PD-L1 contributes to cancer progression by creating an immunosuppressive microenvironment. When PD-L1 binds to PD-1 receptors on T-cells, it sends an inhibitory signal that reduces T-cell activity. This leads to decreased immune surveillance and allows cancer cells to proliferate without being attacked by the body's natural defenses. The presence of PD-L1 on cancer cells is often associated with more aggressive forms of cancer and poorer prognosis.