Vet Pathol 45:563–575 (2008) Review Paper: Preclinical Models of Psoriasis D. M. DANILENKO Genentech, Inc., Department of Pathology, South San Francisco, CA Abstract. Psoriasis is the most common autoimmune disease in man and is characterized by focal to coalescing raised cutaneous plaques with consistent scaling and variable erythema. The specific pathogenesis of psoriasis is not completely understood, but the underlying mechanisms involve a complex interplay between epidermal keratinocytes, T lymphocytes as well as other leukocytes (including dendritic cells and other antigen presenting cells [APCs]), and vascular endothelium. Mirroring the complexity of mechanisms that underlie psoriasis, there are a relatively large number of models of psoriasis. Each model is based on a slightly different pathogenic mechanism, and each has its similarities to psoriasis as well as its limitations. In general, psoriasis models can be very broadly divided on the basis of the pathogenic mechanisms that interplay to cause psoriasis, with the addition of several relatively poorly defined spontaneous murine mutant models. Other than the spontaneous mutant models, murine models of psoriasis can be divided into those that are genetically engineered (transgenic and knockout—with manipulation of either the epidermis, leukocytes, or the endothelium), and those that are induced (either by immune transfer or by xenotransplantation of skin from psoriatic patients). In addition to the murine models, in vitro human epidermal models have recently become more widely utilized. While no one single model of psoriasis is ideal, many have proven to be extremely valuable in investigating and better understanding the molecular mechanisms that underlie the complex interplay between epidermal keratinocytes, the innate and adaptive immune system, and the vascular endothelium in psoriasis. Key words: Animal disease models; human epidermis; in vitro models; mouse; psoriasis. Introduction been much recent progress in understanding many of its complex underlying mechanisms, which Psoriasis is a common inflammatory condition of involve an interplay between epidermal keratino- human skin characterized by focal to coalescing cytes, leukocytes (including dendritic cells and raised cutaneous plaques with consistent scaling other antigen presenting cells [APCs]), and vascular and variable erythema.26 Typical histologic features endothelium (Fig. 1 illustrates some of the key of psoriasis include epidermal hyperplasia (acan- features underlying the pathogenesis of psoria- thosis) with elongated rete ridges, a less discrete sis).26,29 Originally, psoriasis was considered to be epidermal granular layer (hypogranulosis), para- primarily a disorder of disregulated epidermal keratosis, and leukocytic infiltration of the dermis proliferation and differentiation, and indeed, and epidermis.26,29,34 The cellular composition of agents such as retinoids and vitamin D analogs the psoriatic leukocytic infiltrate is variable but that treat epidermal differentiation defects have very consistently contains both CD4+ and CD8+ T shown some efficacy in the treatment of psoria- lymphocytes, with CD4+ T lymphocytes predomi- sis.2,24,32 With the discovery that immunosuppres- nating in the dermis, and CD8+ T lymphocytes, sive agents such as cyclosporin A and corticoste- particularly those expressing the aEb7 integrin, roids are often effective in treating psoriasis, the preferentially infiltrating the epidermis.26,38 Psoria- pendulum has swung toward psoriasis being sis is now recognized as the most common considered primarily a disorder of the immune autoimmune disease in man, with a prevalence of system, with T cells and specifically CD4+ T cells 2–4% worldwide.26,29 While the specific etiology of felt to play a central role.35,51 Initially, psoriasis was psoriasis is unknown, a genetic basis has been believed to primarily be a Th1 T helper cell– suspected for some time, and a number of different mediated process, driven by IFNc and related psoriasis susceptibility gene clusters, designated cytokines.26,29 More recently it has been shown that PSORS1, PSORS2 . . . PSORS6, have been identi- IL-23, a cytokine involved in the development of fied, underscoring the heterogeneous nature of the newly defined Th17 T helper cell subset,36 plays psoriasis.26,34 Similarly, the specific pathogenesis of a major role in psoriasis.5,22,27,33 Even more recent psoriasis is still not fully understood, but there has data suggest that psoriasis is caused by an 563 564 Danilenko Vet Pathol 45:4, 2008 Fig. 1. Schematic diagram illustrating some of the key features that make up our current understanding of the pathogenesis of psoriasis. Psoriasis in genetically susceptible individuals is believed to be triggered by specific stimuli, such as trauma or bacterial infections, which in turn induce resident dendritic cells (DC), macrophages, and T cells to produce cytokines that initiate a cascade of events leading to the hallmarks of psoriasis: immune cell activation, disregulated epidermal acanthosis, and angiogenesis. IL-12 produced by DC and macrophages induces a Th1 immune response, characterized by production of IFNc, while IL-23 from DC and macrophages induces a Th17 immune response, characterized by production of IL-17 and IL-22. These cytokines, in turn, induce the changes characteristic of psoriatic epidermal keratinocytes, including upregulation of NF-kB, induction of nuclear pStat3, and induction of psoriasin. Macrophages activated by TNF-a produce additional cytokines that induce psoriatic epidermal changes, as well as VEGF, which stimulates angiogenesis. Activated macrophages and psoriatic keratinocytes also produce chemokines that recruit leukocytes from the vasculature to infiltrate the developing psoriatic plaque. DC 5 dendritic cell; IFNc 5 interferon gamma; IL 5 interleukin; NF-kB 5 nuclear factor-kappa B; pStat3 5 phospho-signal transducer and activator of transcription 3; Th 5 helper T cell; TNF-a 5 tumor necrosis factor-a; VEGF 5 vascular endothelial growth factor. interaction between epidermal keratinocytes and lated epidermal acanthosis, dermal and epidermal the immune system,32,39,41 and that one possible leukocytic infiltration, and dilation of dermal candidate linking the immune system and epider- blood vessels—lesions that are maintained by mal keratinocytes is IL-22, a T-cell-derived cyto- the complex interplay between T cells and their kine that is produced by Th17 polarized T cells cytokines, other leukocytes, vascular endothelium, that are stimulated by IL-23,56 but that acts and epidermal keratinocytes.26,29 As noted above, on epidermal keratinocytes to induce acanthosis epidermal keratinocytes as well as vascular endo- and differentiation toward a psoriatic pheno- thelial cells are active participants in the psori- type.7,33,40 Regardless of the specific underlying atic inflammatory process via secretion of cyto- pathogenesis, psoriasis is characterized by disregu- kines and growth factors, and the upregulation Vet Pathol 45:4, 2008 Psoriasis Models 565 of signaling and adhesion molecules on their lesions that do respond to corticosteroids, the surfaces.26,29 pathogenesis of these lesions is unknown, and they Mirroring the complexity of mechanisms that do not express all of the features of psoriasis, such underlie psoriasis, there are a relatively large as lack of an epidermal T-cell infiltrate and the lack number of models of psoriasis. While most of of hyperproliferative keratin expression.47 In addi- these are murine models, there are also several non- tion, these mice have a limited useful lifespan murine and a few in vitro human epidermal models because of nonregenerative anemia and the massive (Table 1). Each model is based on a slightly papillomatous hyperplasia of their forestomachs.47 different pathogenic mechanism, and each has its Another spontaneous mouse model that has been strong points/similarities to psoriasis, as well as its relatively well described is the chronic proliferative limitations, not the least of which are the funda- dermatitis mouse (Sharpincpdm/Sharpincpdm), a mental morphologic differences between human mouse that develops marked eosinophilic inflam- psoriatic skin and murine psoriasis models (except- mation in a number of tissues, including the skin, ing xenotransplantation models).29,34,42 Morpholog- which leads to marked acanthosis.17,18,45 Unlike ic differences in murine models versus psoriatic psoriasis, however, the inflammation in this model skin include lack of parakeratotic scaling and is driven by Th2 cytokines such as IL-4, IL-5, and epidermal hypogranulosis, and marked follicular IL-13, and lesions respond to IL-12 treatments.18 hyperplasia rather than true exaggeration of epi- For these reasons, this model is not felt to be very dermal rete ridges (Figs. 2, 3). In general, murine representative of psoriasis, which is predominantly models of psoriasis can be broadly divided on the a Th1- and Th17-driven disease, and which has basis of the pathogenic mechanisms that interplay been shown to respond to an anti-IL-12 (and anti- to cause psoriasis, with the addition of a few IL-23) p40 MAb.22,49 relatively poorly defined spontaneous mutant models. Aside from these spontaneous models, Genetically Engineered Models murine models of psoriasis can be divided into Genetically engineered mice and rats are the those that are genetically engineered (transgenic largest category of psoriasis models and include and knockout—with manipulation of the epider- both transgenics and knockouts. In this review, I mis, leukocytes, or the endothelium), and those have divided these into 3 broad categories: those that are induced (either by immune cell transfer or that target epidermal keratinocytes, those that by xenotransplantation). In general, to be consid- target leukocytes, and those that target vascular ered as a useful model of psoriasis, the model has to endothelium. The CD18 hypomorphic mouse is a either share some histopathologic features with genetically engineered model that targets leuko- psoriasis, have a pathogenesis and/or disease cytes. This mouse does not completely lack the mechanism that is similar to psoriasis, and/or CD18 b2 integrin, but rather has decreased respond similarly to therapeutic agents that psori- expression of this common b2 chain of the asis has been demonstrated to respond to. The ideal leukointegrin adhesion molecule complex. When psoriasis model would have all 3 features. Howev- on a PL/J strain background, these CD18 hypo- er, as mentioned earlier, psoriasis is a heteroge- morphic mice, PL.129S7-Itgb2tm1Bay, develop a neous disease with a complex pathogenesis, There- psoriasiform inflammatory skin condition with a fore, identifying a single model of psoriasis that predominantly lymphocytic infiltration.3,10,23 While completely mirrors all aspects of psoriasis is very lesions in these mice are responsive to dexameth- likely not possible. A discussion of the most widely asone,10 their pathogenesis has not been well accepted models of psoriasis follows in the characterized. In addition, unlike psoriasis, these subsequent sections. mice exhibit nonpsoriasiform epidermal hyperplasia with lack of hyperproliferative keratin expression. Spontaneous Models Therefore, this model has not been widely used for There are a number of different spontaneously efficacy testing of potential therapeutic agents. occurring mouse models of psoriasis, none of which Another genetically engineered mouse model are very well understood mechanistically.42 Flaky that develops cutaneous inflammation with some skin mice (Ttcfsn/Ttcfsn) are probably the best features similar to psoriasis is the p40 keratin 14 described, and have a spontaneous mutation that (K14) transgenic mouse, Tg(KRT14-Il12b)1Tsk, in induces proliferation and hyperkeratosis of strati- which the p40 subunit of both IL-12 and IL-23 is fied squamous epithelia, including the nonglandu- overexpressed via the K14 promoter in basal lar forestomach.46 While these mice have interest- epidermal keratinocytes.24,25 These mice develop ing squamous proliferative and inflammatory cutaneous inflammation that is felt to be more 566 Danilenko Vet Pathol 45:4, 2008 Table 1. Animal and in vitro models of psoriasis. Increased Leukocytic Model Acanthosis Altered Epidermal Differentiation Vascularization Infiltration Spontaneous Models Flaky Skin (Ttcfsn/Ttcfsn) Yes Focal parakeratosis Yes Yes Chronic Proliferative Dermatitis Yes Focal parakeratosis Yes Yes (Sharpincpdm/Sharpincpdm) Genetically Engineered Models Targeting the Immune System PL/J CD18 b2 Integrin Yes Focal parakeratosis Yes Yes Hypomorphic K14.p40 (IL-12 and IL-23) Yes No Yes Yes Transgenic HLA-B27/b2 Microglobulin Yes No Yes Yes Transgenic Rat aE (CD103) Knockout Yes No Yes Yes Targeting Vascular Endothelium K14.VEGF Transgenic Yes Epidermal expression of Yes Yes hyperproliferative keratins pTek-tTA.Tie2 Double Yes No data Yes Yes Transgenic Targeting Epidermal Keratinocytes K14.TGF-a Transgenic Yes Parakeratosis; altered No data In some differentiation animals K14.KGF Transgenic Yes Epidermal expression of Yes No hyperproliferative keratins K5.TGFb1 Transgenic Yes Parakeratosis Yes Yes K14.IL-20 Transgenic Yes Epidermal expression of No No hyperproliferative keratins IKK2 Epidermal Deletion Yes Epidermal expression of Yes Yes hyperproliferative keratins K5.Stat3 Transgenic Yes Epidermal expression of Yes Yes hyperproliferative keratins JunB/c-Jun Inducible Yes Parakeratosis; upregulation of Yes Yes Epidermal Deletion S100A8 and S100A9 Immune Transfer Model CD45RBHi + LPS & IL-12 Yes Epidermal expression of Yes Yes in SCID Mice hyperproliferative keratins Xenotransplantation Models SCID Xenotransplantation Yes Epidermal expression of Yes Yes hyperproliferative keratins AGR129 Xenotransplantation Yes Epidermal expression of Yes Yes hyperproliferative keratins In vitro Models Reconstituted Human Yes Epidermal expression of No No Epidermis (RHE) Using hyperproliferative keratins Psoriatic Keratinocytes RHE Using Normal Human Yes when Epidermal expression of No No Keratinocytes treated with hyperproliferative keratins specific and S100A7 (psoriasin) when cytokines or treated with IL-20 subfamily growth factors cytokines or oncostatin-M Vet Pathol 45:4, 2008 Psoriasis Models 567 Table 1. Extended. Reflects Human T-cell Infiltration Pathogenesis Response to Therapy Situation References 46 Strain-dependent Uncertain Only steroids No 17,18 No data Uncertain; Th2 cytokine driven Steroids, IL-12 No Yes Uncertain; CD4+ T-cell mediated Only steroids No 3,10,23 24,25 Yes Overexpression of IL-23 in No data No epidermis 47,52 Yes Overexpression of HLA linked No data No to spondylo-arthropathies 44 Yes Uncertain, possibly increased sus- No data No ceptibility to bacterial infection 12,53 No data VEGF overexpression Only VEGFR1-VEGFR2-Ig No fusion protein; no data for other 50 Yes Tie2 overexpression Cyclosporin A No 49 In some animals TGF-a overexpression No data No 15 No KGF overexpression No data No 28 Yes TGFb1 overexpression No data No 6 No IL-20 overexpression; neonatal No data No lethal 37,45 Yes Disregulation of epidermal Soluble TNF receptor fusion Possibly cytokine regulatory network. protein T cells not required Yes pStat3 overexpression. CD4+ Only Stat3 Decoy Oligo, no Possibly 41 cells required data for other 55 Yes Induction of Jun regulated No data Possibly cytokines in keratinocytes; T cells not required Yes Activated Th1 polarized CD4+ Steroids and cyclosporin A No 19,20,43 T cells traffic to sites of immune stimulation 4,35,51,54 Yes Human psoriatic skin with Anti-CD11a Yes activated human CD4+ T cells 9,31 Yes Human psoriatic skin with Anti-TNF and soluble TNF Yes proliferation of resident receptor fusion protein human CD4+ T cells 1 No In vitro manipulation No data Partially 7,8,13,40 No In vitro manipulation No data Partially similar to eczema and atopic dermatitis, lacking a (CD103) knockout mouse, Itgaetm1Cmp, also devel- significant cutaneous CD8+ T-cell infiltration, and ops a cutaneous inflammatory disease. The aE so are not considered to be a particularly useful integrin complexes with the b7 chain and is thought model for psoriasis. Similarly, the aE integrin to play a role in cutaneous and mucosal immunity 568 Danilenko Vet Pathol 45:4, 2008 Fig. 2. Skin; human psoriatic skin illustrating exaggerated epidermal rete ridges exhibiting epidermal hypogranulosis (asterisks) with extensive parakeratotic scaling (PS). HE. Bar 5 100 mm in a and 50 mm in b. Fig. 3. Skin; CD45RBHi CD252 T-cell immune transfer mouse model. In contrast to the human psoriatic skin illustrated in Fig. 1, the epidermis in this mouse model has follicular hyperplasia rather than exaggerated epidermal rete ridges, exhibits epidermal hypergranulosis (arrows) rather than hypogranulosis, and lacks the parakeratotic scaling that is one of the hallmarks of a psoriatic plaque. HE. Bar 5 100 mm in a and 50 mm in b. through interaction with its counter receptor, E Tg(HLA-B*2705, B2M)33-3Trg.48,53 This rat over- cadherin.44 In addition, aEb7 is preferentially expresses a form of human leukocyte antigen expressed by the CD8+ T cells that infiltrate the (HLA)-B27 that has been linked to spondyloar- psoriatic epidermis.38 However, the cutaneous thropathies.21 HLA-B27 transgenic rats develop inflammatory condition that aE knockout mice epidermal acanthosis with epidermal infiltration of develop is not felt to mimic psoriasis closely both CD4+ and CD8+ T cells, as well as immune- enough, having significant ulceration and very mediated arthritis and inflammatory bowel dis- few cutaneous CD8+ T cells, for these mice to be ease.48,53 The occurrence of psoriatic skin lesions is a useful psoriasis model. Still, these mice have a less consistent than is the occurrence of the other phenotype that does shed some light on the immune-mediated disorders,48,53 and there have potential functions of the aEb7 integrin complex been no published reports of therapeutic efficacy in cutaneous inflammatory disease.44 testing for the psoriatic lesions. Therefore, while Another genetically engineered animal model this model has attained some utility as a model of that targets the immune system more broadly is the HLA-B27 associated autoimmune disease, it has HLA-B27/human b2 microglobulin transgenic rat, not gained widespread use as a model of psoriasis. Vet Pathol 45:4, 2008 Psoriasis Models 569 Genetically engineered mouse models of psoria- as a model of psoriasis.6 Recently however, several sis that target vascular endothelium include VEGF genetically engineered mouse models that target K14 transgenic mice and Tie2 transgenic mice. epidermal keratinocytes have put the epidermal Mice overexpressing VEGF epidermally via a K14 keratinocyte back into a central role in the promoter, Tg(KRT14-Vegfa)1Gdy, develop a very pathogenesis of psoriasis. These models appear to vasculocentric cutaneous inflammatory disease fulfill most of the criteria necessary for a model of with hyperplastic and inflamed dermal vasculature, psoriasis to be representative and useful. The first of and psoriasiform epidermal acanthosis.12,54 Al- these models targeting epidermal keratinocytes is though these mice have many vascular, epidermal the epidermal-specific IKK2 knockout mouse, and inflammatory features that resemble psoriasis, Tg(KRT14-cre)1Cgn 3 Ikbkbtm1Mpa, in which the lesions appear to be largely vascular-based, and deletion of the IKK2 catalytic subunit of the IkB there is marked dermal infiltration by mast cells. In kinase complex (necessary for NF-kB activation by addition, other than responding to treatment with a inflammatory stimuli) caused mice to develop soluble VEGFR1-VEGFR2-Ig fusion protein, data psoriasiform cutaneous inflammation that shared demonstrating response to therapeutic candidates many features of psoriasis, including dependence on in this model are lacking.12,54 Therefore, there are a intact TNF signaling, but which was T-cell inde- number of caveats that limit the usefulness of the pendent.37,46 A second model targeting epidermal VEGF K14 transgenic mouse model. Tie2 trans- keratinocytes is the JunB/c-Jun epidermal inducible genic mice, Tg(Tek-tTA)1Dmt 3 Tg(TetOS- double knockout mouse, Tg(Krt1-5cre/ERT)1lpc 3 Tek)1Dmt, were constructed using a driver trans- Junbtm3Wag 3 Juntm4Wag.56 JunB is a component of gene, pTek-tTA, which localizes Tie2, the receptor the AP-1 transcription factor and has been localized for angiopoietin-1 and angiopoietin-2, to vascular to the PSORS6 psoriasis susceptibility locus, while endothelium as well as to keratinocytes in the c-Jun is felt to be an antagonist to JunB. Inducible epidermis and hair follicles.51 Tie2 transgenic mice deletion of both JunB and c-Jun leads to psoriasi- also develop a very vasculocentric cutaneous form cutaneous inflammation, as well as to arthritis inflammatory disease with vascular hyperplasia in some mice. The cutaneous inflammation is not and epidermal acanthosis with inflammation. dependent on T cells, nor is it dependent on TNF While many of the same caveats that hold for the signaling, as both Rag2 knockout mice and TNFR1 VEGF K14 transgenic mice also pertain for the knockout mice still developed cutaneous lesions.56 Tie2 transgenic mice (vasculocentric inflammation Interestingly, however, the TNFR1 knockout mice with many mast cells and incomplete characteriza- did not develop arthritis.55 A final recent transgenic tion of epidermal changes), cutaneous lesions in the model targeting epidermal keratinocytes is the K5- Tie2 transgenic mice have at least been shown to be Stat3 transgenic mouse, Tg(KRT5-stat3*A661C* responsive to a therapeutic that has shown efficacy N663C)1Jdg, in which Stat3, a transcription factor in psoriasis, namely cyclosporin A.50 implicated as playing a major role in signal There have been several genetically engineered transduction in psoriatic keratinocytes, is overex- mice, all transgenics, that have targeted epithelial pressed in epidermal keratinocytes.41 In this trans- growth factors such as TGF-a, KGF, and TGF-b genic mouse model, mice develop psoriasiform via K14 or K5 promoters (Tg(TGFa)1Efu, epidermal acanthosis that is most pronounced in Tg(FGF7)2Efu, and Tg(KRT5-TGF-B1)F2020Xjw, areas of friction, such as the tail head, and that is respectively) to the basal epidermis, with resulting accentuated by tape stripping and wounding, all phenotypes that somewhat mimic the epidermal features that are similar to psoriasis.41 In addition, acanthosis of psoriasis because of the disruption of these mice have a cutaneous lymphocytic infiltrate normal epidermal growth and differentiation, while that is predominantly CD4+ in the dermis, and CD8+ generally lacking the inflammatory component of in the epidermis, another feature that is similar to psoriasis.15,28,50 While these growth factor trans- psoriasis.41 Finally, both Stat3 transgenic skin and genics are not good models of psoriasis, they have the injection of activated lymphocytes, specifically proven to be useful in the study of epidermal growth CD4+ T cells, are necessary to generate a psoriatic and differentiation.15,28,50 A relatively recently dis- phenotype in transplanted SCID mice,41 a finding covered cytokine in the IL-10 family, IL-20, also that firmly establishes a link between keratinocytes induces a similar psoriasiform phenotype in K14 and CD4+ T lymphocytes in the pathogenesis of promoter epidermal targeted transgenic mice, psoriasis.39 Since phosphorylated Stat3 overexpres- Tg(KRT14-Il20)1Yac, but again these mice lack sion has also been detected in the nuclei of psoriatic cutaneous inflammation, and in addition exhibit keratinocytes, the link between the K5 Stat3 neonatal lethality, essentially rendering them useless transgenic mouse and psoriasis is even further 570 Danilenko Vet Pathol 45:4, 2008 strengthened.39,41 Cutaneous lesions in these trans- skin transplanted onto immunodeficient mice. genic mice respond to Stat3 decoy oligonucleotides, However, these models are also the most difficult suggesting that therapies aimed at this signaling to utilize, as they rely on having a steady supply pathway may be of potential clinical benefit.41 A of human psoriatic skin available. Hence, only a number of IL-10 family members, including the few laboratories are able to run these models with previously mentioned IL-20, as well as IL-6 family any consistency.4,9,31,35,52,55 In the prototype xe- members, such as oncostatin M, induce Stat3 notransplantation model, the epidermis and phosphorylation, and are being investigated as dermis from a patient with psoriasis is trans- potential therapeutic targets by a number of planted onto the flank of a CB.17-Prkdcscid/ investigators.7,8,33,40 Prkdcscid SCID mouse. As was the case with the Stat3 transgenic mice, psoriatic lesions are only Immune Transfer and Transplantation Models induced when activated T cells, and specifically CD4+ T cells but not CD8+ T cells, are injected This category of models includes the CD45RBHi into the transplanted SCID mice.35,52 The trans- CD252 T-cell immune transfer model and the planted epidermis then undergoes psoriasiform human psoriatic skin xenotransplantation model, acanthosis with the induction of proliferation both using SCID mice as the transplant recipients. markers such as keratin 16. In addition, trans- Both of these models have been relatively exten- planted skin becomes infiltrated by lymphocytes, sively validated in efficacy studies, and thus are with primarily CD4+ T cells in the dermis and frequently favored by investigators hoping to test a primarily CD8+ T cells in the epidermis.35,52 novel potential therapeutic entity.43,55 Intraepidermal microabscesses (Munro’s micro- In the CD45RBHi T-cell immune transfer model, abscesses) are also sometimes present.52 An anti- MHC minor mismatched CD4+ CD45RBHi CD11a MAb (efalizumab), which has demon- CD252 naıve T cells (lacking in regulatory T cells) ¨ strated efficacy in human psoriasis,14,34 has also are injected into ICR-Prkdcscid/Prkdcscid SCID been shown to have efficacy in this mouse mice, and target areas of antigenic stimulation, model.54 A variant of this model using symptom- such as the lower intestinal tract, skin, and lung. less psoriatic skin transplanted onto AGR129 When these CD252 naıve T cells are stimulated ¨ mice, 129-Ifnar1tm1Agt 3 Ifngr1tm1Agt 3 with either LPS or IL-12 (SDI mouse, BioSeek, Rag2tm1Fwa, which lack both interferon type 1 Burlingame, CA), the predominant disease is and type 2 receptors as well as RAG2, has also chronic persistent cutaneous inflammation that recently been described.9,31 In this model, trans- has many features resembling psoriasis19,20,43 planted symptomless psoriatic skin spontaneous- (Fig. 3). The underlying pathogenesis of the ly becomes psoriatic without the addition of cutaneous inflammation is Th1-driven, similar to activated T cells because of the proliferation of psoriasis (although psoriasis is now felt to have a resident human T cells within the xenograft.9,31 major Th17-driven component), but unlike psori- An anti-TNF MAb (infliximab) and a soluble asis, IFNc does not appear to play a major role in TNF receptor fusion protein (etanercept), both of the development of lesions in this model.19,20 In which have been shown to have efficacy in human addition, since all of the transferred lymphocytes psoriasis,11,30,34 also demonstrate efficacy in in- are CD4+, no CD8+ lymphocytes are present in the hibiting the development of psoriatic lesions in inflammatory infiltrate, which is unlike psoriasis. this model.9 To address this limitation, we and other investiga- tors have added back different lymphocyte subsets In Vitro Models to the cells being immune transferred with relative- ly good results, such that CD8+ T cells can be As an alternative to the in vivo models, several found in the epidermal infiltrate with the mainte- laboratories, including our own, have begun to use nance of the severity of the cutaneous inflamma- reconstituted human epidermal culture model tion (Khattri and Danilenko, unpublished data). systems whereby epidermal keratinocytes are As mentioned above, cutaneous lesions in this grown to the air-liquid interface and differentiate model have been shown to respond to a number of and stratify to mimic the morphology of normal therapeutic agents, including cyclosporin A, corti- stratified squamous epidermis. Epidermal kerati- costeroids, and anti-IL-12.19,20,43 nocytes in this model can be obtained from The final set of animal models of psoriasis are individuals with psoriasis1 or from normal individ- probably the most faithful of all to the human uals and then treated with a variety of cytokines condition, since these models use human psoriatic and/or growth factors (e.g., IL-20, IL-22, oncosta- Vet Pathol 45:4, 2008 Psoriasis Models 571 Fig. 4. Skin; in vitro reconstituted human epidermis. Epidermal hyperplasia, induction of epidermal S100A7 (psoriasin) expression, and induction of pStat3 in epidermal keratinocyte nuclei following 4 days in vitro treatment with IL-20 (b,e,h) and IL-22 (c,f,i) compared with media control (a,d,g). IL-22-treated epidermis also exhibits hypogranulosis (asterisks in c), a feature frequently evident in psoriatic epidermis. HE (a,b,c), S100A7 (psoriasin) IHC (d,e,f), pStat3 IHC (g,h,i). Bars 5 50 mm. tin-M) such that the resulting reconstituted human pStat37,8,40 (Figs. 3–5), one of the major signal epidermal model develops phenotypic characteris- transducers in psoriatic epidermis, as described tics that can mimic psoriatic epidermis7,8,13,40 previously.41 While these in vitro model systems (Fig. 4). In both the psoriatic keratinocyte-derived have obvious limitations in that they lack leuko- model and the normal human keratinocyte-derived cytes and blood vessels, they have still proven model treated with either IL-20, IL-22, or onco- useful in studying many aspects of the psoriatic statin-M, keratinocytes exhibit many of the same epidermis, including keratinocyte differentiation phenotypic features evident in psoriasis, such as and response to stimuli. upregulation of chemokines IL-8 and GRO-a, and induction of hyperproliferative keratin 16.1,7,8,13,40 Recommendations and Future Directions Many additional features in common with psoriatic epidermis are seen in the normal keratinocyte While there are a relatively large number of reconstituted epidermis model treated with cyto- animal models purported to mimic psoriasis, there kines such as IL-20, IL-22, or oncostatin-M, are actually only a small number that meet the including upregulation of S100 family members criteria of resembling psoriasis clinically and such as S100A7/psoriasin, and activation of pathologically, having a pathogenic mechanism 572 Danilenko Vet Pathol 45:4, 2008 that is known to play a significant role in psoriasis, plantation models more faithfully mimic human and having been validated by showing a response psoriasis, our laboratory has used the SCID to therapies that psoriasis is known to respond to. CD45RBHi CD252 T-cell immune transfer model Only two sets of models currently meet these more widely for initial screening, as greater criteria: the ICR-Prkdcscid/Prkdcscid SCID mouse numbers of mice can be used, and more treatment CD45RBHi CD252 T-cell immune transfer model, effect parameters can be evaluated. Once a and the 2 human psoriatic skin xenotransplanta- potential therapeutic has shown evidence of effica- tion onto immunodeficient mouse models, CB.17- cy in this model, we will then generally attempt to Prkdcscid/Prkdcscid and AGR129. Of these 2, the confirm this effect in one or both of the xenotrans- human psoriatic skin xenotransplantation models plantation models. are the most faithful to human psoriasis, but the While only the 2 sets of models described above difficulties in obtaining human psoriatic skin for are widely used for efficacy testing of potential transplantation limits the widespread utility of therapeutics, several recently described genetically these models. The CD45RBHi CD252 immune engineered mouse models hold promise for studying transfer model has gained relatively widespread use many aspects of psoriasis pathogenesis and may also but has several limitations: As is the case for all eventually turn out to be useful models for efficacy mouse models, the morphology of lesions in this testing (this will only be determined after repeated model does not completely mimic the epidermal evaluations). The K5-Stat3 transgenic mouse, changes in psoriasis; in addition, the leukocytic Tg(KRT5-stat3*A661C*N663C)1Jdg, while cur- infiltrate lacks CD8+ T cells unless they are rently not meeting the criterion of having been specifically added back in. validated via efficacy testing with a therapeutic Despite these limitations, both models are known to affect psoriasis, nonetheless shows much relatively commonly used for evaluation of poten- promise as a potentially useful model of psoriasis, tial therapeutic agents, as human therapeutics with particularly as it appears to link keratinocytes and demonstrated efficacy in psoriasis have also shown infiltrating T-lymphocytes in the pathogenesis of efficacy in these models (both small molecules such psoriatic lesions.39,41 In addition, while having the as cyclosporin A and biologics such as anti-TNFs limitations imposed by the differences inherent and anti-CD11a).9,43,55 While both sets of models between mouse and human skin, the morphology have been used for efficacy testing of potential of lesions in this model nonetheless appear to therapeutics, the pros and cons of each somewhat relatively faithfully mimic those evident in psoriatic balance each other out. The xenotransplantation skin.41 While this model does hold promise patho- models are more biologically faithful to psoriasis, logically and mechanistically, it still remains to be but the difficulties in obtaining human psoriatic validated by testing whether its cutaneous lesions skin to transplant generally limits the number of will respond to additional therapeutic agents, such animals that can be used in a study. In contrast, the as cyclosporin A, anti-LFA-1, anti-TNFs, and number of SCID CD45RBHi CD252 T-cell im- others. Another recently described mouse model, mune transfer model mice that can be used in a the epidermal JunB/c-Jun epidermal inducible dou- study can be much greater (we generally try to use ble knockout mouse, Tg(Krt1-5cre/ERT)1lpc 3 10–12 mice per treatment group), thereby allowing Junbtm3Wag 3 Juntm4Wag, also develops psoriasiform for much more meaningful statistical analysis of cutaneous inflammation, as well as arthritis in some differences in treatment effects. In the xenotrans- mice.55 As is the case for the Stat3 knockout mice, plantation models, only the transplanted skin can this model has the same limitations imposed by be evaluated for treatment effects, and generally differences between mouse and human skin, and also only histologic evaluation is done (epidermal remains to be validated by testing whether its thickness is the primary measurement that has cutaneous lesions will respond to antipsoriatic been used).9,55 In contrast, in the SCID CD45RBHi therapeutic agents. Still, it is also a valuable model CD252 T-cell immune transfer model treatment for studying the mechanisms common to the effects are evaluated from a number of different development of both epidermal and joint lesions in sites (ears, nasal planum, paws) and evaluation of psoriasis, as well as for investigating the role that the both gross appearance (degree of alopecia, scaling, epidermal keratinocyte plays in the pathogenesis of hyperemia) as well as histologic appearance (epi- psoriasis independent of infiltrating activated T cells. dermal thickness, degree of inflammatory infiltra- Thus, in summary, new models of psoriasis tion and vascular hyperplasia)9,20,43 can be evaluat- continue to be developed even as existing ones are ed (also Khattri and Danilenko, unpublished refined. While no single model of psoriasis is ideal, observations). Therefore, while the xenotrans- many have proven to be extremely valuable in Vet Pathol 45:4, 2008 Psoriasis Models 573 investigating and better understanding the molec- 8 Boniface K, Diveu C, Morel F, Pedretti N, Froger J, ular pathogenic mechanisms that underlie the Ravon E, Garcia M, Venereau E, Preisser L, complex interplay between epidermal keratino- Guignouard E, Guillet G, Dagregorio G, Pene J, ` cytes, the innate and adaptive immune system, Moles JP, Yssel H, Chevalier S, Bernard FX, Gascan and the vascular endothelium in psoriasis. H, Lecron JC: Oncostatin M secreted by skin infiltrating T lymphocytes is a potent keratinocyte Acknowledgements activator involved in skin inflammation. J Immunol 178:4615–4622, 2007 I would like to sincerely thank Roli Khattri, Steve 9 Boyman O, Hefti HP, Conrad C, Nickoloff BJ, Suter Hurst, and Susan Sa of Genentech for their review of M, Nestle FO: Spontaneous development of psori- and comments on this manuscript. I would also like to asis in a new animal model shows an essential role thank Chris Harrison and Heather Abanto (Gardiner- for resident T cells and tumor necrosis factor-alpha. Caldwell Pacific, San Bruno, CA) for their technical J Exp Med 199:731–736, 2004 expertise in the illustration of Fig. 1. Portions of this 10 Bullard DC, Scharffetter-Kochanek K, McArthur manuscript were presented at the 56th Annual Meeting MJ, Chosay JG, McBride ME, Montgomery CA, of the American College of Veterinary Pathologists Beaudet AL: A polygenic mouse model of psoriasi- (December 2005), and were published in extended form skin disease in CD18-deficient mice. Proc Natl abstract form on pages 88–91 of the Conference Acad Sci USA 93:2116–2121, 1996 Proceedings. 11 Chaudhari U, Romano P, Mulcahy LD, Dooley LT, Baker DG, Gottlieb AB: Efficacy and safety References of infliximab monotherapy for plaque-type pso- 1 Barker CL, McHale MT, Gillies AK, Waller J, riasis: a randomised trial. Lancet 357:1842–1847, Pearce DM, Osborne J, Hutchinson PE, Smith GM, 2001 Pringle JH: The development and characterization of 12 Detmar M, Brown LF, Schon MP, Elicker BM, an in vitro model of psoriasis. J Invest Dermatol Velasco P, Richard L, Fukumura D, Monsky W, 123:892–901, 2004 Claffey KP, Jain RK: Increased microvascular 2 Barker JN: The pathophysiology of psoriasis. Lancet density and enhanced leukocyte rolling and adhesion 338:227–230, 1991 in the skin of VEGF transgenic mice. J Invest 3 Barlow SC, Collins RG, Ball NJ, Weaver CT, Dermatol 111:1–6, 1998 Schoeb TR, Bullard DC: Psoriasiform dermatitis 13 Gazel A, Rosdy M, Bertino B, Tornier C, Sahuc F, susceptibility in Itgb2(tm1Bay) PL/J mice requires Blumenberg M: A characteristic subset of psoriasis- low-level CD18 expression and at least two addi- associated genes is induced by oncostatin-M in tional loci for progression to severe disease. reconstituted epidermis. J Invest Dermatol Am J Pathol 163:197–202, 2003 126:2647–2657, 2006 4 Bhagavathula N, Nerusu KC, Fisher GJ, Liu G, 14 Gottlieb AB, Miller B, Lowe N, Shapiro W, Hudson Thakur AB, Gemmell L, Kumar S, Xu ZH, Hinton C, Bright R, Ling M, Magee A, McCall CO, Rist T, P, Tsurushita N, Landolfi NF, Voorhees JJ, Varani Dummer W, Walicke P, Bauer RJ, White M, J: Amphiregulin and epidermal hyperplasia: amphi- Garovoy M: Subcutaneously administered efalizu- regulin is required to maintain the psoriatic pheno- mab (anti-CD11a) improves and signs and symp- type of human skin grafts on severe combined toms of moderate to severe plaque psoriasis. J Cut immunodeficient mice. Am J Pathol 166:1009– Med Surg 7:198–207, 2003 1016, 2005 15 Guo L, Yu Q-C, Fuchs E: Targeting expression of 5 Blauvelt A: New concepts in the pathogenesis and keratinocyte growth factor to keratinocytes elicits treatments of psoriasis: key roles for IL-23, IL-17A striking changes in epithelial differentiation in and TGF-b1. Expert Rev Dermatol 2:69–78, 2007 transgenic mice. EMBO J 12:973–986, 1993 6 Blumberg H, Conklin D, Xu WF, Grossmann A, 16 Harrington LE, Hatton RD, Mangan PR, Turner H, Brender T, Carollo S, Eagan M, Foster D, Halde- Murphy TL, Murphy KM, Weaver CT: Interleukin man BA, Hammond A, Haugen H, Jelinek L, Kelly 17-producing CD4+ effector T cells develop via a JD, Madden K, Maurer MF, Parrish-Novak J, lineage distinct from the T helper type 1 and 2 Prunkard D, Sexson S, Sprecher C, Waggie K, West lineages. Nature Immunol 6:1123–1132, 2005 J, Whitmore TE, Yao L, Kuechle MK, Dale BA, 17 HogenEsch H, Gijbels MJ, Offerman E, van Hooft Chandrasekher YA: Interleukin 20: discovery, re- J, van Bekkum DW, Zurcher C: A spontaneous ceptor identification, and role in epidermal function. mutation characterized by chronic proliferative Cell 104:9–19, 2001 dermatitis in C57BL mice. Am J Pathol 7 Boniface K, Bernard FX, Garcia M, Gurney AL, 143:972–982, 1993 Lecron JC, Morel F: IL-22 inhibits epidermal 18 HogenEsch H, Torregrosa SE, Boggess D, Sundberg differentiation and induces proinflammatory gene BA, Carroll J, Sundberg JP: Increased expression of expression and migration of human keratinocytes. type 2 cytokines in chronic proliferative dermatitis J Immunol 174:3695–3702, 2005 (cpdm) mutant mice and resolution of inflammation 574 Danilenko Vet Pathol 45:4, 2008 following treatment with IL-12. Eur J Immunol through interferon-a production. J Exp Med 31:734–742, 2001 202:135–143, 2005 19 Hong K, Chu A, Ludviksson BR, Berg EL, Ehrhardt 32 Nickoloff BJ: Keratinocytes regain momentum as RO: IL-12, independently of IFN-gamma, plays a instigators of cutaneous inflammation. Trends Mol crucial role in the pathogenesis of a murine psoriasis- Med 12:102–106, 2006 like skin disorder. J Immunol 162:7480–7491, 1999 33 Nickoloff BJ: Cracking the cytokine code in 20 Hong K, Berg EL, Ehrhardt RO: Persistence of psoriasis. Nature Med 13:242–244, 2007 pathogenic CD4+ Th1-like cells in vivo in the 34 Nickoloff BJ, Nestle FO: Recent insights into the absence of IL-12 but in the presence of autoantigen. immunopathogenesis of psoriasis provide new ther- J Immunol 166:4765–4772, 2001 apeutic opportunities. J Clin Invest 113:1664–1675, 21 Inman RD, Scofield RH: Etiopathogenesis of 2004 ankylosing spondylitis and reactive arthritis. Curr 35 Nickoloff BJ, Wrone-Smith T: Injection of pre- Opin Rheumatol 6:360–370, 1994 psoriatic skin with CD4+ T cells induces psoriasis. 22 Kauffman CL, Aria N, Toichi E, McCormick TS, Am J Pathol 155:145–158, 1999 Cooper KD, Gottlieb AB, Everitt DE, Frederick B, 36 Park H, Yang XO, Chang SH, Nurieva R, Wang Zhu Y, Graham MA, Pendley CE, Mascelli MA: A YH, Wang Y, Hood L, Zhu Z, Tian Q, Dong C: A phase I study evaluating the safety, pharmacokinet- distinct lineage of CD4 T cells regulates tissue ics, and clinical response of a human IL-12 p40 inflammation by producing interleukin 17. Nature antibody in subjects with plaque psoriasis. J Invest Immunol 6:1133–1141, 2005 Dermatol 123:1037–1044, 2004 37 Pasparakis M, Courtois G, Hafner M, Schmidt- 23 Kess D, Peters T, Zamek J, Wickenhauser C, Supprian M, Nenci A, Toksoy A, Krampert M, Tawadros S, Loser K, Varga G, Grabbe S, Nischt Goebeler M, Gillitzer R, Israel A, Krieg T, R, Sunderkotter C, Muller W, Krieg T, Scharffetter- Rajewsky K, Haase I: TNF-mediated inflammatory Kochanek K: CD4+ T cell-associated pathophysiol- skin disease in mice with epidermis-specific deletion ogy critically depends on CD18 gene dose effects in a of IKK2. Nature 417:861–866, 2002 murine model of psoriasis. J Immunol 171:5697– 38 Pauls K, Schon M, Kubitza RC, Homey B, Wiesen- 5706, 2003 born A, Lehmann P, Ruzicka T, Parker CM, Schon 24 Kopp T, Kieffer JD, Rot A, Strommer S, Stingl G, MP: Role of integrin alphaE(CD103)beta7 for Kupper TS: Inflammatory skin disease in K14/p40 tissue-specific epidermal localization of CD8+ T transgenic mice: evidence for interleukin-12-like lymphocytes. J Invest Dermatol 117:569–575, 2001 activities of p40. J Invest Dermatol 117:618–626, 39 Pittelkow MR: Psoriasis: more than skin deep. Nat 2001 Med 11:17–18, 2005 25 Kopp T, Lenz P, Bello-Fernandez C, Kastelein RA, Kupper TS, Stingl G: IL-23 production by cosecre- 40 Sa S, Valdez PA, Wu J, Jung K, Zhong F, Hall L, tion of endogenous p19 and transgenic p40 in keratin Kasman I, Winer J, Modrusan Z, Danilenko DM, 14/p40 transgenic mice: evidence for enhanced Ouyang W: The effects of IL-20 subfamily cytokines cutaneous immunity. J Immunol 170:5438–5444, on reconstituted human epidermis suggest potential 2003 roles in cutaneous innate defense and pathogenic 26 Krueger JG, Bowcock A: Psoriasis pathophysiology: adaptive immunity in psoriasis. J Immunol current concepts of pathogenesis. Ann Rheum Dis 178:2229–2240, 2007 64 (Suppl 2): ii30–ii36, 2005 41 Sano S, Chan KS, Carbajal S, Clifford J, Peavey M, 27 Lee E, Trepicchio WL, Oestreicher JL, Pittman D, Kiguchi K, Itami S, Nickoloff BJ, DiGiovanni J: Wang F, Chamian F, Dhodapkar M, Krueger JG: Stat3 links activated keratinocytes and immuno- Increased expression of interleukin 23 p19 and p40 in cytes required for development of psoriasis in a lesional skin of patients with psoriasis vulgaris. J Exp novel transgenic mouse model. Nat Med 11:43–49, Med 199:125–130, 2004 2005 28 Li AG, Wang D, Feng XH, Wang XJ: Latent 42 Schon MP: Animal models of psoriasis - what can TGFb1 overexpression in keratinocytes results in a we learn from them? J Invest Dermatol 112:405–410, severe psoriasis-like skin disorder. EMBO J 1999 23:1770–1781, 2004 43 Schon MP, Detmar M, Parker CM: Murine psori- 29 Lowes MA, Bowcock AM, Krueger JG: Pathogen- asis-like disorder induced by naive CD4+ T cells. Nat esis and therapy of psoriasis. Nature 445:866–873, Med 3:183–188, 1997 2007 44 Schon MP, Schon M, Warren HB, Donohue JP, 30 Mease PJ, Goffe BS, Metz J, VanderStoep A, Finck Parker CM: Cutaneous inflammatory disorder in B, Burge DJ: Etanercept in the treatment of psoriatic integrin alphaE (CD103)-deficient mice. J Immunol arthritis and psoriasis: a randomised trial. Lancet 165:6583–6589, 2000 356:385–390, 2000 45 Seymour RE, Hasham MG, Cox GA, Shultz LD, 31 Nestle FO, Conrad C, Tun-Kyi A, Homey B, HogenEsch H, Roopenian DC, Sundberg JP: Spon- Gombert M, Boyman O, Burg G, Liu Y-J, Gilliet taneous mutations in the mouse Sharpin gene result M: Plasmacytoid predendritic cells initiate psoriasis in multiorgan inflammation, immune system dysreg- Vet Pathol 45:4, 2008 Psoriasis Models 575 ulation and dermatitis. Genes Immun 8:416–421, Dagnino L, Arbiser J, Klement G, Dumont DJ: 2007 A cyclosporine-sensitive psoriasis-like disease pro- 46 Stratis A, Pasparakis M, Rupec RA, Markur D, duced in Tie2 transgenic mice. Am J Pathol 166: Hartmann K, Scharffetter-Kochanek K, Peters T, 843–855, 2005 van Rooijen N, Krieg T, Haase I: Pathogenic role 52 Wrone-Smith T, Nickoloff BJ: Dermal injection of for skin macrophages in a mouse model of kerati- immunocytes induces psoriasis. J Clin Invest nocyte-induced psoriasis-like skin inflammation. 98:1878–1887, 1996 J Clin Invest 116:2094–2104, 2006 53 Yanagisawa H, Richardson JA, Taurog JD, Ham- 47 Sundberg JP, France M, Boggess D, Sundberg BA, mer RE: Characterization of psoriasiform and Jenson AB, Beamer WG, Shultz LD: Development alopecic skin lesions in HLA-B27 transgenic rats. and progression of psoriasiform dermatitis and Am J Pathol 147:955–964, 1995 systemic lesions in the flaky skin (fsn) mouse mutant. 54 Xia YP, Li B, Hylton D, Detmar M, Yancopoulos Pathobiology 65:271–286, 1997 GD, Rudge JS: Transgenic delivery of VEGF to 48 Taurog JD, Maika SD, Simmons WA, Breban M, mouse skin leads to an inflammatory condition Hammer RE: Susceptibility to inflammatory disease resembling human psoriasis. Blood 102:161–168, 2003 in HLA-B27 transgenic rat lines correlates with the 55 Zeigler M, Chi Y, Tumas DB, Bodary S, Tang H, level of B27 expression. J Immunol 150:4168–4178, Varani J: Anti-CD11a ameliorates disease in the 1993 human psoriatic skin-SCID mouse transplant model: 49 Toichi E, Torres G, McCormick TS, Chang T, comparison of antibody to CD11a with cyclosporin Mascelli MA, Kauffman CL, Aria N, Gottlieb AB, A and clobetasol propionate. Lab Invest 81:1253– Everitt DE, Frederick B, Pendley CE, Cooper KD: 1261, 2001 An anti-IL-12p40 antibody down-regulates type 1 56 Zenz R, Eferl R, Kenner L, Florin L, Hummerich L, cytokines, chemokines and IL-12/IL-23 in psoriasis. Mehic D, Scheuch H, Angel P, Tschachler E, J Immunol 177:4917–4926, 2006 Wagner EF: Psoriasis-like skin disease and arthritis 50 Vassar R, Fuchs E: Transgenic mice provide new caused by inducible epidermal deletion of Jun insights into the role of TGF-alpha during epidermal proteins. Nature 437:369–375, 2005 development and differentiation. Genes Dev 57 Zheng Y, Danilenko DM, Valdez P, Kasman I, 5:714–727, 1991 Eastham-Anderson J, Wu J, Ouyang W: Interleukin- 51 Voskas D, Jones N, Van Slyke P, Sturk C, Chang W, 22, a TH17 cytokine, mediates IL-23-induced dermal Haninec A, Babichev YO, Tran J, Master Z, Chen S, inflammation and acanthosis. Nature 445:648–651, Ward N, Cruz M, Jones J, Kerbel RS, Jothy S, 2007 Request reprints from D. M. Danilenko, Genentech, Inc., Department of Pathology, One DNA Way, MS 72B, South San Francisco, CA 94080 (USA). E-mail: email@example.com.