Microenvironment-mediated immune escape and drug resistance in B-cell lymphoproliferative disorders
Multiple Myeloma (MM) and Chronic Lymphocytic Leukemia (CLL) are paradigm diseases in which tumor cells manipulate the local microenvironment to proliferate, to resist chemotherapy, and to escape immune cell recognition. Other similarities are that both diseases are preceded by a premalignant phase, named monoclonal gammopathy of undetermined significance (MGUS) in MM and monoclonal B-cell lymphocytosis (MBL) in CLL, and that myeloma and CLL cells can be spontaneously recognized and killed in vitro by Vg9Vd2 T cells. The latter are a unique subset of non-conventional T cells with a natural inclination to react against malignant B cells via the TCR-dependent recognition of phosphoantigens (pAgs) generated in the mevalonate (Mev) pathway [such as isopentenyl-pyrophosphate (IPP)], and via the the NKR-dependent recognition of stress-induced self-ligands. It is possible to boost IPP production and increase antitumor Vg9Vd2 T-cell activation by targeting the farnesyl pyrophosphate synthase in the Mev pathway of tumor cells or dendritic cells (DC) with zoledronic acid (ZA). However, bone marrow (BM) Vg9Vd2 T cells of MM patients and peripheral blood (PB) Vg9Vd2 T cells of CLL patients constantly fail to proliferate to ZA stimulation even when challenged with ZA-treated DC. Thus, Vg9Vd2 T cells that have tightly been exposed to tumor cells are impaired in their TCR-dependent pAg recognition. Interestingly, the microenvironment of MM patients is inhabited by tumor cells and bystander cells (e.g. stromal cells) with an extremely accelerated Mev pathway activity leading to the over-production of Mev metabolites and high concentrations of extra-cellular IPP. The long-term exposure of Vg9Vd2 T cells to supra-physiological levels of IPP in the absence of appropriate costimulation may contribute to the chronic exhaustion and pAg anergy of Vg9Vd2 T cells rather than trigger and sustain their antitumor activity. This microenvironment-mediated immune dysfunction of Vg9Vd2 T cells undermines the possibility to fully exploit their natural anti-B-cell activity and may explain why Vg9Vd2 T-cell-based immune interventions have so far fallen short of clinical expectations.
An accelerated Mev activity is also a metabolic signature of multidrug resistance (MDR), a major challenge to durable tumor cell eradication by chemotherapy. The membrane transporter P-glycoprotein (Pgp) plays a key role in MDR and several data indicate that Pgp activity in myeloma and CLL cells is fostered by an accelerated Mev activity.
Aims: 1) To unravel some of the mechanisms through which tumor cells in cooperation with the local microenvironment invalidate the immune competence of BM Vg9Vd2 T cells with special regard to the over-production of IPP, the over-expression of inhibitory pathways (e.g. PD-1/PD-L1 and BLTA/HVEM), and the recruitment of suppressor cells [e.g. T-regulatory (Treg) cells and myeloid-derived suppressor cells (MDSC)] in the BM of MM patients;
2) To assess the activity of the Mev pathway in primary myeloma cells and myeloma cell lines and correlate the chemoresistance to proteasome inhibitors such as bortezomib and carfilzomib in the presence of pro-survival bystander cells and Mev pathway inhibitors such as ZA and statins.