Local immune checkpoint blockade therapy by an adenovirus encoding a novel PD-L1 inhibitory peptide inhibits the growth of colon carcinoma in immunocompetent mice

•A novel inhibitory peptide interfering with the PD-L1/PD-1 immune checkpoint pathway, dubbed PD-L1ip3, was designed.•The affinity of PD-L1ip3 for PD-L1 was only a few times weaker than that of its natural ligand, PD-1.•Direct treatment with PD-L1ip3 enhanced the ability of CD8+ T cells primed with cancer antigens to kill cancer cells in culture.•A combination treatment including transduction into cancer cells of a gene encoding PD-L1ip3 coupled with direct administration of PD-L1ip3 in peptide form significantly attenuated the growth of murine colon carcinoma in mice. A novel peptide that interferes with the PD-1/PD-L1 immune checkpoint pathway, termed PD-L1 inhibitory peptide 3 (PD-L1ip3), was computationally designed, experimentally validated for its specific binding to PD-L1, and evaluated for its antitumor effects in cell culture and in a mouse colon carcinoma syngeneic murine model. In several cell culture studies, direct treatment with PD-L1ip3, but not a similar peptide with a scrambled sequence, substantially increased death of CT26 colon carcinoma cells when co-cultured with murine CD8+ T cells primed by CT26 cell antigens. In a syngeneic mouse tumor model, the growth of CT26 tumor cells transduced with the PD-L1ip3 gene by an adenovirus vector was significantly slower than that of un-transduced CT26 cells in immunocompetent mice. This tumor growth attenuation was further enhanced by the coadministration of the peptide form of PD-L1ip3 (10 mg/kg/day). The current study suggests that this peptide can stimulate host antitumor immunity via blockade of the PD-1/PD-L1 pathway, thereby increasing CD8+ T cell-induced death of colon carcinoma cells. The tumor site-specific inhibition of PD-L1 by an adenovirus carrying the PD-L1ip3 gene, together with direct peptide treatment, may be used as a local immune checkpoint blockade therapy to inhibit colon carcinoma growth.