Genetically modified non-human animal with human or chimeric CD137

1. A genetically-modified mouse whose genome comprises at least one chromosome comprising, at an endogenous CD137 gene locus, a replacement of a nucleic acid sequence encoding a portion of the extracellular region of an endogenous CD137 protein with a nucleic acid sequence encoding a portion of the extracellular region of a human CD137 protein, wherein the mouse detectably expresses a chimeric CD137 protein comprising a humanized CD137 extracellular region and an endogenous CD137 cytoplasmic region on the surface of activated T cells, wherein the sequence encoding the chimeric CD137 protein is operably linked to an endogenous regulatory element at the endogenous CD137 gene locus in the at least one chromosome, wherein the expressed chimeric CD137 protein can be recognized by an anti-human CD137 antibody, and wherein the anti-human CD137 antibody can increase immune response in the mouse, wherein the chimeric CD137 protein comprises SEQ ID NO: 24.

2. The mouse of claim 1 , wherein the mouse does not express endogenous CD137.

3. The mouse of claim 1 , wherein the mouse further comprises a sequence encoding an additional human or chimeric protein.

4. The mouse of claim 3 , wherein the additional human or chimeric protein is programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), Lymphocyte Activating 3 (LAG-3), B And T Lymphocyte Associated (BTLA), Programmed Cell Death 1 Ligand 1 (PD-L1), CD27, CD28, CD47, T-Cell Immunoreceptor With Ig And ITIM Domains (TIGIT), T-cell Immunoglobulin and Mucin-Domain Containing-3 (TIM-3), Glucocorticoid-Induced TNFR-Related Protein (GITR), or TNF Receptor Superfamily Member 4 (0X40).

5. The genetically-modified, mouse of claim 1 , wherein the mouse has a tumor, and the anti-human CD137 antibody inhibits tumor growth in the mammal.

6. A method of determining effectiveness of an anti-CD137 antibody for treating cancer, comprising:

administering the anti-CD137 antibody to the mouse of claim 1 , wherein the mouse has a tumor; and

determining inhibitory effects of the anti-CD137 antibody to the tumor.

7. The method of claim 6 , wherein the tumor comprises one or more human tumor cells that are injected into the mouse.

8. A genetically-modified, non-human mammal that is made by a method comprising:

(1) providing a plasmid comprising a human CD137 gene fragment, flanked by a 5′ homology arm and a 3′ homology arm, wherein the 5′ and 3′ homology arms target the endogenous CD137 gene, wherein the human CD137 gene fragment comprises SEQ ID NO: 21;

(2) modifying the genome of a fertilized egg or an embryonic stem cell by contacting the genome of the fertilized egg or the embryonic stem cell with the plasmid of step (1);

(3) transplanting the fertilized egg obtained in step (2) into the oviduct of a pseudo-pregnant female non-human mammal or transplanting the embryonic stem cell obtained in step (2) into a blastocyst, which is then transplanted into the oviduct of a pseudo-pregnant female non-human to produce a genetically-modified non-human that functionally expresses a chimeric CD137 protein comprising a humanized CD137 extracellular region and an endogenous CD137 cytoplasmic region on the surface of activated T cells, wherein the sequence encoding the chimeric CD137 protein is operably linked to an endogenous regulatory element at the endogenous CD137 gene locus; and

(4) mating the mammal obtained in step (3) to obtain a homozygote.

9. The genetically-modified, non-human mammal of claim 8 , wherein the 5′ homology arm comprises SEQ ID NO: 19.

10. The genetically-modified, non-human mammal of claim 8 , wherein the 3′ homology arm comprises SEQ ID NO: 20.

11. The genetically-modified, non-human mammal of claim 8 , wherein the mammal is a mouse.