O22 Programmed cell death-1 receptor is a checkpoint in Tbet+ Th17 cells within the tumour microenvironment

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Abstract 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.

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