Enhanced Formation and Survival of Regulatory T Cells in CLL.

Background. In healthy individuals it has been shown that regulatory T cells (Treg) differentiate from rapidly proliferating memory T cells. Rapid proliferation in combination with high susceptibility to apoptosis, make the Treg population highly dynamic. Tregs are thought to play an important role in immune evasion by malignancies. High Treg numbers in patients with malignancies are often associated with poor prognosis. An important question to be answered is whether tumor cells can promote the expansion and activation of Tregs. Recently it has been described that chronic lymphocytic leukemia (CLL) patients have increased numbers of Treg, with highest Treg frequencies in progressive patients¹. The mechanism of this expansion however remains unknown. In the present study, we analysed the mechanism behind Treg expansion in CLL.

Results. Neither analysis of the T cell receptor (TCR) repertoire nor CD45 isoform expression of Treg from CLL patients provided evidence for chronic (tumor) antigenic stimulation as a possible cause for Treg expansion in CLL. Furthermore, we found no correlation with CMV serology, which has been demonstrated to influence the CD8⁺ T cell repertoire in CLL. However, in line with recent studies demonstrating that CD70⁺ NHL cells are strong inducers of Treg, we found evidence for increased formation of Treg in CLL via CD70 costimulation. We observed that CD40 ligand activated CLL cells (which might be considered a model for lymph node CLL cells²) strongly induced CD70-dependent formation of Treg. RT-MLPA expression analysis of 34 apoptosis-regulating genes showed that in comparison to other CD4⁺ T cells, Treg of both healthy individuals and CLL patients had a high expression of pro-apoptotic Noxa and a low expression of anti-apoptotic Bcl-2. Strikingly, Bcl-2 levels of Treg in CLL patients were significantly higher than in healthy individuals. Finally, the different apoptotic profile resulted in differences at the functional level, since Treg from CLL patients were more resistant to drug-induced apoptosis than Treg from healthy individuals.

Conclusion. Treg in CLL may accumulate both by increased formation, facilitated by CD27-CD70 interaction in the lymph node proliferation centres, and decreased sensitivity to apoptosis due to a shifted Noxa-Bcl-2 balance. Since high Treg numbers might negatively affect the course of disease, targeting these mechanisms may provide additional therapeutic approaches in CLL.