γδ T cells: pleiotropic immune effectors with therapeutic potential in cancer

Abstract

The potential of cancer immunotherapy relies on the mobilization of immune cells capable of producing antitumour cytokines and effectively killing tumour cells. These are major attributes of γδ T cells, a lymphoid lineage that is often underestimated despite its major role in tumour immune surveillance, which has been established in a variety of preclinical cancer models. This situation notwithstanding, in particular instances the tumour microenvironment seemingly mobilizes γδ T cells with immunosuppressive or tumour-promoting functions, thus emphasizing the importance of regulating γδ T cell responses in order to realize their translation into effective cancer immunotherapies. In this Review we outline both seminal work and recent advances in our understanding of how γδ T cells participate in tumour immunity and how their functions are regulated in experimental models of cancer. We also discuss the current strategies aimed at maximizing the therapeutic potential of human γδ T cells, on the eve of their exploration in cancer clinical trials that may position them as key players in cancer immunotherapy.

Fig. 1. Timeline of developments in the research of γδ T cell function in cancer and their exploitation for immunotherapy.

BTN3A1, butyrophilin subfamily 3 member A1; CAR, chimeric antigen receptor; DOT, Delta One ^ T protocol; IL-17, interleukin-17; TEGs, T cells engineered with defined Yδ T cell receptors^–.

Fig. 2. Antitumour γδ T cell functions and their regulation.

γδ T cells directly recognize tumour cells through the T cell receptor (TCR) and natural killer cell receptors (NKRs). Tumour cell killing can be mediated by the expression of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), FAS or the granule exocytosis pathway (leading to the secretion of perforin and granzyme). Moreover, γδ T cells can target tumour cells through antibody-dependent cellular cytotoxicity upon treatment with tumour-specific antibodies. Alternatively, γδ T cells induce antitumour immune responses through interferon-γ (IFNγ) production and through antigen-presenting cell functions that lead to αβ T cell activation, while 4-1BB ligand (4-1BBL) expression stimulates NK cells. In addition, γδ T cells induce antibody class switching in B cells, contributing to a protective humoral response. The antitumour features of γδ T cells are mainly potentiated by interleukin-15 (IL-15) and IL-2, while the expression of programmed cell death protein 1 (PD1), the presence of secreted major histocompatibility complex class I polypeptide related sequence A (sMICA) or treatment with the DNA methylation inhibitor decitabine and histone deacetylase (HDAC) inhibitors dampens their killing capacity. Other immune cell subsets, including regulatory T (T_reg) cells and neutrophils, can also inhibit antitumour γδ T cell features, through the production of either IL-10 and transforming growth factor β (TGFβ) or Arginase-1 and reactive oxygen species (ROS), respectively. DC, dendritic cell; FASL, FAS ligand; FcγRIII, Fcγ receptor III; GM-CSF, granulocyte-macrophage colony-stimulating factor; HLA-DR, human leukocyte antigen-DR; LDL, low-density lipoprotein; LDL-R, LDL receptor; MHC, major histocompatibility complex; NKG2D, natural killer group 2D; sTRAIL, secreted TRAIL; TRAIL-R, TRAIL receptor.

Fig. 3. Pro-tumour γδ T cell functions and their regulation.

The pro-tumour functions of γδ T cells are mainly associated with interleukin-17A (IL-17A; (shortened here to IL-17) production, which has several roles, including stimulation of tumour cell proliferation, induction of angiogenesis and mobilization of pro-inflammatory or immunosuppressive myeloid cells. Commensal bacteria, 27-hydroxycholesterol (27-HC) or IL-17 itself can mobilize myeloid cells, which produce IL-17-promoting cytokines including IL-1β and IL-23. Both IL-1β and IL-6 can induce the expression of nitric oxide synthase 2 (NOS2), which promotes IL-17⁺ γδ T cell responses. IL-7 is another factor involved in the survival and proliferation of IL-17-producing γδ T cells. Other tumour-promoting roles of γδ T cells include the inhibition of dendritic cell (DC) maturation; the suppression of T cell responses through galectin, programmed cell death protein 1 ligand 1 (PDL1), and IL-4 expression; and the induction of tumour-cell proliferation by IL-22 and amphiregulin production. Inhibition of IL-17-producing γδ T cells can be achieved through reactive oxygen species (ROS) generated by neutrophils or by the E3 ubiquitin ligase ITCH, which targets retinoic-acid-receptor-related orphan receptor-γt (RORγt) for degradation. P, phosphorylation; STAT3, signal transducer and activator of transcription 3; TGFβ, transforming growth factor β.

Fig. 4. Current strategies for therapeutic manipulation of human γδ T cells.

Current strategies for the therapeutic use of human γδ T cells involve both Vδ1 and Vδ2 subsets. Vδ1 can be isolated from tissues and expanded in vitro, or from peripheral blood and expanded with the Delta One T (DOT) cell-generating protocol (a 3-week clinical-grade protocol involving T cell receptor (TCR) and cytokine stimulation), which gives rise to Vδ1⁺ T cells expressing the natural killer (NK) cell receptors NKp30 and NKp44 and the ability to target both solid and haematological tumours. Vδ2-based strategies also involve peripheral blood extraction and in vitro activation with phosphoantigens (PAg). Another strategy relies on the generation of T cells engineered with defined γδ TCRs (TEGs), which consists of the cloning and transfer of Vγ9Vδ2 TCRs into αβ T cells. CAR, chimeric antigen receptor; NKG2D, natural killer group 2D; PBL, peripheral blood lymphocyte.