PDCD1 and neoplasm: To this end, Liu and colleagues demonstrated that interferon-gamma (IFNγ) produced by cytotoxic CD8+ T cells stimulates Kyn release by tumor-repopulating cells (TRCs) and that TRC-derived Kyn is subsequently taken up by CD8+ T cells through TCR-mediated upregulation of solute carrier family 7 member 8 (SLC7A8) and Solute Carrier Family 36 Member 4 (SLC36A4, also known as PAT4), resulting in Kyn-AhR-mediated upregulation of programmed cell death protein 1 (PD-1) (33).