To determine the role of PD-1 on tumor-infiltrating Treg cells, in the regulation of the CD8+ T cell response, we compared the suppressive activity of Treg expressing high- and low-levels of PD-1 (PD-1hi Treg cells from lung TM 3 weeks after TC-1 injection vs. PD-1lo Treg cells from the spleen of the same TM-bearing mice). Here, PDCD1 is linked to neoplasm.