Enhanced formation and survival of CD4+CD25hi Foxp3+T-cells in chronic lymphocytic leukemia

Recently, it has been described that patients with chronic lymphocytic leukemia (CLL) have increased numbers of regulatory T (Treg) cells. In the present study, we analysed the mechanism behind Treg cells expansion in CLL. Neither analysis of the T-cell receptor repertoire nor CD45 isoform expression of Treg cells from patients with CLL provided evidence for chronic (tumor) antigenic stimulation as a possible cause for Treg cells expansion in CLL. We found evidence however for increased formation of Treg cells via CD70 costimulation, because we observed that CD40 ligand activated CLL cells (which might be considered a model of lymph node CLL cells) strongly induced CD70-dependent formation of Treg cells. Reverse transcription-multiplex ligation-dependent probe amplification assay expression analysis of 34 apoptosis-regulating genes showed that in comparison with other CD4+ T-cells, Treg cells from both healthy individuals (HD) and patients with CLL had a high expression of pro-apoptotic Noxa and a low expression of anti-apoptotic Bcl-2. Strikingly, Bcl-2 levels of Treg cells in patients with CLL were significantly higher than in HD. Finally, the different apoptotic profile resulted in differences at the functional level, because Treg cells from patients with CLL were more resistant to drug-induced apoptosis than Treg cells from HD. In conclusion, Treg cells in CLL may accumulate both by increased formation, facilitated by CD27-CD70 interaction in the lymph node proliferation centres, and decreased sensitivity to apoptosis because of a shifted Noxa-Bcl-2 balance.