Atezolizumab blocks the interaction of PD-L1 with programmed cell death protein 1 (PD-1) and CD80 receptors (B7-1Rs). PD-L1 can be highly expressed on certain tumors, which is thought to lead to reduced activation of immune cells (cytotoxic T-cells in particular) that might otherwise recognize and attack the cancer.Inhibition of PD-L1 by atezolizumab can remove this inhibitor effect and thereby engender an anti-tumor response. It is one of several ways to block inhibitory signals related to T-cell activation, a more general strategy known as "immune checkpoint inhibition."
For some cancers (notably bladder) the probability of benefit is related to PD-L1 expression, but most cancers with PD-L1 expression still do not respond, and many (about 15%) without PD-L1 expression do respond.
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