D. 21 days post-implantation, proportions of FOXP3+ Tregs in the CD4+ compartment of all MC38 tumor-bearing mice in each experiment were found by flow cytometry. Suspensions of cells obtained from spleen, tumor-draining lymph nodes, and tumor-infiltrating leukocytes (TILs) were stained for surface CD4 and intracellular FOXP3 prior to analysis by flow cytometry. 