AU2012327206B2 - Humanized IL-6 and IL-6 receptor - Google Patents

Abstract

(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (19) World Intellectual Property Organization International Bureau (10) International Publication Number (43) International Publication Date W O 2013/063556 Al 2 May 2013 (02.05.2013) W I PO I P CT (51) International Patent Classification: (81) Designated States (unless otherwise indicated, for every C07K14/54 (2006.01) A01K 67/027 (2006.01) kind of national protection available): AE, AG, AL, AM, C07K14/715 (2006.01) AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW, BY, ) .a BZ, CA, CH, CL, CN, CO, CR, CU, CZ, DE, DK, DM, (21) International Application Number: DO, DZ, EC, EE, EG, ES, Fl, GB, GD, GE, GH, GM, GT, PCT/US2012/062379 HN, HR, HU, ID, IL, IN, IS, JP, KE, KG, KM, KN, KP, (22) International Filing Date: KR, KZ, LA, LC, LK, LR, LS, LT, LU, LY, MA, MD, 29 October 2012 (29.10.2012) ME, MG, MK, MN, MW, MX, MY, MZ, NA, NG, NI, NO, NZ, OM, PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, (25) Filing Language: English RW, SC, SD, SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, (26) Publication Language: English TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW. (30) Priority Data: 61/552,900 28 October 2011 (28.10.2011) US (84) Designated States (unless otherwise indicated, for every 61/556,579 7 November 2011 (07.11.2011) US kind of regional protection available): ARIPO (BW, GH, GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, SZ, TZ, (71) Applicant: REGENERON PHARMACEUTICALS, UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU, TJ, INC. [US/US]; 777 Old Saw Mills River Road, Tarrytown, TM), European (AL, AT, BE, BG, CH, CY, CZ, DE, DK, NY 10591 (US). EE, ES, Fl, FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV, (72) Inventors: WANG, Li-hsien; 184 Brianwood Drive, MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK, SM, Somers, NY 10589 (US). DORE, Anthony, T.; 485 TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW, Gramatan Avenue, Mount Vernon, NY 10552 (US). ML, MR, NE, SN, TD, TG). STEVENS, Sean; 355 Berry Street, #413, San Francisco, Published: CA 94158 (US). MURPHY, Andrew, J.; 10 Newton _ with international search report (Art. 21(3)) Court, Croton-on-hudson, NY 10520 (US). - before the expiration of the time limit for amending the (74) Agent: SMELAND, Tor; Regeneron Pharmaceuticals, claims and to be republished in the event of receipt of Inc., 777 Old Saw Mills River Road, Tarrytown, NY 10591 amnets(ue4.(h (US). - with sequence listing part of description (Rule 5.2(a)) (54) Title: HUMANIZED IL-6 AND IL-6 RECEPTOR (57) Abstract: Mice that comprise a replacement of endogenous mouse IL-6 and/or IL-6 receptor genes are described, and methods f4 for making and using the mice. Mice comprising a replacement at an endogenous IL-6Ra locus of mouse ectodomain- encoding se quence with human ectodomain-encoding sequence is provided. Mice comprising a human IL-6 gene under control of mouse IL-6 regulatory elements is also provided, including mice that have a replacement of mouse IL-6-encoding sequence with human IL-6-en coding sequence at an endogenous mouse IL-6 locus.

Description

WO 2013/063556 PCT/US2012/062379 HUMANIZED IL-6 AND IL-6 RECEPTOR FIELD OF INVENTION [0001] Non-human animals having a replacement of the endogenous non-human animal IL-6 and/or IL-6 receptor genes are provided. IL-6 and/or IL-6 receptor genes of the non-human animal are replaced, at the endogenous non-human loci, with human IL-6 and/or humanized IL-6 receptor genes comprising human sequence. Non-human animals that have human IL-6 and/or humanized IL-6 receptor genes, wherein the non-human animals do not exhibit one or more pathologies that are characteristic of non-human animals transgenic for human IL-6. BACKGROUND [0002] Mice transgenic for a human IL-6 gene are known in the art. However, random insertion of a human IL-6 transgene into the mouse genome results in poorly regulated expression of the human IL-6 protein, which manifests itself in a variety of pathologies in such transgenic mice, including, but not limited to, plasmacytosis and glomerulonephritis. As a result, these mice have limited usefulness. [0003] There is a need for non-human animals, e.g., mice and rats, the express human or humanized IL-6 and/or human or humanized IL-6 receptor. There is a need for such humanized mice that do not exhibit one or more pathologies exhibited by transgenic hlL-6 mice. SUMMARY [0004] In one aspect, genetically modified non-human animals are provided that comprise a replacement at an endogenous IL-6 and/or IL-6 receptor locus of a gene encoding an endogenous IL-6 and/or IL-6 receptor with a gene encoding a human or humanized IL-6 and/or IL-6 receptor. Murine animals are provided that comprise a replacement of an endogenous IL-6 gene, at an endogenous murine IL-6 locus, with a human IL-6 gene; and/or that comprise a replacement of an endogenous IL-6 receptor gene (or nucleotide sequence encoding an ectodomain thereof) with a human IL-6 receptor gene (or nucleotide sequence encoding an ectodomain thereof). [0005] In one aspect, genetically modified murine animals are provided that express a human IL-6 gene under the control of endogenous murine promoter and/or endogenous murine regulatory elements, from an endogenous murine IL-6 locus. [0006] In one aspect, genetically modified murine animals are provided that express a human IL-6 receptor gene (or a gene encoding a human ectodomain and mouse 1 WO 2013/063556 PCT/US2012/062379 transmembrane and intracellular domains) under the control of endogenous murine promoter and/or endogenous murine regulatory elements, from an endogenous murine IL-6 receptor locus. [0007] In one aspect, a genetically modified animal (e.g., a murine animal, e.g., a mouse or rat) is provided that expresses a human IL-6 protein, wherein the non human animal does not exhibit a pathology selected from plasmacytosis, glomerulonephritis, glomerulosclerosis, mesangio-proliferative glomerulonephritis, intestinal lymphoma, kidney lymphoma, splenomegaly, lymph node enlargement, liver enlargement, megakaryocytes in bone marrow, compacted abnormal plasma cells, infiltration of plasma cells into lung or liver or kidney, mesangial cell proliferation in kidney, cerebral overexpression of IL-6, ramified microglial cells in white matter, reactive astrocytosis in brain, kidney failure, elevated megakaryocytes in spleen, muscle wasting (e.g., gastrocnemius muscle wasting), elevated muscle cathepsins B and B+L (e.g., around 20-fold and 6-fold), and a combination thereof. [0008] In one embodiment, the non-human animal comprises a normal B cell population. In one embodiment, the normal B cell population is approximately the same in number and immunophenotype as a wild-type animal, e.g., a wild-type mouse. [0009] In one embodiment, the non-human animal is murine (e.g., a mouse or rat) and expresses human IL-6 (hlL-6) in serum at a level below about 800 pg/mL, below about 700, 600, 500, 400, 300, or 200 pg/mL. In a specific embodiment, the murine animal expresses hlL-6 in serum at a level of about 50 to about no more than 200 pg/mL, in another embodiment about 75-125 pg/mL, in another embodiment at about 100 pg/mL. [0010] In one aspect, a non-human animal is provided that expresses hlL-6 and/or hlL-6R, wherein the non-human animal expresses hlL-6 and/or hlL-6R from an endogenous non-human IL-6 locus and/or an endogenous non-human hlL-6R locus. In a specific embodiment, the non-human animal is murine (e.g., mouse or rat). [0011] In one aspect, a genetically modified mouse is provided that expresses hlL-6 from an endogenous mouse IL-6 locus, wherein the endogenous mouse IL-6 gene has been replaced with a hlL-6 gene. [0012] In one embodiment, the mouse comprises a cell that expresses an IL-6 receptor (IL-6R) that comprises a human ectodomain on the surface of the cell. In one embodiment, the cell is a lymphocyte. In one embodiment, the lymphocyte is a B cell. 2 WO 2013/063556 PCT/US2012/062379 [0013] In one embodiment, about 6.8 kb at the endogenous mouse IL-6 locus, including exons 1 through 5 and a 3' untranslated sequence, is deleted and replaced with about 4.8 kb of human IL-6 gene sequence comprising exons 1 through 5 of the human IL-6 gene. In a specific embodiment, the human IL-6 gene comprises exons 1 through 5 of the human IL-6 gene of human BAC CTD-2369M23. [0014] In one aspect, a genetically modified mouse is provided that expresses IL-6 from a human IL-6 gene, wherein the mouse expresses human IL-6 in its serum. [0015] In one embodiment, the mouse serum exhibits a serum concentration of human IL-6 of about 25 to about 300 pg/mL, 50 to about 250 pg/mL, 75 to about 200 pg/mL, or 100 to about 150 pg/mL. In a specific embodiment, the level of human IL-6 in the serum of the mouse is about 100 pg/mL. [0016] In one embodiment, the level of a pan B cell-specific marker in bone marrow of the mouse is about the same as that of a wild-type mouse. In one embodiment, the level of a pan B cell-specific marker in spleen is about the same as that of a wild type mouse. In one embodiment, the pan B cell-specific marker is selected from B220, CD19, CD20, CD22, CD79a, CD79b, L26, and Pax-5 (BSAP). [0017] In one aspect, a genetically modified mouse is provided that expresses hlL6, wherein the mouse does not exhibit a feature selected from plasmacytosis, splenomegaly, lymph node enlargement, compacted abnormal plasma cells, and a combination thereof. [0018] In one embodiment, the mouse comprises a spleen that is about the same weight (per body weight) as a wild-type mouse. In one embodiment, the lymph nodes of the mouse are about the same weight (per body weight) as a wild-type mouse. In one embodiment, plasma cells of the mouse do not exhibit plasmocytosis characteristic of mice that overexpress human IL-6. [0019] In one embodiment, the mouse does not exhibit glomerulonephritis. [0020] In one embodiment, the mouse exhibits a mesangial cell level comparable to a wild-type mouse. [0021] In one aspect, a genetically modified mouse is provided that expresses hlL6 from an endogenous mouse IL-6 locus, wherein the endogenous mouse IL-6 gene has been replaced with a hlL-6 gene, wherein the mouse does not exhibit a feature selected from a morphologically detectable neuropathology, a reactive astrocytosis, and a combination thereof. In one embodiment, the mouse comprises a brain that is morphologically indistinct from a wild-type mouse brain. In one embodiment, the mouse comprises brain tissue that exhibits a level of reactive astrocytosis that is no higher than that of a wild-type mouse. 3 WO 2013/063556 PCT/US2012/062379 [0022] In one embodiment, the mouse does not express human IL-6 in neurons. In one embodiment, the mouse comprises activated astrocyte levels that are comparable to activated astrocyte levels in a wild-type mouse. [0023] In one embodiment, the mouse comprises ramified microglial cells in its white matter, wherein the ramified microglial cells are present in an amount equivalent to an amount of ramified microglial cells in a wild-type mouse. [0024] In one embodiment, the mouse does not exhibit a reactive atrocytosis. In one embodiment, the white matter of the mouse is morphologically indistinct from the white matter of a wild-type mouse. In one embodiment, the white matter of the mouse is histologically indistinct from a wild-type mouse white matter with respect to histochemical staining of reactive astrocytes. [0025] In one embodiment, the mouse comprises a brain that is morphologically indistinct from a wild-type mouse brain. In one embodiment, the mouse comprises brain tissue that exhibits a level of reactive astrocytosis that is no higher than that of a wild-type mouse. [0026] In one aspect, a genetically modified mouse is provided that expresses hlL6 from an endogenous mouse IL-6 locus, wherein the endogenous mouse IL-6 gene has been replaced with a hlL-6 gene, wherein the mouse does not exhibit a feature selected from a life span shortened by about 50% or more, kidney failure, hypergammaglobulinemia, elevated megakaryocytes in spleen, elevated megakaryocytes in bone marrow, plasmacytosis of spleen, plasmacytosis of thymus, plasmacytosis of lymph nodes, glomerulonephritis, glomerulosclerosis, and a combination thereof. [0027] In one embodiment, the mice have a life span that exceeds 20 weeks. In one embodiment, the mice have a life span that exceeds 30 weeks, 40 weeks, or 50 weeks. In one embodiment, the mice exhibit a life span about equal to that of a wild type mouse of the same strain. [0028] In one embodiment, the mice exhibit a level of megakaryocytes in spleen that is no more than about the splenic megakaryocyte level of a wild-type mouse [0029] In one embodiment, the mice comprise lymphoid organs that are essentially devoid of abnormal and compactly arranged plasmacytoid cells. [0030] In one embodiment, the mice exhibit gamma globulin serum levels equivalent to gamma globulin serum levels in wild-type mice. In one embodiment, the levels of al- and p-globulin in serum of the mice are equivalent to a1- and p-globulin serum levels of wild-type mice of the same strain. 4 WO 2013/063556 PCT/US2012/062379 [0031] In one aspect, a genetically modified mouse is provided that expresses human IL-6 from an endogenous mouse IL-6 locus, wherein the endogenous mouse IL-6 gene has been replaced with a hlL-6 gene, wherein the mouse does not exhibit a feature selected from muscle wasting, an elevated cathepsin B level as compared with a wild-type mouse of the same strain, an elevated cathepsin A+B level as compared with a wild-type mouse of the same strain, an increased liver weight as compared with a wild-type mouse of the same strain, and a combination thereof. [0032] In one embodiment, the weight of the liver of the mouse is about 800-900 mg at 12 weeks. [0033] In one embodiment, the mouse exhibits a cathepsin B level throughout its life span that is no more than about the level observed in a wild-type mouse. In one embodiment, the mouse exhibits a cathepsin A+B level throughout its life span that is no more than about the level observed in a wild-type mouse. [0034] In one embodiment, the mouse as an adult exhibits a gastrocnemus muscle weight that is within about 10% of the weight of a wild-type mouse of the same strain. In one embodiment, the mouse as an adult exhibits a gastrocnemus muscle weight that is about the same as that of a wild-type mouse. [0035] In one aspect a mouse is provided that comprises a nucleotide sequence encoding a human IL-6 protein, wherein the nucleotide sequence encoding the human IL-6 protein replaces in whole or in part an endogenous nucleotide sequence encoding and endogenous mouse IL-6 protein. [0036] In one aspect, a mouse is provided that comprises a replacement at an endogenous mouse IL-6 receptor locus of mouse IL-6Ra ectodomain with an ectodomain sequence of a human IL-6Ra to form a chimeric human/mouse IL-6Ra gene. [0037] In one embodiment, the chimeric IL-6Ra gene is under the control of a mouse promoter and/or mouse regulatory elements at the endogenous mouse IL-6Ra locus. [0038] In one embodiment, about 35.4 kb of mouse IL-6Ra ectodomain-encoding sequence is replaced with about 45.5 kb of human IL-6R ectodomain-encoding sequence. [0039] In one embodiment, the human IL-6R ectodomain-encoding sequence encompasses the first (ATG) codon in exon 1 through exon 8. [0040] In one embodiment, the mouse IL-6Ra sequence that is replaced includes a contiguous sequence that encompasses exons 1 through 8. In a specific embodiment, exons 1 through 8 and a portion of intron 8 is deleted. 5 [0041] In one aspect, a genetically modified mouse is provided, comprising a replacement at an endogenous mouse IL-6 locus of a mouse gene encoding IL-6 with a human gene encoding human IL-6, wherein the human gene encoding human IL-6 is under control of endogenous mouse regulatory elements at the endogenous mouse IL-6 locus. [0042] In one embodiment, the human gene encoding human IL-6 is a human IL-6 gene of BAC ID CTD-2369M23. [0043] In one embodiment the mouse expresses a mouse IL-6Ra. In one embodiment, the mouse expresses a human IL-6Ra. In one embodiment, the humanized IL-6Ra comprises a human ectodomain In one embodiment, the humanized IL-6Ra comprises a mouse transmembrane domain and a mouse cytoplasmic domain. In one embodiment, the mouse expresses a humanized IL-6Ra that comprises a humanization of ectodomain but not transmembrane and/or cytosolic domain. (0044] In one embodiment, the mouse does not exhibit a feature selected from plasmocytosis, glomerulosclerosis, glomerulonephritis, kidney failure, hypergammaglobulinemia, elevated megakaryocytes in spleen, elevated megakaryocytes in bone marrow, splenomegaly, lymph node enlargement, compacted abnormal plasma cells, and a combination thereof. [0045] In one aspect, a genetically modified mouse is provided, comprising a humanization of an endogenous mouse IL-6Ra gene, wherein the humanization comprises a replacement of mouse IL-6Ra ectodomain-encoding sequence with human IL-6iRa ectodomain-encoding sequence at the endogenous mouse IL-6Ra locus, and wherein the humanized IL-6Ra gene is under control of endogenous mouse regulatory elements. [0046] In one embodiment, a contiguous mouse sequence comprising mouse exons I through 8 is replaced with a contiguous genomic fragment of human IL-6Ra sequence encoding a human IL 6Ra ectodomain. In one embodiment, the contiguous genomic fragment of human IL-6Ra sequence encoding the ectodomain is from BAC CTD-2192J23. [0047] In one embodiment, the mouse further comprises a humanized IL-6 gene. In one embodiment, the mouse comprises a replacement at an endogenous mouse IL-6 locus of a mouse IL-6 gene with a human IL-6 gene. In one embodiment, the humanized IL-6 gene is under control of endogenous mouse regulatory elements. [0048] In one aspect, a method is provided for making a humanized mouse, comprising replacing a mouse gene sequence encoding mouse IL-6 with a human gene encoding human IL-6 so that the human IL-6 gene is under control of endogenous mouse regulatory elements. [0049] In one embodiment, the replacement is at an endogenous mouse IL-6 locus, and the human gene encoding human IL-6 is operably linked to endogenous mouse regulatory sequences, [0050] In one aspect, a method for making a humanized mouse is provided, comprising replacing all mouse exons encoding ectodomain sequences of mouse IL-6Ra with a human genomic 6 fragment encoding human IL-6Ra ectodomain to form a humanized IL-6Ra gene, wherein the humanized IL-6Ra gene is under control of endogenous mouse regulatory elements. [00511 In one embodiment, the replacement is at an endogenous mouse IL-6Ra locus, and the humanized IL-6Ra gene is operably linked to endogenous mouse regulatory sequences. [0052] In one aspect, a genetically modified mouse is provided, comprising a humanized IL-6Ra gene comprising a replacement of mouse ectodomain-encoding sequence with human ectodomain sequence, wherein the humanized IL-6Ra gene comprises a mouse transmembrane sequence and a mouse cytoplasmic sequence, wherein the mouse further comprises a gene encoding a human IL-6, wherein the genes encoding a human IL-6 and humanized IL-6Ra are under control of endogenous mouse regulatory elements. [00531 In one embodiment, the mouse is incapable of expressing a fully mouse IL-6Ra and incapable of expressing a mouse IL-6. [00541 In various aspects, the genetically modified mice described herein comprise the genetic modifications in their germline. [0055] In one aspect, a tissue, cell, or membrane fragment from a mouse as described herein is provided. (0056] In one embodiment, the tissue or cell is from a mouse that expresses a human IL-6 protein, but that does not express a mouse iL-6 protein. In one embodiment, the tissue or cell is from a mouse that expresses a humanized IL-6Ra protein, but not a mouse IL-6Ra protein. In one embodiment, the humanized IL-6Ra protein comprises a human ectodomain and a mouse transmembrane domain and a mouse cytosolic domain. In one embodiment, the tissue or cell is from a mouse that expresses a human IL-6, a humanized IL-6Ra, and that does not express a mouse IL-6 and does not express an IL-6Ra that comprises a mouse ectodomain. [0057] In one aspect, an ex vivo complex of a mouse cell bearing a humanized IL-6Ra (human ectodomain and mouse transmembrane and mouse cytoplasmic domain) and a human IL-6 is provided. [0058] In one aspect, a mouse embryo comprising a genetic modification as described herein is provided. 7 WO 2013/063556 PCT/US2012/062379 [0059] In one aspect, a mouse host embryo is provided that comprises a donor cell that comprises a genetic modification as described herein. [0060] In one aspect, a pluripotent or totipotent non-human animal cell comprising a genetic modification as described herein is provided. In one embodiment, the cell is a murine cell. In one embodiment, the cell is an ES cell. [0061] In one aspect, a mouse egg is provided, wherein the mouse egg comprises an ectopic mouse chromosome, wherein the ectopic mouse chromosome comprises a genetic modification as described herein. [0062] In one aspect, the mouse, embryo, egg, or cell that is genetically modified to comprise a human IL-6 gene or human or humanized IL-6Ra gene is of a mouse that is 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/Svlm), 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 a specific embodiment, the genetically modified mouse is a mix of an aforementioned 129 strain and an aforementioned C57BL/6 strain. In another specific embodiment, the mouse is a mix of aforementioned 129 strains, or a mix of aforementioned BL/6 strains. In a specific 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 one embodiment, the mouse is Swiss or Swiss Webster mouse. [0063] Each of the aspects and embodiments described herein are capable of being used together, unless excluded either explicitly or clearly from the context of the embodiment or aspect. BRIEF DESCRIPTION OF THE DRAWINGS [0064] FIG. 1 provides an illustration, not to scale, of the human (top) and mouse (bottom) IL-6 genomic loci. Exons 1, 11, 111, IV, and V (in both human and mouse) are indicated by closed boxes to the right in the figure. Selected putative regulatory regions are indicated by open boxes to the left in the figure. 8 WO 2013/063556 PCT/US2012/062379 [0065] FIG. 2 shows acute phase response (mSAA level) in the presence or absence of turpentine in wild-type mice, humanized ectodomain IL-6R mice, and mice with humanized IL-6 and IL-6R genes. [0066] FIG. 3 shows turpentine-dependent acute phase response (SAA) in wild-type mice the absence or presence of anti-mouse IL-6R antibody (left); and turpentine dependent acute phase response in humanized IL-6/IL-6R mice in the absence or present of anti-human IL-6R antibody (right). [0067] FIG. 4 shows FACS analysis for splenic B cells of wild-type and humanized IL-6 mice; pan B cell marker. [0068] FIG. 5 shows FACS analysis for splenic T cells of wild-type an humanized IL 6 mice; T helper cells and cytotoxic T cells. [0069] FIG. 6 shows FACS analysis for splenic cells of wild-type and humanized IL-6 mice; Ly6G/C(Grl). [0070] FIG. 7 shows FACS analysis for splenic cells of wild-type and humanized IL-6 mice; NK cells and granulocytes (Ly6Ghi+/CD1 bhi+). [0071] FIG. 8 shows FACS analysis for blood B cells of wild-type and humanized IL 6 mice; pan B cell marker. [0072] FIG. 9 shows FACS analysis for blood T cells of wild-type and humanized IL 6 mice; T helper cells and cytotoxic T cells. [0073] FIG. 10 shows FACS analysis for blood myeloid cells of wild-type and humanized IL-6 mice; Gr1* cells. [0074] FIG. 11 shows FACS analysis for blood myeloid cells of wild-type and humanized IL-6 mice; CD11b vs. Ly6G/C(Grl). [0075] FIG. 12 shows FACS analysis for blood myeloid cells of wild-type and humanized IL-6 mice; DX5 vs CD1 lb cells. [0076] FIG. 13 shows FACS analysis of bone marrow IgM/CD24/B220 for wild-type and humanized IL-6 mice. Top: normal progression in bone marrow. Bottom: FACS analysis for wild-type, hlL-6 heterozygotes, and hlL-6 homozygotes (IgM staining). [0077] FIG. 14 shows FACS analysis of bone marrow IgM/CD24/B220 for wild-type and humanized IL-6 mice. Top: normal progression in bone marrow. Bottom: FACS analysis for wild-type, hlL-6 heterozygotes, and hlL-6 homozygotes (CD24 staining). [0078] FIG. 15 shows FACS analysis of bone marrow CD43 and B220 for wild-type and humanized IL-6 mice. Top: normal progression in bone marrow. Bottom: FACS analysis for wild-type, hlL-6 heterozygotes, and hlL-6 homozygotes (CD43 staining). 9 WO 2013/063556 PCT/US2012/062379 DETAILED DESCRIPTION IL-6 and IL-6R [0079] The IL-6 receptor (IL-6R) was long characterized as a receptor for a B cell stimulatory factor (BSF-2, or B cell Stimulatory Factor 2; also, BCDF, or B Cell Differentiation Factor) responsible for inducing B cells to synthesize immunoglobulin (Yamasaki et al. (1988) Cloning and Expression of the Human Interleukin-6(BSF 2/IFNs 2) Receptor, Science 241:825-828). IL-6 was first described as interferon-p2 as the result of its discovery during a search for a virally-induced protein termed interfereon-p, by treating human fibroblasts with dsRNA poly(I)poly(C) to induce an anti-viral response (Weissenbach et al. (1980) Two interferon mRNAs in human fibroblasts: In vitro translation and Escherichia coli cloning studies, Proc. Natl Acad. Sci. USA 77(12):7152-7156; Keller et al. (1996) Molecular and Cellular Biology of Interleukin-6 and Its Receptor, Frontiers in Bioscience 1:d340-357). [0080] The human cDNA encodes a 468 amino acid protein having a 19-mer signal sequence and a cytoplasmic domain of about 82 amino acids that lacks a tyrosine kinase domain (see, Id.). The N-terminal (ectodomain) of the protein has an Ig superfamily domain of about 90 amino acids, a 250-amino acid domain between the Ig superfamily domain and the membrane, a transmembrane span of about 28 amino acids (see, Id.). The ectodomain of the receptor binds its ligand IL-6, which triggers association with gpl30 in the membrane and it is this complex that conducts signal transduction; the cytoplasmic domain reportedly does not transduce signal (Taga et al. (1989) Interleukin-6 Triggers the Association of Its Receptor with a Possible Signal Transducer, gp130, Cell 58:573-581). Indeed, a soluble form of IL-6R lacking a cytoplasmic domain can associate with IL-6 and bind gp130 on the surface of a cell and effectively transduce signal (/d.). [0081] The homology of hlL-6R and mlL-6R at the protein level is only about 54%; the transmembrane domain has a homology of about 79%, whereas the cytoplasmic domain has a homology of about 54% (Sugito et al. (1990)). [0082] The natural ligand for the IL-6R, IL-6, was first isolated from cultures of HTLV 1-transformed T cells (see, Hirano et al. (1985) Purification to homogeneity and characterization of human B cell differentiation factor (BCDF or BSFp-2), Proc. NatI. Acad. Sci. USA 82:5490-5494). A human cDNA for the IL-6 gene was cloned at least twice, once as BSF-2 (see, Hirano et al. (1086) Complementary DNA fro a novel human interleukin (BSF-2) that induces B lymphocytes to produce immunoglobulin, Nature 324:73-76) and once as IFNs 2 (see, Zilberstein et al. (1986) Structure and expression of cDNA and genes for human interferon-beta-2, a distinct 10 WO 2013/063556 PCT/US2012/062379 species inducible by growth-stimulatory cytokines, EMBO 5:2529-2537), although it has since been demonstrated that recombinant human IL-6 exhibits no detectable IFN activity. [0083] Human IL-6 is a 184-amino acid protein that exhibits only about 42% homology with mouse IL-6, although the genomic organization of the human and mouse genes are basically the same, and the promoter regions of the human and mouse genes share a 400-bp stretch that is highly conserved (see, Tanabe et al. (1988) Genomic Structure of the Murine IL-6 Gene: High Degree Conservation of Potential Regulatory Sequences between Mouse and Human, J. Immunol. 141(11):3875-3881). [0084] The human IL-6 gene is about 5 kb (Yasukawa et al. (1987) Structure and expression of human B cell stimulatory factor-2 (BSC-2/IL-6) gene, EMBO J. 6(10):2939-2945), whereas the mouse IL-6 gene is about 7 kb (Tanabe et al. (1988) Genomic Structure of the Murine IL-6 Gene: High Degree Conservation of Potential Regulatory Sequences between Mouse and Human, J. Immunol. 141(11):3875 3881). The mouse and human IL-6 genes reportedly share highly conserved 5' flanking sequence important to regulation. A schematic diagram of the human and mouse IL-6 genomic loci is shown in FIG. 1 (not to scale). Exons 1, 11, 111, IV, and V (in both human and mouse) are indicated by closed boxes to the right in the figure. Selected putative regulatory regions are indicated by open boxes to the left in the figure. The putative regulatory regions for humans are, from left to right, a glucocorticoid element from -557 to -552; an IFN enhancer core sequence from -472 to -468; a glucocorticoid element from -466 to -461; an AT-rich region from -395 to 334, a consensus AP-1 binding site from -383 to -277; an IFN enhancer core sequence from -253 to -248; a GGAAA-containing motif from -205 to -192; a c-fos SRE homology sequence from -169 to -82 containing an IFN enhancer core sequence, a cAMP-response element, a GGAAA motif, a CCAAT box, and a GC-rich region; and AP-1 binding site from -61 to -55; and a CCAAT box from -34 to -30. The putative regulatory regions for mouse are, from left to right, a GC rich region from 553 to -536, a glucocorticoid element from -521 to -516 and from -500 to -495; a Z DNA stretch from -447 to -396; an AP-1 binding site overlapping an IFN enhancer core sequence from -277 to -288, a GGAAA motif overlapping an IFN enhancer core sequence from -210 to -195; a c-fos SRE homology region from -171 to -82 containing a cAMP response element, a GGAAA motif overlapping an IFN enhancer core sequence, and a GC-rich region; and, an AP-1 binding site from -61 to -55. Mouse codons I-V have lengths 19, 185, 114, 150, and 165, respectively. Mouse intron lengths are: 1-1I, 162 bp; II-Ill, 1253 bp; Ill-IV, 2981 bp; IV-V, 1281 bp. Human 11 WO 2013/063556 PCT/US2012/062379 codons I-V have lengths 19, 191, 114, 147, and 165. Human intron lengths are I-II, 154; II-Ill, 1047; Ill-IV, 706; IV-V, 1737. Genomic organization data are from Tanabe et al. (1988), and Yasukawa et al. (1987) Structure and expression of human B cell stimulatory factor-2 (BSF-2/IL-6) gene, EMBO J. 9(10):2939-2945. [0085] It might be reasonable to presume that the mouse and human IL-6 genes appear to be similarly regulated based on the similarity of their 5'-flanking sequence. A variety cell types exhibit enhanced IL-6 expression in response to IL-1, TNF, PDGF, IFNp, serum, poly(I)poly(C), and cycloheximide (see, Tanabe et al. (1988). IL-6 in humans mediates the acute phase response, hematopoiesis, B cell differentiation, T cell activation, growth and/or differentiation and/or activation of a variety of cell types (e.g., hepatocytes, fibroblasts, endothelial cells, neurons, pituitary cells, lymphomas, myelomas, breast carcinomas, NK cells, macrophages, osteoclasts, etc.) (reviewed in, e.g., Heinrich et al. (1990), Kishimoto et al. (1989), and Keller et al. (1996); Sugita et al. (1990) Functional Murine Interleukin Receptor with Intracisternal A Particle Gene Product at its Cytoplasmic Domain, J. Exp. Med. 171:2001-2009). [0086] In practice, however, mice transgenic for human IL-6 exhibit a panoply of substantial and debilitating pathologies, reflecting a significant pleiotropy of the IL-6 gene. Transgenic mice comprising a 6.6-kb fragment containing the human IL-6 gene and a p enhancer (Ep) produce high concentrations of hlL-6 and extremely high IgG1 levels (120- to 400-fold over wild-type mice), reflecting an IL-6 deregulation that is accompanied by plasmacytosis, mesangio-proliferative glomerulonephritis, and high bone marrow megakaryocyte levels (Suematsu et al. (1989) IgG1 plasmacytosis in interleukin 6 transgenic mice, Proc. Natl Acad. Sci. USA 86:7547-7551). Aberrant regulation of IL-6 and/or IL-6R is associated with myelomas, plastocytomas, rheumatoid arthritis, Castleman's disease, mesangial proliferative glomerulonephritis, cardiac myxoma, plams cell neoplasias, psoriasis, and other disorders (see, Kishimoto, T. (1989) The Biology of Interleukin-6, Blood 74(1):1-10; Sugita et al. (1990); also, Hirano et al. (1990) Biological and clinical aspects of interleukin 6, Immunology Today 11(12):443-449)). IL-6 is also implicated in sustaining levels of intra-prostatic androgens during androgen deprivation treatment of prostate cancer patients by a paracrine and/or autocrine mechanism, potentially providing castration-resistant prostate tumor growth (Chun et al. (2009) Interleukin-6 Regulates Androgen Synthesis in Prostate Cancer Cells, Clin. Cancer Res. 15:4815-4822). 12 WO 2013/063556 PCT/US2012/062379 [0087] The human protein is encoded as a 212 amino acid protein, in mature form a 184 amino acid protein following cleavage of a 28 amino acid signal sequence. It contains two N-glycosylation and two 0-glycosylation sites, and human IL-6 is phosphorylated in some cells. The mouse protein is encoded as a 211 amino acid protein, in mature form a 187 amino acid protein following cleavage of a 23 amino acid signal sequence. 0-glycosylation sites are present, but not N-glycosylation sites. (See reviews on IL-6, e.g., Heinrich et al. (1990) Interleukin-6 and the acute phase response, Biochem. J. 265:621-636.) [0088] IL-6 function is pleiotropic. The IL-6 receptor is found on activated B cells but reportedly not on resting B cells. In contrast, IL-6R is found on resting T cells and can reportedly promote T cell differentiation, activation, and proliferation, including the differentiation of T cells into cytotoxic T lymphocytes in the presence of IL-2. Humanized IL-6/IL-6R Ectodomain Mice and IL-6-Mediated Acute Phase Response [0089] In humans, IL-6 induces the acute phase response. Early studies with human hepatocytes established that IL-6 induces acute phase proteins such as, e.g., C reactive protein (CRP) and serum amyloid A (SAA) in a dose-dependent and time dependent manner (reviewed in Heinrich et al. (1990) Interleukin-6 and the acute phase response, Biochem. J. 265:621-636). Non-human animals, e.g., mice or rats, comprising humanized IL-6 and IL-6R genes are therefore useful systems for measuring the acute phase response mediated by human IL-6. Such animals are also useful for determining whether a substance induces an IL-6-mediated acute phase response, by exposing a humanized IL-6/IL-6R animal as described herein to the substance, and measuring a level of one or more acute phase response proteins (or RNAs). In one embodiment, the humanized animal is exposed to the substance in the presence of an antagonist of a human IL-6R, and a level of one or more acute phase response proteins (or RNAs) is measured, wherein a reduction in a level of an acute phase response protein (or RNA) in the presence of the human IL-6R antagonist indicates a human IL-6R-mediated acute phase response. [0090] Human IL-6 can bind both human IL-6R and mouse IL-6R; mouse IL-6 binds mouse IL-6R but not human IL-6R (no binding of mlL-6 to hlL-6R detectable, whereas hlL-6 can compete with mlL-6 for binding mlL-6R; Coulie et al. (1989) High and low-affinity receptors for murine interleukin 6. Distinct distribution on B and T cells, Eur. J. Immunol. 19:2107-211); see also, e.g., Peters et al. (1996) The Function of the Soluble Interleukin 6 (IL-6) Receptor In Vivo: Sensitization of Human 13 WO 2013/063556 PCT/US2012/062379 Soluble IL-6 Receptor Transgenic Mice Towards IL-6 and Prolongation of the Plasma Half-life of IL-6, J. Exp. Med. 183:1399-1406). Thus, human cells that bear hlL-6R in a mouse (e.g., in a xenogenic transplant) cannot rely on endogenous mlL-6 to carry out IL-6-mediated functions, including but not limited to the role of IL-6 blood cell or lymphocyte development (e.g., hematopoiesis, B cell activation, T cell activation, etc.). [00911 n a mixed in vivo system comprising a wild-type mouse IL-6 gene and a human IL-6R gene (but no mouse IL-6R gene), an acute phase response inducer is not expected to induce detectable levels of acute phase proteins that would indicate an acute phase response. However, a humanized mouse as described herein, comprising a humanized IL-6 gene and an IL-6R gene comprising a humanized ectodomain sequence will respond to an acute phase response inducer and exhibit acute phase response proteins in serum. Mice wild-type for IL-6/IL-6R tested for acute phase proteins in the presence or absence of the acute phase inducer turpentine showed a turpentine-dependent increase in acute phase proteins. Mice with humanized IL-6 gene, but not IL-6R, showed no acute phase response in the presence of turpentine. But mice bearing both a human IL-6 gene and an IL-6R gene with a humanized ectodomain exhibited a strong acute phase response (FIG. 2). The IL-6-mediated acute phase response was IL-6 dependent in both wild-type mice (FIG. 3, top) and in humanized IL-6/IL-6R ectodomain mice (FIG. 3, bottom), as evidenced by the ability of the appropriate anti-IL-6R antibody to abrogate the acute phase response at a sufficiently high antibody dose. Thus, a double humanization of IL-6 and IL-6R recapitulates the wild-type IL-6-mediated acute phase response with respect to serum acute phase proteins. Genetically Modified Mice [0092] Genetically modified mice are provided that express a human IL-6 and/or a humanized IL-6 receptor from endogenous mouse loci, wherein the endogenous mouse IL-6 gene and/or the endogenous mouse IL-6 receptor gene have been replaced with a human IL-6 gene and/or a human sequence comprising a sequence that encodes an ectodomain of a human IL-6 receptor. The genetically modified mice express the human IL-6 and/or humanized IL-6 receptor from humanized endogenous loci that are under control of mouse promoters and/or mouse regulatory elements. The replacement(s) at the endogenous mouse loci provide non-human animals that express human IL-6 and a humanized IL-6 receptor in a manner that does not result in the panoply of substantial pathologies observed in IL-6 transgenic mice known in the art. 14 WO 2013/063556 PCT/US2012/062379 [0093] Transgenic mice that express human IL-6 are known in the art. However, they generally suffer from significant pathologies that severely limit their usefulness. Humanized mice as described herein express a human IL-6 and/or humanized IL-6 receptor under the control of endogenous mouse regulatory elements at endogenous mouse IL-6 and IL-6Ra loci. These mice, in contrast, exhibit expression patterns with respect to these genes that are different from transgenic mice known in the art. [0094] Replacement of non-human genes in a non-human animal with homologous or orthologous human genes or human sequences, at the endogenous non-human locus and under control of endogenous promoters and/or regulatory elements, can result in a non-human animal with qualities and characteristics that may be substantially different from a typical knockout-plus-transgene animal. In the typical knockout-plus-transgene animal, an endogenous locus is removed or damaged and a fully human transgene is inserted into the animal's genome and presumably integrates at random into the genome. Typically, the location of the integrated transgene is unknown; expression of the human protein is measured by transcription of the human gene and/or protein assay and/or functional assay. Inclusion in the human transgene of upstream and/or downstream human sequences are apparently presumed to be sufficient to provide suitable support for expression and/or regulation of the transgene no matter where in the animal's genome the transgene winds up. But in many cases the transgene with human regulatory elements expresses in a manner that is unphysiological or otherwise unsatisfactory, and can be actually detrimental to the animal. In contrast, the inventors demonstrate that a replacement with human sequence at an endogenous locus under control of endogenous regulatory elements provides a physiologically appropriate expression pattern and level that results in a useful humanized animal whose physiology with respect to the replaced gene are meaningful and appropriate and context of the humanized animal's physiology. [0095] Fertilized mouse eggs injected with a construct having the MHC class I promoter H2 and a p-globin intron driving expression of a 695-bp mouse IL-6 gene reportedly produce mice that constitutively express mouse IL-6 at relatively high levels (as compared with wild-type mice) (see, Woodrofe et al. (1992) Long-Term Consequences of Interleukin-6 Overexpression in Transgenic Mice, DNA and Cell Biology 11 (8):587-592). But these mice are prone to develop lymphomas associated with the intestines, lymph nodes, and kidney, as well as splenic amyloid deposits. They also exhibit abnormal B cell maturation (see, Woodrofe et al., Id.), so that studies of B cell function are compromised. In contrast, mice as described herein 15 WO 2013/063556 PCT/US2012/062379 that comprise a replacement of the mouse IL-6 gene with a human IL-6 gene at the mouse IL-6 locus are not prone to develop these lymphomas, and the mice exhibit apparently normal B cell populations. [0096] Mice (C57BL/6) transgenic for hlL-6 due to a random insertion of a 6.6-kb (BamHI-Pvu || fragment) length of human DNA containing the hlL-6 gene coupled with an IgM enhancer have been reported (see, Suematsu et al. (1989) IgG1 plasmocytosis in interleukin 6 transgenic mice, Proc. Natl. Acad. Sci. USA 86:7547 7551). The mice express hlL-6 at between 800 pg/mL and 20,000 pg/mL in serum, where wild-type mice typically express only about 100 pg/mL IL-6. The mice exhibit an increase in serum Ig (120 to 400-fold over wild-type mice) and a decrease in albumin as they age. The mice suffer from a massive plasmacytosis, exhibit splenomegaly and lymph node enlargement, as well as exhibiting plasma cells and increased megakaryocytes in bone marrow. Upon inspection, what appear to be enlarged lymph nodes are instead massed of compacted abnormal plasma cells. Both spleen and thymus exhibit massive proliferation of plasma cells, which also infiltrate portions of the lung, liver, and kidney. Kidney in these mice also exhibits IL 6-stimulated mesangial cell proliferation typical of mesangio-proliferative glomerulonephritis. Similarly, mice (BALB/c) transgenic for a trimmed hlL-6 cDNA driven by a mouse H-2Ld promoter randomly inserted into the genome display severe plasmacytosis (see, Suematsu et al. (1992) Generation of plasmacytomas with the chromosomal translocation t(12;15) in interleukin 6 transgenic mice, Proc. Natl. Acad. Sci. USA 89:232-235). Although C57BL/6 mice that overexpress hlL-6 do not develop transplantable plasmacytomas (they do exhibit plasmacytosis), transgenic BL/6 mice back-crossed into BALB/c mice reportedly do. [0097] Random transgenesis of a hlL-6 cDNA driven by a glial fibrillary acidic protein (GFAP) gene promoter reportedly results in hlL-6 overexpression in the mouse central nervous system, which also leads to significant pathologies (see, Campbell et al. (1993) Neurologic disease induced in transgenic mice by cerebral overexpression of interleukin 6, Proc. Natl. Acad. Sci. USA 90:10061-10065). These mice exhibit extensive neuropathology and reactive astrocytosis resulting from IL-6 expression in the CNS due to loss of control as the result of random integration of an IL-6 transgene at an apparently CNS-permissive transcriptional locus. Although expression of hlL-6 cDNA linked to a p-globin 3'-UTR and driven by a neuron-specific enolase promoter microinjected into fertilized mouse eggs (F1 C57BL/6 x BALB/c) produced mice with a normal lifespan and without apparent neurological defects that expressed hlL-6 in neurons but not elsewhere (see, Fattor et al. (1994) IL-6 16 WO 2013/063556 PCT/US2012/062379 Expression in Neurons of Transgenic Mice Causes Reactive Astrocytosis and Increase in Ramified Microglial Cells But No Neuronal Damage, Eur. J. Neuroscience 7:2441-2449), the mice exhibited high levels (20- to 30-fold higher than wild-type) of activated and enlarged astrocytes with increased processes throughout the brain, as well as a 10- to 15-fold increase in ramified microglial cells in white matter. Thus, brain expression of IL-6 reportedly leads to conditions that range from reactive astrocytosis to frank and profound neuropathology. [0098] Microinjection into fertilized eggs of an F1 cross of C57BL/6x"DBAII" mice of a 639-bp hlL-6 cDNA linked to a p-globin 3'-UTR and a mouse MT-1 promoter reportedly produced a transgenic mouse in which the hlL-6 gene was randomly integrated produced a weakened and diseased mouse that dies young of kidney failure (see Fattori et al. (1994) Blood, Development of Progressive Kidney Damage and Myeloma Kidney in Interleukin-6 Transgenic Mice, Blood 63(9):2570-2579). Transgenic mice expired at 12-20 weeks and exhibited elevated levels of a1 and p globulins in plasma, hypergammaglobulinemia, elevated megakaryocytes in spleen (3-fold higher than wild-type) and bone marrow, plasmacytosis of lymphoid organs (spleen, thymus, and lymph nodes) characterized by abnormal and compactly arranged plasmocytoid cells, and glomerulonephritis leading to glomerulosclerosis similar to multiple myeloma. [0099] Microinjection into fertilized eggs of a C57BL/6J mouse of a H-2Ld-driven hIL 6 cDNA caused IL-6-dependent muscle wasting in mice, characterized in part by a significantly lower gastrocnemius muscle weight in transgenic mice as compared to weight-matched controls, a difference that was ameliorated by treatment with an IL-6 antagonist (see, Tsujinaka et al. (1996) Interleukin 6 Receptor Antibody Inhibits Muscle Atrophy and Modulates Proteolytic Systems in Interleukin 6 Transgenic Mice, J. Clin. Invest. 97(1):244-249). At 12 weeks these mice displayed serum hlL-6 levels of more than 600,000 pg/mL. The transgenic mice also had livers that weighed about 1,242 mg, as compared to control livers that were about 862 mg. Transgenic mice treated with IL-6 antagonist had livers that weighed about 888 mg. Muscle cathepsins B and B+L were significantly higher (20-fold and 6.2-fold) in transgenic mice than in controls, a phenomenon that was eliminated in transgenic mice treated with an IL-6 antagonist. cathepsin B and L mRNAs were estimated to be about 277% and 257%, respectively, as compared with wild-type mice; the difference was significantly reduced with IL-6 antagonist treatment. [00100] Mice comprising a hlL-6 minigene driven by a mouse MHC class I H 2Ld promoter and a hlL-6R minigene driven by a chicken p-actin promoter, and a 17 WO 2013/063556 PCT/US2012/062379 gp130 gene, exhibited pathologies typical of hlL-6 transgenic mice (e.g., hepergammaglobulinemia, splenomegaly, mesangial proliferative glomerulonephritis, lung lymphoid infiltration) as well as ventricular hypertrophy (Hirota et al. (1995) Continuous activation of gp130, a signal-transducing receptor component for interleukin 6-related cytokines, causes myocardial hypertrophy in mice, Proc. Natl Acad. Sci. USA 92:4862-4866). The ventricular hypertrophy is believed to be mediated by a continuous activation of gpl 30 (Id.). The role of IL-6 is reportedly to help strengthen the cytokine receptor complex and induce dimerization of gpl 30, which is the signal transducing component responsible for transducing the IL-6 signal (Paonessa et al. (1995) Two distinct and independent sites on IL-6 trigger gpl 30 dimer formation and signalling, EMBO J. 14(9):1942-1951). The activated complex is believed to be a hexamer composed of two IL-6, each IL-6 bound to one IL-6Ra and two gpl30 (each IL-6 contains two independent gpl30-binding sites) exhibiting a 2:2:2 stoichiometry, wherein the dimerization of gp130 causes activation of JAK-Tyk tyrosine kinases, phosphorylation of gp130 and STAT family transcription factors and other intracellular substrates (/d.; Stahl, N. (1994) Association and Activation of Jak Tyk Kinases by CNTF-LIF-OSM-IL-6 P Receptor Components, Science 263:92-95), consistent with a general model of cytokine receptor complex formation (see, Stahl, N. and Yancopoulos, G. (1993) The Alphas, Betas, and Kinases of Cytokine Receptor Complexes, Cell 74:587-590; Davis et al. (1993) LIFRp and gpl30 as Heterodimerizing Signal Transducers of the Tripartite CNTF Receptor, Science 260:1805-1808; Murakami et al. (1993) IL-6-Induced Homodimerization of gpl30 and Associated Activation of a Tyrosine Kinase, Science 2601808-1810). [00101] Mice transgenic for human slL-6R driven by a rat PEP carboxykinase promoter and human IL-6 driven by a mouse metallothionein-1 promoter are reportedly markedly smaller that mice transgenic for human IL-6 alone or human sIL 6R alone (Peters et al. (1997) Extramedullary Expansion of Hematopoietic Progenitor Cells in Interleukin(IL-)-6-slL-6R Double Transgenic Mice, J. Exp. Med. 185(4):755 766), reflected in reduced body fat and reduced weight (20-25 g vs. 40 g). Double transgenic mice reportedly also exhibit spleen (5-fold) and liver (2-fold) enlargement as compared with reportedly normal organ weights for single transgenic mice, apparently due to extramedullary proliferation of hematopoeitic cells of spleena and liver but not bone marrow, as well as elevated megakaryocytes in spleen and plasmacellular infiltrates in all parenchymal organs (/d.). Double transgenics also exhibit livers with an increase of about 200- to about 300-fold in granulocytes, macrophages, progenitor cells, and B cells as compared with single transgenics; in 18 WO 2013/063556 PCT/US2012/062379 contrast, IL-6 single transgenic mice exhibited lesser increases in macrophages (15 fold) and B cells (45-fold) (/d.). The extraordinary findings are presumably due to stimulation of growth and differentiation of hematopoietic progenitor cells by activating gp130 signal transduction (/d.). [00102] Further, double transgenic (mouse metallothionine promoter-driven hlL-6/rat PEP carboxykinase promoter-driven hlL-6R) mice exhibit a hepatocellular hyperplasia that is reportedly identical to human nodular regenerative hyperplasia with sustained hepatocyte proliferation that strongly suggests that IL-6 is responsible for both hepatocyte proliferation and pathogenic hepatocellular transformation (Maione et al. (1998) Coexrpession of IL-6 and soluble IL-6R causes nodular regenerative hyperplasia and adenomas of the liver, EMBO J. 17(19):5588-5597). Because hepatocellular hyperplasia is reportedly not observed in single transgenic hlL-6 mice and hlL-6 can bind mlL-6R, the finding may appear paradoxical until it is considered that the double transgenic may result in higher levels of hlL-6 complexed to soluble IL-6R (here, soluble hlL-6R), which complex is a more potent inhibitor that IL-6 alone (/d.). [00103] In contrast to mice that are transgenic for human IL-6, humanized IL-6 mice that comprise a replacement at an endogenous mouse IL-6 locus, which retain mouse regulatory elements but comprise a humanization of IL-6-encoding sequence, do not exhibit the severe pathologies of prior art mice. Genetically modified mice that were heterozygous or homozygous for hlL-6 were grossly normal. [00104] Mice with a humanized IL-6 gene (MAID 760) as described in the Examples were immunophenotyped and found to have normal B cell numbers in FACS analyses (lymphocyte-gated) of spleen B cells using a pan B cell marker (CD445R(B220)) (FIG.4). For spleen, wild-type mice exhibited 63% B cells; hlL-6 heterozygote mice exhibited 63% B cells; and mice homozygous for hlL-6 at the endogeous mouse locus exhibited 63% B cells. B cell numbers for homozygous hIL 6 mice immunized with TNP-KLH were also normal (65% for wild-type, and 61% for hlL-6 homozygotes). [00105] Splenic T cells were also about the same as wild-type (FIG.5). Percentages of splenic T cells for Thelper/Tcytoxic were, for wild-type 20%/40% (ratio of 1.4:1); for hlL-6 heterozygotes 23%/14% (ratio of 1.6:1); for hlL-6 homozygotes 21%/15% (ratio of 1.4:1) (markers were CD8a-APC; CD4-FITC). Homozygous hlL-6 mice immunized with TNP-KLH exhibited similar splenic T cell numbers to wild-type mice, i.e., Thelper/Tcytotoxic were 22%/20% (ratio of 1.1:1) as compared with 21%/19% for wild-type (also a ratio of 1.1:1). 19 WO 2013/063556 PCT/US2012/062379 [001061 Humanized IL-6 mice also exhibited about normal levels of splenic NK cells on FACS analysis (CD1 1b and DX5) (FIG. 7). hlL-6 heterozygotes exhibited 2.2% NK cells, and hlL-6 homozygotes exhibited 1.8% NK cells, whereas wild-type mice exhibited 2.4% NK cells. Following immunization with TNP-KLH, homozygotes exhibited 1.6% splenic NK cells, whereas wild-type mice exhibited 2.1% splenic NK cells. [00107] Humanized IL-6 mice also exhibited normal levels of splenic Ly6G/C(Grl) cells (FIG. 6). hlL-6 heterozygotes exhibited 7.0% GR1* cells (1.3% Gr1hi); homozygotes exhibited 6.8% Gr1* cells (0.9% Gr1hi), whereas wild-type mice exhibited 8.0% Gr1* cells (1.8%Grlhi). Immunized IL-6 homozygotes (immunized with TNP-KLH) exhibited 11% Gr1+ cells (4.0% Gr1hi), whereas wild-type mice exhibited 10% Gr1* cells (3.0% Gr1hi) [001081 Humanized IL-6 mice also exhibited normal blood B and T cell numbers in FACS analysis (FIG. 8 and FIG. 9). FACs with a pan B cell marker (CD445R(B220)) revealed that homozygous hlL-6 mice exhibied 52% B cell as compared with wild-type 53%; heterozygotes exhibited 38% (an average of two different stainings of 29% and 47%). Homozygous hlL-6 mice immunized with TNP KLH gave similar B cell numbers (43%, as compared with 45% for wild-type mice). [00109] Humanized IL-6 mice exhibited normal blood T cell numbers in FACS analysis as measured by CD8a and CD4 staining. Heterozygous hlL-6 mice exhibited Thelper/Tcytotoxic numbers of 39%/26% (ratio of 1.5:1); homozygous hIL 6 mice exhibited Th/Tc numbers of 24%/20% (ratio of 1.2:1), whereas wild-type mice exhibited Th/Tc numbers of 26%/20% (ratio of 1.3:1). Homozygous hlL-6 mice immunized with TNP-KLH had Th/Tc numbers of 29%/21% (ratio of 1.4:1), whereas wild-type immunized mice had Th/Tc numbers of 28%/23% (1.2:1). [00110] Humanized IL-6 mice also exhibited myeloid cell numbers in blood that were similar to wild-type mice as measured by FACS analysis of naive and immunized mouse blood stained with Ly6G/C(Grl) and CD11 b, as well as CD11 b and DX5 (FIG. 10, FIG. 11, and FIG 12). Heterozygous hlL-6 mice exhibited %Gr+ cells of 10.8%, homozygotes 6.9%, whereas wild-type mice exhibited 9.7%. Immunized hlL-6 homozygotes exhibited M1(Ly6G/C(Gr) of 101_104) / M2(Ly6G/C(Gr) staining of about 102-1 03) numbers of 43%/34%, whereas wild-type mice had numbers of 45%/38%. FACS plots of CD1 1b (vertical axis) vs. Ly6G/C (horizontal axis) for immunized homozygous hlL-6 mice showed cell percentage in quadrants (upper left/upper right/lower right) of 16%/8%/3%, which were identical to immunized wild-type quadrant numbers. 20 WO 2013/063556 PCT/US2012/062379 [00111] Homozygous TNP-KLH-immunized humanized IL-6 mice exhibited CD11 b vs. DX5(NK) staining FACS plots that were similar to immunized wild-type mice. Quadrant analysis blood FACS plots (CD1 1b vertical axis, DX5(NK) horizontal axis) revealed upper left/upper right/lower right numbers of 9.5%/17%/i 0% for hlL-6 homozygotes and 6.5%/17.3%/14% for wild-type mice. [00112] Humanized IL-6 mice exhibited an isotype response that was essentially the same as observed in wild-type mice. Early and final IgG1, IgG2a, IgG2b, IgG3, IgA, IgE, and IgM levels were about the same as observed in wild-type mice. In one experiment, final IgM was slightly higher in humanized mice; final IgG3 was also elevated in humanized mice. [00113] B cell development in naive hlL-6 mice was essentially indistinguishable from development in wild-type mice based on FACS analysis of bone marrow IgM/CD24/B220 staining (FIG. 13). Immunophenotyping of immune mice revealed that marker populations for various cell types in the B cell development progression were essentially normal in hlL-6 mice. Progression of cells from hematopoietic stem cells, common lymphoid progenitors, ProB cells, PreB cells, and immature and mature B cells is normal in hlL-6 mice (FIG. 14 and FIG. 15) EXAMPLES Example 1: Replacement of Endogenous Mouse IL-6 Gene with hlL-6 Gene [00114] The 4.8-kb human IL-6 gene containing exons 1 through 4 of the human IL-6 gene replaced 6.8 kb of the murine IL-6 gene locus. [00115] A targeting construct for replacing the mouse with the human IL-6 gene in a single targeting step was constructed using VELOCIGENE@ genetic engineering technology (see, Valenzuela et al. (2003) High-throughput engineering of the mouse genome coupled with high-resolution expression analysis, Nature Biotech, 21(6):652-659). Mouse and human IL-6 DNA were obtained from bacterial artificial chromosome (BAC) RPCI-23 clone 368C3, and from BAC CTD clone 2369M23, respectively. Briefly, a Notl linearized targeting construct generated by gap repair cloning containing mouse IL-6 upstream and downstream homology arms flanking a 4.8 kb human IL-6 sequence extending from ATG in exon 1 to exon 5 with 16 nucleotides of 3' downstream sequence (genomic coordinates: NCBlh37.1: ch7:22,766,882 to 22,771,637) and a floxed neo selection cassette, was electroporated into F1 H4 mouse embryonic stem (ES) cells (C57BL/6 x 129 F1 hybrid). Correctly targeted ES cells (MAID 790) were further electroporated with a transient Cre-expressing vector to remove the drug selection cassette. Targeted ES 21 WO 2013/063556 PCT/US2012/062379 cell clones without drug cassette (MAID 1428) were introduced into an 8-cell stage mouse embryo by the VELOCIMOUSE® method (see, US Pat. No. 7,294,754, 7,576,259, 7,659,442, and Poueymirou et al. (2007) FO 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@ (FO mice fully derived from the donor ES cell) bearing the humanized IL-6 gene were identified by genotyping for loss of mouse allele and gain of human allele using a modification of allele assay (see, Valenzuela et al. (2003)). [001161 Correctly targeted ES cell clones were identified by a loss-of-native allele (LONA) assay (Valenzuela et al. 2003) in which the number of copies of the native, unmodified 116 gene were determined by two TaqMan T M quantitative polymerase chain reactions (qPCRs) specific for sequences in the mouse 116 gene that were targeted for deletion. The qPCR assays comprised the following primer probe sets (written 5' to 3'): upstream forward primer, TTGCCGGTTT TCCCTTTTCT C (SEQ ID NO:1); upstream reverse primer, AGGGAAGGCC GTGGTTGTC (SEQ ID NO:2); upstream probe, FAM-CCAGCATCAG TCCCAAGAAG GCAACT-BHQ (SEQ ID NO:3); downstream forward primer, TCAGAGTGTG GGCGAACAAA G (SEQ ID NO:4); downstream reverse primer, GTGGCAAAAG CAGCCTTAGC (SEQ ID NO:5); downstream probe, FAM-TCATTCCAGG CCCTTCTTAT TGCATCTG-BHQ (SEQ ID NO:6); in which FAM refers to the 5-carboxyfluorescein fluorescent probe and BHQ refers to the fluorescence quencher of the black hole quencher type (Biosearch Technologies). DNA purified from ES cell clones that that have taken up the targeting vector and incorporated in their genomes was combined with TaqMan T M Gene Expression Master Mix (Life Technologies) according to the manufacturer's suggestions in a 384-well PCR plate (MicroAmp TM Optical 384-Well Reaction Plate, Life Technologies) and cycled in an Applied Biosystems Prism 7900HT, which collects fluorescence data during the course of the PCRs and determines a threshold cycle (Ct), the fractional PCR cycle at which the accumulated fluorescence reaches a pre-set threshold. The upstream and downstream //6-specific qPCRs and two qPCRs for non-targeted reference genes were run for each DNA sample. The differences in the Ct values (ACt) between each //6-specific qPCR and each reference gene qPCR were calculated, and then the difference between each ACt and the median ACt for all samples assayed was calculated to obtain AACt values for each sample. The copy number of the /16 gene in each sample was calculated from the following formula: copy number = 2 - 2 AACt. A correctly targeted clone, having lost one of its native copies, will has an 116 gene copy number equal to one. Confirmation that the human 22 WO 2013/063556 PCT/US2012/062379 /L6 gene sequence replaced the deleted mouse 116 gene sequence in the humanized allele was confirmed by a TaqMan T M qPCR assay that comprises the following primer-probe sets (written 5' to 3'): the human forward primer, CCCCACTCCACTGGAATTTG (SEQ ID NO:7); the human reverse primer, GTTCAACCACAGCCAGGAAAG (SEQ ID NO:8); and the human probe, FAM AGCTACAACTCATTGGCATCCTGGCAA-BHQ (SEQ ID NO:9). [00117] The same LONA assay was used to assay DNA purified from tail biopsies for mice derived from the targeted ES cells to determine their 116 genotypes and confirm that the humanized 116 allele had transmitted through the germline. Two pups heterozygous for the replacement are bred to generate a mouse that is homozygous for the replacement of the endogenous mouse IL-6 gene by the human IL-6 gene. Pups that are homozygous for the replacement are used for phenotyping. [001181 The upstream junction of the murine locus and the sequence containing the hlL-6 gene is designed to be within 5'-AATTAGAGAG TTGACTCCTA ATAAATATGA GACTGGGGAT GTCTGTAGCT CATTCTGCTC TGGAGCCCAC CAAGAACGAT AGTCAATTCC AGAAACCGCT ATGAACTCCT TCTCCACAAG TAAGTGCAGG AAATCCTTAG CCCTGGAACT GCCAGCGGCG GTCGAGCCCT GTGTGAGGGA GGGGTGTGTG GCCCAGG (SEQ ID NO:10), wherein the final mouse nucleotide prior to the first nucleotide of the human gene is the "T" in CCGCT, and the first nucleotide of the human sequence is the first "A" in ATGAA. The downstream junction of the sequence containing the hlL-6 gene and the murine locus is designed to be within 5'-TTTTAAAGAA ATATTTATAT TGTATTTATA TAATGTATAA ATGGTTTTTA TACCAATAAA TGGCATTTTA AAAAATTCAG CAACTTTGAG TGTGTCACGC TCCCGGGCTC GATAACTATA ACGGTCCTAA GGTAGCGACT CGAGATAACT T-3' (SEQ ID NO:1 1), wherein the final nucleotide of the human sequence is with the final "G" in TCACG and the first nucleotide of the mouse sequence is the first "C" in CTCCC; the downstream junction region also contained a loxP site at the 3' end (the beginning of which is shown) for removal of a floxed ubiquitin promoter-driven neo cassette. The junction of the neo cassette with the mouse IL-6 locus is designed to be within 5'-TATACGAAGT TATCCTAGGT TGGAGCTCCT AAGTTACATC CAAACATCCT CCCCCAAATC AATAATTAAG CACTTTTTAT GACATGTAAA GTTAAATAAG AAGTGAAAGC TGCAGATGGT GAGTGAGA (SEQ ID NO:12), where the final "C" of AGCTC is the final nucleotide of the neo cassette; the first nucleotide of the mouse genome following the cassette is the initial "C" of CTAAG. 23 WO 2013/063556 PCT/US2012/062379 Example 2: Immunophenotyping of Naive and Immunized hlL-6 Mice: B Cells [00119] Mice homozygous for the hlL-6 gene replacement were analyzed for B cells (DC445R(B220). Lymphocyte-gated fractions from spleen cell preparations of naive and immunized (TNP-KLH) hlL-6 mice were stained and immunophenotyped using flow cytometry. FACS analysis showed that the percentage of B cells of the spleen cell preparation as measured by CD45R(B220)-FITC staining were about the same (63% of cells) for preparations from naive wild-type, hlL-6 heterozygotes, and hlL-6 homozygotes. For immunized mice, B cells accounted for about 65% of total cells of the spleen cell preparation in wild-type mice, and about 61% of total cells in hlL-6 homozygotes. Spleens of hlL-6 mice (both naive and immunized) contain a population of B cells that is about the same size as the splenic B cell population in wild-type mice. [00120] Bone marrow of wild-type, hlL-6 heterozygotes, and hlL-6 homozygotes was stained with B cell markers (CD45R(B220)-APC, CD24(HSA)-PE, or CD43 conjugated to a dye and/or IgM (IgM-FITC). B cell development in bone marrow of normal mice will be reflected in surface markers as cells progress from stem cells to early pro-B cells to late pro-B cells, to large pre-B cells to small pre-B cells to immature B cells and finally, to mature B cells. Common lymphocyte progenitor pro-B cells will express CD45R, and in later stages will express IgM as immature and later as mature B cells. Thus, CD45R-stained and anti-IgM-stained B cells should reveal a pattern characteristic of B cell development. Bone marrow of hlL-6 heterozygotes and homozygotes displayed a pattern of CD45R(B220)-APC and anit-IgM-FITC staining that was essentially indistinguishable from wild-type bone marrow, showing populations of B cells that stained positive for CD45R(B220) and IgM, or CD45R(B220) alone. B cell sub-populations in bone marrow of hlL-6 mice revealed by FACS staining were similar to those in wild-type mice (Table 1; see also, FIG. 13). [00121] Table 1. B Cells in Bone Marrow of Naive Mice hlL-6 Mouse Wild-type Mouse (%) Heterozygote Homozygote (%) (%) CLP-ProB 40 29 32 PreB-ImmatureB 12.3 19.3 15.3 Mature B 6.4 6.5 6.7 24 WO 2013/063556 PCT/US2012/062379 [00122] Staining for CD24 (see FIG. 14) revealed the (normal) pattern shown in Table 2, indicating normal development in bone marrow. Table 2. B Cells in Bone Marrow of Naive Mice hlL-6 Mouse Wild-type Mouse (%) Heterozygote Homozygote (%) (%) Developing HSC- 46.6 46 43 CLP Mature CLP/early 10.2 9.0 10.1 ProB Late ProB, PreB, 7.2 11.6 10.7 Immature B Mature B 14.1 14.9 17 [00123] Staining for CD43 (see FIG. 15) revealed the (normal) pattern shown in Table 3, indicating normal development in bone marrow. Table 3. B Cells in Bone Marrow of Naive Mice hlL-6 Mouse Wild-type Mouse (%) Heterozygote Homozygote (%) (%) PreBil-Immature 28.4 21.4 21.2 B cells Mature B cells 8.1 11.5 8.0 ProB-PreBI 3.4 4.3 4.7 [00124] Thus, immunophenotyping of naive hlL-6 mice revealed that B cell development in such mice is essentially normal. Example 3: Replacement of Endogenous Mouse IL-6Ra Ectodomain Gene Sequence with hlL-6Ra Ectodomain Gene Sequence [00125] The 45 kb human IL-6Ra gene containing exons 1 through 8 of the human IL-6Ra gene replaced 35.4 kb of the murine IL-6Ra gene locus. Mouse exons 9 and 10 were retained; only exons 1-8 were humanized. In total, 35,384 nt of mouse sequence was replaced by 45,047 nt of human sequence. [00126] A targeting construct for replacing the mouse with the human IL-6Ra gene in a single targeting step was constructed using VELOCIGENE@ genetic engineering technology (see, Valenzuela et al. (2003) High-throughput engineering of the mouse genome coupled with high-resolution expression analysis, Nature 25 WO 2013/063556 PCT/US2012/062379 Biotech, 21(6):652-659). Mouse and human IL-6Ra DNA were obtained from bacterial artificial chromosome (BAC) RPCI-23 clone 125J8, and from BAC CTD clone 2192J23, respectively. Briefly, a Notl linearized targeting construct generated by gap repair cloning containing mouse IL-6Ra upstream and downstream homology arms flanking a 45 kb human IL-6Ra sequence extending from ATG in exon 1 to exon 8 with 69 nucleotides of 3' downstream sequence and a floxed neo selection cassette, was electroporated into F1 H4 mouse embryonic stem (ES) cells (C57BL/6 x 129 F1 hybrid). Correctly targeted ES cells (MAID 794) were further electroporated with a transient Cre-expressing vector to remove the drug selection cassette. Targeted ES cell clones without drug cassette (MAID 1442) were introduced into an 8-cell stage mouse embryo by the VELOCIMOUSE® method (see, US Pat. No. 7,294,754, 7,576,259, 7,659,442, and Poueymirou et al. (2007) FO 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@ (FO mice fully derived from the donor ES cell) bearing the humanized IL-6Ra gene were identified by genotyping for loss of mouse allele and gain of human allele using a modification of allele assay (see, Valenzuela et al. (2003)). [00127] Correctly targeted ES cell clones were identified by a loss-of-native allele (LONA) assay (Valenzuela et al. 2003) in which the number of copies of the native, unmodified 116 gene were determined by two TaqMan T M quantitative polymerase chain reactions (qPCRs) specific for sequences in the mouse 116 gene that were targeted for deletion. The qPCR assays comprised the following primer probe sets (written 5' to 3'): upstream forward primer, GCCCTAGCAT GCAGAATGC (SEQ ID NO:13); upstream reverse primer, AAGAGGTCCC ACATCCTTTG C (SEQ ID NO:14); upstream probe, CCCACATCCA TCCCAATCCT GTGAG (SEQ ID NO:15); downstream forward primer, GAGCTTGCCC CCAGAAAGG (SEQ ID NO:16); downstream reverse primer, CGGCCACATC TCTGGAAGAC (SEQ ID NO:17); downstream probe, CATGCACTGC CCCAAGTCTG GTTTCAGT (SEQ ID NO:18). DNA purified from ES cell clones that that have taken up the targeting vector and incorporated in their genomes was combined with TaqMan T M Gene Expression Master Mix (Life Technologies) according to the manufacturer's suggestions in a 384-well PCR plate (MicroAmp TM Optical 384-Well Reaction Plate, Life Technologies) and cycled in an Applied Biosystems Prism 7900HT, which collects fluorescence data during the course of the PCRs and determines a threshold cycle (Ct), the fractional PCR cycle at which the accumulated fluorescence reaches a pre set threshold. The upstream and downstream IL-6Ra-specific qPCRs and two 26 WO 2013/063556 PCT/US2012/062379 qPCRs for non-targeted reference genes were run for each DNA sample. The differences in the Ct values (ACt) between each IL-6Ra-specific qPCR and each reference gene qPCR were calculated, and then the difference between each ACt and the median ACt for all samples assayed was calculated to obtain AACt values for each sample. The copy number of the 116 gene in each sample was calculated from the following formula: copy number = 2 - 2-AACt. A correctly targeted clone, having lost one of its native copies, will have an IL-6Ra gene copy number equal to one. Confirmation that the human IL-6Ra gene sequence replaced the deleted mouse IL 6Ra gene sequence in the humanized allele was confirmed by a TaqMan T M qPCR assay that comprises the following primer-probe sets (written 5' to 3'): the human forward primer, GGAGAGGGCA GAGGCACTTA C (SEQ ID NO:19); the human reverse primer, GGCCAGAGCC CAAGAAAAG (SEQ ID NO:20); and the human probe, CCCGTTGACT GTAATCTGCC CCTGG (SEQ ID NO:21). [00128] The same LONA assay was used to assay DNA purified from tail biopsies for mice derived from the targeted ES cells to determine their IL-6Ra genotypes and confirm that the humanized IL-6Ra allele had transmitted through the germline. Pups heterozygous for the replacement are bred to generate a mouse that is homozygous for the replacement of the endogenous mouse IL-6Ra gene by the human IL-6Ra (ectodomain) gene. Pups that are homozygous for the replacement are used for phenotyping. [00129] The upstream junction of the murine locus and the sequence containing the hlL-6Ra gene is designed to be within 5'-CGAGGGCGAC TGCTCTCGCT GCCCCAGTCT GCCGGCCGCC CGGCCCCGGC TGCGGAGCCG CTCTGCCGCC CGCCGTCCCG CGTAGAAGGA AGCATGCTGG CCGTCGGCTG CGCGCTGCTG GCTGCCCTGC TGGCCGCGCC GGGAGCGGCG CTGGCCCCAA GGCGCTGCCC TGCGCAGGGT AAGGGCTTCG G (SEQ ID NO:22), wherein the final mouse nucleotide prior to the first nucleotide of the human gene is the "C" in GAAGC, and the first nucleotide of the human sequence is the first "A" in ATGCT. The downstream junction of the sequence containing the hlL-6 gene and the murine locus is designed to be within 5'-CAAGATTATT GGAGTCTGAA ATGGAATACC TGTTGAGGGA AATCTTTATT TTGGGAGCCC TTGATTTCAA TGCTTTTGAT TCCCTATCCC TGCAAGACCC GGGCTCGATA ACTATAACGG TCCTAAGGTA GCGACTCGAG ATAACTTC-3' (SEQ ID NO:23), wherein the final nucleotide of the human sequence is with the final "A" in CAAGA and the first nucleotide of the mouse sequence is the first "C" in CCCGG; the downstream junction region also contained a loxP site at the 3' end for removal of a floxed ubiquitin promoter-driven neo cassette. 27 The first nucleotide of the loxp site is the first "A" in ATAAC. The junction of the neo cassette with the mouse IL-6Ra locus is designed to be within 5'-TATACGAAGT TATCCTAGGT TGGAGCTCTA CTCCATATGC TCACTTGCCG TTGTTTGCTA CGATACGGTG AGGCCCGTGC GAAGAGTGGC ACAGATCAGG AGGCTTATGT GGTCAGTCCA CAGTATGGC (SEQ ID NO:24), where the final "C" of AGCTC is the final nucleotide of the neo cassette; the first nucleotide of the mouse genome following the cassette is the initial "T" of TACTC. [001301 The discussion of documents, acts, materials, devicesarticles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. [00131] Throughout the description and claims of this specification, the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps. 28

Claims (18)

1. A genetically modified mouse comprising a replacement at an endogenous mouse IL-6 locus of a mouse gene encoding IL-6 with a human gene encoding human IL-6, wherein the human gene encoding human IL-6 is under control of endogenous mouse regulatory elements at the endogenous mouse IL-6 locus.

2. The genetically modified mouse of claim 1, wherein the human gene encoding human IL-6 comprises exons 1 through 5 of the human IL-6 gene found in the CTD-2369M23 bacterial artificial chromosome.

3. The genetically modified mouse of claim 1, wherein the mouse expresses a mouse IL-6Ra.

4. The genetically modified mouse of claim 1, wherein the mouse expresses a humanized IL-6RCt, wherein the endogenous mouse IL-6Ra gene has been replaced with a human sequence comprising a sequence that encodes an ectodomain of a human IL-6Ra.

5. The genetically modified mouse of any one of claims 1 to 4, wherein the mouse does not exhibit a feature selected from plasmocytosis, glomerulosclerosis, glomerulonephritis, kidney failure, hypergammag lobulinemia, elevated megakaryocytes in spleen, elevated megakaryocytes in bone marrow, splenomegaly, lymph node enlargement, compacted abnormal plasma cells, and a combination thereof.

6. The genetically modified mouse of claim 4 or claim 5, wherein the humanized IL-6Ra comprises a human ectodomain.

7. The genetically modified mouse of claim 6, wherein the humanized IL-6Ra comprises a mouse transmembrane domain and a mouse cytoplasmic domain.

8. The genetically modified mouse of claim 1, wherein the mouse comprises a cell that expresses an IL-6Ra that comprises a human ectodomain on the surface of the cell.

9. A genetically modified mouse, comprising a humanization of an endogenous mouse IL-6Ra gene, wherein the humanization comprises a replacement of mouse IL-6Rc ectodomain-encoding sequence with human IL-6Ra ectodomain- encoding sequence at the endogenous mouse IL-6Ra locus, and wherein the humanized IL-6Ra gene is under control of endogenous mouse regulatory elements, 29

10. The genetically modified mouse of claim 9, wherein a contiguous mouse sequence comprising mouse exons 1 through 8 is replaced with a contiguous genomic fragment of human IL-6Ra sequence encoding a human IL6Ra ectodomain.

11. The genetically modified mouse of claim 10, wherein the contiguous genomic fragment of human IL-6Ra sequence encoding the ectodomain is found in the CTD-2192J23 bacterial artificial chromosome.

12. The genetically modified mouse of claim 9, further comprising a humanized IL-6 gene comprising a replacement at an endogenous mouse IL-6 locus of a mouse IL-6 gene with a human gene encoding human IL-6.

13. The genetically modified mouse of claim 12, wherein the humanized IL-6 gene is under control of endogenous mouse regulatory elements.

14. A method for making a humanized mouse, comprising replacing a mouse gene sequence encoding mouse :L-6 with a human gene encoding human IL-6 so that the human IL-6 gene is under control of endogenous mouse regulatory elements.

15. A method for making a humanized mouse, comprising replacing all mouse exons encoding ectodomain sequences of mouse IL-6Ra with a human genomic fragment encoding human IL-6Ra ectodomain to form a humanized IL-6Ra gene, wherein the humanized IL-6Ra gene is under control of endogenous mouse regulatory elements.

16. A genetically modified mouse comprising a humanized IL-6Ra gene comprising a replacement of rnouse ectodomain-encoding sequence with human ectodomain sequence, wherein the humanized IL-6Ra gene comprises a mouse transmembrane sequence and a mouse cytoplasmic sequence, wherein the mouse further comprises a gene encoding a human IL-6, wherein the genes encoding a human IL-6 and humanized 1L-6Ra are under control of endogenous mouse regulatory elements.

17. The genetically modified mouse of claim 16, wherein the mouse does not express a mouse IL 6Ra and does not express a mouse IL-6.

18. A genetically modified mouse of any one of claims 1, 9 and 16, or a method of claim 14 or claim 15, substantially as herein described with reference to any one or more of the Examples and/or Figures. 30