Genetically modified non-human animal with human or chimeric LAG3

1. A genetically-modified rodent whose genome comprises at least one chromosome comprising a sequence encoding a chimeric Lymphocyte-associated gene 3 (LAG3) protein, wherein the sequence comprises a humanized LAG3 exon 2, a human LAG3 exon 3, a human LAG3 exon 4, a human LAG3 exon 5, a human LAG3 exon 6, a humanized LAG3 exon 7, and an endogenous LAG3 exon 8; wherein the rodent has one or more cells expressing the chimeric LAG3 protein, wherein an anti-human LAG3 antibody can bind to the expressed chimeric LAG3 protein and block the interaction of the chimeric LAG3 protein with a LAG3 ligand, thereby increasing immune response, wherein the chimeric LAG3 protein comprises the amino acid sequence of SEQ ID NO: 44.

2. The rodent of claim 1 , wherein the sequence encoding the chimeric LAG3 protein is operably linked to an endogenous regulatory element at the endogenous LAG3 gene locus in the at least one chromosome.

3. The rodent of claim 1 , wherein the rodent is a mouse.

4. The rodent of claim 1 , wherein the rodent does not express an endogenous LAG3 protein.

5. The rodent of claim 1 , wherein the one or more cells comprise activated T cells, natural killer cells, or regulatory T cells (Tregs).

6. A genetically-modified rodent, wherein the genome of the rodent comprises a replacement of a nucleotide sequence comprising a contiguous sequence starting from within exon 2 and ending within exon 7 of an endogenous LAG3 gene with a nucleotide sequence comprising a contiguous sequence starting from within exon 2 and ending within exon 7 of a human LAG3 gene at an endogenous LAG3 gene locus thereby generating a chimeric LAG3 gene, wherein the rodent has one or more cells expressing a chimeric LAG3 protein encoded by the chimeric LAG3 gene, wherein an anti-human LAG3 antibody can bind to the expressed chimeric LAG3 protein and block the interaction of the chimeric LAG3 protein with a LAG3 ligand, thereby increasing immune response, wherein the chimeric LAG3 protein comprises the amino acid sequence of SEQ ID NO: 44.

7. The rodent of claim 6 , wherein the replaced nucleotide sequence comprises a sequence encoding the extracellular region of LAG3.

8. The rodent of claim 6 , wherein the rodent is homozygous with respect to the chimeric LAG3 gene.

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

10. The rodent of claim 9 , wherein the additional human or chimeric protein is programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), B And T Lymphocyte Associated (BTLA), Programmed Cell Death 1 Ligand 1 (PD-L1), CD27, CD28, CD40, CD47, CD137, CD154, 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), Signal regulatory protein a(SIRPα) or TNF Receptor Superfamily Member 4 (0X40).

11. A method of determining effectiveness of an anti-LAG3 antibody for treating cancer, comprising:

administering the anti-LAG3 antibody to the rodent of claim 1 , wherein the rodent has a tumor; and

determining inhibitory effects of the anti-LAG3 antibody to the tumor.

12. The method of claim 11 , wherein the tumor comprises one or more cancer cells that are injected into the rodent.

13. A genetically-modified rodent whose genome comprises at least one chromosome comprising a sequence encoding a chimeric LAG3 protein, wherein the chimeric LAG3 protein comprises the amino acid sequence of SEQ ID NO: 9, wherein the rodent has one or more cells expressing the chimeric LAG3 protein, wherein an anti-human LAG3 antibody can bind to the expressed chimeric LAG3 protein and block the interaction of the chimeric LAG3 protein with a LAG3 ligand, thereby increasing immune response.

14. The rodent of claim 13 , wherein the rodent is a mouse.

15. The rodent of claim 1 , wherein the chimeric LAG3 protein comprises a chimeric LAG3 transmembrane region.