Disclosed is the new, useful, and unexpected finding that immunization to placental endothelial cells stimulated with interferon gamma result in antibodies to the checkpoint inhibitor PD-L1. In one embodiment, the invention teaches the use of ValloVax™ to induce immunological hyperresponsiveness and reduction of costimulatory need for T cell activation. In another embodiment the invention teaches means of selecting placental populations for enhanced expression of PD-L1 in order to augment immunity towards checkpoint inhibitors.
