In vitro killing assay.

For the in vitro killing assay 50,000 MC38-GFP cells were stained with CellTrace Violet (Invitrogen, Thermo Fisher Scientific), mixed with 50,000 MC-38-OVA-GFP cells stained with CellTrace Yellow (Invitrogen, Thermo Fisher Scientific), and incubated for 24 hours at 37°C. Cells were then kept alone or cocultured with enriched CD8⁺ T cells from spleens of either Ptpn2^fl/fl Cd4^Cre– OT I or Ptpn2^fl/fl Cd4^Cre+/– OT I mice for 24 hours in a 37°C incubator. CD8⁺ T cells were enriched using the EasySep CD8⁺ T Cell Enrichment Kit (STEMCELL Technologies). Apoptosis was then detected using the Annexin V Kit – DY-634 Antibody (Abcam) and the Zombie NIR Fixable Viability Kit (BioLegend).