We found that tumors from PPA1-DOX-treated mice showed obviously increased infiltration of both CD4+ and CD8+ T cells, as compared with RNA-DOX-, DOX-, or 5-FU-treated mice, respectively (Figure 5D), suggesting that PPA1-DOX can enhance immune response in colon cancer due to disruption of the inhibitory PD-1/PD-L1 immune checkpoint by the PD-L1 targeted polypeptide PPA1. Here, CD4 is linked to malignant colon neoplasm.