Vγ9Vδ2 T cell-based immunotherapy in hematological malignancies: from bench to bedside

Abstract

Many hematological malignancies consist of tumor cells that are spontaneously recognized and killed by Vγ9Vδ2 T cells. These tumor cells generate high amounts of intracellular phosphorylated metabolites mimicking the natural ligands and display a wide range of stress-induced self-ligands that are recognized by Vγ9Vδ2 T cells via TCR-dependent and TCR-independent mechanisms. The intrinsic features of Vγ9Vδ2 T cells and that of tumor cells of hematological origin constitute an ideal combination from which to develop Vγ9Vδ2 T cell-based immune interventions. In this review, we will discuss the rationale, preclinical and clinical data in favor of this therapeutic strategy and the future perspectives of its development.

Fig. 1

Tumor cell-induced activation of Vγ9Vδ2 T cells. The Mev pathway of tumor cells can generate supra-physiological amounts of phosphorylated Mev pathway metabolites, such as IPP, that mimick the natural p-Ags recognized by Vγ9Vδ2 T cells via their TCR. The Mev pathway is particularly active in tumor cells of hematopoietic origin. Other molecules that are recognized via TCR include ecto-F1-ATPase, a form of the mitochondrial ATP synthase ectopically expressed with ApoA-1 on the cell surface of tumor cells, including those of hematological origin. The ecto-F1-ATPase/Apo-1 complex is probably involved in the presentation of endogenous phosphorylated Mev metabolites, considering its capacity to bind ApppI. ApppI is an IPP-containing metabolite which is naturally produced in Daudi cells and formed in NBP-treated cells when intracellular IPP levels exceed a critical threshold as a consequence of FPPS inhibition. Tumor cells also display a wide range of cell surface proteins that are recognized via TCR-independent mechanisms, including a restricted set of endogenous stress determinants that are recognized via KARs, such as NKG2D, and KIRs. The expression of MHC class I molecules on tumor cells is generally sensed by Vγ9Vδ2 T cells as an inhibitory signal. The net balance between the expression of KARs and KIRs on Vγ9Vδ2 T cells, and the expression of their corresponding ligands on tumor cells, fine-tune the activation threshold and antitumor activity of Vγ9Vδ2 T cells. Another set of molecules that facilitate Vγ9Vδ2 T cell activation is represented by adhesion molecules such as LFA-1, CD6, CD2, and CD226. The interactions of these molecules with the corresponding ligands on tumor cells (ICAM-1, LFA-3, CD166 and others) help to stabilize the immunological synapse and deliver co-stimulatory signals. After productive interaction with tumor cells, fully activated Vγ9Vδ2 T cells proliferate, release cytokines (such as IFN-γ, TNF-α), chemokines (such as MIP-1α, MI-1β), and exert direct and indirect cytotoxic activity againt tumor cells, either alone or in association with other innate and adaptive immune effector cells (such as NK cells, CTL) and molecules (such as ADCC). Considering the immune adjuvant properties of Vγ9Vδ2 T cells, the immune performances of other effector cells are generally improved by their concurrent activation

Fig. 2

Ranking of human tumor cells in function of their intrinsic or inducible susceptibility to Vγ9Vδ2 T cell recognition and killing. At the top of the list are tumor cells of B cell origin, such as B cell lymphomas (NHL) and MM. These cells are the only ones immunogenic enough to fully activate unprimed Vγ9Vδ2 T cells. Once activated by these tumor cells, Vγ9Vδ2 T cells also recognize and kill tumor cells unrelated to the inducing ones. Data obtained with cell lines indicate that an hierarchical order also exist among B cell tumors. The activity rate of the Mev pathway and the reciprocal expression of cell surface ligands for KARs and KIRs of Vγ9Vδ2 T cells may determine their final immunogenicity and immune susceptibility. CML tumor cells display an intermediate susceptibility to Vγ9Vδ2 T cells. ZA-induced IPP accumulation is required to convert them into highly susceptible targets. Most solid tumor cells behave as CML cells, being unable to activate Vγ9Vδ2 T cells unless they are manipulated to maximize Vγ9Vδ2 T cell immune reactivity. However, there are some cell lines derived from solid cancer that never become susceptible to Vγ9Vδ2 T cells irrespective of any manipulation. So far, there are no data available that tumor cells other those of human origin are susceptible to Vγ9Vδ2 T cells, even if derived from B cells and treated with NBPs to increase their intracellular IPP content. This hierarchical rank of susceptibility is not irrevocable and it is possible to scale up the position of certain tumor cells in the list by manipulating the Mev pathway to generate an excess of IPP or modulating the expression of KIRs and KARs and their ligands on the cell surface of Vγ9Vδ2 T cells and tumor cells, respectively

Fig. 3

Therapeutic strategies for Vγ9Vδ2 T cell-based immune interventions. Two main strategies are under clinical investigation for the treatment of patients with hematological malignancies and other cancers: in vivo activation by administration of Vγ9Vδ2 T cells agonists (left) or adoptive transfer of ex vivo activated Vγ9Vδ2 T cells (right). In vivo stimulation of Vγ9Vδ2 T cells can be pursued with s-pAgs such as BrHPP and 2M3B1-PP or NBPs in association with IL-2. Other cytokines and growth factors can be useful, but they are not yet available for clinical studies. Ex vivo expansion of Vγ9Vδ2 T cells is induced with the same compounds. Each approach has its own pros and cons. Pros of in vivo stimulation are low cost, feasibility, and the possibility to exploit the pleiotropic direct and indirect antitumor activities of NBPs. Cons are the microenvironment interference and the immune dysfunctions eventually affecting Vγ9Vδ2 T cells in cancer patients. Pros of ex vivo activation are the possibility to generate large numbers of Vγ9Vδ2 T cells, and to improve their immune performances by appropriate stimulation in a controlled setting. On the other hand, adoptive therapies are expensive, time-consuming, and strictly regulated by GMP requirements. Vγ9Vδ2 T cell-based immunotherapy can be gainfully combined with therapeutic mAbs. mAbs can be administered in vivo or used ex vivo to load activated Vγ9Vδ2 T cells