Utility of CD26 in flow cytometric immunophenotyping of T-cell lymphomas in tissue and body fluid specimens

Background: CD26 is expressed by most CD4+ T cells in normal peripheral blood specimens. Neoplastic T cells are frequently CD26- in mycosis fungoides/Sezary syndrome involving the peripheral blood. However, CD26 expression by reactive and neoplastic T cells in solid tissues and body fluids has not been fully characterized by flow cytometry (FC). Methods: Solid tissue and body fluid specimens were assayed for CD26 expression using four-color FC immunophenotyping, by qualitative assessment of population clusters, and by quantitation with comparison with isotype controls. Benign T cells were studied in reactive tissues and in the background of other malignancies. Results: Many T-cell lymphomas were dim or negative for CD26, whereas a few were brightly positive. In the majority of T-cell lymphomas, CD26 expression could potentially help identify aberrant population clusters. T cells in reactive tissue specime-s and tumor-infiltrating T cells were commonly dim to negative for CD26. Conclusions: Both T-cell lymphomas and reactive T cells in tissue and body fluid specimens often show low levels of CD26 expression. Therefore, quantitative methods may not reliably distinguish benign from neoplastic T cells in these specimens. However, CD26, in combination with other T-cell markers, can be helpful for identifying aberrant population clusters in T-cell lymphomas.