Comprehensive insights into the effects and regulatory mechanisms of immune cells expressing programmed death-1/programmed death ligand 1 in solid tumors

The programmed cell death-1 (PD-1)/programmed cell death ligand 1 (PD-L1) signaling pathway is an important mechanism in tumor immune escape, and expression of PD-L1 on tumor cells has been reported more frequently. However, accumulating evidence suggests that PD-1/PD-L1 is also widely expressed on immune cells, and that regulation is also critical for tumor immune responses. In this review, we emphasized that under solid tumor conditions, the immunoregulatory effects of immune cells expressing PD-1 or PD-L1, affected the prognoses of cancer patients. Therefore, a better understanding of the mechanisms that regulate PD-1 or PD-L1 expression on immune cells would provide clear insights into the increased efficacy of anti-PD antibodies and the development of novel tumor immunotherapy strategies.