Background: Adoptive T cell therapy has been proven to be a promising modality for the treatment of cancer patients in recent years. However, the increased expression of inhibitory receptors could negatively regulate the function and persistence of transferred T cells which mediates T cell anergy, exhaustion, and tumor regression. In this study, we investigated increased cytotoxic activity after the blockade of PD-1 for effective immunotherapy.
Methods: The cytotoxic function of expanded CD8⁺ CTLs and interactions with tumor cells investigated after blocking of PD-1. Ex vivo expanded CD8⁺ CTLs were co-cultured with mismatch repair (MMR) stable or deficient (high microsatellite instability [MSI-H]) EpCAM⁺ tumor cells. The levels of IFN-γ and GrB were detected by enzyme-linked immunosorbent spot assay. Flow cytometry and confocal microscopy were used to assess CD107a mobilization, cytosolic uptake, and cell migration. 
Results: A dramatic increase in PD-1 expression on the surface of CD8⁺ CTLs during ex vivo expansion was observed. PD-1 level was downregulated by approximately 40% after incubation of the CD8⁺ CTLs with monoclonal antibody which enhanced the secretion of IFN-γ, GrB, and CD107a. Additionally, PD-1 blockade enhanced cell migration and cytosolic exchange between CD8⁺ CTLs and MMR deficient (MSI-H) EpCAM⁺PD-L1⁺ tumor cells.
Conclusion: The blockade of PD-1 enhanced the cytotoxic efficacy of CD8⁺ CTLs toward MMR deficient tumor cells. In conclusion, we propose that blocking of PD-1 during the expansion of CD8⁺ CTLs may improve the clinical efficacy of cell-based adoptive immunotherapy.
Keywords: PD-1, CTLs, MSI-H, EpCAM⁺PD-L1⁺, cancer immunotherapy