The present disclosure provides proteins, such as antibodies, that include an antigen binding portion that specifically binds to Programmed Death 1 Ligand 1 (PD-L1). Also provided are nucleic acids encoding the proteins, and cells (e.g., genetically modified cytotoxic lymphocytes) that include such nucleic acids. In some embodiments, a subject method includes reducing the interaction between PD-L1 on a first-cell and PD-1 on a second cell. In some cases, the contacting is in vivo. For example, the methods and compositions provided can be used in the treatment of viral infection and cancer, such as the treatment of solid tumors via ACT or via administration of a subject protein that specifically binds to PD-L1.