A population of cytotoxic lymphocytes for use in the treatment of a tumor in a subject, wherein the population of cytotoxic lymphocytes is a population of tumor infiltrating lymphocytes (TILs) and obtainable from a method comprising: genetically modifying a cytotoxic lymphocyte previously isolated from a tumor of the subject by introducing into the cytotoxic lymphocyte a nucleic acid encoding a protein that specifically binds to PD-L1, wherein the genetically modified cytotoxic lymphocyte expresses and secretes the protein that specifically binds to PD-L1; expanding the genetically modified cytotoxic lymphocyte to generate the population of genetically modified cytotoxic lymphocytes wherein the genetically modified cytotoxic lymphocytes is a population of genetically modified tumor infiltrating lymphocytes (TILs); wherein the population of genetically modified cytotoxic lymphocytes is for administration to the subject to treat the tumor, wherein the nucleic acid sequence encodes a protein that specifically binds to PD-L1 and wherein said protein that specifically binds to PD-L1 is selected from one of the following proteins: a) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:1, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO:5; and b) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:9, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO: 13 wherein the protein is selected from an antibody, a maxibody and a single-chain antibody (scFv), wherein the single-chain antibody comprises the first and second polypeptides of (a) fused directly or via a linker to one another or the first and second polypeptides of (b) fused directly or via a linker to one another, and wherein the maxibody comprises an scFv fused directly or via a linker to an Fc domain. genetically modifying a cytotoxic lymphocyte previously isolated from a tumor of the subject by introducing into the cytotoxic lymphocyte a nucleic acid encoding a protein that specifically binds to PD-L1, wherein the genetically modified cytotoxic lymphocyte expresses and secretes the protein that specifically binds to PD-L1; expanding the genetically modified cytotoxic lymphocyte to generate the population of genetically modified cytotoxic lymphocytes wherein the genetically modified cytotoxic lymphocytes is a population of genetically modified tumor infiltrating lymphocytes (TILs); wherein the population of genetically modified cytotoxic lymphocytes is for administration to the subject to treat the tumor, wherein the nucleic acid sequence encodes a protein that specifically binds to PD-L1 and wherein said protein that specifically binds to PD-L1 is selected from one of the following proteins: a) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:1, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO:5; and b) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:9, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO: 13 a) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:1, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO:5; and b) a protein wherein the first polypeptide comprises the amino acid sequence set forth in SEQ ID NO:9, and the second polypeptide comprises the amino acid sequence set forth in SEQ ID NO: 13 wherein the protein is selected from an antibody, a maxibody and a single-chain antibody (scFv), wherein the single-chain antibody comprises the first and second polypeptides of (a) fused directly or via a linker to one another or the first and second polypeptides of (b) fused directly or via a linker to one another, and wherein the maxibody comprises an scFv fused directly or via a linker to an Fc domain.