-therapy management. In addition to reviewing the diagnostic, prognostic, and therapeutic utility of flow cytometric immunophenotyping in several of specific T and NK cell entities, we will also cover some of the new immunophenotypic markers. Furthermore, we will touch upon incorporation of flow cytometry in the final diagnosis, including correlation with other ancillary tests. © 2019 International Clinical Cytometry Society.

Leukocytosis

An increased white blood cell count, or leukocytosis, is a common laboratory finding. Appropriate specimen evaluation depends on which lineages are increased and the morphologic findings on peripheral blood smear review to guide further testing. The presence of blasts is concerning for acute leukemia and may require bone marrow biopsy. Lymphocytosis may be morphologically divided into polymorphic and monomorphic populations. Polymorphic lymphocytosis is most consistent with a reactive process, while monomorphic populations are concerning for lymphoproliferative neoplasm. The differential can be further narrowed based on morphologic findings. Myeloid leukocytosis can occur in a number of reactive conditions as well as myeloid malignancies. The types of cells present and morphology can help to guide additional workup. This study provides guidance for the appropriate evaluation and further workup of leukocytosis.