Ablation of PD-1 Reduces the Stability and Lipid Metabolism of Regulatory T Cells in the Tumor Microenvironment

Regulatory T cells (T regcells) are responsible for immune homeostasis and highly express PD-1 in the tumor microenvironment (TME). However, the role of PD-1 in tumor-infiltrating (TI) T regcells remains controversial. Indeed, PD-1 expression in T regcells was even higher than in CD8 +T cells and conventional CD4 +T cells. Here, we identified that conditional deletion of PD-1 in T regcells delayed tumor growth by reducing TI T regcell pool and amplifying the functionality of TI CD8 +and CD4 +T cells. In Pdcd1 fl/flFoxp3 eGFP-Cre-ERT2(+/−)mice, in which both PD-1 WTand PD-1 KOT regcells coexisted, TI PD-1 KOT regcells exhibited the impaired proliferative and suppressive capacity compared to TI PD-1 WTT regcells. Additionally, exT regcells, which lost their Foxp3 expression, were more abundant in PD-1 KOT regcells than PD-1 WTT regcells. In TC-1 lung cancer, PD-1 antibody therapy was effective in reducing TI T regcell pool. Single-cell analysis identified that PD-1 signaling promoted various pathways related to lipid metabolism, proliferation, and suppression in TI T regcells. Single-cell TCR sequencing revealed that the clonal expansion of TI T regcells was enriched in PD-1 WTT regcells compared to PD-1 KOT regcells. We also showed that conditional deletion of PD-1 in T regcells reduced lipid uptake and mitochondrial mass of T regcells in TME. These results suggest that PD-1 ablation or blockade can enhance antitumor immunity by exacerbating T regcell stability and metabolic fitness in the TME. This study was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (2017R1A5A1014560, 2019M3A9B6065221 to S-.J.H.; 2018R1A5A2025079, 2019M3A9B6065192 to I.L.). This study was also supported by the Korean Health Technology R&D Project (HV20C0144, HN21C1410 to S-.J.H.) through the Korean Health Industry Development Institute (KHIDI) funded by the Ministry of Health & Welfare. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.