γδ T Cells for Leukemia Immunotherapy: New and Expanding Trends

Abstract

Recently, many discoveries have elucidated the cellular and molecular diversity in the leukemic microenvironment and improved our knowledge regarding their complex nature. This has allowed the development of new therapeutic strategies against leukemia. Advances in biotechnology and the current understanding of T cell-engineering have led to new approaches in this fight, thus improving cell-mediated immune response against cancer. However, most of the investigations focus only on conventional cytotoxic cells, while ignoring the potential of unconventional T cells that until now have been little studied. γδ T cells are a unique lymphocyte subpopulation that has an extensive repertoire of tumor sensing and may have new immunotherapeutic applications in a wide range of tumors. The ability to respond regardless of human leukocyte antigen (HLA) expression, the secretion of antitumor mediators and high functional plasticity are hallmarks of γδ T cells, and are ones that make them a promising alternative in the field of cell therapy. Despite this situation, in particular cases, the leukemic microenvironment can adopt strategies to circumvent the antitumor response of these lymphocytes, causing their exhaustion or polarization to a tumor-promoting phenotype. Intervening in this crosstalk can improve their capabilities and clinical applications and can make them key components in new therapeutic antileukemic approaches. In this review, we highlight several characteristics of γδ T cells and their interactions in leukemia. Furthermore, we explore strategies for maximizing their antitumor functions, aiming to illustrate the findings destined for a better mobilization of γδ T cells against the tumor. Finally, we outline our perspectives on their therapeutic applicability and indicate outstanding issues for future basic and clinical leukemia research, in the hope of contributing to the advancement of studies on γδ T cells in cancer immunotherapy.

Keywords: cell transplantation; clinical trials; gamma-delta T cells; leukemic microenvironment; off-the-shelf cell therapy.

Figure 1

Crosstalk between γδ T cells and the leukemic microenvironment. Upon infiltrating the TME, γδ T cells are exposed to several persistent inflammatory and/or suppressive signals. Pathways implicated in crosstalk with the leukemic microenvironment can be classified into three general categories (center and inner circle): (i) cell-to-cell signals including antigen recognition by γδ T cell receptor (TCR), stimulatory or inhibitory molecules and/or tumor-sensing molecules; (ii) soluble factors such as cytokines and chemokines that will drive changes in expression levels of (iii) homing receptors and adhesion molecules. Several stromal and/or immune cells could be the source of many of these changes (outer circle). Among these, tumor-associated macrophages (TAM), myeloid-derived suppressor cells (MDSC), regulatory T (Treg) cells and dendritic cells (DC) retain their reprogramming potential into the TME by regulating inflammation or suppression through Th1, Th2 and Th17 cytokines. In addition, the hematopoietic niche can regulate hypoxia, responsible for supporting leukemic cells (LC) survival. Mesenchymal stromal cells (MSC) and endothelial cells can also express many factors that attract antitumor cells, such as γδ T cells, αβ T cells and NK cells which can exert cytotoxicity or undergo cell exhaustion after infiltrating the leukemic microenvironment. CCL, CC-chemokine ligand; CTLA4, cytotoxic T lymphocyte antigen 4; CXCL, CXC-chemokine ligand; PD1, programmed cell death protein 1.

Figure 2

Antileukemic roles of γδ T cells and their regulation. γδ T cells kill leukemic cells (LC) via direct and indirect mechanisms. When identifying LCs through γδ TCR and co-receptors such as natural killer cell receptors (NKR), they secrete high levels of perforins and granzymes, mediating direct target killing. Additionally, γδ T cells produce interferon (IFN)-γ and tumor necrosis factor (TNF), which can increase MHC class I expression in LCs, and enhance αβ T cell-mediated cytotoxicity. IFN-γ release also allows NK cell activation, which can enhance tumor killing via NKG2D. Alternatively, γδ T cell-derived granulocyte-macrophage colony-stimulating factor (GM-CSF) can induce dendritic cell (DC) maturation, which in turn potentiates antitumor responses via interleukin (IL)-2, IL-12, IL-15 and IL-18. Thus, αβ or γδ T cells and NK cells can be recruited for exerting cytotoxicity in many compartments. Moreover, γδ T cells display APC functions and support αβ T cell and NK cell polarization towards an antitumor phenotype. In contrast, their cytotoxicity can be decreased by regulatory T (Treg) cells and immunosuppressive myeloid cells (IMC), since they produce several inhibitory factors such as IL-10, transforming growth factor β (TGF-β), reactive oxygen species (ROS) and Arginase-1. Finally, PD1-PD1L axis expression can regulate γδ T cell antitumor activities. APC, antigen-presenting cell; FasL, Fas ligand; Fas-R, Fas receptor; PD1, programmed cell death protein 1; PD1-L, PD1 ligand; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand; TRAIL-R, TRAIL receptor.

Figure 3

Translating γδ T cells into clinical strategies against leukemia. γδ T cells exert antitumor responses in different compartments and expressing distinct TCR patterns. Vδ1 and Vδ3 subtypes have been implicated as cytotoxic mediators in bone marrow (A), while Vγ9Vδ2 cells have been shown to respond mainly in peripheral blood (B). However, strategies are directed towards Vδ1 and Vδ2 subtypes, as they are the best known (C). Granulocyte colony-stimulating factor (G-CSF) was shown to be a potential adjuvant to mobilize γδ T cells for peripheral blood and enrich the graft. Additionally, Vδ1 cells can be isolated from UCB or PB and expanded in vitro using some approaches, such as the DOT protocol, already reviewed here. In vivo stimulation with Vδ1 TCR ligands may be a good alternative, but it remains poorly investigated. In parallel, Vδ2 cells can be isolated from PB and activated and/or expanded in vitro using pAgs. A new therapeutic concept consists in the cloning and transfer of γδ TCRs into αβ T cells (TEGs) and can enhance antileukemic responses. The fact is that many of these strategies give rise to γδ T cells that express several recognition receptors and have a higher capacity to target leukemic cells (LC), which can be further improved with chimeric antigen receptor (CAR) transduction. Moreover, the use of therapeutic antibodies (Abs), such as immune checkpoint inhibitors (ICI), anti-CD19 and anti-CD20 Abs, can also provide improved efficiency in potential approaches, since γδ T cells have unique features and an attractive degree of safety for their translation into clinical trials. CRS, cytokine release syndrome; DOT, Delta One T; PB, peripheral blood; IC, immune checkpoint; TEGs, T cells engineered to express a defined γδTCRs; UCB, umbilical cord blood.