Cancer immunotherapy with γδ T cells: many paths ahead of us

Abstract

γδ T cells play uniquely important roles in stress surveillance and immunity for infections and carcinogenesis. Human γδ T cells recognize and kill transformed cells independently of human leukocyte antigen (HLA) restriction, which is an essential feature of conventional αβ T cells. Vγ9Vδ2 γδ T cells, which prevail in the peripheral blood of healthy adults, are activated by microbial or endogenous tumor-derived pyrophosphates by a mechanism dependent on butyrophilin molecules. γδ T cells expressing other T cell receptor variable genes, notably Vδ1, are more abundant in mucosal tissue. In addition to the T cell receptor, γδ T cells usually express activating natural killer (NK) receptors, such as NKp30, NKp44, or NKG2D which binds to stress-inducible surface molecules that are absent on healthy cells but are frequently expressed on malignant cells. Therefore, γδ T cells are endowed with at least two independent recognition systems to sense tumor cells and to initiate anticancer effector mechanisms, including cytokine production and cytotoxicity. In view of their HLA-independent potent antitumor activity, there has been increasing interest in translating the unique potential of γδ T cells into innovative cellular cancer immunotherapies. Here, we discuss recent developments to enhance the efficacy of γδ T cell-based immunotherapy. This includes strategies for in vivo activation and tumor-targeting of γδ T cells, the optimization of in vitro expansion protocols, and the development of gene-modified γδ T cells. It is equally important to consider potential synergisms with other therapeutic strategies, notably checkpoint inhibitors, chemotherapy, or the (local) activation of innate immunity.

Keywords: Adoptive T cell transfer; Antibody constructs; Cytokines; Immunotherapy; Leukemia; Lymphoma; Solid tumors; gamma/delta T cells.

Fig. 1

Role of BTN2A1 and BTN3A1 in the activation of human Vγ9Vδ2 γδ T cells. The butyrophilin members BTN2A1 and BTN3A1 are loosely associated on the surface of target cells. a In the homeostatic “resting” state, the intracellular B30.2 signaling domain does not associate with endogenous (tumor-derived IPP) or exogenous (microbe-derived HMBPP) phosphoantigens (pAgs). However, BTN2A1 binds to germ-line-encoded regions of the Vγ9 chain in the homeostatic state. There is also evidence that the CDR3 region of the TCR δ chain interacts with another currently unidentified ligand. b In infected cells and tumor cells, exogenous (HMBPP) or endogenous (IPP) pAgs bind to the B30.2 domain and thereby induces a conformational change in the BTN2A1–BTN3A1 complex, resulting in TCR-dependent activation of Vγ9Vδ2 T cells. This step may involve other as yet unidentified CDR3 ligands.^, c Agonistic anti-BTN3A antibodies such as clone 20.1 mimic the activity of pAgs by inducing a conformational change in the BTN molecules, leading to γδ T cell activation. The depicted model is based on refs. ^–

Fig. 2

Major receptor–ligand interactions between Vδ2/non-Vδ2 γδ T cells and tumor cells/antigen-presenting cells. Left side: The best characterized ligands for the human Vγ9Vδ2 TCR are phosphoantigens (pAgs), which are recognized in a BTN2A1-/BTN3A1-dependent manner. Other ligands for this TCR include the ectopically expressed DNA repair protein human MutS homologue 2 (hMSH2) and ectopically expressed F1-ATPase in conjunction with apolipoprotein A-I. Tumor antigen–TCR crosslinking bispecific T cell engagers (BiTEs) also activate Vδ2 T cells via the TCR. Therapeutically used antibodies against tumor (associated) antigens can activate γδ T cells via CD16/FcγRIII-dependent ADCC. Right side: Ligands for non-Vδ2 γδ T cells. Some ligands for specific subsets of human non-Vδ2 γδ T cells have been identified: endothelial protein C receptor (EPCR) on CMV-infected and intestinal tumor cells (Vγ4Vδ5), butyrophilin-like molecules BTNL3/BTNL8 on intestinal epithelia (Vγ4/Vδ1 or Vδ3), Annexin A2 (Anx A2) (Vγ8Vδ3), MICA (Vδ1), and lipids bound to CD1d (Vδ1). MICA/B and ULBP molecules are ligands for the activating NKG2D receptor expressed on both Vδ2 and non-Vδ γδ T cells. In addition to NKG2D, other NK receptors (NKp30, Nkp44, NKp46) can be expressed as well

Fig. 3

How to enhance the efficacy of γδ T cell immunotherapy in vitro and in vivo. a Genetic engineering is used to transduce chimeric antigen receptors (CARs) into γδ T cells, to transfect αβ T cells with selected high affinity γδ TCRs (TEGs), or to render γδ T cells drug-resistant (drug-resistant immunotherapy, DRI). In vitro expansion of γδ T cells can be optimized by selected cytokines (e.g., IL-15, TGF-β), specific medium supplements (e.g., vitamin C), or the selection of novel activators (ZOL derivatives, novel pAgs). b Multiple strategies are available to activate γδ T cells in vivo, to target γδ T cells to tumor antigens, to target tumor-intrinsic suppressive pathways, or to increase the local inflammatory milieu