Leukemic T cells are specifically enriched in a unique CD3dimCD7low subpopulation of CD4+ T cells in acute‐type adult T‐cell leukemia

The morphological discrimination of leukemic from non‐leukemic T cells is often difficult in adult T‐cell leukemia (ATL) as ATL cells show morphological diversity, with the exception of typical “flower cells.” Because defects in the expression of CD3 as well as CD7 are common in ATL cells, we applied multi‐color flow cytometry to detect a putative leukemia‐specific cell population in the peripheral blood from ATL patients. CD4+CD14− cells subjected to two‐color analysis based on a CD3 vs CD7 plot clearly demonstrated the presence of a CD3dimCD7low subpopulation in each of nine patients with acute‐type ATL. The majority of sorted cells from this fraction showed a flower cell‐like morphology and carried a high proviral load for the human T‐cell leukemia virus type 1 (HTLV‐I). Genomic integration site analysis (inverse long‐range PCR) and analysis of the T cell receptor Vβ repertoire by flow cytometry indicated that the majority of leukemia cells were included in the CD3dimCD7low subpopulation. These results suggest that leukemic T cells are specifically enriched in a unique CD3dimCD7low subpopulation of CD4+ T cells in acute‐type ATL. (Cancer Sci 2011; 102: 569–577)