Patent Publication Number: US-10314296-B2

Title: Genetically modified major histocompatibility complex mice

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/830,464, filed Aug. 19, 2015, which application claims benefit under 35 U.S.C. § 119(e) of U.S. provisional patent application Ser. No. 62/039,333, filed Aug. 19, 2014 and is a continuation-in-part application of U.S. application Ser. No. 14/185,316, filed Feb. 20, 2014, which claims the benefit under 35 § 119(e) of U.S. Provisional Patent Application Ser. No. 61/767,811, filed Feb. 22, 2013, all of which are hereby incorporated by reference in their entirety. 
    
    
     SEQUENCE LISTING 
     A Sequence Listing in the form of a text file entitled, “2015-08-19-T0036US01-SEQ-LIST_ST25,” created on Aug. 19, 2015, (size 9.20 Kb) is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a genetically modified non-human animal, e.g., a rodent (e.g., a mouse or a rat), that expresses a human or humanized Major Histocompatibility Complex (MHC) class I and a human or humanized MHC class II molecules. The invention also relates to a genetically modified non-human animal, e.g., a mouse or a rat, that expresses a human or humanized MHC I protein (e.g., MHC I α chain) and a human or humanized MHC II protein (e.g., MHC II α and MHC II β chains), and further expresses a human or humanized β2 microglobulin; as well as embryos, tissues, and cells expressing the same. The invention further provides methods for making a genetically modified non-human animal that expresses both human or humanized MHC class I and class II proteins, and/or β2 microglobulin. Also provided are methods for identifying and evaluating peptides in the context of a humanized cellular immune system in vitro or in a genetically modified non-human animal, and methods of modifying an MHC locus of a non-human animal, e.g., a mouse or a rat, to express a human or humanized MHC I and a human or humanized MHC II proteins. 
     BACKGROUND OF THE INVENTION 
     In the adaptive immune response, foreign antigens are recognized by receptor molecules on B lymphocytes (e.g., immunoglobulins) and T lymphocytes (e.g., T cell receptor or TCR). These foreign antigens are presented on the surface of cells as peptide fragments by specialized proteins, generically referred to as major histocompatibility complex (MHC) molecules. MHC molecules are encoded by multiple loci that are found as a linked cluster of genes that spans about 4 Mb. In mice, the MHC genes are found on chromosome 17, and for historical reasons are referred to as the histocompatibility 2 (H-2) genes. In humans, the genes are found on chromosome 6 and are called human leukocyte antigen (HLA) genes. The loci in mice and humans are polygenic; they include three highly polymorphic classes of MHC genes (class I, II and III) that exhibit similar organization in human and murine genomes (see  FIG. 2  and  FIG. 3 , respectively). 
     MHC loci exhibit the highest polymorphism in the genome; some genes are represented by &gt;300 alleles (e.g., human HLA-DRβ and human HLA-B). All class I and II MHC genes can present peptide fragments, but each gene expresses a protein with different binding characteristics, reflecting polymorphisms and allelic variants. Any given individual has a unique range of peptide fragments that can be presented on the cell surface to B and T cells in the course of an immune response. 
     Both humans and mice have class I MHC genes (see  FIG. 2  and  FIG. 3 , respectively). In humans, the classical class I genes are termed HLA-A, HLA-B and HLA-C, whereas in mice they are H-2K, H-2D and H-2L. Class I molecules consist of two chains: a polymorphic α-chain (sometimes referred to as heavy chain) and a smaller chain called β2-microglobulin (also known as light chain), which is generally not polymorphic ( FIG. 1 , left). These two chains form a non-covalent heterodimer on the cell surface. The α-chain contains three domains (α1, α2 and α3). Exon 1 of the α-chain gene encodes the leader sequence, exons 2 and 3 encode the α1 and α2 domains, exon 4 encodes the α3 domain, exon 5 encodes the transmembrane domain, and exons 6 and 7 encode the cytoplasmic tail. The α-chain forms a peptide-binding cleft involving the α1 and α2 domains (which resemble Ig-like domains) followed by the α3 domain, which is similar to β2-microglobulin. 
     β2 microglobulin is a non-glycosylated 12 kDa protein; one of its functions is to stabilize the MHC class I α-chain. Unlike the α-chain, the β2 microglobulin does not span the membrane. The human β2 microglobulin locus is on chromosome 15, while the mouse locus is on chromosome 2. The β2 microglobulin gene consists of 4 exons and 3 introns. Circulating forms of β2 microglobulin are present in serum, urine, and other body fluids; non-covalently MHC I-associated β2 microglobulin can be exchanged with circulating β2 microglobulin under physiological conditions. 
     Class I MHC molecules are expressed on all nucleated cells, including tumor cells. They are expressed specifically on T and B lymphocytes, macrophages, dendritic cells and neutrophils, among other cells, and function to display peptide fragments (typically 8-10 amino acids in length) on the surface to CD8+ cytotoxic T lymphocytes (CTLs). CTLs are specialized to kill any cell that bears an MHC I-bound peptide recognized by its own membrane-bound TCR. When a cell displays peptides derived from cellular proteins not normally present (e.g., of viral, tumor, or other non-self origin), such peptides are recognized by CTLs, which become activated and kill the cell displaying the peptide. 
     Both humans and mice also have class II MHC genes (see  FIGS. 2 and 3 , respectively). In humans, the classical MHC II genes are termed HLA-DP, HLA-DQ, and HLA-DR, whereas in mice they are H-2A and H-2E (often abbreviated as I-A and I-E, respectively). Additional proteins encoded by genes in the MHC II locus, HLA-DM and HLA-DO in humans, and H-2M and H-2O in mice, are not found on the cell surface, but reside in the endocytic compartment and ensure proper loading of MHC II molecules with peptides. Class II molecules consist of two polypeptide chains: α chain and β chain. The extracellular portion of the α chain contains two extracellular domains, α1 and α2; and the extracellular portion of the β chain also contains two extracellular domains, β1 and β2 (see  FIG. 1 , right). The α and the β chains are non-covalently associated with each other. 
     MHC class II molecules are expressed on antigen-presenting cells (APCs), e.g., B cells, macrophages, dendritic cells, endothelial cells during a course of inflammation, etc. MHC II molecules expressed on the surface of APCs typically present antigens generated in intracellular vesicles to CD4+ T cells. In order to participate in CD4+ T cell engagement, the MHC class II complex with the antigen of interest must be sufficiently stable to survive long enough to engage a CD4+ T cell. When a CD4+ T helper cell is engaged by a foreign peptide/MHC II complex on the surface of APC, the T cell is activated to release cytokines that assist in immune response to the invader. 
     Not all antigens will provoke T cell activation due to tolerance mechanisms. However, in some diseases (e.g., cancer, autoimmune diseases) peptides derived from self-proteins become the target of the cellular component of the immune system, which results in destruction of cells presenting such peptides. There has been significant advancement in recognizing antigens that are clinically significant (e.g., antigens associated with various types of cancer). However, in order to improve identification and selection of peptides that will provoke a suitable response in a human T cell, in particular for peptides of clinically significant antigens, there remains a need for in vivo and in vitro systems that mimic aspects of human immune system. Thus, there is a need for biological systems (e.g., genetically modified non-human animals and cells) that can display components of a human immune system. 
     SUMMARY OF THE INVENTION 
     A biological system for generating or identifying peptides that associate with human MHC class I proteins and chimeras thereof and bind CD8+ T cells, as well as peptides that associate with human MHC class II proteins and chimeras thereof and bind to CD4+ T cells, is provided. Non-human animals comprising non-human cells that express humanized molecules that function in the cellular immune response are provided. Humanized rodent loci that encode humanized MHC I and MHC II proteins are also provided. Humanized rodent cells that express humanized MHC molecules are also provided. In vivo and in vitro systems are provided that comprise humanized rodent cells, wherein the rodent cells express one or more humanized immune system molecules. 
     In various embodiments, provided herein is a non-human animal comprising at an endogenous MHC locus a first nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide, wherein a human portion of the chimeric MHC I polypeptide comprises an extracellular domain of a human MHC I polypeptide; a second nucleotide sequence encoding a chimeric human/non-human MHC II α polypeptide, wherein a human portion of the chimeric human/non-human MHC II α polypeptide comprises an extracellular domain of a human MHC II a polypeptide; and a third nucleotide sequence encoding a chimeric human/non-human MHC II polypeptide, wherein a human portion of the chimeric human/non-human MHC II β polypeptide comprises an extracellular domain of a human MHC II β polypeptide, wherein the non-human animal expresses functional chimeric human/non-human MHC I and MHC II proteins from its endogenous non-human MHC loci. In one embodiment, the animal does not express functional endogenous MHC I, II α, and/or II β polypeptides from the endogenous non-human MHC loci. In some embodiments, the animal is not capable of expressing functional endogenous MHC I, II α, and/or II β polypeptides, since for example, genes encoding endogenous MHC I and MHC II proteins may be functionally inactivated, e.g., are incapable of being transcribed and translated into endogenous MHC I and MHC II proteins, respectively. In various embodiments, all endogenous genes of the non-human animal encoding MHC molecules that are expressed on the cell surface (e.g., present antigen to and associate with T cell receptor) are functionally inactivated such that the animal does not express endogenous cell-surface MHC I, II α, and/or II β polypeptides. In some embodiments, genes may be functionally inactivated with a mutation (e.g., inversion), replacement of the gene (in whole or in part) and/or deletion of the gene (in whole or in part). In some embodiments, a non-human MHC class I gene may be functionally inactivated by the replacement of an endogenous sequence encoding α1, α2, and α3 domains of an endogenous MHC class I polypeptide with a sequence encoding α1, α2, and α3 domains of a human MHC class I polypeptide; a non-human MHC class II α gene may be inactivated by the replacement of an endogenous sequence encoding α1 and α2 domains of an endogenous MHC class II α polypeptide with a sequence encoding α1 and α2 domains of a human MHC class II α polypeptide; or a non-human MHC class II β gene may be inactivated by the replacement of an endogenous sequence encoding β1 and β2 domains of an endogenous MHC class II β polypeptide with a sequence encoding β1 and β2 domains of a human MHC class II β polypeptide. 
     In one aspect, the first nucleotide sequence is located at the endogenous non-human MHC I locus, the second nucleotide sequence is located at the endogenous non-human MHC II α locus, and the third nucleotide sequence is located at the endogenous non-human MHC II β locus. In one aspect, the first, second and/or third nucleotide sequence(s) are operably linked to endogenous non-human regulatory elements. In one aspect, the first nucleotide sequence is operably linked to endogenous non-human MHC I promoter and regulatory elements, the second nucleotide sequence is operably linked to endogenous non-human MHC II α promoter and regulatory elements, and the third nucleotide sequence is operably linked to endogenous non-human MHC β promoter and regulatory elements. 
     In one embodiment, the human portion of a chimeric MHC I polypeptide comprises α1, α2, and α3 domains of the human MHC I polypeptide. In one aspect, a non-human portion of the chimeric MHC I polypeptide comprises transmembrane and cytoplasmic domains of an endogenous non-human MHC I polypeptide. The human MHC I polypeptide may be selected from the group consisting of HLA-A, HLA-B, and HLA-C. In one embodiment, the human MHC I polypeptide is HLA-A2. In another aspect, the human MHC I polypeptide is HLA-A3, HLA-B7, HLA-B27, HLA-Cw6, or any other MHC I molecule expressed in a human population. In an additional embodiment, a non-human animal of the invention further comprises at its endogenous non-human β2 microglobulin locus a nucleotide sequence encoding a human or humanized β2 microglobulin polypeptide, wherein the animal expresses the human or humanized β2 microglobulin polypeptide. 
     In one embodiment, the human MHC II α extracellular domain comprises human MHC II α1 and α2 domains. In another embodiment, the human MHC II β extracellular domain comprises human MHC II β1 and β2 domains. In one aspect, the non-human portion of a chimeric human/non-human MHC II α polypeptide comprises transmembrane and cytoplasmic domains of an endogenous non-human MHC II α polypeptide. In one aspect, the non-human portion of a chimeric human/non-human MHC II β polypeptide comprises transmembrane and cytoplasmic domains of an endogenous non-human MHC II β polypeptide. In one embodiment, the human portions of a chimeric human/mouse MHC II α and β polypeptides are derived from a human HLA class II protein selected from the group consisting of HLA-DR, HLA-DQ, and HLA-DP. In one specific embodiment, the human portions of chimeric human/non-human MHC II α and β polypeptides are derived from a human HLA-DR2 protein. Alternatively, the human portions of chimeric human/non-human MHC II α and β polypeptides may be derived from human MHC II protein selected from HLA-DR4, HLA-DQ2.5, HLA-DQ8, or any other MHC II molecule expressed in a human population. 
     In some aspects, a provided animal comprises two copies of the MHC locus containing the first, the second, and the third nucleotide sequences, while in other aspects, a provided animal comprises one copy of the MHC locus containing the first, the second, and the third nucleotide sequences. Thus, the animal may be homozygous or heterozygous for the MHC locus containing nucleotide sequences encoding chimeric human/non-human MHC I, MHC II α, and MHC II β polypeptides. In some embodiments of the invention, the genetically modified MHC locus, comprising nucleotide sequences encoding chimeric human/non-human MHC I, MHC II α, and MHC II β polypeptides described herein, is in the germline of the non-human animal. 
     Also provided herein is an MHC locus comprising a first nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide, wherein a human portion of the chimeric MHC I polypeptide comprises an extracellular domain of a human MHC I polypeptide; a second nucleotide sequence encoding a chimeric human/non-human MHC II α polypeptide, wherein a human portion of the chimeric human/non-human MHC II α polypeptide comprises an extracellular domain of a human MHC II α polypeptide; and a third nucleotide sequence encoding a chimeric human/non-human MHC II β polypeptide, wherein a human portion of the chimeric human/non-human MHC II β polypeptide comprises an extracellular domain of a human MHC II β polypeptide. In some aspects, non-human portions of the chimeric MHC I, II α, and II β polypeptides comprise transmembrane and cytoplasmic domains of non-human MHC I, II α, and II β, respectively. 
     In one embodiment, the genetically engineered non-human animal is a rodent. In one embodiment, the rodent is a rat or a mouse. In one embodiment, the rodent is a mouse. Thus, in one aspect, the first nucleotide sequence encodes a chimeric human/mouse MHC I polypeptide, and the mouse portion of the chimeric MHC I polypeptide is derived from H-2K, H-2D, or H-2L. In one specific embodiment, the mouse portion of the chimeric MHC I polypeptide is derived from H-2K. In one aspect, the second nucleotide sequence encodes a chimeric human/mouse MHC II α polypeptide, the third nucleotide sequence encodes a chimeric human/mouse MHC II β polypeptide, and the mouse portions of the chimeric MHC II α and β polypeptides are derived from H-2E or H-2A. In yet another embodiment, the mouse does not express any functional endogenous MHC II α and MHC II β polypeptides on a cell surface, and the only MHC II α and MHC II β polypeptides expressed on a cell surface are chimeric human/mouse MHC II α and MHC II β polypeptides. In a specific embodiment, the mouse portions of the chimeric MHC II polypeptides are derived from H-2E. In some embodiments, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In some embodiments, the mouse does not express any endogenous MHC I and II polypeptides on a cell surface; in some embodiments, all endogenous mouse MHC I and II polypeptides that are expressed on a cell surface are functionally inactivated or fully or partially deleted. 
     Thus, also provided herein is a genetically engineered mouse comprising at an endogenous MHC locus a first nucleotide sequence encoding a chimeric human/mouse MHC I polypeptide, wherein the human portion of the chimeric MHC I polypeptide comprises an extracellular domain of a human MHC I polypeptide; a second nucleotide sequence encoding a chimeric human/mouse MHC II α polypeptide, wherein the human portion of the chimeric human/non-human MHC II α polypeptide comprises an extracellular domain of a human MHC II α polypeptide; and a third nucleotide sequence encoding a chimeric human/mouse MHC II β polypeptide, wherein the human portion of the chimeric human/non-human MHC II β polypeptide comprises an extracellular domain of a human MHC II β polypeptide; wherein the mouse expresses functional chimeric human/mouse MHC I and MHC II proteins from its endogenous mouse MHC locus. In one specific embodiment, the first nucleotide sequence encodes a chimeric HLA-A2/H-2K polypeptide, the second nucleotide sequence encodes an a chain of a chimeric HLA-DR/H-2E polypeptide (e.g., HLA-DR2/H-2E polypeptide), and the third nucleotide sequence encodes a β chain of a chimeric HLA-DR/H-2E polypeptide (e.g., HLA-DR2/H-2E polypeptide), and the mouse expresses functional HLA-A2/H-2K and HLA-DR/H-2E (e.g., HLA-DR2/H-2E) proteins. In an additional embodiment, the mouse further comprises at an endogenous β2 microglobulin locus a nucleotide sequence encoding a human or humanized β2 microglobulin polypeptide. In one embodiment, the mouse does not express functional endogenous MHC polypeptides from its endogenous MHC locus. In some embodiments, the mouse does not express functional MHC I or II polypeptides on a cell surface; in some embodiments, the only MHC I and MHC II polypeptides expressed on a cell surface are chimeric human/mouse MHC I and II polypeptides. In some embodiments, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In yet another embodiment, the endogenous H-2D is deleted or functionally inactivated. 
     Also provided herein are methods for generating a genetically modified non-human animal (e.g., rodent, e.g., mouse or rat) described herein. Thus, in one aspect, the invention provides a method of generating a genetically modified non-human animal comprising replacing at an endogenous non-human MHC II locus a nucleotide sequence encoding a non-human MHC II complex with a nucleotide sequence encoding a chimeric human/non-human MHC II complex to generate a first non-human animal; and replacing at an endogenous non-human MHC I locus a nucleotide sequence encoding a non-human MHC I polypeptide with a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide to generate a second non-human animal. In one aspect, the steps of replacing nucleotide sequences comprise homologous recombination in non-human ES cells, and the second non-human animal is generated by homologous recombination in ES cells bearing nucleotide sequences encoding chimeric human/non-human MHC II complex. The chimeric MHC II complex comprises chimeric human/non-human MHC II α and β polypeptides. In some embodiments, ES cells are mouse ES cells engineered to lack all or a portion of an endogenous H-2D locus. In other embodiments, the ES cells are engineered to have a functionally inactivated H-2D locus. In yet another embodiments, the ES cells lack or are engineered to lack or have functional inactivation of all or a portion of endogenous H-2A, H-2E, H-2K, H-2D, and H-2L loci. 
     In an alternative embodiment, the invention provides a method of generating a genetically modified non-human animal comprising replacing at an endogenous non-human MHC I locus a nucleotide sequence encoding a non-human MHC I polypeptide with a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide to generate a first non-human animal; and replacing at an endogenous non-human MHC II locus a nucleotide sequence encoding a non-human MHC II complex with a nucleotide sequence encoding a chimeric human/non-human MHC II complex to generate a second non-human animal. In one aspect, the steps of replacing nucleotide sequences comprise homologous recombination in non-human ES cells, and the second non-human animal is generated by homologous recombination in ES cells bearing a nucleotide sequence encoding chimeric human/non-human MHC I polypeptide. 
     Also provided herein are cells, e.g., isolated antigen-presenting cells, derived from the non-human animals (e.g., rodents, e.g., mice or rats) described herein. Tissues and embryos derived from the non-human animals described herein are also provided. 
     Any of the embodiments and aspects described herein can be used in conjunction with one another, unless otherwise indicated or apparent from the context. Other embodiments will become apparent to those skilled in the art from a review of the ensuing detailed description. The following detailed description includes exemplary representations of various embodiments of the invention, which are not restrictive of the invention as claimed. The accompanying figures constitute a part of this specification and, together with the description, serve only to illustrate embodiments and not to limit the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of the MHC I (left panel) and MHC II (right panel) class molecules expressed on the surface of a cell. The gray circles represent peptides bound in the peptide-binding clefts. 
         FIG. 2  is a schematic representation (not to scale) of the relative genomic structure of the human HLA, showing class I, II and III genes. 
         FIG. 3  is a schematic representation (not to scale) of the relative genomic structure of the mouse MHC, showing class I, II and III genes. 
         FIG. 4  depicts the strategy for generating a humanized MHC locus comprising humanized MHC I and MHC II genes. In the particular embodiment depicted, the MHC locus of the generated mouse comprises chimeric HLA-A2/H-2K and HLA-DR2/H-2E sequences (H2-K +/1666  MHC-II +/6112 ) and lacks H2-D sequence (H2-D +/delete ) and H-2A sequence (the genetic engineering scheme also results in a deletion of H-2A, see Example 1.2). Large Targeting Vectors introduced into ES cells at each stage of humanization are depicted to the right of the arrows. 
         FIG. 5  is a schematic diagram (not to scale) of the targeting strategy used for making a chimeric H-2K locus that expresses an extracellular region of a human HLA-A2 protein. Mouse sequences are represented in black and human sequences are represented in white. L=leader, UTR=untranslated region, TM=transmembrane domain, CYT=cytoplasmic domain, HYG=hygromycin. 
         FIG. 6  is a schematic diagram of an exemplary HLA-DR2/H-2E large targeting vector. 
         FIG. 7  is a schematic representation of exemplary genotypes of MHC loci (**) represents H-2L gene that is not present in all mouse strains), where endogenous mouse H-2K and H-2E loci were replaced by chimeric human/mouse loci, and H-2A and H-2D loci were deleted. 
         FIG. 8  depicts contour plots of percentage of mouse CD3+ T cells and CD19+ B cells in spleens from 3 wild-type (VVT) control mice and 3 chimeric A2 and DR2 mice. Cells were gated on live cells discriminated based on FSC/SSC gating. HLA-A2/H-2K, HLA-DR2/H-2E, H-2A-del, H-2D-del HET*=mice created in the genetic engineering scheme depicted in  FIG. 4 , also referred to in this and remaining figures as “chimeric A2 and DR2 mice.” 
         FIG. 9  depicts histograms of mouse MHC class I (H2D), human MHC class I (HLA-A2), and human MHC class II (HLA-DR) expression levels on CD19+ B cells from spleens of 3 wild-type (VVT) control mice and 3 chimeric A2 and DR2 mice. 
         FIG. 10  depicts histograms of mouse MHC class I (H2D), human MHC class I (HLA-A2), and human MHC class II (HLA-DR) expression levels on CD14+ monocytes from spleens of 3 wild-type (WT) control mice and 3 chimeric A2 and DR2 mice. 
         FIG. 11  depicts histograms of mouse MHC class I (H2D), human MHC class I (HLA-A2), and human MHC class II (HLA-DR) expression levels on CD3+ T cells from spleens of 3 wild-type (VVT) control mice and 3 chimeric A2 and DR2 mice. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Definitions 
     The present invention provides genetically modified non-human animals (e.g., mice, rats, rabbits, etc.) that express both human or humanized MHC I and MHC II proteins; embryos, cells, and tissues comprising the same; methods of making the same; as well as methods of using the same. Unless defined otherwise, all terms and phrases used herein include the meanings that the terms and phrases have attained in the art, unless the contrary is clearly indicated or clearly apparent from the context in which the term or phrase is used. 
     The term “conservative,” when used to describe a conservative amino acid substitution, includes substitution of an amino acid residue by another amino acid residue having a side chain R group with similar chemical properties (e.g., charge or hydrophobicity). Conservative amino acid substitutions may be achieved by modifying a nucleotide sequence so as to introduce a nucleotide change that will encode the conservative substitution. In general, a conservative amino acid substitution will not substantially change the functional properties of interest of a protein, for example, the ability of MHC I or MHC II to present a peptide of interest. Examples of groups of amino acids that have side chains with similar chemical properties include aliphatic side chains such as glycine, alanine, valine, leucine, and isoleucine; aliphatic-hydroxyl side chains such as serine and threonine; amide-containing side chains such as asparagine and glutamine; aromatic side chains such as phenylalanine, tyrosine, and tryptophan; basic side chains such as lysine, arginine, and histidine; acidic side chains such as aspartic acid and glutamic acid; and, sulfur-containing side chains such as cysteine and methionine. Conservative amino acids substitution groups include, for example, valine/leucine/isoleucine, phenylalanine/tyrosine, lysine/arginine, alanine/valine, glutamate/aspartate, and asparagine/glutamine. In some embodiments, a conservative amino acid substitution can be a substitution of any native residue in a protein with alanine, as used in, for example, alanine scanning mutagenesis. In some embodiments, a conservative substitution is made that has a positive value in the PAM250 log-likelihood matrix disclosed in Gonnet et al. ((1992) Exhaustive Matching of the Entire Protein Sequence Database, Science 256:1443-45), hereby incorporated by reference. In some embodiments, the substitution is a moderately conservative substitution wherein the substitution has a nonnegative value in the PAM250 log-likelihood matrix. 
     Thus, also encompassed by the invention is a genetically modified non-human animal whose genome comprises a nucleotide sequence encoding a human or humanized MHC I and II polypeptides, wherein MHC I or MHC II the polypeptide comprises conservative amino acid substitutions in the amino acid sequence described herein. 
     One skilled in the art would understand that in addition to the nucleic acid residues encoding a human or humanized MHC I or MHC II polypeptide described herein, due to the degeneracy of the genetic code, other nucleic acids may encode the polypeptide of the invention. Therefore, in addition to a genetically modified non-human animal that comprises in its genome a nucleotide sequence encoding MHC I and MHC II polypeptides with conservative amino acid substitutions, a non-human animal whose genome comprises a nucleotide sequence that differs from that described herein due to the degeneracy of the genetic code is also provided. 
     The term “identity” when used in connection with sequence includes identity as determined by a number of different algorithms known in the art that can be used to measure nucleotide and/or amino acid sequence identity. In some embodiments described herein, identities are determined using a ClustalW v. 1.83 (slow) alignment employing an open gap penalty of 10.0, an extend gap penalty of 0.1, and using a Gonnet similarity matrix (MacVector™ 10.0.2, MacVector Inc., 2008). The length of the sequences compared with respect to identity of sequences will depend upon the particular sequences. In various embodiments, identity is determined by comparing the sequence of a mature protein from its N-terminal to its C-terminal. In various embodiments when comparing a chimeric human/non-human sequence to a human sequence, the human portion of the chimeric human/non-human sequence (but not the non-human portion) is used in making a comparison for the purpose of ascertaining a level of identity between a human sequence and a human portion of a chimeric human/non-human sequence (e.g., comparing a human ectodomain of a chimeric human/mouse protein to a human ectodomain of a human protein). 
     The terms “homology” or “homologous” in reference to sequences, e.g., nucleotide or amino acid sequences, means two sequences which, upon optimal alignment and comparison, are identical in at least about 75% of nucleotides or amino acids, e.g., at least about 80% of nucleotides or amino acids, e.g., at least about 90-95% nucleotides or amino acids, e.g., greater than 97% nucleotides or amino acids. One skilled in the art would understand that, for optimal gene targeting, the targeting construct should contain arms homologous to endogenous DNA sequences (i.e., “homology arms”); thus, homologous recombination can occur between the targeting construct and the targeted endogenous sequence. 
     The term “operably linked” refers to a juxtaposition wherein the components so described are in a relationship permitting them to function in their intended manner. As such, a nucleic acid sequence encoding a protein may be operably linked to regulatory sequences (e.g., promoter, enhancer, silencer sequence, etc.) so as to retain proper transcriptional regulation. In addition, various portions of the chimeric or humanized protein of the invention may be operably linked to retain proper folding, processing, targeting, expression, and other functional properties of the protein in the cell. Unless stated otherwise, various domains of the chimeric or humanized protein of the invention are operably linked to each other. 
     The term “MHC I complex” or the like, as used herein, includes the complex between the MHC I α chain polypeptide and the β2-microglobulin polypeptide. The term “MHC I polypeptide” or the like, as used herein, includes the MHC I α chain polypeptide alone. The terms “MHC II complex,” “MHC II protein,” or the like, as used herein, include the complex between an MHC II α polypeptide and an MHC II β polypeptide. The term “MHC II α polypeptide” or “MHC II β polypeptide” (or the like), as used herein, includes the MHC II α polypeptide alone or MHC II β polypeptide alone, respectively. Similarly, the terms “HLA-DR4 complex”, “HLA-DR4 protein,” “H-2E complex,” “H-2E” protein,” or the like, refer to complex between α and β polypeptides. Typically, the terms “human MHC” and “HLA” are used interchangeably. 
     The term “replacement” in reference to gene replacement refers to placing exogenous genetic material at an endogenous genetic locus, thereby replacing all or a portion of the endogenous gene with an orthologous or homologous nucleic acid sequence. As demonstrated in the Examples below, nucleic acid sequence of endogenous MHC locus was replaced by a nucleotide sequence comprising sequences encoding a portion of human MHC I polypeptide, specifically, encoding the extracellular portion of the MHC I polypeptide; as well as portions of human MHC II α and β polypeptides, specifically, encoding the extracellular portions of the MHC II α and β polypeptides. 
     “Functional” as used herein, e.g., in reference to a functional polypeptide, refers to a polypeptide that retains at least one biological activity normally associated with the native protein. For example, in some embodiments of the invention, a replacement at an endogenous locus (e.g., replacement at an endogenous non-human MHC locus) results in a locus that fails to express a functional endogenous polypeptide, e.g., MHC I or MHC II polypeptide. Likewise, the term “functional” as used herein in reference to functional extracellular domain of a protein, refers to an extracellular domain that retains its functionality, e.g., in the case of MHC I or MHC II, ability to bind an antigen, ability to bind a T cell co-receptor, etc. In reference to genes, an endogenous gene may be functionally inactivated by various techniques such that the whole gene or a portion of the gene that encodes an endogenous protein is still present at its locus but the gene is incapable of encoding an endogenous protein that is biologically active. In some embodiments of the invention, a replacement at the endogenous MHC locus results in a locus that fails to express an extracellular domain (e.g., a functional extracellular domain) of an endogenous MHC while expressing an extracellular domain (e.g., a functional extracellular domain) of a human MHC. 
     Genetically Modified MHC Animals 
     In various embodiments, the invention generally provides genetically modified non-human animals that comprise in their genome a nucleotide sequence encoding a human or humanized MHC I and MHC II polypeptides; thus, the animals express a human or humanized MHC I and MHC II polypeptides. 
     MHC genes are categorized into three classes: class I, class II, and class III, all of which are encoded either on human chromosome 6 or mouse chromosome 17. A schematic of the relative organization of the human and mouse MHC classes is presented in  FIGS. 2 and 3 , respectively. The MHC genes are among the most polymorphic genes of the mouse and human genomes. MHC polymorphisms are presumed to be important in providing evolutionary advantage; changes in sequence can result in differences in peptide binding that allow for better presentation of pathogens to cytotoxic T cells. 
     MHC class I protein comprises an extracellular domain (which comprises three domains: α 1 , α 2 , and α 3 ), a transmembrane domain, and a cytoplasmic tail. The α 1  and α 2  domains form the peptide-binding cleft, while the α 3  interacts with β2-microglobulin. 
     In addition to its interaction with β2-microglobulin, the α 3  domain interacts with the TCR co-receptor CD8, facilitating antigen-specific activation. Although binding of MHC class I to CD8 is about 100-fold weaker than binding of TCR to MHC class I, CD8 binding enhances the affinity of TCR binding. Wooldridge et al. (2010) MHC Class I Molecules with Superenhanced CD8 Binding Properties Bypass the Requirement for Cognate TCR Recognition and Nonspecifically Activate CTLs, J. Immunol. 184:3357-3366. Interestingly, increasing MHC class I binding to CD8 abrogated antigen specificity in CTL activation. Id. 
     CD8 binding to MHC class I molecules is species-specific; the mouse homolog of CD8, Lyt-2, was shown to bind H-2D d  molecules at the α3 domain, but it did not bind HLA-A molecules. Connolly et al. (1988) The Lyt-2 Molecule Recognizes Residues in the Class I α3 Domain in Allogeneic Cytotoxic T Cell Responses, J. Exp. Med. 168:325-341. Differential binding was presumably due to CDR-like determinants (CDR1- and CDR2-like) on CD8 that was not conserved between humans and mice. Sanders et al. (1991) Mutations in CD8 that Affect Interactions with HLA Class I and Monoclonal Anti-CD8 Antibodies, J. Exp. Med. 174:371-379; Vitiello et al. (1991) Analysis of the HLA-restricted Influenza-specific Cytotoxic T Lymphocyte Response in Transgenic Mice Carrying a Chimeric Human-Mouse Class I Major Histocompatibility Complex, J. Exp. Med. 173:1007-1015; and, Gao et al. (1997) Crystal structure of the complex between human CD8αα and HLA-A2, Nature 387:630-634. It has been reported that CD8 binds HLA-A2 in a conserved region of the α3 domain (at position 223-229). A single substitution (V245A) in HLA-A reduced binding of CD8 to HLA-A, with a concomitant large reduction in T cell-mediated lysis. Salter et al. (1989), Polymorphism in the α3 domain of HLA-A molecules affects binding to CD8, Nature 338:345-348. In general, polymorphism in the α3 domain of HLA-A molecules also affected binding to CD8. Id. In mice, amino acid substitution at residue 227 in H-2D d  affected the binding of mouse Lyt-2 to H-2D d , and cells transfected with a mutant H-2D d  were not lysed by CD8+ T cells. Potter et al. (1989) Substitution at residue 227 of H-2 class I molecules abrogates recognition by CD8-dependent, but not CD8-independent, cytotoxic T lymphocytes, Nature 337:73-75. 
     Therefore, due to species specificity of interaction between the MHC class I α3 domain and CD8, an MHC I complex comprising a replacement of an H-2K α3 domain with a human HLA-A2 α3 domain was nonfunctional in a mouse (i.e., in vivo) in the absence of a human CD8. In animals transgenic for HLA-A2, substitution of human α3 domain for the mouse α3 domain resulted in restoration of T cell response. Irwin et al. (1989) Species-restricted interactions between CD8 and the α3 domain of class I influence the magnitude of the xenogeneic response, J. Exp. Med. 170:1091-1101; Vitiello et al. (1991), supra. 
     The transmembrane domain and cytoplasmic tail of mouse MHC class I proteins also have important functions. One function of MHC I transmembrane domain is to facilitate modulation by HLA-A2 of homotypic cell adhesion (to enhance or inhibit adhesion), presumably as the result of cross-linking (or ligation) of surface MHC molecules. Wagner et al. (1994) Ligation of MHC Class I and Class II Molecules Can Lead to Heterologous Desensitization of Signal Transduction Pathways That Regulate Homotypic Adhesion in Human Lymphocytes, J. Immunol. 152:5275-5287. Cell adhesion can be affected by mAbs that bind at diverse epitopes of the HLA-A2 molecule, suggesting that there are multiple sites on HLA-A2 implicated in modulating homotypic cell adhesion; depending on the epitope bound, the affect can be to enhance or to inhibit HLA-A2-dependent adhesion. Id. 
     The cytoplasmic tail, encoded by exons 6 and 7 of the MHC I gene, is reportedly necessary for proper expression on the cell surface and for LIR1-mediated inhibition of NK cell cytotoxicity. Gruda et al. (2007) Intracellular Cysteine Residues in the Tail of MHC Class I Proteins Are Crucial for Extracellular Recognition by Leukocyte Ig-Like Receptor 1, J. Immunol. 179:3655-3661. A cytoplasmic tail is required for multimerizaton of at least some MHC I molecules through formation of disulfide bonds on its cysteine residues, and thus may play a role in clustering and in recognition by NK cells. Lynch et al. (2009) Novel MHC Class I Structures on Exosomes, J. Immunol. 183:1884-1891. 
     The cytoplasmic domain of HLA-A2 contains a constitutively phosphorylated serine residue and a phosphorylatable tyrosine, although—in Jurkat cells—mutant HLA-A2 molecules lacking a cytoplasmic domain appear normal with respect to expression, cytoskeletal association, aggregation, and endocytic internalization. Gur et al. (1997) Structural Analysis of Class I MHC Molecules: The Cytoplasmic Domain Is Not Required for Cytoskeletal Association, Aggregation, and Internalization, Mol. Immunol. 34(2):125-132. Truncated HLA-A2 molecules lacking the cytoplasmic domain are apparently normally expressed and associate with β2 microglobulin. Id. 
     However, several studies have demonstrated that the cytoplasmic tail is critical in intracellular trafficking, dendritic cell (DC)-mediated antigen presentation, and CTL priming. A tyrosine residue encoded by exon 6 was shown to be required for MHC I trafficking through endosomal compartments, presentation of exogenous antigens, and CTL priming; while deletion of exon 7 caused enhancement of anti-viral CTL responses. Lizee et al. (2003) Control of Dendritic Cross-Presentation by the Major Histocompatibility Complex Class I Cytoplasmic Domain, Nature Immunol. 4:1065-73; Basha et al. (2008) MHC Class I Endosomal and Lysosomal Trafficking Coincides with Exogenous Antigen Loading in Dendritic Cells, PLoS ONE 3: e3247; and Rodriguez-Cruz et al. (2011) Natural Splice Variant of MHC Class I Cytoplasmic Tail Enhances Dendritic Cell-Induced CD8+ T-Cell Responses and Boosts Anti-Tumor Immunity, PLoS ONE 6:e22939. 
     MHC class II complex comprises two non-covalently associated domains: an α chain and a β chain, also referred herein as an α polypeptide and a β polypeptide ( FIG. 1 , right). The protein spans the plasma membrane; thus it contains an extracellular domain, a transmembrane domain, and a cytoplasmic domain. The extracellular portion of the α chain includes α1 and α2 domains, and the extracellular portion of the β chain includes β1 and β2 domains. The α1 and β1 domains form a peptide-binding cleft on the cell surface. Due to the three-dimensional conformation of the peptide-binding cleft of the MHC II complex, there is theoretically no upper limit on the length of the bound antigen, but typically peptides presented by MHC II are between 13 and 17 amino acids in length. 
     In addition to its interaction with the antigenic peptides, the peptide-binding cleft of the MHC II molecule interacts with invariant chain (Ii) during the processes of MHC II complex formation and peptide acquisition. The α/β MHC II dimers assemble in the endoplasmic reticulum and associate with Ii chain, which is responsible for control of peptide binding and targeting of the MHC II into endocytic pathway. In the endosome, Ii undergoes proteolysis, and a small fragment of Ii, Class II-associated invariant chain peptide (CLIP), remains at the peptide-binding cleft. In the endosome, under control of HLA-DM (in humans), CLIP is exchanged for antigenic peptides. 
     MHC II interacts with T cell co-receptor CD4 at the hydrophobic crevice at the junction between α2 and β2 domains. Wang and Reinherz (2002) Structural Basis of T Cell Recognition of Peptides Bound to MHC Molecules, Molecular Immunology, 38:1039-49. When CD4 and T cell receptor bind the same MHC II molecule complexed with a peptide, the sensitivity of a T cell to antigen is increased, and it requires 100-fold less antigen for activation. See, Janeway&#39;s Immunobiology, 7 th  Ed., Murphy et al. eds., Garland Science, 2008, incorporated herein by reference. 
     Numerous functions have been proposed for transmembrane and cytoplasmic domains of MHC II. In the case of cytoplasmic domain, it has been shown to be important for intracellular signaling, trafficking to the plasma membrane, and ultimately, antigen presentation. For example, it was shown that T cell hybridomas respond poorly to antigen-presenting cells (APCs) transfected with MHC II β chains truncated at the cytoplasmic domain, and induction of B cell differentiation is hampered. See, e.g., Smiley et al. (1996) Truncation of the class II β-chain cytoplasmic domain influences the level of class II/invariant chain-derived peptide complexes, Proc. Natl. Acad. Sci. USA, 93:241-44. Truncation of Class II molecules seems to impair cAMP production. It has been postulated that deletion of the cytoplasmic tail of MHC II affects intracellular trafficking, thus preventing the complex from coming across relevant antigens in the endocytic pathway. Smiley et al. (supra) demonstrated that truncation of class II molecules at the cytoplasmic domain reduces the number of CLIP/class II complexes, postulating that this affects the ability of CLIP to effectively regulate antigen presentation. 
     It has been hypothesized that, since MHC II clustering is important for T cell receptor (TCR) triggering, if MHC II molecules truncated at the cytoplasmic domain were prevented from binding cytoskeleton and thus aggregating, antigen presentation to T cells would be affected. Ostrand-Rosenberg et al. (1991) Abrogation of Tumorigenicity by MHC Class II Antigen Expression Requires the Cytoplasmic Domain of the Class II Molecule, J. Immunol. 147:2419-22. In fact, it was recently shown that HLA-DR truncated at the cytoplasmic domain failed to associate with the cytoskeleton following oligomerization. El Fakhy et al. (2004) Delineation of the HLA-DR Region and the Residues Involved in the Association with the Cytoskeleton, J. Biol. Chem. 279:18472-80. Importantly, actin cytoskeleton is a site of localized signal transduction activity, which can effect antigen presentation. In addition to association with cytoskeleton, recent studies have also shown that up to 20% of all HLA-DR molecules constitutively reside in the lipid rafts of APCs, which are microdomains rich in cholesterol and glycosphingolipids, and that such localization is important for antigen presentation, immune synapse formation, and MHC II-mediated signaling. See, e.g., Dolan et al. (2004) Invariant Chain and the MHC II Cytoplasmic Domains Regulate Localization of MHC Class II Molecules to Lipid Rafts in Tumor Cell-Based Vaccines, J. Immunol. 172:907-14. Dolan et al. suggested that truncation of cytoplasmic domain of MHC II reduces constitutive localization of MHC II to lipid rafts. 
     In addition, the cytoplasmic domain of MHC II, in particular the β chain, contains a leucine residue that is subject to ubiquitination by ubiquitin ligase, membrane-associated RING-CH I (MARCH I), which controls endocytic trafficking, internalization, and degradation of MHC II; and it has been shown that MARCH-mediated ubiquitination ceases upon dendritic cell maturation resulting in increased levels of MHC II at the plasma membrane. Shin et al. (2006) Surface expression of MHC class II in dendritic cells is controlled by regulated ubiquitination, Nature 444:115-18; De Gassart et al. (2008) MHC class II stabilization at the surface of human dendritic cells is the result of maturation-dependent MARCH I down-regulation, Proc. Natl. Acad. Sci. USA 105:3491-96. 
     Transmembrane domains of α and β chains of MHC II interact with each other and this interaction is important for proper assembly of class II MHC complex. Cosson and Bonifacino (1992) Role of Transmembrane Domain Interactions in the Assembly of Class II MHC Molecules, Nature 258:659-62. In fact, MHC II molecules in which the transmembrane domains of the α and β chains were replaced by the α chain of IL-2 receptor were retained in the ER and were barely detectable at the cell surface. Id. Through mutagenesis studies, conserved Gly residues at the α and β transmembrane domains were found to be responsible for MHC II assembly at the cell surface. Id. Thus, both transmembrane and cytoplasmic domains are crucial for the proper function of the MHC II complex. 
     In various embodiments, provided herein is a genetically modified non-human animal, e.g., rodent (e.g., mouse or rat) comprising in its genome a nucleotide sequence encoding a human or humanized MHC I polypeptide and a nucleotide sequence encoding human or humanized MHC II protein. The MHC I nucleotide sequence may encode an MHC I polypeptide that is partially human and partially non-human, e.g., chimeric human/non-human MHC I polypeptide, and the MHC II nucleotide sequence may encode an MHC II protein that is partially human and partially non-human, e.g., chimeric human/non-human MHC II protein (e.g., comprising chimeric human/non-human MHC II α and β polypeptides). In some aspects, the animal does not express endogenous MHC I and II polypeptides, e.g., functional endogenous MHC I and II polypeptides. In some embodiments, the only MHC I and MHC II molecules expressed on a cell surface of the animal are chimeric MHC I and II molecules. 
     A genetically modified non-human animal comprising in its genome, e.g., at the endogenous locus, a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide is disclosed in U.S. Patent Application Publication Nos. 20130111617 and 20130185819, which applications are incorporated herein by reference in their entireties. A genetically modified non-human animal comprising in its genome, e.g., at the endogenous locus, a nucleotide sequence encoding humanized, e.g., chimeric human/non-human MHC II polypeptides is disclosed in U.S. Pat. Nos. 8,847,005 and 9,043,996, which applications are incorporated herein by reference in their entireties. A genetically modified non-human animal comprising in its genome, e.g., at the endogenous locus, a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide and comprising in its genome, e.g., at the endogenous locus, a nucleotide sequence encoding humanized, e.g., chimeric human/non-human MHC II polypeptides, is disclosed in U.S. Patent Application Publication No. 20140245467, which application is incorporated herein by reference in its entirety. 
     In various embodiments provided herein is a genetically modified non-human animal comprising in its genome, e.g., at endogenous MHC locus, a first nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide, wherein a human portion of the chimeric MHC I polypeptide comprises an extracellular domain of a human MHC I polypeptide; a second nucleotide sequence encoding a chimeric human/non-human MHC II α polypeptide, wherein a human portion of the chimeric MHC II α polypeptide comprises an extracellular domain of a human MHC II α polypeptide; and a third nucleotide sequence encoding a chimeric human/non-human MHC II β polypeptide, wherein a human portion of the chimeric MHC II β polypeptide comprises an extracellular domain of a human MHC II β polypeptide; wherein the non-human animal expresses functional chimeric human/non-human MHC I and MHC II proteins from its endogenous non-human MHC locus. In one embodiment, the first, second, and/or third nucleotide sequences are located the endogenous non-human MHC locus. In one embodiment, wherein the non-human animal is a mouse, the first, second, and/or third nucleotide sequences are located at the endogenous mouse MHC locus on mouse chromosome 17. In one embodiment, the first nucleotide sequence is located at the endogenous non-human MHC I locus. In one embodiment, the second nucleotide sequence is located at the endogenous non-human MHC II α locus. In one embodiment, the third nucleotide sequence is located at the endogenous non-human MHC II β locus. 
     In one embodiment, the non-human animal only expresses the chimeric human/non-human MHC I, MHC II α and/or MHC β II polypeptides and does not express endogenous non-human MHC polypeptides (e.g., functional endogenous MHC I, II α and/or II β polypeptides) from the endogenous non-human MHC locus. In one embodiment, the animal described herein expresses a functional chimeric MHC I and a functional chimeric MHC II on the surface of its cells, e.g., antigen presenting cells, etc. In one embodiment, the only MHC I and MHC II expressed by the animal on a cell surface are chimeric MHC I and chimeric MHC II, and the animal does not express any endogenous MHC I and MHC II on a cell surface. 
     In one embodiment, the chimeric human/non-human MHC I polypeptide comprises in its human portion a peptide binding domain of a human MHC I polypeptide. In one aspect, the human portion of the chimeric polypeptide comprises an extracellular domain of a human MHC I. In this embodiment, the human portion of the chimeric polypeptide comprises an extracellular domain of an α chain of a human MHC I. In one embodiment, the human portion of the chimeric polypeptide comprises α1 and α2 domains of a human MHC I. In another embodiment, the human portion of the chimeric polypeptide comprises α1, α2, and α3 domains of a human MHC I. 
     In one aspect, a human portion of the chimeric MHC II α polypeptide and/or a human portion of the chimeric MHC II β polypeptide comprises a peptide-binding domain of a human MHC II α polypeptide and/or human MHC II β polypeptide, respectively. In one aspect, a human portion of the chimeric MHC II α and/or β polypeptide comprises an extracellular domain of a human MHC II α and/or β polypeptide, respectively. In one embodiment, a human portion of the chimeric MHC II α polypeptide comprises α1 domain of a human MHC II α polypeptide; in another embodiment, a human portion of the chimeric MHC II α polypeptide comprises α1 and α2 domains of a human MHC II α polypeptide. In an additional embodiment, a human portion of the chimeric MHC II β polypeptide comprises β1 domain of a human MHC II β polypeptide; in another embodiment, a human portion of the chimeric MHC II β polypeptide comprises β1 and β2 domains of a human MHC II β polypeptide. 
     In some embodiments, the human or humanized MHC I polypeptide may be derived from a functional human HLA molecule encoded by any of HLA-A, HLA-B, HLA-C, HLA-E, HLA-F, or HLA-G loci. The human or humanized MHC II polypeptide may be derived from a functional human HLA molecule encoded by an of HLA-DP, -DQ, and -DR loci. A list of commonly used HLA antigens and alleles is described in Shankarkumar et al. ((2004) The Human Leukocyte Antigen (HLA) System, Int. J. Hum. Genet. 4(2):91-103), incorporated herein by reference. Shankarkumar et al. also present a brief explanation of HLA nomenclature used in the art. Additional information regarding HLA nomenclature and various HLA alleles can be found in Holdsworth et al. (2009) The HLA dictionary 2008: a summary of HLA-A, -B, -C, -DRB1/3/4/5, and DQB1 alleles and their association with serologically defined HLA-A, -B, -C, -DR, and -DQ antigens, Tissue Antigens 73:95-170, and a recent update by Marsh et al. (2010) Nomenclature for factors of the HLA system, 2010, Tissue Antigens 75:291-455, both incorporated herein by reference. In some embodiments, the MHC I or MHC II polypeptides may be derived from any functional human HLA-A, B, C, DR, or DQ molecules. Thus, the human or humanized MHC I and/or II polypeptides may be derived from any functional human HLA molecules described therein. In some embodiments, all MHC I and MHC II polypeptides expressed on a cell surface comprise a portion derived from human HLA molecules. 
     Of particular interest are human HLA molecules, specific polymorphic HLA alleles, known to be associated with a number of human diseases, e.g., human autoimmune diseases. In fact, specific polymorphisms in HLA loci have been identified that correlate with development of rheumatoid arthritis, type I diabetes, Hashimoto&#39;s thyroiditis, multiple sclerosis, myasthenia gravis, Graves&#39; disease, systemic lupus erythematosus, celiac disease, Crohn&#39;s disease, ulcerative colitis, and other autoimmune disorders. See, e.g., Wong and Wen (2004) What can the HLA transgenic mouse tell us about autoimmune diabetes?, Diabetologia 47:1476-87; Taneja and David (1998) HLA Transgenic Mice as Humanized Mouse Models of Disease and Immunity, J. Clin. Invest. 101:921-26; de Bakker et al. (2006), A high-resolution HLA and SNP haplotype map for disease association studies in the extended human MHC, Nature Genetics 38:1166-72 and Supplementary Information; and International MHC and Autoimmunity Genetics Network (2009) Mapping of multiple susceptibility variants within the MHC region for 7 immune-mediated diseases, Proc. Natl. Acad. Sci. USA 106:18680-85. Thus, the human or humanized MHC I and/or II polypeptides may be derived from a human HLA molecule known to be associated with a particular disease, e.g., autoimmune disease. 
     In one specific aspect, the human or humanized MHC I polypeptide is derived from human HLA-A. In a specific embodiment, the HLA-A polypeptide is an HLA-A2 polypeptide (e.g., and HLA-A2.1 polypeptide). In one embodiment, the HLA-A polypeptide is a polypeptide encoded by an HLA-A*0201 allele, e.g., HLA-A*02:01:01:01 allele. The HLA-A*0201 allele is commonly used amongst the North American population. Although the present Examples describe this particular HLA sequence, any suitable HLA-A sequence is encompassed herein, e.g., polymorphic variants of HLA-A2 exhibited in human population, sequences with one or more conservative or non-conservative amino acid modifications, nucleic acid sequences differing from the sequence described herein due to the degeneracy of genetic code, etc. 
     In another specific aspect, the human portion of the chimeric MHC I polypeptide is derived from human MHC I selected from HLA-B and HLA-C. In one aspect, it is derived from HLA-B, e.g., HLA-B27. In another aspect, it is derived from HLA-A3, -B7, -Cw6, etc. 
     In one specific aspect, the human portions of the humanized MHC II α and β polypeptides described herein are derived from human HLA-DR, e.g., HLA-DR2. Typically, HLA-DR α chains are monomorphic, e.g., the α chain of HLA-DR complex is encoded by HLA-DRA gene (e.g., HLA-DRα*01 gene). On the other hand, the HLA-DR β chain is polymorphic. Thus, HLA-DR2 comprises an α chain encoded by HLA-DRA gene and a β chain encoded by HLA-DR1β*1501 gene. Although the present Examples describe these particular HLA sequences, any suitable HLA-DR sequences are encompassed herein, e.g., polymorphic variants exhibited in human population, sequences with one or more conservative or non-conservative amino acid modifications, nucleic acid sequences differing from the sequences described herein due to the degeneracy of genetic code, etc. 
     The human portions of the chimeric MHC II α and/or β polypeptide may be encoded by nucleotide sequences of HLA alleles known to be associated with common human diseases. Such HLA alleles include, but are not limited to, HLA-DRB1*0401, -DRB1*0301, -DQA1*0501, -DQB1*0201, -DRB1*1501, -DRB1*1502, -DQB1*0602, -DQA1*0102, -DQA1*0201, -DQB1*0202, -DQA1*0501, and combinations thereof. For a summary of HLA allele/disease associations, see de Bakker et al. (2006), supra, incorporated herein by reference. 
     In one aspect, the non-human portion of a chimeric human/non-human MHC I, MHC II α and/or MHC II β polypeptide(s) comprises transmembrane and/or cytoplasmic domains of an endogenous non-human (e.g., rodent, e.g., mouse, rat, etc.) MHC I, MHC II α and/or MHC II polypeptide(s), respectively. Thus, the non-human portion of the chimeric human/non-human MHC I polypeptide may comprise transmembrane and/or cytoplasmic domains of an endogenous non-human MHC I polypeptide. The non-human portion of a chimeric MHC II α polypeptide may comprise transmembrane and/or cytoplasmic domains of an endogenous non-human MHC II α polypeptide. The non-human portion of a chimeric human/non-human MHC II β polypeptide may comprise transmembrane and/or cytoplasmic domains of an endogenous non-human MHC II β polypeptide. In one aspect, the non-human animal is mouse, and a non-human portion of the chimeric MHC I polypeptide is derived from a mouse H-2K protein. In one aspect, the animal is a mouse, and non-human portions of the chimeric MHC II α and β polypeptides are derived from a mouse H-2E protein. Thus, a non-human portion of the chimeric MHC I polypeptide may comprise transmembrane and cytoplasmic domains derived from a mouse H-2K, and non-human portions of the chimeric MHC II α and β polypeptides may comprise transmembrane and cytoplasmic domains derived from a mouse H-2E protein. Although specific H-2K and H-2E sequences are contemplated in the Examples, any suitable sequences, e.g., polymorphic variants, conservative/non-conservative amino acid substitutions, etc., are encompassed herein. In one aspect, the non-human animal is a mouse, and the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In other aspects, the mouse does not express any functional endogenous mouse MHC I and MHC II on a cell surface. 
     A chimeric human/non-human polypeptide may be such that it comprises a human or a non-human leader (signal) sequence. In one embodiment, the chimeric MHC I polypeptide comprises a non-human leader sequence of an endogenous MHC I polypeptide. In one embodiment, the chimeric MHC II α polypeptide comprises a non-human leader sequence of an endogenous MHC II α polypeptide. In one embodiment, the chimeric MHC II β polypeptide comprises a non-human leader sequence of an endogenous MHC II β polypeptide. In an alternative embodiment, the chimeric MHC I, MHC II α and/or MHC II β polypeptide(s) comprises a non-human leader sequence of MHC I, MHC II α and/or MHC II β polypeptide(s), respectively, from another non-human animal, e.g., another rodent or another mouse strain. Thus, the nucleotide sequence encoding the chimeric MHC I, MHC II α and/or MHC II β polypeptide may be operably linked to a nucleotide sequence encoding a non-human MHC I, MHC II α and/or MHC II β leader sequence, respectively. In yet another embodiment, the chimeric MHC I, MHC II α and/or MHC II β polypeptide(s) comprises a human leader sequence of human MHC I, human MHC II α and/or human MHC II β polypeptide, respectively (e.g., a leader sequence of human HLA-A2, human HLA-DRA and/or human HLA-DRβ1*1501, respectively). 
     A chimeric human/non-human MHC I, MHC II α and/or MHC II β polypeptide may comprise in its human portion a complete or substantially complete extracellular domain of a human MHC I, human MHC II α and/or human MHC II β polypeptide, respectively. Thus, a human portion may comprise at least 80%, preferably at least 85%, more preferably at least 90%, e.g., 95% or more of the amino acids encoding an extracellular domain of a human MHC I, human MHC II α and/or human MHC II β polypeptide (e.g., human HLA-A2, human HLA-DRA and/or human HLA-DRβ1*1501). In one example, substantially complete extracellular domain of the human MHC I, human MHC II α and/or human MHC II β polypeptide lacks a human leader sequence. In another example, the chimeric human/non-human MHC I, chimeric human/non-human MHC II α and/or the chimeric human/non-human MHC II β polypeptide comprises a human leader sequence. 
     Moreover, the chimeric MHC I, MHC II α and/or MHC II β polypeptide may be operably linked to (e.g., be expressed under the regulatory control of) endogenous non-human promoter and regulatory elements, e.g., mouse MHC I, MHC II α and/or MHC II β regulatory elements, respectively. Such arrangement will facilitate proper expression of the chimeric MHC I and/or MHC II polypeptides in the non-human animal, e.g., during immune response in the non-human animal. 
     The genetically modified non-human animal may be selected from a group consisting of a mouse, rat, rabbit, pig, bovine (e.g., cow, bull, buffalo), deer, sheep, goat, chicken, cat, dog, ferret, primate (e.g., marmoset, rhesus monkey). For the non-human animals where suitable genetically modifiable ES cells are not readily available, other methods are employed to make a non-human animal comprising the genetic modification. Such methods include, e.g., modifying a non-ES cell genome (e.g., a fibroblast or an induced pluripotent cell) and employing nuclear transfer to transfer the modified genome to a suitable cell, e.g., an oocyte, and gestating the modified cell (e.g., the modified oocyte) in a non-human animal under suitable conditions to form an embryo. 
     In one aspect, the non-human animal is a mammal. In one aspect, the non-human animal is a small mammal, e.g., of the superfamily Dipodoidea or Muroidea. In one embodiment, the genetically modified animal is a rodent. In one embodiment, the rodent is selected from a mouse, a rat, and a hamster. In one embodiment, the rodent is selected from the superfamily Muroidea. In one embodiment, the genetically modified animal is from a family selected from Calomyscidae (e.g., mouse-like hamsters), Cricetidae (e.g., hamster, New World rats and mice, voles), Muridae (true mice and rats, gerbils, spiny mice, crested rats), Nesomyidae (climbing mice, rock mice, white-tailed rats, Malagasy rats and mice), Platacanthomyidae (e.g., spiny dormice), and Spalacidae (e.g., mole rates, bamboo rats, and zokors). In a specific embodiment, the genetically modified rodent is selected from a true mouse or rat (family Muridae), a gerbil, a spiny mouse, and a crested rat. In one embodiment, the genetically modified mouse is from a member of the family Muridae. In one embodiment, the animal is a rodent. In a specific embodiment, the rodent is selected from a mouse and a rat. In one embodiment, the non-human animal is a mouse. 
     In one embodiment, the non-human animal is a rodent that is a mouse of a C57BL strain selected from C57BL/A, C57BL/An, C57BL/GrFa, C57BL/KaLwN, C57BL/6, C57BL/6J, C57BL/6ByJ, C57BL/6NJ, C57BL/10, C57BL/10ScSn, C57BL/10Cr, and C57BL/Ola. In another embodiment, the mouse is a 129 strain selected from the group consisting of a strain that is 129P1, 129P2, 129P3, 129X1, 129S1 (e.g., 129S1/SV, 129S1/SvIm), 129S2, 129S4, 129S5, 129S9/SvEvH, 129S6 (129/SvEvTac), 129S7, 129S8, 129T1, 129T2 (see, e.g., Festing et al. (1999) Revised nomenclature for strain 129 mice, Mammalian Genome 10:836, see also, Auerbach et al (2000) Establishment and Chimera Analysis of 129/SvEv- and C57BL/6-Derived Mouse Embryonic Stem Cell Lines). In one embodiment, the genetically modified mouse is a mix of an aforementioned 129 strain and an aforementioned C57BL/6 strain. In another embodiment, the mouse is a mix of aforementioned 129 strains, or a mix of aforementioned BL/6 strains. In one embodiment, the 129 strain of the mix is a 129S6 (129/SvEvTac) strain. In another embodiment, the mouse is a BALB strain, e.g., BALB/c strain. In yet another embodiment, the mouse is a mix of a BALB strain and another aforementioned strain. In some embodiments, the mouse strain is such that a genetically wild-type mouse of that strain only expresses MHC I polypeptides derived from H-2K and H-2D on a cell surface and lacks endogenous H-2L gene. 
     In one embodiment, the non-human animal is a rat. In one embodiment, the rat is selected from a Wistar rat, an LEA strain, a Sprague Dawley strain, a Fischer strain, F344, F6, and Dark Agouti. In one embodiment, the rat strain is a mix of two or more strains selected from the group consisting of Wistar, LEA, Sprague Dawley, Fischer, F344, F6, and Dark Agouti. 
     Thus, in one embodiment, the invention relates to a genetically modified mouse that comprises in its genome a first nucleotide sequence encoding a chimeric human/mouse MHC I, a second nucleotide sequence encoding a chimeric human/mouse MHC II α, and a third nucleotide sequence encoding a chimeric human/mouse MHC II β polypeptides. A human portion of the chimeric MHC I, MHC II α, and MHC II β may comprise an extracellular domain of a human MHC I, MHC II α, and MHC II β, respectively. In one embodiment, the mouse expresses functional chimeric human/mouse MHC I, MHC II α, and MHC II β polypeptides from its endogenous mouse MHC locus. In one embodiment, the mouse does not express functional mouse MHC polypeptides, e.g., functional mouse MHC I, MHC II α, and MHC II β polypeptides, from its endogenous mouse MHC locus. In one embodiment, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In other embodiments, the only MHC I and MHC II expressed by the mouse on a cell surface are chimeric MHC I and II. 
     In one embodiment, a human portion of the chimeric human/mouse MHC I polypeptide comprises a peptide binding domain or an extracellular domain of a human MHC I (e.g., human HLA-A, e.g., human HLA-A2, e.g., human HLA-A2.1). In some embodiments, the mouse does not express a peptide binding or an extracellular domain of an endogenous mouse MHC I polypeptide from its endogenous mouse MHC I locus. The peptide binding domain of the chimeric human/mouse MHC I may comprise human α1 and α2 domains. Alternatively, the peptide binding domain of the chimeric human/mouse MHC I may comprise human α1, α2, and α3 domains. In one aspect, the extracellular domain of the chimeric human/mouse MHC I comprises an extracellular domain of a human MHC I α chain. In one embodiment, the endogenous mouse MHC I locus is an H-2K (e.g., H-2Kb) locus, and the mouse portion of the chimeric MHC I polypeptide comprises transmembrane and cytoplasmic domains of a mouse H-2K (e.g., H-2Kb) polypeptide. Thus, in one embodiment, the mouse of the invention comprises at its endogenous mouse MHC I locus a nucleotide sequence encoding a chimeric human/mouse MHC I, wherein a human portion of the chimeric polypeptide comprises an extracellular domain of a human HLA-A2 (e.g., HLA-A2.1) polypeptide and a mouse portion comprises transmembrane and cytoplasmic domains of a mouse H-2K (e.g., H-2Kb) polypeptide, and a mouse expresses a chimeric human/mouse HLA-A2/H-2K protein. In other embodiment, the mouse portion of the chimeric MHC I polypeptide may be derived from other mouse MHC I, e.g., H-2D, H-2L, etc.; and the human portion of the chimeric MHC I polypeptide may be derived from other human MHC I, e.g., HLA-B, HLA-C, etc. In one aspect, the mouse does not express a functional endogenous H-2K polypeptide from its endogenous mouse H-2K locus. In one embodiment, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In other embodiments, the only MHC I polypeptides expressed by the mouse on a cell surface are chimeric human/mouse MHC I polypeptides. 
     In one embodiment, a human portion of the chimeric human/mouse MHC II α polypeptide comprises a human MHC II α peptide binding or extracellular domain and a human portion of the chimeric human/mouse MHC II β polypeptide comprises a human MHC II β peptide binding or extracellular domain. In some embodiments, the mouse does not express a peptide binding or an extracellular domain of endogenous mouse α and/or β polypeptide from an endogenous mouse locus (e.g., H-2A and/or H-2E locus). In some embodiments, the mouse comprises a genome that lacks a gene that encodes a functional MHC class II molecule comprising an H-2Ab1, H-2Aa, H-2Eb1, H-2Eb2, H-2Ea, and a combination thereof. In some embodiments, the only MHC II polypeptides expressed by the mouse on a cell surface are chimeric human/mouse MHC II polypeptides. The peptide-binding domain of the chimeric human/mouse MHC II α polypeptide may comprise human α1 domain and the peptide-binding domain of the chimeric human/mouse MHC II β polypeptide may comprise a human β1 domain; thus, the peptide-binding domain of the chimeric MHC II complex may comprise human α1 and β1 domains. The extracellular domain of the chimeric human/mouse MHC II α polypeptide may comprise human α1 and α2 domains and the extracellular domain of the chimeric human/mouse MHC II β polypeptide may comprise human β1 and β2 domains; thus, the extracellular domain of the chimeric MHC II complex may comprise human α1, α2, β1 and β2 domains. In one embodiment, the mouse portion of the chimeric MHC II complex comprises transmembrane and cytosolic domains of mouse MHC II, e.g. mouse H-2E (e.g., transmembrane and cytosolic domains of mouse H-2E α and β chains). Thus, in one embodiment, the mouse of the invention comprises at its endogenous mouse MHC II locus a nucleotide sequence encoding a chimeric human/mouse MHC II α, wherein a human portion of the chimeric MHC II α polypeptide comprises an extracellular domain derived from an α chain of a human MHC II (e.g., α chain of HLA-DR2) and a mouse portion comprises transmembrane and cytoplasmic domains derived from an α chain of a mouse MHC II (e.g., H-2E); and a mouse comprises at its endogenous mouse MHC II locus a nucleotide sequence encoding a chimeric human/mouse MHC II β, wherein a human portion of the chimeric MHC II β polypeptide comprises an extracellular domain derived from a β chain of a human MHC II (e.g., β chain of HLA-DR2) and a mouse portion comprises transmembrane and cytoplasmic domains derived from a β chain of a mouse MHC II (e.g., H-2E); wherein the mouse expresses a chimeric human/mouse HLA-DR2/H-2E protein. In other embodiment, the mouse portion of the chimeric MHC II protein may be derived from other mouse MHC II, e.g., H-2A, etc.; and the human portion of the chimeric MHC II protein may be derived from other human MHC II, e.g., HLA-DQ, etc. In one aspect, the mouse does not express functional endogenous H-2A and H-2E polypeptides from their endogenous mouse loci (e.g., the mouse does not express H-2Ab1, H-2Aa, H-2Eb1, H-2Eb2, and H-2Ea polypeptides). In one aspect, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In some embodiments, the mouse lacks expression of any endogenous MHC I or MHC II molecule on a cell surface. 
     In a further embodiment, a non-human animal of the invention, e.g., a rodent, e.g., a mouse, comprises at an endogenous β2 microglobulin locus a nucleotide sequence encoding a human or humanized β2 microglobulin. β2 microglobulin or the light chain of the MHC class I complex (also abbreviated “(β2M”) is a small (12 kDa) non-glycosylated protein, that functions primarily to stabilize the MHC I α chain. Generation of human or humanized microglobulin animals is described in detail in U.S. Patent Publication No. 20130111617, and is incorporated herein by reference. A mouse comprising a humanized MHC locus as described in the present disclosure, and a human or humanized β2 microglobulin locus as described in U.S. Patent Publication No. 20130111617, may be generated by any methods known in the art, e.g., breeding, or alternatively, using homologous recombination in ES cells. 
     Various other embodiments of a genetically modified non-human animal, e.g. rodent, e.g., rat or mouse, would be evident to one skilled in the art from the present disclosure and from the disclosure of U.S. Patent Publication No. 20130111617 and U.S. Pat. No. 8,847,005 incorporated herein by reference. 
     In various aspects of the invention, the sequence(s) encoding a chimeric human/non-human MHC I and MHC II polypeptides are located at an endogenous non-human MHC locus (e.g., mouse H-2K and/or H-2E locus). In one embodiment, this results in a replacement of an endogenous MHC gene(s) or a portion thereof with a nucleotide sequence(s) encoding a human or humanized MHC I or MHC II polypeptides. Since the nucleotide sequences encoding MHC I, MHC II α and MHC II β polypeptides are located in proximity to one another on the chromosome, in order to achieve the greatest success in humanization of both MHC I and MHC II in one animal, the MHC I and MHC II loci should be targeted sequentially. Thus, also provided herein are methods of generating a genetically modified non-human animal comprising nucleotide sequences encoding chimeric human/non-human MHC I, MHC II α and MHC II β polypeptides as described herein. 
     In some embodiments, the method utilizes a targeting construct made using VELOCIGENE® technology, introducing the construct into ES cells, and introducing targeted ES cell clones into a mouse embryo using VELOCIMOUSE® technology, as described in the Examples. 
     The nucleotide constructs used for generating non-human animals described herein are also provided. In one aspect, the nucleotide construct comprises: 5′ and 3′ non-human homology arms, a human DNA fragment comprising human MHC gene sequences (e.g., human HLA-A2 or human HLA-DR4 gene sequences), and a selection cassette flanked by recombination sites. In one embodiment, the human DNA fragment is a genomic fragment that comprises both introns and exons of a human MHC gene (e.g., human HLA-A2 or HLA-DR2 gene). In one embodiment, the non-human homology arms are homologous to a non-human MHC locus (e.g., MHC I or MHC II locus). 
     A selection cassette is a nucleotide sequence inserted into a targeting construct to facilitate selection of cells (e.g., ES cells) that have integrated the construct of interest. A number of suitable selection cassettes are known in the art. Commonly, a selection cassette enables positive selection in the presence of a particular antibiotic (e.g., Neo, Hyg, Pur, CM, Spec, etc.). In addition, a selection cassette may be flanked by recombination sites, which allow deletion of the selection cassette upon treatment with recombinase enzymes. Commonly used recombination sites are loxP and Frt, recognized by Cre and Flp enzymes, respectively, but others are known in the art. In one embodiment, the selection cassette is located at the 5′ end the human DNA fragment. In another embodiment, the selection cassette is located at the 3′ end of the human DNA fragment. In another embodiment, the selection cassette is located within the human DNA fragment. In another embodiment, the selection cassette is located within an intron of the human DNA fragment. 
     In one embodiment, the 5′ and 3′ non-human homology arms comprise genomic sequence at 5′ and 3′ locations, respectively, of an endogenous non-human (e.g., murine) MHC class I or class II gene locus (e.g., 5′ of the first leader sequence and 3′ of the α3 exon of the mouse MHC I gene, or upstream of mouse H-2Ab1 gene and downstream of mouse H-2Ea gene). In one embodiment, the endogenous MHC class I locus is selected from mouse H-2K, H-2D and H-2L. In a specific embodiment, the endogenous MHC class I locus is mouse H-2K. In one embodiment, the endogenous MHC II locus is selected from mouse H-2E and H-2A. In one embodiment, the engineered MHC II construct allows replacement of both mouse H-2E and H-2A genes. In one embodiment, the mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the mouse is engineered to lack all or a portion of an endogenous H-2D locus. In another embodiment, the mouse does not express any functional endogenous MHC I and MHC II polypeptides on a cell surface. In one embodiment, the only MHC I and MHC II expressed by the mouse on a cell surface are chimeric human/mouse MHC I and MHC II. 
     Thus, in one embodiment, provided herein is a method of generating a genetically engineered non-human animal (e.g., rodent, e.g., rat or mouse) capable of expressing humanized MHC I and II proteins comprising replacing at an endogenous non-human MHC II locus a nucleotide sequence encoding a non-human MHC II complex with a nucleotide sequence encoding a chimeric human/non-human MHC II complex to generate a first non-human animal; and replacing at an endogenous non-human MHC I locus a nucleotide sequence encoding a non-human MHC I polypeptide with a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide to generate a second non-human animal. In one embodiment, the steps of replacing nucleotide sequences comprise homologous recombination in ES cells. In one embodiment, the second non-human animal is generated by homologous recombination in ES cells bearing nucleotide sequences encoding chimeric human/non-human MHC II complex. In one embodiment, the non-human animal is a mouse, and the first or second mouse does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the first or second mouse is engineered to lack all or a portion of an endogenous H-2D locus. Alternatively, also provided herein is a method of generating a genetically engineered non-human animal (e.g., rodent, e.g., rat or mouse) capable of expressing humanized MHC I and II proteins comprising replacing at an endogenous non-human MHC I locus a nucleotide sequence encoding a non-human MHC I polypeptide with a nucleotide sequence encoding a chimeric human/non-human MHC I polypeptide to generate a first non-human animal; and replacing at an endogenous non-human MHC II locus a nucleotide sequence encoding a non-human MHC II complex with a nucleotide sequence encoding a chimeric human/non-human MHC II complex to generate a second non-human animal. In such embodiment, the second non-human animal is generated by homologous recombination in ES cells bearing a nucleotide sequence encoding chimeric human/non-human MHC I polypeptide. 
     Upon completion of gene targeting, ES cells or genetically modified non-human animals are screened to confirm successful incorporation of exogenous nucleotide sequence of interest or expression of exogenous polypeptide. Numerous techniques are known to those skilled in the art, and include (but are not limited to) Southern blotting, long PCR, quantitative PCT (e.g., real-time PCR using TAQMAN®), fluorescence in situ hybridization, Northern blotting, flow cytometry, Western analysis, immunocytochemistry, immunohistochemistry, etc. In one example, non-human animals (e.g., mice) bearing the genetic modification of interest can be identified by screening for loss of mouse allele and/or gain of human allele using a modification of allele assay described in Valenzuela et al. (2003) High-throughput engineering of the mouse genome coupled with high-resolution expression analysis, Nature Biotech. 21(6):652-659. Other assays that identify a specific nucleotide or amino acid sequence in the genetically modified animals are known to those skilled in the art. 
     In one aspect, a cell that expresses a chimeric human/non-human MHC I and MHC II proteins (e.g., HLA-A2/H-2K and HLA-DR2/H-2E proteins) is provided. In one aspect, the cell is a mouse cell that does not express functional endogenous MHC polypeptides from its H-2D locus. In some embodiments, the cell is a mouse cell is engineered to lack all or a portion of an endogenous H-2D locus. In some embodiments, the cell is a mouse cell that does not express any endogenous MHC I and MHC II polypeptide on its surface. In one embodiment, the cell comprises an expression vector comprising a chimeric MHC class I sequence and chimeric MHC class II sequence as described herein. In one embodiment, the cell is selected from CHO, COS, 293, HeLa, and a retinal cell expressing a viral nucleic acid sequence (e.g., a PERC.6™ cell). 
     A chimeric MHC II complex comprising an extracellular domain of HLA-DR2 described herein may be detected by anti-HLA-DR antibodies. Thus, a cell displaying chimeric human/non-human MHC II polypeptide may be detected and/or selected using anti-HLA-DR antibody. The chimeric MHC I complex comprising an extracellular domain of HLA-A2 described herein may be detected using anti-HLA-A, e.g., anti-HLA-A2 antibodies. Thus, a cell displaying a chimeric human/non-human MHC I polypeptide may be detected and/or selected using anti-HLA-A antibody. Antibodies that recognize other HLA alleles are commercially available or can be generated, and may be used for detection/selection. 
     Although the Examples that follow describe a genetically engineered animal whose genome comprises a replacement of a nucleotide sequence encoding mouse H-2K, and H-2A and H-2E proteins with a nucleotide sequence encoding a chimeric human/mouse HLA-A2/H-2K and HLA-DR2/H-2E protein, respectively, one skilled in the art would understand that a similar strategy may be used to introduce chimeras comprising other human MHC I and II genes (other HLA-A, HLA-B, and HLA-C; and other HLA-DR, HLA-DP and HLA-DQ genes). Such animals comprising multiple chimeric human/non-human (e.g., human/rodent, e.g., human/mouse) MHC I and MHC II genes at endogenous MHC loci are also provided. 
     In various embodiments of the invention, the mouse that comprises chimeric human/mouse MHC I and MHC II loci and expresses only chimeric human/mouse MHC I and MHC II on a cell surface (and lacks cell surface expression of any endogenous MHC I and MHC II) displays essentially normal B to T cell ratio, e.g., B to T cell ratio in the spleen. In some embodiments of the invention, the mouse described herein displays normal T and B cell development, expression levels, and expression patterns. In some embodiments of the invention, the mouse described herein expresses chimeric human/mouse MHC II only on antigen presenting cells of the mouse. In some embodiments, a mouse described herein elicits an immune response, e.g., a cellular immune response, to one or more human antigens. In some embodiments, a mouse described herein elicits a T cell response to one or more human antigens. 
     In various embodiments, the genetically modified non-human animals described herein make cells, e.g., APCs, with human or humanized MHC I and II on the cell surface and, as a result, present peptides as epitopes for T cells in a human-like manner, because substantially all of the components of the complex are human or humanized. The genetically modified non-human animals of the invention can be used to study the function of a human immune system in the humanized animal; for identification of antigens and antigen epitopes that elicit immune response (e.g., T cell epitopes, e.g., unique human cancer epitopes), e.g., for use in vaccine development; for evaluation of vaccine candidates and other vaccine strategies; for studying human autoimmunity; for studying human infectious diseases; and otherwise for devising better therapeutic strategies based on human MHC expression. 
     EXAMPLES 
     The invention will be further illustrated by the following nonlimiting examples. These Examples are set forth to aid in the understanding of the invention but are not intended to, and should not be construed to, limit its scope in any way. The Examples do not include detailed descriptions of conventional methods that would be well known to those of ordinary skill in the art (molecular cloning techniques, etc.). Unless indicated otherwise, parts are parts by weight, molecular weight is average molecular weight, temperature is indicated in Celsius, and pressure is at or near atmospheric. 
     Example 1: Engineering a Mouse Comprising Humanized MHC I and MHC II Loci 
     The various steps involved in engineering a mouse comprising humanized MHC I and MHC II loci, with corresponding and additional endogenous MHC I and MHC II loci deletions (HLA-A2/H-2K, HLA-DR2/H-2E, H-2A-del, H-2D-del) are depicted in  FIG. 4 . Detailed description of the steps appears below. 
     Example 1.1: Engineering Mouse ES Cells Comprising Humanized MHC I Gene 
     The mouse H-2K gene was humanized in a single step by construction of a unique targeting vector from human and mouse bacterial artificial chromosome (BAC) DNA using VELOCIGENE® technology (see, e.g., U.S. Pat. No. 6,586,251 and Valenzuela et al. (2003) High-throughput engineering of the mouse genome coupled with high-resolution expression analysis. Nat. Biotech. 21(6): 652-659). DNA from mouse BAC clone RP23-173k21 (Invitrogen) was modified by homologous recombination to replace the genomic DNA encoding the α1, α2 and α3 domains of the mouse H-2K gene with human genomic DNA encoding the α1, α2 and α3 subunits of the human HLA-A2 gene ( FIG. 5 ). Detailed steps for construction of BAC DNA can be found in U.S. Patent Application Publication No. 2013-0111617, incorporated herein by reference. The targeted BAC DNA was used to electroporate mouse F1H4 ES cells to create modified ES cells for generating mice that express a chimeric MHC class I protein on the surface of nucleated cells (e.g., T and B lymphocytes, macrophages, neutrophils). ES cells containing an insertion of human HLA sequences were identified by a the quantitative PCR assay using TAQMAN™ (Lie and Petropoulos (1998) Curr. Opin. Biotechnology 9:43-48). Detailed steps for construction of mouse ES cells comprising humanized MHC I gene as well as screening can be also found in the same U.S. Patent Application Publication No. 2013-0111617. 
     ES cells comprising the chimeric HLA-A2/H-2K were utilized in further genetic engineering steps detailed below and in  FIG. 4 . 
     Example 1.2: Engineering Mouse ES Cells Comprising Deletion of MHC II Loci 
     The targeting vector for introducing a deletion of the endogenous MHC class II H-2Ab1, H-2Aa, H-2Eb1, H-2Eb2, and H-2Ea genes was made using VELOCIGENE® genetic engineering technology (see, e.g., U.S. Pat. No. 6,586,251 and Valenzuela et al., supra). Bacterial Artificial Chromosome (BAC) RP23-458i22 (Invitrogen) DNA was modified to delete the endogenous MHC class II genes H-2Ab1, H-2Aa, H-2Eb1, H-2Eb2, and H-2Ea. 
     Briefly, upstream and downstream homology arms were derived by PCR of mouse BAC DNA from locations 5′ of the H-2Ab1 gene and 3′ of the H-2Ea gene, respectively. These homology arms were used to make a cassette that deleted ˜79 kb of RP23-458i22 comprising genes H-2Ab1, H-2Aa, H-2Eb1, H-2Eb2, and H-2Ea of the MHC class II locus by bacterial homologous recombination (BHR). This region was replaced with a neomycin cassette flanked by lox2372 sites. The final targeting vector included from 5′ to 3′ a 26 kb homology arm comprising mouse genomic sequence 5′ to the H-2Ab1 gene of the endogenous MHC class II locus, a 5′ lox2372 site, a neomycin cassette, a 3′ lox2372 site and a 63 kb homology arm comprising mouse genomic sequence 3′ to the H-2Ea gene of the endogenous MHC class II locus (MAID 5092). 
     The BAC DNA targeting vector (described above) was used to electroporate mouse ES cells comprising humanized MHC I locus (from Example 1.1 above) to create modified ES cells comprising a deletion of the endogenous MHC class II locus (both H-2A and H-2E were deleted). Positive ES cells containing a deleted endogenous MHC class II locus were identified by the quantitative PCR assay using TAQMAN™ probes (Lie and Petropoulos (1998) Curr. Opin. Biotechnology 9:43-48). The upstream region of the deleted locus was confirmed by PCR using primers 5111U F (CAGAACGCCAGGCTGTAAC; SEQ ID NO:1) and 5111U R (GGAGAGCAGGGTCAGTCAAC; SEQ ID NO:2) and probe 5111U P (CACCGCCACTCACAGCTCCTTACA; SEQ ID NO:3), whereas the downstream region of the deleted locus was confirmed using primers 5111D F (GTGGGCACCATCTTCATCATTC; SEQ ID NO:4) and 5111D R (CTTCCTTTCCAGGGTGTGACTC; SEQ ID NO:5) and probe 5111D P (AGGCCTGCGATCAGGTGGCACCT; SEQ ID NO:6). The presence of the neomycin cassette from the targeting vector was confirmed using primers NEOF (GGTGGAGAGGCTATTCGGC; SEQ ID NO:7) and NEOR (GAACACGGCGGCATCAG; SEQ ID NO:8) and probe NEOP (TGGGCACAACAGACAATCGGCTG; SEQ ID NO:9). The nucleotide sequence across the upstream deletion point (SEQ ID NO:10) included the following, which indicates endogenous mouse sequence upstream of the deletion point (contained within the parentheses below) linked contiguously to cassette sequence present at the deletion point: (TTTGTAAACA AAGTCTACCC AGAGACAGAT GACAGACTTC AGCTCCAATG CTGATTGGTT CCTCACTTGG GACCAACCCT) ACCGGTATAA CTTCGTATAA GGTATCCTAT ACGAAGTTAT ATGCATGGCC TCCGCGCCGG. The nucleotide sequence across the downstream deletion point (SEQ ID NO:11) included the following, which indicates cassette sequence contiguous with endogenous mouse sequence downstream of the deletion point (contained within the parentheses below): CGACCTGCAG CCGGCGCGCC ATAACTTCGT ATAAGGTATC CTATACGAAG TTATCTCGAG (CACAGGCATT TGGGTGGGCA GGGATGGACG GTGACTGGGA CAATCGGGAT GGAAGAGCAT AGAATGGGAG TTAGGGAAGA). 
     Subsequently to generation of the ES cells comprising both the MHC I humanization and endogenous MHC II deletion described above, the loxed neomycin cassette was removed using CRE. Specifically, a plasmid encoding Cre recombinase was electroporated into ES cells to remove the neomycin cassette. 
     Example 1.3: Generation of Mouse ES Cells Comprising H2-D Locus Deletion 
     To delete mouse H-2D locus, BHR was used to modify mouse BAC clone bMQ-218H21 (Sanger Institute), replacing 3756 bp of the H2-D gene (from the ATG start codon to 3 bp downstream of the TGA stop codon, exons 1-8 of mouse H-2D) with a 6,085 bp cassette containing from 5′ to 3′: a LacZ gene in frame with a 5′ loxp site, UbC promoter, Neomycin gene, and 3′ loxp site. 
     The BAC DNA targeting vector (described above) was used to electroporate mouse ES cells comprising humanized MHC I locus and a deletion of mouse MHC II, described in Example 1.2 above. Positive ES cells containing a deleted endogenous H-2D locus were identified by the quantitative PCR assay, as described above. Table 1 contains primers and probes used for the quantitative PCR assay. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 TAQMAN ™ Loss of Allele Assay Primers and Probes 
               
            
           
           
               
               
               
               
            
               
                 Name 
                   
                   
                   
               
               
                 (loca- 
                 Forward 
                 Reverse 
                   
               
               
                 tion) 
                 Primer 
                 Primer 
                 Probe 
               
               
                   
               
               
                 5152 mTU 
                 CGAGGAGCCCCG 
                 AAGCGCACGAACTC 
                 CTCTGTCGGCTAT 
               
               
                 (up- 
                 GTACA 
                 CTTGTT 
                 GTGG 
               
               
                 stream) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 12) 
                 NO: 13) 
                 NO: 14) 
               
               
                   
               
               
                 5152 mTD 
                 GGACTCCCAGAAT 
                 GAGTCATGAACCAT 
                 TGGTGGGTTGCTG 
               
               
                 (down- 
                 CTCCTGAGA 
                 CACTGTGAAGA 
                 GAA 
               
               
                 stream) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 15) 
                 NO: 16) 
                 NO: 17) 
               
               
                   
               
            
           
         
       
     
     Example 1.4: Generation of Mouse ES Cells Comprising Humanized MHC II Locus 
     To generate a vector comprising humanized HLA-DR2/H-2E, first, mouse H-2Ea gene was modified in accordance with the description in US Patent Application Publication No. US 2013-0111616, incorporated herein by reference, to generate a vector comprising sequence encoding a chimeric H-2Ea/HLA-DRA1*01 protein. 
     For mouse H-2Eb gene, synthesized human HLA-DR2 β chain (DRB1*1501) was used to generate a vector comprising DRβ1*02(1501) exons and introns, and swapped using bacterial homologous recombination into the vector comprising chimeric H-2Ea/HLA-DRA1*01 protein. H-2Eb gene was modified essentially as described in U.S. Patent Application Publication Nos. US 2013-0111616 and US 2013-0185820, incorporated herein by reference. A hygromycin selection cassette was used. 
     The resulting HLA-DR2/H-2E LTVEC is depicted in  FIG. 6 . The various nucleotide sequence junctions of the resulting LTVECs (e.g., mouse/human sequence junctions, human/mouse sequence junctions, or junctions of mouse or human sequence with selection cassettes) are summarized below in Table 2 and listed in the Sequence Listing; their locations are indicated in the schematic diagram of  FIG. 6 . In Table 2 below, with the exception of sequences marked with asterisks (*, see Table legend) the mouse sequences are in regular font; the human sequences are in parentheses; the Lox sequences are italicized; and the restriction sites introduced during cloning steps and other vector-based sequences (e.g., multiple cloning sites, etc.) are bolded. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Nucleotide Sequence Junctions 
               
            
           
           
               
               
            
               
                 SEQ 
                   
               
               
                 ID 
                   
               
               
                 NO: 
                 Nucleotide Sequence 
               
               
                   
               
               
                 18 
                 CTGTTTCTTC CCTAACTCCC ATTCTATGCT CTTCCATCCC  
               
               
                   
                 GA  CCGCGG (CCCA ATCTCTCTCC ACTACTTCCT 
               
               
                   
                 GCCTACATGT ATGTAGGT) 
               
               
                   
               
               
                 19 
                 (CAAGGTTTCC TCCTATGATG CTTGTGTGAA ACTCGG)  
               
               
                   
                   GGCC GGCC  AGCATTTAAC AGTACAGGGA TGGGAGCACA 
               
               
                   
                 GCTCAC 
               
               
                   
               
               
                 20* 
                 (GAAAGCAGTC TTCCCAGCCT TCACACTCAG AGGTACAAAT)  
               
               
                   
                 CCCCATTTTC ATATTAGCGA TTTTAATTTA TTCTAGCCTC 
               
               
                   
               
               
                 21* 
                 TCTTCCCTAA CTCCCATTCT ATGCTCTTCC ATCCCGA  CCG    
               
               
                   
                   CGG  (CCCAATC TCTCTCCACT ACTTCCTGCC TACATGTATG) 
               
               
                   
               
               
                 22 
                 GAGTTCCTCCATCACTTCACTGGGTAGCACAGCTGTAACTGT 
               
               
                   
                 CCAGCCTG(TCCTGGGCTGCAGGTGGTGGGCGTTGCGGGTGG 
               
               
                   
                 GGCCGGTTAAGGTTCCA) 
               
               
                   
               
               
                 23 
                 (TCCCACATCCTATTTTAATTTGCTCCATGTTCTCATCTCCA 
               
               
                   
                 TCAGCACAG) CTCGAG   ATAACTTCGTATAATGTATGCTATA   
               
               
                   
                   CGAAGTTAT   ATGCATGGCC   
               
               
                   
               
               
                 24 
                   ATACGAAGTTAT   GCTAGTAACTATAACGGTCCTAAGGTAGC   
               
               
                   
                   GAGTGGCTT  ACAGGTAGGTGCGTGAAGCTTCTACAAGCACA 
               
               
                   
                 GTTGCCCCCTGGGAAGCA 
               
               
                   
               
               
                 *Sequences marked with asterisk are C57BL/6-BALB/c junction sequences where C57BL/6 sequences are in parentheses. During cloning of the chimeric H-2Ea gene, exon 1 and the remainder of intron 1 of the C57BL/6 allele of H-2Ea was replaced with the equivalent 2616 bp region from the BALB/c allele of H-2Ea. This was done because exon 1 of the C57BL/6 allele of H-2Ea contains a deletion which renders the gene nonfunctional, while exon 1 of BALB/c allele of H-2Ea is functional. For a more detailed description, see U.S. patent application Publication No. US 2013-0111616, incorporated herein by reference. 
               
            
           
         
       
     
     The targeted BAC DNA described above was used to electroporate mouse ES cells comprising humanized MHC I (HLA-A2), as well as MHC II and H-2D deletion to create modified ES cells for generating mice that express chimeric MHC I and MHC II genes and lack functional endogenous mouse H-2E, H-2A, H-2K, and H-2D loci. ES cells containing an insertion of human HLA sequences were identified by a quantitative PCR (TAQMAN™) assay, using primers and probes in Table 3 and the procedure described above. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 TAQMAN ™ Primer and Probe Sequences 
               
            
           
           
               
               
               
               
            
               
                 Name 
                 Forward 
                 Reverse 
                   
               
               
                 (location) 
                 Primer 
                 Primer 
                 Probe 
               
               
                   
               
               
                 Hyg  
                 TGCGGCCGATC 
                 TTGACCGATTCC 
                 ACGAGCGGGTT 
               
               
                 cassette 
                 TTAGCC 
                 TTGCGG 
                 CGGCCCATTC 
               
               
                   
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 25) 
                 NO: 26) 
                 NO: 27) 
               
               
                   
               
               
                 7092 hTUP1 
                 CCCCACAGCAC 
                 CGTCCCATTGAA 
                 TGGCAGCCTAA 
               
               
                 (Exon 2 of 
                 GTTTCCT 
                 GAAATGACACT 
                 GAGG 
               
               
                 DRB1*1501) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 28) 
                 NO: 29) 
                 NO: 30) 
               
               
                   
               
               
                 7092 hTUP2 
                 CCCCACAGCAC 
                 ACCCGCTCCGTC 
                 AGCCTAAGAGG 
               
               
                 (Exon 2 of 
                 GTTTCCT 
                 CCATT 
                 GAGTGTC 
               
               
                 DRB1*1501) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 31) 
                 NO: 32) 
                 NO: 33) 
               
               
                   
               
               
                 7092 hTDP1 
                 AGACCCTGGTG 
                 CGCTTGGGTGCT 
                 TCGAAGTGGAG 
               
               
                 (Exon 3 of 
                 ATGCTGGAA 
                 CCACTT 
                 AGGTTTA 
               
               
                 DRB1*1501) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 34) 
                 NO: 35) 
                 NO: 36) 
               
               
                   
               
               
                 7092 hTDP2 
                 TGGAATGGAGT 
                 GCACGGTCCCCT 
                 TGACTTCCTAAA 
               
               
                 (exon 3 of 
                 GAGCAGCTTT 
                 TCTTAGTG 
                 TTTCTC 
               
               
                 DRB1*1501) 
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                   
                 NO: 37) 
                 NO: 38) 
                 NO: 39) 
               
               
                   
               
               
                 hDRAIU 
                 CTGGCGGCTTG 
                 CATGATTTCCAG 
                 CGATTTGCCAGC 
               
               
                 (exon 2 of 
                 AAGAATTTGG 
                 GTTGGCTTTGTC 
                 TTTGAGGCTCAA 
               
               
                 DRA) 
                 (SEQ ID 
                 (SEQ ID 
                 GG 
               
               
                   
                 NO: 40) 
                 NO: 41) 
                 (SEQ ID 
               
               
                   
                   
                   
                 NO: 42) 
               
               
                   
               
               
                 1751jxn2 1   
                 CCTCACTTGGGA 
                 TTGTCCCAGTCA 
                 TGCATCTCGAGC 
               
               
                 (loss-of- 
                 CCAACCCTA 
                 CCGTCCAT 
                 ACAGGCATTTGG 
               
               
                 allele  
                 (SEQ ID 
                 (SEQ ID 
                 (SEQ ID 
               
               
                 assay, 
                 NO: 43) 
                 NO: 44) 
                 NO: 45) 
               
               
                 sequence 
                   
                   
                   
               
               
                 present  
                   
                   
                   
               
               
                 in H- 
                   
                   
                   
               
               
                 2A and  
                   
                   
                   
               
               
                 H-2E 
                   
                   
                   
               
               
                 delete  
                   
                   
                   
               
               
                 only) 
               
               
                   
               
               
                   1 A11 sequences except this one are used in the gain-of-allele assay. 
               
            
           
         
       
     
     The selection cassette may be removed by methods known by the skilled artisan. For example, ES cells bearing the chimeric human/mouse MHC class I locus may be transfected with a construct that expresses Cre in order to remove the “loxed” selection cassette introduced by the insertion of the targeting construct. The selection cassette may optionally be removed by breeding to mice that express Cre recombinase. Optionally, the selection cassette is retained in the mice. 
     Targeted ES cells containing all of the modifications described in Examples 1.1-1.4 (H2-K +/1666  MHC-II +/6112  H2-D +/delete  of  FIG. 4 ) were verified using a quantitative TAQMAN® assay described above using the primer/probe sets described herein for individual modifications. An additional primer/probe set was used to determine that during cassette-deletion step, no inverted clone was created due to lox sites present in opposing orientation. 
     Targeted ES cells described above were used as donor ES cells and introduced into an 8-cell stage mouse embryo by the VELOCIMOUSE® method (see, e.g., U.S. Pat. No. 7,294,754 and Poueymirou et al. (2007) F0 generation mice that are essentially fully derived from the donor gene-targeted ES cells allowing immediate phenotypic analyses Nature Biotech. 25(1):91-99). VELOCIMICE® (F 0  mice fully derived from the donor ES cell) independently bearing a chimeric MHC class I and MHC II genes were identified by genotyping using a modification of allele assay (Valenzuela et al., supra) that detects the presence of the unique human gene sequences. A schematic representation of the genotype of MHC loci in the resulting mice is depicted in  FIG. 7  (** represents H-2L gene which is not present in all mouse strains). 
     Example 1.5: Characterization of Mice Comprising Chimeric MHC I and II Genes 
     Spleens from WT or heterozygous humanized HLA-A2/HLA-DR2/H-2D-del mice (see mice generated on the bottom of  FIG. 4  (“HLA-A2/H-2K, HLA-DR2/H-2E, H-2A-del, H-2D-del HET” or “H-2K +/1666  MHC-II +/6112  H2-D +/delete ” or “chimeric A2 and DR2”) were harvested, and single cell suspensions were prepared. Cell surface expression of mouse MHC class I (H2D), human MHC class I (HLA-A2), and human MHC class II (HLA-DR) on CD3+ T cells, CD19+ B cells, and CD14+ monocytes were analyzed by FACS. As shown in  FIG. 8 , there was no significant disruption in T to B cell ratio or no significant effect on T and B cell development. Expression of human HLA-A2 and HLA-DR were detectable on the surface of CD19+ B cells and monocytes ( FIGS. 9 and 10 ). Expression of human HLA-A2, but not HLA-DR, was detectable on the surface of CD3+ T cells ( FIG. 11 ), as MHC II is only expressed on antigen-presenting cells. 
     Equivalents 
     Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. 
     Entire contents of all non-patent documents, patent applications and patents cited throughout this application are incorporated by reference herein in their entirety.