The metabolism of mevalonate as a signaling pathway influencing tumor-host interactions and disease progression in chronic lymphocytic leukemia (CLL)

Background. The mevalonate (Mev) pathway is essential for the post-translational modification of proteins involved in cell growth and differentiation. It also generates intermediate metabolites that are specifically recognized by the Vd2 gd T-cell subset. Vd2 T cells play an important role in immune responses against tumor cells. They represent less than 5% of peripheral blood lymphocytes but can be efficiently expanded in vitro upon exposure to Zoledronic acid (Zol). Aims. The aim of this work was to evaluate whether the Mev pathway might be regarded as a signaling pathway influencing tumor-host interactions in CLL. Methods. Sixtyseven previously untreated CLL patients were evaluated for in vitro Vd2 T cells expansion upon stimulation with Zol + interleukin-2 (IL-2). The IgVH mutational status was analyzed by cDNA PCR amplification and sequencing. The level of activity of the Mev pathway was determined by 1) the analysis of gene expression profile (GEP) data 2) the biochemical quantification of the intermediate metabolite Isopentenyl piro-phosphate (IPP). gd T cells subset distribution and natural killer receptors (NKRs) profile were evaluated by multicolor flowcytometry. Results. Based on the in vitro Vd2 proliferative response to Zol+IL2, 60% of CLL patients were defined as responders (R) and 30% as non-responsders (NR). Interestingly, our data have shown a strict and significant correlation among the Vdelta2 proliferative response to Zol + IL-2 and the IgVH mutational status of the leukemic cell. Indeed, 100% of R patients were mutated (M), whereas 70% of NR patients were unmutated (UM). To gain further insight into this association, we evaluated the activity of the Mev pathway in tumor cells of M and UM patients. Both the GEP data and the biochemical analysis have shown a significantly higher activity of the Mev pathway in UM CLL cells. The higher amount of IPP produced by UM CLL cells most likely determines a chronic stimulation of Vd2 T cells in vivo. Indeed, a predominance of effector memory and terminally differentiated effector memory (TEMRA) Vd2 T cells expressing the inhibitory NKR ILT2 was observed in NR patients. The expression of ILT2 on Vd2 T cells was paralleled by high level of expression of the corresponding ligand HLA-G on CLL cells. HLA-G expression is associated with an unfavorable outcome in CLL. The association of R/NR status with biological markers of disease aggressiveness (i.e., IgVH mutational status and HLA-G expression) prompted us to determine whether R/NR status might associate to other features of disease aggressiveness and to predict tumor progression in CLL patients. Results from this analysis showed a significantly higher frequency of poor-risk cytogenetic abnormalities and a significantly shorter time to first treatment in NR as compared to R patients. Conclusions. Based on these data, we propose that the high rate of activity of the Mev pathway in UM CLL cells might determine a chronic stimulation of Vd2 T cells thus leading to their in vivo differentiation into functionally impaired TEMRA. Therefore, we have identified a novel mechanism of immune escape which is at least partially contributing to disease progression in UM CLL.