Innate lymphoid cells (ILCs) play a key role in tissue-mediated immunity and can be controlled by coreceptor signaling. Here, we define a subset of ILCs that are Tbet + NK1.1 − and are present within the tumor microenvironment (TME). We show programmed death-1 receptor (PD-1) expression on ILCs within TME is found in Tbet + NK1.1 − ILCs. PD-1 significantly controlled the proliferation and function of Tbet + NK1.1 − ILCs in multiple murine and human tumors. We found tumor-derived lactate enhanced PD-1 expression on Tbet + NK1.1 − ILCs within the TME, which resulted in dampened the mammalian target of rapamycin (mTOR) signaling along with increased fatty acid uptake. In line with these metabolic changes, PD-1-deficient Tbet + NK1.1 − ILCs expressed significantly increased IFNγ and granzyme B and K. Furthermore, PD-1-deficient Tbet + NK1.1 − ILCs contributed toward diminished tumor growth in an experimental murine model of melanoma. These data demonstrate that PD-1 can regulate antitumor responses of Tbet + NK1.1 − ILCs within the TME.

Programmed cell death-1 (PD-1) receptor is critical for immune regulation and is expressed on a variety of immune cells, including effector T cells. In tumours, PD-1 engagement dampens immune activation and increases tumour progression. However, it is unclear how PD-1 affects T helper (Th) cell differentiation, namely Th17 cells. Previously, we have demonstrated that PD-1 maintains T regulatory (Treg) cell plasticity, and Tbet+ Th1 cell plasticity to Treg cells in inflammation and cancer. Therefore, it is clear that PD-1 is indispensable in T-cell lineage maintenance, with limited understanding on its role in Th17 differentiation, function and lineage stability. We hypothesized that as PD-1 blockade augments Th17 responses in cancer, PD-1 could control Th17 function and lineage stability within the tumour microenvironment. We established a B16 F10 melanoma model in wild-type (WT) and Pdcd1–/– (PD-1) mice, and isolated Thy1+ tumour-infiltrating lymphocytes (TILs) from tumours. Single-cell analysis was conducted, and protein and gene expression analysed within the RORc cluster. Our results demonstrated that WT and PD1–/– mice possessed RORc-expressing cells within the TILs, with PD1–/– mice showing an increased number of Th17+ TILs, revealing a role for PD-1 in regulating Th17 responses within TILs. We next investigated the RORc cluster for function and found expression of Il17a and Ifng. Of note, we found that Th17 differentiation genes [Dusp2 (P = 0.004); Ddx58 (P = 0.009)] were significantly regulated by PD-1. Although an increase in interleukin (IL)-12RB1 (P = 0.001) and IL-17RB protein (P = 0.004) was noted, suggesting a plasticity towards either Th1 or Th2 phenotypes, a significant downregulation of Tbet (Tbx21, P = 0.003) was observed in the PD1–/– population. These data suggest that PD-1 is required for RORc+ cells to acquire functional plasticity within tumours, thereby highlighting a role for PD-1 in Th17 plasticity. In summary, our study is the first to show the importance of PD-1 in driving Tbet+ Th17 cell differentiation within the TME.