Differentiating reactive and neoplastic gamma‐delta (γδ) T‐cell expansions in the peripheral blood and bone marrow

The clinical and immunophenotypic attributes of reactive γδ T-cell expansions are less well characterized than their malignant counterparts, which can pose diagnostic challenges. This study aims to investigate the characteristics and long-term clinical outcomes of reactive γδ T-cell expansions. A retrospective review was performed to identify patients with expanded γδ T-cell population (>15% of T-cells) by flow cytometry in peripheral blood and/or bone marrow specimens over a 17-year period. The cases were divided into reactive and malignant categories and their clinical and immunophenotypic findings were compared. Clinical follow-up was performed. 97 patients were identified including 19 malignant and 78 reactive cases with a variety of underlying conditions. The median absolute γδ T-cell count and median percentage of γδ T-cells per total T-cells were significantly lower in reactive vs. malignant cases (p = 0.0001 and p < 0.00001, respectively). Reactive cases showed more frequent brighter surface CD3 expression (87.1% vs. 42.1%; p < 0.0001), no discrete loss of CD7 (0% vs. 36.9%; p < 0.0001), less frequent lack of CD5 (25.7% vs. 42.4%; p < 0.0001), and no homogeneous CD56 expression (0% vs. 31.6%; p > 0.0001) as compared with malignant cases. Upon long-term follow-up, none of the reactive cases showed clinical evidence of malignant evolution. Reactive expansions of γδ T-cells can be seen in a variety of conditions including hematologic neoplasms, autoimmune and post-transplant states, and infections. Such cases have significantly lower γδ T-cell counts and percentages and no discrete loss of CD7. Lack of CD5 on its own is not an indication of immunophenotypic aberrancy in γδ T-cells. Upon long-term clinical follow-up, such reactive expansions show no evidence of evolution to γδ T-cell malignancies.