Antitumor effects of the silencing of programmed cell death ligand 1 in colorectal cancer via immunoregulation

Activation of programmed cell death 1 (PD-1)/PD-ligand 1 (PD-L1) can promote immune suppression of the tumor microenvironment. However, the effects and mechanisms of PD-L1 silencing on colorectal cancer growth are largely unknown. In the present study, PD-L1 expression was compared in colorectal cancer and paracancerous tissues by immunofluorescence. A stable colorectal carcinoma cell line encoding PD-L1 short hairpin RNA (shRNA) was established. Thereafter, inoculated tumors were modeled in C57B/L6 mice. Experiments were divided into 3 groups: Control group, vector group, and PD-L1 silencing group (inoculated with the stable CT26 cell line encoding PD-L1 shRNA). Following decapitation of the mice, tumors were weighed and apoptosis of tumor cells was detected. The number and viability of cluster of differentiation (CD)4(+) and CD8(+) T cells were analyzed by flow cytometry and a cell counting kit assay, respectively. Compared with paracancerous tissue, colorectal cancer tissue extensively expressed PD-L1, RAC-α serine/threonine-protein kinase (AKT), and phosphatidylinositol 3-kinase (PI3K). Lymphocyte-activating gene 3 (LAG-3) expression was observed at the edge of tumor tissue, but rarely observed in paracancerous tissue. A stable CT26 cell line encoding PD-L1 shRNA was established, and lack of PD-L1 expression was confirmed by reverse transcription-polymerase chain reaction and western blotting. Compared with the control, the shPD-L1 group demonstrated reduced tumor growth, a high level of apoptosis in tumor cells, a low level of PI3K and AKT expression, and an increased number of cells and greater activity of CD4(+) T and CD8(+) T cells. Taken together, PD-L1 silencing promoted tumor cell apoptosis, at least in part, through the activation of CD4(+) and CD8(+) T cells.