igney & zollner_trends pharmacol sci 2004

Techniques: Species’ finest blend –humanized mouse models ininflammatory skin disease researchFrederik H. Igney, Khusru Asadullah and Thomas M. Zollner

Schering AG, CRBA Dermatology, Berlin, Germany

Differences between humans and mice often hamper

the transfer of promising results from the bench to the

clinic. For ethical reasons, research that involves

patients is limited, and so there is an urgent need for

models that mimic the human situation as closely as

possible. In recent years, there has been considerable

progress in generating humanized mouse models, and

their application to drug discovery has proved fruitful.

So, how can mice be humanized, and how can human-

ized mice be employed in immunology research and drug

discovery? In this article, we answer these questions,

focusing on T-cell-mediated skin diseases as an example.

Inflammatory diseases have a high prevalence in Westerncountries.Hence, pharmaceutical companies spend increas-ing amounts of money to develop drugs for these disorders.Themost expensive phase of drug discovery is clinical trialsbut new compounds fail frequently at this stage. Often,results from animal experiments and clinical outcome donot correlate because of significant differences in humanand murine immunity [1]. In addition, the complex patho-physiology of human inflammatory diseases is representedonly partially in classical animal models. Moreover,research that involves patients is limited; in particular,it is not possible to induce diseases for scientific purposes.Thus, there is an urgent need for more-predictive andreliable animal models. Humanized mouse models seem tobe the answer to this problem because they combine theadvantages of small-animal models with better correlationin the clinic. Here, we review how mice can be humanizedand discuss the applications of humanized mice inimmunology research and drug discovery, focusing onT-cell-mediated skin diseases.

How to humanize a mouse

The most important way to humanize animals is to pro-duce chimeras by xenotransplantation. In general, thisinvolves transplanting human grafts into immunodefi-cient mice. A special kind of humanization can also beobtained by replacing a murine gene with its humanhomolog. Both techniques can be combined, for example bygrafting cells into mice that express a human growthfactor. Interspecific chimeras either between sheep and

Corresponding author: Frederik H. Igney ([email protected]).Available online 21 August 2004

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goats or between mice and rats have been generated bycombining embryonic cells of the respective species.However, for ethical reasons this technique has notbeen and probably will never be developed for human–mouse chimeras.

Recipient mice

In immune-competent mice, foreign tissue is recognizedand rejected by immune cells. Thus, only immunodeficientmice can be used for xenotransplantation. The first mousestrains used were nude, severe-combined-immunodefi-ciency (SCID) or recombination activating gene 1-knock-out (Rag1K/K) and Rag2K/K mice, each of which havedeficiencies in adaptive immunity [2,3]. Superior hosts forxenotransplantation are obtained by combining severalimmunological defects. Today’s standard models are non-obese-diabetic/SCID and SCID/beige double mutant mice.In addition to lacking functional T and B cells, these micehave deficiencies in natural killer (NK) cells and othercomponents of innate immunity. Possibly superioralternatives are mice that possess deletions of both Rag2and the common cytokine receptor g chain (Rag2K/K/gc

K/K

mice) and BNX mice, which possess three separatemutations: the beige, nude and x-linked immunodefi-ciency (also known as xid or Bruton agammaglobulinemiatyrosine kinase) mutations [2,3]. For detailed informationon mouse strains see Mouse Genome Informatics(http://www.informatics.jax.org/). Several other combi-nations of immune defects are also available. The optimalmouse strain for xenotransplantationmight depend on thespecific application. In general, deficiency in the adaptiveand the innate immune response seems to be beneficial,and the rule ‘the more defects, the better’ seems to bevalid. However, in practical terms, mice with more‘complete’ immunodeficiencies tend to be less robust,which, in turn, increases the risk of them dying as theexperiment progresses.

Human grafts

It is possible to transplant virtually every tissue of thehuman body into immunodeficient mice [4]. In initialattempts, the human adaptive immune system has beenreconstituted by transplantation of immune cells andlymphoid organs (Table 1). Careful characterization ofthese models reveals an amazing consistency with thehuman immune system [5,6]. However, each model shows

Review TRENDS in Pharmacological Sciences Vol.25 No.10 October 2004

. doi:10.1016/j.tips.2004.08.004

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Table 1. Reconstitution of the human immune system in micea

Nameb Mouse strain Human graft Important characteristics Refs

huPBL-SCID SCIDc PBLs Functional immune system; signs of GVHD [6]

SCID-huThy/Liv SCID Fetal thymus and liver Continued repopulation of lymphoid and myeloid

lineages; no peripheral B cells; tolerance to

murine background

[5]

HID Beige/nude/xid Bone marrow Sustained active hematopoiesis [5]

SCID-hu-bone SCID Fetal bone Sustained active hematopoiesis; no peripheral

T cells

[5]

SCID-huBM/T SCID Fetal thymus and bone Generation of all leukocyte lineages and Ig

classes

[8]

SCID-huIC SCID/beige Fetal bone, thymus, skin

and lymph nodes

Primary, antigen-specific T-cell and B-cell

responses

[8]

Rag2-gc-CD34C Newborn

Rag2K/K/gcK/K

CD34C cord-blood cells B-cell, T-cell and dendritic-cell development;

structured thymus, spleen, lymph nodes;

functional immune responses

[9]

aAbbreviations: GVHD, graft-versus-host disease; PBL, peripheral blood lymphocyte; Rag2, recombination activating gene 2; SCID, severe combined immunodeficiency; xid,

x-linked immunodeficiency; gc, common cytokine receptor g chain.bNames are either used by the original authors or established in the literature and, in general, represent an abbreviation of the mouse strain and human (hu) grafts used.cTo date, either non-obese-diabetic (NOD)/SCID or SCID/beige mice are usually used, which seem to be superior to SCID mice for reconstitution with human PBLs.


its unique deviations from normal immunity, and induc-ing primary immune responses is difficult in mostmodels [7,8]. Recently, in a sophisticated model calledSCID–huIC, which uses SCID/beige mice transplantedwith human fetal bone, thymus, skin and autologousmesenteric lymph nodes, immunization led to a primaryantigen-specific T-cell and B-cell response [8]. Intra-hepatic injection of CD34C cord-blood cells into newbornRag2K/K/gc

K/Kmice resulted in the development of B cells,T cells and dendritic cells, and in the formation of struc-tured lymphoid organs [9]. Functional immune responsescould be induced with Epstein-Barr virus and tetanustoxoid. One complication is the reaction of humanlymphocytes against the murine host. Clear signs of axenogeneic graft-versus-host disease (GVHD) have beenfound, particularly in the huPBL–SCID model [in whichhuman peripheral blood lymphocytes (huPBLs) areinjected into SCID mice] [6]. Many engrafted T cellsseem to correlate with significant GVHD. By contrast, inother models, T cells develop tolerance to the murinebackground [5].

In addition to the immune system, human skin isthe most frequently transplanted human xenograft [10].Both healthy and diseased human skin has been used.Co-engraftment of human immune cells and human skinoffers the opportunity to study their interactions in vivo.Moreover, engraftment of artificial, human-skin equiva-lent has been investigated [11]. Further immunologicallyimportant grafts are human bronchi for the study ofasthma, synovial tissue for the study of rheumatoidarthritis, vaginal and neural tissue for the study ofhuman immunodeficiency virus (HIV) infection, andthyroid grafts for the study of Grave’s disease [4].

Employing humanized mice in inflammation research

Humanized mouse models are used in research into allaspects of immunology. An interesting approach is theproduction of human antibodies in mice with a humanizedimmune system [12]. Vaccination can be studied byimmunizing mice with a reconstituted human immunesystem [8,9], hematopoiesis by transferring human stemcells [9,13], and allograft rejection by co-transplantingallogeneic immune cells and grafts [14,15]. In addition to

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these basic immunological questions, humanized micecan be used to investigate diseases by transferring therespective ‘diseased’ tissues and cells. These disordersinclude HIV infection, lymphomas, autoimmune diseasessuch as multiple sclerosis, lupus and thyreoditis, andinflammatory diseases such as asthma and rheumatoidarthritis [4,13,16]. Moreover, they have been used success-fully in studies of T-cell-mediated dermatoses. Thesediseases share features with immunological disorders inother organ systems. The skin can, thus, be regarded asmodel organ for the investigation of immunologicaldisorders and for the development of new therapeuticstrategies. Therefore, we focus on skin diseases thatinvolve T cells and related pathophysiological processes.

T-cell migration to the skin

A crucial step in T-cell-mediated skin and other diseasesis the recruitment of T cells to the respective target organ[17]. Humanized mouse models are employed to furtherelucidate the recruitment of immune cells to inflammatorysites and to find therapies that interfere specifically withthis process in humans. Initially, either human immunecells or human skin were transferred to mice, whichprovided important insights into the trafficking of immunecells in response to chemokines [10,18]. Of greater value,is the combined transplant of both blood and skin(Figure 1a). Starting w1 week after injection, healthyhuman skin is infiltrated and rejected by allogeneicimmune cells. Interestingly, this occurs without signs ofxenogeneic GVHD. This model has been used to analyzethe mechanism of allograft rejection [14,15] and to test theeffect of suppressive therapies such as cyclosporine,rapamycin, anti-human lymphocyte-function-associatedantigen 3 (LFA3), human LFA3–IgG1 and interleukin 11(IL-11) [19–21].

Rapid T-cell infiltration can be induced in this systemby intradermal injection of chemokines before the onset ofthe alloimmune response. For example, tumor necrosisfactor a (TNF-a), CCL3 (macrophage inflammatory pro-tein 1a) and CCL2 (monocyte chemoattractant protein 1)attracted high numbers of CD45ROC CD45RAK T cells,whereas CCL5 [RANTES (regulated upon activation,normal T cells, expressed and presumably secreted)],


TRENDS in Pharmacological Sciences

InfiltrationSkin

T cells

Therapy

TherapyLesionalpso skin

(a)

HumanPBMCs

Activation+/– therapy

No therapy

Therapy

No therapy

No therapy

SCID

(b)

Induction

Non-lesionalpso skin SCID

(c)

4 weeks

ChemokinesSCID

(i)

(ii)

(i)

(ii)

(i)

(ii)

Figure 1. Humanized mouse models. (a) Humanized mouse model of T-cell migration. Human skin from a healthy donor is transplanted onto immunodeficient mice such as

severe combined immunodeficiency (SCID) mice. After engraftment, mice are injected with either autologous or allogeneic T cells. Intradermal injection of chemokines

induces infiltration of immune cells and allows testing of therapies that interfere with this process. Usually, T-cell infiltration is determined by immunohistology. Sections

show CD3 staining of human skin with (i) and without (ii) T-cell infiltration. Scale barsZ 100 mm. (b) Humanized mouse model of psoriasis (pso). Lesional skin from a patient

with pso is transplanted onto immunodeficient mice, and either topical or systemic therapy is studied. Scale bars Z 250 mm. (c) Induction of pso in non-lesional skin. Non-

lesional skin from a psoriasis patient is transplanted onto immunodeficient mice. After engraftment pso can be induced by intradermal injection of pre-activated autologous

immunocytes or superantigen. Therapy can be applied either to the immunocytes before injection or directly to the lesional skin. Scale barsZ 250 mm. Sections in (b) and (c)

show hematoxylin and eosin staining of human lesional psoriatic (i) and non-lesional (ii) skin on SCID mice. Abbreviation: PBMCs, peripheral blood mononuclear cells.

Review TRENDS in Pharmacological Sciences Vol.25 No.10 October 2004 545

CXCL12 (stromal cell-derived factor 1) and CXCL10(interferon g-inducible protein 10) attracted only lownumbers [22]. In experiments that used autologous skinand peripheral blood mononuclear cells (PBMCs),CXCL10, CCL22 (macrophage-derived chemokine),CCL11 (eotaxin) and CCL5 differentially recruited subsetsof immune cells [23]. Preferential recruitment of T helper 1(Th1) and Th2-associated cells indicates potential value asa model for diseases associated with Th1 and Th2 cells.

By using T cells from patients with skin diseases, thesemodels can be used to delineate the molecules that areessential for T-cell homing in the respective diseases, andfor testing specific therapies (see below).

Atopic dermatitis

In atopic dermatitis (AD) and type I hypersensitivityreactions, Th2 cells and IgE provoke a prolonged inflam-matory response in the skin. In classical animal models,wild-type mice are sensitized by applying chemicalallergens such as ovalbumin and trimellitic anhydrideand are challenged with the same hapten several days


later. This results in an acute inflammatory response, butnot a chronic Th2-cell response with pronounced skininflammation. Moreover, although they have a majorimpact on the course of the disease, the allergens appliedand their routes of administration (e.g. subcutaneousimplantation and the use of adjuvants) are often not rele-vant for human pathophysiology. Thus, translating resultsfrom animal experiments to humans is a crucial issue.

Several aspects of AD and type I hypersensitivityreactions can be mimicked in humanized mousemodels. Transferring PBLs from atopic patients leads toIL-4-dependent production of IgE in SCID mice [24].Human IgE is also produced in SCID mice after recon-stitution with PBLs from allergic patients and immuniz-ation with the respective allergen [25,26]. Ex vivostimulation of splenocytes with allergen, IL-2 andantigen-presenting cells gives rise to Th2-like T cells[26]. After intraperitoneal and intradermal injection ofPBMCs from AD patients into SCID mice, topicalstimulation with superantigen and a relevant allergeninduced weak, epidermal inflammation that resembled



AD [27]. SCID mice reconstituted with human serumthat contains anti-allergen IgE antibodies have positiveimmediate-type skin test responses to intradermal injec-tion of allergen [28]. These mice also develop increasedairway responsiveness.

To determine the molecules involved in skin homingof T cells in AD, and to interfere specifically with thisprocess, human PBMCs and Th2 clones derived fromeither lesions or blood of patients with AD have beentransferred into mice transplanted with allogeneic humanskin (Figure 1a) [29,30]. Th2 clones were selectivelyrecruited to the skin by injection of CCL17 and CCL22,which are ligands of the Th2-associated chemokinereceptor CCR4, and by CCL2, but not by CXCL10, CCL1or CCL11. Normal human PBMCswere attracted by CCL2and CXCL10. Infiltration in response to CCL22 wasdependent on E-selectin (CD62E) and was inhibited byeither an anti-CD62E antibody or a CD62E antagonist.Infiltration can also be blocked by an anti-leukocytefunction-associated molecule 1 (LFA-1) antibody and anLFA-1 inhibitor [29,30]. By contrast, results from otheranimal experiments and clinical observations indicatethat blocking CD62E alone is not sufficient for therapeuticefficacy [29,30]

In addition, skin from allergic patients transplanted toSCID mice is sensitive to an early-phase skin-prick testwith the relevant allergen [31]. In SCID mice engraftedwith skin and autologous PBMCs from allergic patients, aprofound, allergic, cutaneous reaction is induced by intra-dermal injection of allergen. The skin is infiltrated byCD4C and CD45ROC cells, basophils and murine eosino-phils. The cutaneous reaction is not observed with graftsfrom non-atopic donors. Administering anti-CCR3 anti-body selectively reduces the accumulation of eosinophilsbut not CD4C cells and basophils [31].

Thus, there are humanized mouse models of majoraspects of AD, namely IgE production and migration ofT cells to the skin. However, induction of significantdisease is not yet possible, and therapeutic responsivenessin these models and in humans does not always correlate.

Delayed-type hypersensitivity and allergic contact

dermatitis

Allergic-contact dermatitis (ACD) is a type IV hypersensi-tivity reaction that is mediated by antigen-specific effectorT cells. Several classical mouse models for ACD that areused widely in drug discovery are predictive in certainrespects [32]. However, distinct differences between thesemodels and the human disease exist, particularly withregard to the ratio of naive to primed T cells, cellularinfiltration and therapeutic responsiveness. Furthermore,classical models are mainly models of acute disease,whereas most humans suffer chronic disease.

Few humanized mouse models for type IV hypersensi-tivity reactions have been established. A frequent cause ofACD is allergy to nickel ions. Human skin grafts on SCIDmice injected with autologous, nickel-reactive T-cell lineshave been topically challenged with nickel sulfate, whichleads to a massive accumulation of T cells in the grafts[33]. Moreover, delayed-type hypersensitivity (DTH) toeither tuberculin or tetanus toxoid can be modeled in


humanized mice [34,35]. Thus, SCID mice have beentransplanted with human skin and reconstituted withautologous PBMCs from sensitized donors. Intradermalinjection of tetanus toxoid or tuberculin caused theinfiltration of activated T cells, which is similar to thehuman reaction.

A different kind of humanized mouse model can be usedfor testing human DTH reactions in animals. In thisso-called ‘trans vivo DTH’, human PBMCs were injectedinto either the pinnae or footpads of naive immune-competent or immune-deficient mice. Co-injection ofantigens such as tetanus toxoid and alloantigen induceda DTH-like swelling. This model has been used tocharacterize the immune response of liver-transplantrecipients against donor antigens [36].

None of these models represents the full course of thedisease. In particular, sensitization of human T cells torelevant antigens in the murine host has not beenachieved. Thus, humanized models have no big advantagecompared with classical models for ACD. Recentlydeveloped models with inducible human immunity(Rag2–gc–CD34C and SCID–huIC mice) might prove tobe useful in the future [8,9].

Psoriasis

Psoriasis is a chronic skin disease that is characterized byinfiltration of inflammatory cells and hyperproliferationof keratinocytes. Many cytokines and T cells have aprominent role in the pathogenesis of the disease. T cellsmight be activated by autoantigens and/or by bacterialsuperantigens. In general, psoriasis does not occur spon-taneously in animals, and transgenic and other mousemodels do not represent the full complexity of the disease.Thus, psoriasis-like lesions in these models lack typicalcellular infiltrates of human lesions. Because of the lack ofsuitable animal models there were early attempts totransplant human psoriatic skin onto immunodeficientmice (Figure 1b). Although both lesional and non-lesionalskin grafts change their phenotype in nude mice, they arequite stable in SCID mice [37]. The gradual loss of psori-atic features in SCID mice is rescued by injecting eitherT cells derived from psoriatic lesions or superantigen-stimulated PBMCs from patients with psoriasis [38,39].

Split-thickness or full-thickness grafts from humanpsoriatic lesions transplanted onto SCID or SCID/beigemice have been used widely to test anti-psoriatic treat-ments [40]. To date, drug efficacy observed in the human-ized murine model and in patients is consistent. Thus,established treatments are also successful in the SCIDmouse model (Table 2). Therefore, the humanized mousemodel of psoriasis is valuable for preclinical evaluation ofnovel, topical and systemic therapeutic strategies. Differ-ent classes of drugs, such as proteasome inhibitors [41],selectin inhibitors [42], antibodies [12,43,44] and anti-sense oligonucleotides [45] (Table 2), reduce the severity ofpsoriasis in the mouse xenograft model, measured byepidermal thickness, grade of parakeratosis, and numbersof inflammatory cells and proliferating keratinocytes.Anti-CD11a therapy and the peroxisome proliferator-activated receptor g ligand troglitazone also substantiallyimproved psoriasis in patients [43,46]. By contrast, the


Table 2. Psoriasis treatment tested in SCID mouse modelsa

Class Drugb Efficacyc Refs

Modeld Patient

Established drugs

Calcineurin inhibitor Cyclosporin A C C [43,48]

Glucocorticoids Dexamethasone C C [41,47]

Clobetasol propionate C C [43]

Vitamin D3 1a,25-Dihydroxycholecalciferol C C [48]

Compounds in development

Proteasome inhibitor PS519 C ? [41]

Selectin inhibitor Efomycine M C ? [42]

Leukotriene synthesis inhibitor BAYX1005 K K [47]

PPAR-g ligands Troglitazone C C [46]

NGF receptor blocker K252a C ? [44]

Antibodies Anti-CD11a, efalizumab C C [43]

Anti-IL-15 C ? [12]

Anti-NGF C ? [44]

Antisense oligonucleotides IGF-I C ? [45]

Cytokines IL-10 K (C) [48]aAbbreviations: IGF-I, insulin-like growth factor I; IL-15, interleukin 15; NGF, nerve growth factor; PPAR, peroxisome proliferator-activated receptor; SCID, severe combined

immunodeficiency.bSee Chemical names.cC, good response; (C), moderate response; K, bad or no response; ?, unknown response.dHuman lesional psoriatic skin was transplanted onto immuodeficient mice and drug efficacy determined after either systemic or topical therapy.


therapeutic effect of the leukotriene inhibitor BAYX1005(see Chemical names) is disappointing, in both the mousemodel and in humans [47]. Preliminary results indicatethat IL-10 is successful in humans but inefficient in thehumanized mouse model [48]. The inconsistency might becaused by different biological functions in humans andmice. For example, IL-10 is produced by both Th1 cells andTh2 cells and is a strong growth factor for B cells only inhumans [1].

An alternative to transplanting lesional skin is toinduce psoriasis in engrafted, healthy, non-lesional skinfrom psoriasis patients (Figure 1c). Induction can beachieved by intradermal injection of superantigen-stimu-lated PBMCs from the same patient or by direct injectionof superantigen into the skin [49–51]. More-detailedanalysis reveals that long-term stimulated patientPBMCs reliably induce psoriasis only in skin from thesame patient [51]. Treating activated PBMCs beforeinjection into the skin allows immunomodulatory drugsto be tested [48]. Interestingly, when transplanted ontomice deficient in Rag2 and type I and II interferonreceptors (AGR mice), non-lesional skin from patientsdevelops spontaneously into psoriatic lesions withoutinduction [52].

The humanized SCID mouse model of psoriasis, thus,seems to be a reliable, versatile model for the humandisease that has striking correlations to human patho-physiology and response to therapy.

Other T-cell-mediated skin diseases

Humanized mouse models have also been established foradditional T-cell-mediated skin diseases for which nosuitable classical animal model exist. Alopecia areata is atissue-restricted autoimmune disease of the hair follicle,which results in hair loss and baldness. After transplant-ing human scalp explants from involved areas onto SCIDmice, normal hair regrowth was observed. Injection ofautologous lymphocytes isolated from scalp lesions andstimulated with hair-follicle homogenate reproduces the


changes that are characteristic of alopecia areata [53,54].This model has been used to characterize the pathogeneticT-cell response. Injection of both CD4C and CD8C T cellsis necessary to induce significant hair loss [55]. T cellsactivated by melanocyte peptides also reduce hair growth,which indicates that these epitopes can function as auto-antigens [56]. Immunohistochemical examination of biop-sies reveals that production of CXCL10 by follicularepithelium and interferon g by infiltrating T cells is asso-ciatedwith hair loss,which supports aTh1-like disease [57].

Another skin disease with potential involvement ofT cells is pustulosis palmaris et plantaris (PPP), a chronicrecurring disorder of the palms and soles that ischaracterized by sterile, intra-epidermal pustules. Thereseems to be a relationship between PPP and tonsillar focalinfections. SCID mice reconstituted with human tonsillarmononuclear cells (TMCs) from patients with PPP developskin lesions, including fur loss and eruptions around theircheeks and foreheads [58]. After transplanting uninvolvedpatient skin and immediate intraperitoneal injection ofpatient TMCs, CD3C and CD4C cells infiltrate the graftand intercellular adhesion molecule 1 is upregulated. Thisis not seen after injection with PBLs [58].

Pemphigus foliaceus is an autoimmune, blistering, skindisease caused by pathogenic autoantibodies against theglycoprotein desmoglein-1. Injection of either the IgGfraction from patient serum or related antibodies intoSCID mice transplanted with human skin mimics somefeatures of the disease [59]. In addition, mice engraftedwith artificial human epidermal equivalents and injectedwith patient serum develop pemphigus-like clinicalfeatures [60]. Topical treatment with wheat germ agglu-tinin inhibits autoantibody binding and prevents acantho-lysis and blister formation [60].

In a more general approach, human skin grafted onSCID mice has been transduced with adenoviral vectorscoding for 37 different genes that are potentially involvedin skin diseases [61]. Depending on the gene, infiltration ofinflammatory cells, changes in vascular density, matrix


Chemical names

BAYX1005: (R)-2–4-[(quinolin-2-yl-methoxy) phenyl]-2cyclopentyl

acetic acid

K252a: (9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-

1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3 0,2 0,1 0-kl]pyrrolo[3,4-i][1,6]

benzodiazocine-10-carboxylic acid methyl ester

PS519: [1R-[1S,4R,5S]]-1-(1-hydroxy-2-methylpropyl)-4-n-propyl-6-

oxa-2-azabicyclo[3.2.0]heptane-3,7-dione


formation, proliferation and epidermal hyperplasia havebeen observed.

Concluding remarks

Major progress has been achieved in grafting humantissues into immunocompromisedmice. Transfering eithertissues or cells from patients yields models for complexdiseases that do not occur normally in animals. Thesemodels are invaluable for unraveling the pathophysiologyof diseases and for preclinical testing of novel therapeuticstrategies. Humanized mouse models are one step closerto the patient and promise better correlation with theclinical outcome than classical animal models. It mighteven be possible to test the suitability of a therapy for anindividual patient before clinical application, thus, con-tributing to customized medicine.

However, this closeness to the clinical situation bringssimilar shortcomings. Thus, the number of animals ineach experiment is restricted because patient material islimited and inter-individual variability is relatively high,which makes statistical evaluation difficult. Moreover, theuse of different immunodeficient mouse strains usuallyleads to different results, which can make comparison andinterpretation of the outcomes difficult. There are alsorestrictions with regard to practicability and costs; timelyand sufficient supply of fresh patient material is crucialbut challenging, and housing and handling immunodefi-cient mice is more complex than conventional animalhusbandry.

Therefore, humanized animal models are not appro-priate for high-throughput screening. In drug discoverythey are particularly useful as an intermediate stepbetween late-preclinical research and clinical develop-ment, and for target validation and compound character-ization for complex diseases.

The future will bring more sophisticated models thatmimic some aspects of the human situation more closely.These models will narrow the gap between the bench andthe clinic and, thus, benefit patients.

AcknowledgementsWe apologize for not citing many excellent papers because of spacelimitations.

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Free journals for dev

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The science publishers, Blackwell, Elsevier, the Harcourt Worldwide

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45 Wraight, C.J. et al. (2000) Reversal of epidermal hyperproliferation inpsoriasis by insulin-like growth factor I receptor antisense oligo-nucleotides. Nat. Biotechnol. 18, 521–526

46 Ellis, C.N. et al. (2000) Troglitazone improves psoriasis andnormalizes models of proliferative skin disease: ligands for peroxi-some proliferator-activated receptor-gamma inhibit keratinocyteproliferation. Arch. Dermatol. 136, 609–616

47 Boehncke, W.H. et al. (1999) The SCID-hu xenogeneic transplantationmodel allows screening of anti- psoriatic drugs. Arch. Dermatol. Res.

291, 104–10648 Dam, T.N. et al. (1999) 1alpha,25-dihydroxycholecalciferol and

cyclosporine suppress induction and promote resolution of psoriasisin human skin grafts transplanted on to SCID mice. J. Invest.

Dermatol. 113, 1082–108949 Boehncke, W.H. et al. (1996) Pulling the trigger on psoriasis. Nature

379, 77750 Nickoloff, B.J. and Wrone-Smith, T. (1999) Injection of pre-psoriatic

skin with CD4C Tcells induces psoriasis. Am. J. Pathol. 155, 145–158

51 Gilhar, A. et al. (2002) Psoriasis is mediated by a cutaneous defecttriggered by activated immunocytes: induction of psoriasis by cellswith natural killer receptors. J. Invest. Dermatol. 119, 384–391

52 Boyman, O. et al. (2004) Spontaneous development of psoriasis in anew animal model shows an essential role for resident t cells andtumor necrosis factor-{alpha}. J. Exp. Med. 199, 731–736

53 Gilhar, A. et al. (1998) Autoimmune hair loss (alopecia areata)transferred by T lymphocytes to human scalp explants on SCIDmice. J. Clin. Invest. 101, 62–67

54 Kalish, R.S. and Gilhar, A. (2003) Alopecia areata: autoimmunity–theevidence is compelling. J. Investig. Dermatol. Symp. Proc. 8, 164–167

55 Gilhar, A. et al. (2002) Mediation of alopecia areata by cooperationbetween CD4C and CD8C T lymphocytes: transfer to human scalpexplants on Prkdc(scid) mice. Arch. Dermatol. 138, 916–922

56 Gilhar,A. et al. (2001)Melanocyte-associatedTcell epitopes can functionas autoantigens for transfer of alopecia areata to human scalp explantson Prkdc(scid) mice. J. Invest. Dermatol. 117, 1357–1362

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58 Yamanaka, N. et al. (2001) Engraftment of tonsillar mononuclear cells

in human skin/SCID mouse chimera–validation of a novel xenogeneictransplantation model for autoimmune diseases. Microbiol. Immunol.

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60 Ortiz-Urda, S. et al. (2003) The plant lectin wheat germ agglutinininhibits the binding of pemphigus foliaceus autoantibodies todesmoglein 1 in a majority of patients and prevents pathomechanismsof pemphigus foliaceus in vitro and in vivo. J. Immunol. 171,6244–6250

61 Gruss, C.J. et al. (2003) Stroma formation and angiogenesis byoverexpression of growth factors, cytokines, and proteolytic enzymesin human skin grafted to SCID mice. J. Invest. Dermatol. 120,683–692

eloping countries

ess to Research Initiative, which enables nearly 70 of the world’s

medical literature through the Internet.

STM group, Wolters Kluwer International Health and Science,

eBritishMedical Journal in 2001. Initially,more than 1000 journals

s,medical schools, research and public institutions in developing

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een rich and poor countries’.

ork.net for more information.


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