CD4 and neoplasm: We further found in CD73−/− hosts, anti-4-1BB significantly increased the ratio of T effector cell (CD4+Foxp3-) to Treg (CD4+Foxp3+) cells (Fig. 1g) and induced the higher proliferation of tumor-infiltrating both CD4+ and CD8+ T cells, as indicated by the expression levels of the cell cycle associated protein Ki67 (Fig. 1h, i).