Genetically modified non-human animal with human or chimeric PD-L1

1. A method for making a genetically-modified mouse, comprising:

replacing in the genome of a fertilized egg or an embryonic stem cell, at an endogenous programmed death-ligand 1 (PD-L1) gene locus, a portion of exon 3 of the endogenous PD-L1 gene with a portion of exon 3 of a human PD-L1 gene, thereby forming a chimeric exon 3, wherein exons 1, 2 and 4-7 of the endogenous PD-L1 gene are not modified, wherein the chimeric exon 3 consists of a portion of endogenous exon 3 of mouse PD-L1 gene and a portion of exon 3 of the human PD-L1 gene; and

transplanting the fertilized egg or the embryonic stem cell into a female mouse to produce a genetically-modified mouse, wherein the genetically-modified mouse detectably expresses a humanized PD-L1 protein on the surface of one or more spleen cells after the genetically-modified mouse is challenged by an anti-CD3 antibody.

2. The method of claim 1 , wherein the mouse is homozygous with respect to the replacement.

3. The method of claim 1 , wherein the human PD-L1 gene fragment encodes amino acids 21-128 of SEQ ID NO: 29.

4. The method of claim 1 , wherein exon 3 at the endogenous PD-L1 gene locus is modified by homologous recombination with a vector comprising a 5′ end homologous arm and a 3′ end homologous arm.

5. The method of claim 4 , wherein the sequence of the 5′ end homologous arm is set forth in SEQ ID NO: 34, and the sequence of the 3′ end homologous arm is set forth in SEQ ID NO: 42.

6. The method of claim 1 , wherein the genome of the genetically-modified mouse comprises at least one chromosome comprising a sequence encoding a humanized PD-L1 at the endogenous PD-L1 gene locus, wherein the humanized PD-L1 comprises SEQ ID NO: 33, wherein the sequence encoding the humanized PD-L1 protein is operably linked to an endogenous regulatory element at the endogenous PD-L1 gene locus in the at least one chromosome.

7. The method of claim 1 , wherein the chimeric exon 3 comprises at least 50 contiguous nucleotides of exon 3 of the human PD-L1 gene.

8. A method for making a genetically-modified mouse, comprising:

(1) providing a plasmid comprising a human PD-L1 gene fragment from exon 3 of the human PD-L1 gene, flanked by a 5′ homology arm and a 3′ homology arm;

(2) providing two small guide RNAs (sgRNAs) that target exon 3 of the endogenous PD-L1 gene;

(3) modifying the genome of a fertilized egg by using the plasmid of step (1), the sgRNAs of step (2), and Cas9; and

(4) transplanting the fertilized egg obtained in step (3) into a female mouse to produce a genetically-modified mouse, wherein the genetically-modified mouse detectably expresses a humanized PD-L1 protein on the surface of one or more spleen cells after the genetically-modified mouse is challenged by an anti-CD3 antibody, wherein a portion of exon 3 of the endogenous PD-L1 gene is replaced with a portion of exon 3 of a human PD-L1 gene, wherein the two sgRNAs target a 5′-terminal targeting site and a 3′-terminal targeting site, respectively, wherein the 5′-terminal targeting site is selected from the group consisting of SEQ ID NOS: 1, 2 and 5, and the 3′-terminal targeting site is selected from the group consisting of SEQ ID NOS: 9, 11, 13 and 16.

9. The method of claim 8 , wherein the mouse does not express endogenous PD-L1.

10. The method of claim 8 , wherein the genome of the genetically-modified mouse comprises at least one chromosome comprising a sequence encoding a humanized PD-L1 at the endogenous PD-L1 gene locus, wherein the humanized PD-L1 comprises SEQ ID NO: 33.

11. The method of claim 10 , wherein the sequence encoding the humanized PD-L1 is operably linked to an endogenous regulatory element at the endogenous PD-L1 gene locus in the at least one chromosome.

12. The method of claim 8 , wherein the 5′-terminal targeting site is SEQ ID NO: 1 and the 3′-terminal targeting site is SEQ ID NO: 11.

13. The method of claim 8 , wherein the mouse has a C57BL/6 background.