CD4(+)CADM1(+) cell percentage predicts disease progression in HTLV‐1 carriers and indolent adult T‐cell leukemia/lymphoma

We recently took advantage of the universal expression of cell adhesion molecule 1 (CADM1) by CD4(+) cells infected with HTLV‐1 and the downregulation of CD7 expression that corresponds with the oncogenic stage of HTLV‐1‐infected cells to develop a flow cytometric system using CADM1 versus CD7 plotting of CD4(+) cells. We risk‐stratified HTLV‐1 asymptomatic carriers (AC) and indolent adult T‐cell leukemia/lymphoma (ATL) cases based on the CADM1(+) percentage, in which HTLV‐1‐infected clones are efficiently enriched. AC and indolent ATL cases were initially classified according to their CADM1(+) cell percentage. Follow‐up clinical and flow cytometric data were obtained for 71 cases. In G1 (CADM1(+) ≤ 10%) and G2 (10% < CADM1(+) ≤ 25%) cases, no apparent clinical disease progression was observed. In G3 (25% < CADM1(+) ≤ 50%) cases, five out of nine (55.5%) cases progressed from AC to smoldering‐type ATL. In G4 (50% < CADM1(+)) cases, the cumulative incidence of receiving systemic chemotherapy at 3 years was 28.4%. Our results indicate that the percentage of the CD4(+)CADM1(+) population predicts clinical disease progression: G1 and G2 cases, including AC cases, are stable and considered to be at low risk; G3 cases, including advanced AC cases and smoldering‐type ATL cases based on the Shimoyama criteria, are considered to have intermediate risk; and G4 cases, which are mainly indolent ATL cases, are unstable and at high risk of acute transformation.