PDCD1 and infection: Results of previous studies also demonstrated that infection with oncolytic viruses led to the upregulation of PD-1 in tumor cells and tumor-infiltrating immune cells, and the combination therapy of intratumorally administered oncolytic viruses and systemic PD-1 or PD-L1 blockade resulted in a marked enhancement of the anti-tumor immune effect, leading to rejection of the oncolytic virus-treated and distant, non-infected tumors [25,38].