Identification of TRA‐1‐60‐positive cells as a potent refractory population in follicular lymphomas

Despite receiving rituximab‐combined chemotherapy, follicular lymphoma (FL) patients often suffer tumor recurrence and understand that the cause of relapse in FL would thus significantly ameliorate the tumor therapeutics. In the present study, we show that TRA‐1‐60‐expressing cells are a unique population in FL, converge to the conventional stem cell marker Oct3/4 and ALDH1‐positive population, and resist current B‐lymphoma agents. TRA‐1‐60 expression was observed in scattered lymphoma cells in FL tissues only as well as in resting B‐lymphocytes inside germinal centers. Retrospective comparison between recurrent and cognate primary tissues showed that the number of TRA‐1‐60‐positive cells from rituximab, cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone (R‐CHOP)‐treated FL had increased relative to primary tissue, a finding corroborated by assays on different rituximab‐treated FL cell lines, FL‐18 and DOHH2, wherein TRA‐positive cell numbers increased over 10‐fold compared to the untreated sample. Concordantly, scanty TRA‐1‐60‐positive FL‐18 cells implanted s.c. into mice evinced potent tumor‐initiating capacity in vivo, where tumors were 12‐fold larger in volume (P = 0.0021 < 0.005) and 13‐fold heavier in weight (P = 0.0015 < 0.005) compared to those xenografted from TRA‐negative cells. To explain these results, gene expression profiling and qPCR analysis indicated that TRA‐1‐60‐positive cells defined a distinct population from that of TRA‐negative cells, with upregulation of multiple drug transporters and therapeutic resistance genes. Hence, TRA‐1‐60‐expressing cells in FL are considered to be vigorously intractable against conventional therapeutic agents, which may explain its refractory recurrence.