A polygenic mouse model of psoriasiform skin disease in CD18-deficient mice

The Role of LFA-1 for the Differentiation and Function of Regulatory T Cells-Lessons Learned from Different Transgenic Mouse Models

Regulatory T cells (Treg) are essential for the maintenance of peripheral tolerance. Treg dysfunction results in diverse inflammatory and autoimmune diseases with life-threatening consequences. β2-integrins (CD11a-d/CD18) play important roles in the migration of leukocytes into inflamed tissues and cell signaling. Of all β2-integrins, T cells, including Treg, only express CD11a/CD18, termed lymphocyte function-associated antigen 1 (LFA-1), on their surface. In humans, loss-of-function mutations in the common subunit CD18 result in leukocyte adhesion deficiency type-1 (LAD-1). Clinical symptoms vary depending on the extent of residual β2-integrin function, and patients may experience leukocytosis and recurrent infections. Some patients can develop autoimmune diseases, but the immune processes underlying the paradoxical situation of immune deficiency and autoimmunity have been scarcely investigated. To understand this complex phenotype, different transgenic mouse strains with a constitutive knockout of β2-integrins have been established. However, since a constitutive knockout affects all leukocytes and may limit the validity of studies focusing on their cell type-specific role, we established a Treg-specific CD18-floxed mouse strain. This mini-review aims to delineate the role of LFA-1 for the induction, maintenance, and regulatory function of Treg in vitro and in vivo as deduced from observations using the various β2-integrin-deficient mouse models.

The Experimental Animal Models in Psoriasis Research: A Comprehensive Review

Psoriasis is an autoimmune, chronic, inflammatory skin condition mediated by T cells. It differs from other inflammatory conditions by causing significant alterations in epidermal cell proliferation and differentiation that are both complicated and prominent. The lack of an appropriate animal model has significantly hindered studies into the pathogenic mechanisms of psoriasis since animals other than humans typically do not exhibit the complex phenotypic features of human psoriasis. A variety of methods, including spontaneous mutations, drug-induced mutations, genetically engineered animals, xenotransplantation models, and immunological reconstitution approaches, have all been employed to study specific characteristics in the pathogenesis of psoriasis. Although some of these approaches have been used for more than 50 years and far more models have been introduced recently, they have surprisingly not yet undergone detailed validation. Despite their limitations, these models have shown a connection between keratinocyte hyperplasia, vascular hyperplasia, and a cell-mediated immune response in the skin. The xenotransplantation of diseased or unaffected human skin onto immune-compromised recipients has also significantly aided psoriasis research. This technique has been used in a variety of ways to investigate the function of T lymphocytes and other cells, including preclinical therapeutic studies. The design of pertinent in vivo and in vitro psoriasis models is currently of utmost concern and a crucial step toward its cure. This article outlines the general approach in the development of psoriasis-related animal models, aspects of some specific models, along with their strengths and limitations.

Impaired Treg-DC interactions contribute to autoimmunity in leukocyte adhesion deficiency type 1

Leukocyte adhesion deficiency type 1 (LAD-1) is a rare disease resulting from mutations in the gene encoding for the common β-chain of the β2-integrin family (CD18). The most prominent clinical symptoms are profound leukocytosis and high susceptibility to infections. Patients with LAD-1 are prone to develop autoimmune diseases, but the molecular and cellular mechanisms that result in coexisting immunodeficiency and autoimmunity are still unresolved. CD4+FOXP3+ Treg are known for their essential role in preventing autoimmunity. To understand the role of Treg in LAD-1 development and manifestation of autoimmunity, we generated mice specifically lacking CD18 on Treg (CD18Foxp3), resulting in defective LFA-1 expression. Here, we demonstrate a crucial role of LFA-1 on Treg to maintain immune homeostasis by modifying T cell-DC interactions and CD4+ T cell activation. Treg-specific CD18 deletion did not impair Treg migration into extralymphatic organs, but it resulted in shorter interactions of Treg with DC. In vivo, CD18Foxp3 mice developed spontaneous hyperplasia in lymphatic organs and diffuse inflammation of the skin and in multiple internal organs. Thus, LFA-1 on Treg is required for the maintenance of immune homeostasis.

Mouse Models of Psoriasis: A Comprehensive Review

The use of preclinical animal models of psoriasis has significantly increased over the last three decades, with each model having unique strengths and limitations. Some models translate better to human disease, and many have provided unique insight into psoriasis disease pathogenesis. In this comprehensive review, we present a comparative description and discussion of genetic mouse models, xenograft approaches, and elicited methods using cytokine injections into and topical imiquimod onto mice. We provide an inclusive list of genetically modified animals that have had imiquimod applied to or cytokines injected into their skin and describe the outcomes of these manipulations. This review will provide a valuable resource for those interested in working with psoriasis animal models.

Molecular Insights Into Neutrophil Biology From the Zebrafish Perspective: Lessons From CD18 Deficiency

Neutrophils are key players in innate immunity and originate from the bone marrow of the adult mammalian organism. In mammals, mature neutrophils are released from the bone marrow into the peripheral blood where they circulate until their recruitment to sites of inflammation in a multistep adhesion cascade. Here, adhesion molecules of the β₂ integrin family (CD11/CD18) are critically required for the initial neutrophil adhesion to the inflamed endothelium and several post-adhesion steps allowing their extravasation into the inflamed tissue. Within the mammalian tissue, interstitial neutrophil migration can occur widely independent of β₂ integrins. This is in sharp contrast to neutrophil recruitment in zebrafish larvae (Danio rerio) where neutrophils originate from the caudal hematopoietic tissue and mainly migrate interstitially to sites of lesion upon the early onset of inflammation. However, neutrophils extravasate from the circulation to the inflamed tissue in zebrafish larvae at later-time points. Although zebrafish larvae are a widely accepted model system to analyze neutrophil trafficking in vivo, the functional impact of β₂ integrins for neutrophil trafficking during acute inflammation is completely unknown in this model. In this study, we generated zebrafish with a genetic deletion of CD18, the β subunit of β₂ integrins, using CRISPR/Cas9 technology. Sequence alignments demonstrated a high similarity of the amino acid sequences between zebrafish and human CD18 especially in the functionally relevant I-like domain. In addition, the cytoplasmic domain of CD18 harbors two highly conserved NXXF motifs suggesting that zebrafish CD18 may share functional properties of human CD18. Accordingly, CD18 knock-out (KO) zebrafish larvae displayed the key symptoms of patients suffering from leukocyte adhesion deficiency (LAD) type I due to defects in ITGB2, the gene for CD18. Importantly, CD18 KO zebrafish larvae showed reduced neutrophil trafficking to sites of sterile inflammation despite the fact that an increased number of neutrophils was detectable in the circulation. By demonstrating the functional importance of CD18 for neutrophil trafficking in zebrafish larvae, our findings shed new light on neutrophil biology in vertebrates and introduce a new model organism for studying LAD type I.

New Frontiers in Psoriatic Disease Research, Part I: Genetics, Environmental Triggers, Immunology, Pathophysiology, and Precision Medicine

Psoriasis is a chronic inflammatory condition characterized by systemic immune dysregulation. Over the past several years, advances in genetics, microbiology, immunology, and mouse models have revealed the complex interplay between the heritable and microenvironmental factors that drive the development of psoriatic inflammation. In the first of this two-part review series, the authors will discuss the newest insights into the pathogenesis of psoriatic disease and highlight how the evolution of these scientific fields has paved the way for a more personalized approach to psoriatic disease treatment.

T-Cell Adhesion in Healthy and Inflamed Skin

The diverse populations of tissue-resident and transitory T cells present in the skin share a common functional need to enter, traverse, and interact with their environment. These processes are largely dependent on the regulated expression of adhesion molecules, such as selectins and integrins, which mediate bidirectional interactions between immune cells and skin stroma. Dysregulation and engagement of adhesion pathways contribute to ectopic T-cell activity in tissues, leading to the initiation and/or exacerbation of chronic inflammation. In this paper, we review how the molecular interactions supported by adhesion pathways contribute to T-cell dynamics and function in the skin. A comprehensive understanding of the molecular mechanisms underpinning T-cell adhesion in inflammatory skin disorders will facilitate the development of novel tissue-specific therapeutic strategies.

The importance of immunity in the development of reliable animal models for psoriasis and atopic dermatitis

Psoriasis (PS) and atopic dermatitis (AD) are common inflammatory skin diseases characterized by an imbalance in specific T-cell subsets, resulting in a specific cytokine profile in patients. Obtaining models closely resembling both pathologies along with a relevant clinical impact is crucial for the development of new therapies because of the high prevalence of these diseases. Single-gene mouse models developed until now do not fully reflect the complexity of these disorders, in part not only because of inherent differences between mice and humans but also because of the multifactorial nature of these pathologies. The skin-humanized mouse model developed by our group, based on a tissue engineering approach, has been used to test therapeutic strategies, although this methodology is still technically challenging and not widely available. The skin-humanized mouse models for PS and AD reproduce human skin phenotypes, providing valuable tools for drug development and testing in the preclinical setting. The tissue engineering approach allows the development of personalized medicine, covering the broad genotypic spectrum of these pathologies. This review highlights the main differences between available murine models focusing on the tissue-specific immunity of PS and AD. We discuss their contribution to unravel the complex pathophysiology of these diseases and to translate this knowledge into more accurate therapies.

Animal models of psoriasis-highlights and drawbacks

Research into the pathophysiology of psoriasis remains challenging, because this disease does not occur naturally in laboratory animals. However, specific aspects of its complex immune-pathology can be illuminated through transgenic, knockout, xenotransplantation, immunological reconstitution, drug-induced, or spontaneous mutation models in rodents. Although some of these approaches have already been pursued for more than 5 decades and even more models have been described in recent times, they have surprisingly not yet been systematically validated. As a consequence, researchers regularly examine specific aspects that only partially reflect the complex overall picture of the human disease. Nonetheless, animal models are of great utility to investigate inflammatory mediators, the communication between cells of the innate and the adaptive immune systems, the role of resident cells as well as new therapies. Of note, various manipulations in experimental animals resulted in rather similar phenotypes. These were called "psoriasiform", "psoriasis-like" or even "psoriasis" usually on the basis of some similarities with the human disorder. Xenotransplantation of human skin onto immunocompromised animals can overcome this limitation only in part. In this review, we elucidate approaches for the generation of animal models of psoriasis and assess their strengths and limitations with a certain focus on more recently developed models.

Murine models of psoriasis and its applications in drug development

Psoriasis is an autoimmune skin disease which characteristic of a well-demarcated, erythematous, raised lesion with silvery-white dry scale. Although the mechanism of psoriasis has not been fully understood so far, much progress has been made in understanding many of its complex potential mechanism, particularly the crucial role of the IL-23/Th17 axis. There are a large number of psoriasis models that reflect the complexity of the psoriasis mechanisms. In this review, we summarize various psoriasis mouse models, detail the features and molecular mechanisms of these mouse models, and discuss their strengths and limitations for psoriasis research. The development of mouse models of psoriasis provide an important basis for studying psoriasis pathogenesis and antipsoriatic drugs development. Therefore, the application of various psoriasis mouse models in antipsoriatic drug development are also discussed.

Research Techniques Made Simple: Murine Models of Human Psoriasis

Psoriasis vulgaris is a common, inflammatory skin disease affecting approximately 3% of the population in the United States. The etiology of psoriasis and its associated comorbidities are complex and the result of complicated interactions between the skin, immune system, disease-associated susceptibility loci, and multiple environmental triggers. The modeling of human disease in vivo through the use of murine models represents a powerful, indispensable tool for investigating the immune and genetic mechanisms contributing to a clinical disease phenotype. Nevertheless, modeling a complex, multigenic disease like psoriasis in mice has proven to be extremely challenging and is associated with significant limitations. Over the last four decades, more than 40 unique mouse models for psoriasis have been described. These models can be categorized into three major types: acute (inducible), genetically engineered (transgenic), and xenograft (humanized). The purpose of this Research Techniques Made Simple article is to provide an overview of the common types of psoriasis-like mouse models currently in use and their inherent advantages and limitations. We also highlight the need for improved psoriasis mouse model systems and several key factors to be considered as this field of laboratory science advances.

Psoriasiform skin disease in transgenic pigs with high-copy ectopic expression of human integrins α2 and β1

Psoriasis is a complex human-specific disease characterized by perturbed keratinocyte proliferation and a pro-inflammatory environment in the skin. Porcine skin architecture and immunity are very similar to that in humans, rendering the pig a suitable animal model for studying the biology and treatment of psoriasis. Expression of integrins, which is normally confined to the basal layer of the epidermis, is maintained in suprabasal keratinocytes in psoriatic skin, modulating proliferation and differentiation as well as leukocyte infiltration. Here, we generated minipigs co-expressing integrins α2 and β1 in suprabasal epidermal layers. Integrin-transgenic minipigs born into the project displayed skin phenotypes that correlated with the number of inserted transgenes. Molecular analyses were in good concordance with histological observations of psoriatic hallmarks, including hypogranulosis and T-lymphocyte infiltration. These findings mark the first creation of minipigs with a psoriasiform phenotype resembling human psoriasis and demonstrate that integrin signaling plays a key role in psoriasis pathology.

Summary: A cloned porcine disease model to advance topical treatment in the debilitating skin disorder psoriasis.

Models in the Research Process of Psoriasis

Psoriasis is an ancient, universal chronic skin disease with a significant geographical variability, with the lowest incidence rate at the equator, increasing towards the poles. Insights into the mechanisms responsible for psoriasis have generated an increasing number of druggable targets and molecular drugs. The development of relevant in vitro and in vivo models of psoriasis is now a priority and an important step towards its cure. In this review, we summarize the current cellular and animal systems suited to the study of psoriasis. We discuss the strengths and limitations of the various models and the lessons learned. We conclude that, so far, there is no one model that can meet all of the research needs. Therefore, the choice model system will depend on the questions being addressed.

In vitro skin models and tissue engineering protocols for skin graft applications

In this review, we present a brief introduction of the skin structure, a concise compilation of skin-related disorders, and a thorough discussion of different in vitro skin models, artificial skin substitutes, skin grafts, and dermal tissue engineering protocols. The advantages of the development of in vitro skin disorder models, such as UV radiation and the prototype model, melanoma model, wound healing model, psoriasis model, and full-thickness model are also discussed. Different types of skin grafts including allografts, autografts, allogeneic, and xenogeneic are described in detail with their associated applications. We also discuss different tissue engineering protocols for the design of various types of skin substitutes and their commercial outcomes. Brief highlights are given of the new generation three-dimensional printed scaffolds for tissue regeneration applications.

Cross Talk between Proliferative, Angiogenic, and Cellular Mechanisms Orchestred by HIF-1α in Psoriasis

Psoriasis is a chronic inflammatory skin disease where the altered regulation in angiogenesis, inflammation, and proliferation of keratinocytes are the possible causes of the disease, and the transcription factor “hypoxia-inducible factor 1-alpha” (HIF-1α) is involved in the homeostasis of these three biological phenomena. In this review, the role of HIF-1α in the cross talk between the cytokines and cells of the immunological system involved in the pathogenesis of psoriasis is discussed.

Reactive oxygen species in psoriasis and psoriasis arthritis: relevance to human disease

Psoriasis (Ps) is a chronic, immune-mediated, skin inflammatory disease affecting up to 3% of the population worldwide. Different environmental triggers initiate this complex multifactorial syndrome. Many individuals affected by Ps (6-26%) develop inflammatory disease in other organs, often in the joints as in psoriasis arthritis (PsA). Animal models that reflect the typical Ps syndrome, including both skin and joint pathology as in Ps and PsA, are valuable tools for dissecting disease pathways leading to clinical manifestations. In this context, we developed a new acute Ps and PsA-like disease model that appears after exposure to Saccharomyces cerevisiae mannan in certain mouse strains. The disease was found to be triggered by mannan-activated macrophages, leading to the activation of a pathogenic interleukin-17 pathway involving innate lymphocytes. Interestingly, the production of reactive oxygen species protected the mice from the triggering of this pathway and ameliorated Ps and PsA development.

Genetically Modified Mice as a Tool to Study Inflammatory Skin Diseases

Although mice have a long tradition as models for human skin diseases, they have recently received increasingly more attention. This is because of the rapid advancement of genetic engineering methods which made it possible to create mice with precisely defined genetic changes. Many of these mice develop impressive and sometimes unexpected, puzzling phenotypes. Their interpretation is a major challenge to the basic researcher and can often be ameliorated by the input of an experienced dermatologist. Together with recent examples of genetically modified mouse models of inflammatory skin diseases we give a short overview of the methods used to generate such mice, describe possible ways to analyse them, and discuss problems that arise in the interpretation of the findings.

The integrin adhesome: from genes and proteins to human disease

The adhesive interactions of cells with their environment through the integrin family of transmembrane receptors have key roles in regulating multiple aspects of cellular physiology, including cell proliferation, viability, differentiation and migration. Consequently, failure to establish functional cell adhesions, and thus the assembly of associated cytoplasmic scaffolding and signalling networks, can have severe pathological effects. The roles of specific constituents of integrin-mediated adhesions, which are collectively known as the 'integrin adhesome', in diverse pathological states are becoming clear. Indeed, the prominence of mutations in specific adhesome molecules in various human diseases is now appreciated, and experimental as well as in silico approaches provide insights into the molecular mechanisms underlying these pathological conditions.

Reduced CD18 levels drive regulatory T cell conversion into Th17 cells in the CD18hypo PL/J mouse model of psoriasis

Defective development and function of CD4(+)CD25(high+)Foxp3(+) regulatory T cells (Tregs) contribute to the pathogenesis of psoriasis and other autoimmune diseases. Little is known about the influence of adhesions molecules on the differentiation of Foxp3(+) Tregs into proinflammatory Th17 cells occurring in lesional skin and blood of psoriasis patients. In the CD18(hypo) PL/J mouse model of psoriasis, reduced expression of CD18/β2 integrin to 2-16% of wild-type levels is associated with progressive loss of Tregs, impaired cell-cell contact between Tregs and dendritic cells (DCs), as well as Treg dysfunction as reported earlier. In the present investigation, Tregs derived from CD18(hypo) PL/J mice were analyzed for their propensity to differentiate into IL-17-producing Th17 cells in vivo and in in vitro Treg-DC cocultures. Adoptively transferred CD18(hypo) PL/J Tregs were more inclined toward conversion into IL-17-producing Th17 cells in vivo in an inflammatory as well as noninflammatory environment compared with CD18(wt) PL/J Tregs. Addition of neutralizing Ab against CD18 to Treg-DC cocultures in vitro promoted conversion of CD18(wt) PL/J Tregs to Th17 cells in a dose-dependent manner similar to conversion rates of CD18(hypo) PL/J Tregs. Reduced thymic output of naturally occurring Tregs and peripheral conversion of Tregs into Th17 cells therefore both contribute to the loss of Tregs and the psoriasiform dermatitis observed in CD18(hypo) PL/J mice. Our data overall indicate that CD18 expression levels impact Treg development as well as Treg plasticity and that differentiation of Tregs into IL-17-producing Th17 cells is distinctly facilitated by a subtotal deficiency of CD18.

Animal models of skin disease for drug discovery

INTRODUCTION

Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

AREAS COVERED

In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

EXPERT OPINION

Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.

Psoriasis: what we have learned from mouse models

Psoriasis is a common inflammatory skin disease of unknown etiology, for which there is no cure. This heterogeneous, cutaneous, inflammatory disorder is clinically characterized by prominent epidermal hyperplasia and a distinct inflammatory infiltrate. Crosstalk between immunocytes and keratinocytes, which results in the production of cytokines, chemokines and growth factors, is thought to mediate the disease. Given that psoriasis is only observed in humans, numerous genetic approaches to model the disease in mice have been undertaken. In this Review, we describe and critically assess the mouse models and transplantation experiments that have contributed to the discovery of novel disease-relevant pathways in psoriasis. Research performed using improved mouse models, combined with studies employing human cells, xenografts and patient material, will be key to our understanding of why such distinctive patterns of inflammation develop in patients with psoriasis. Indeed, a combination of genetic and immunological investigations will be necessary to develop both improved drugs for the treatment of psoriasis and novel curative strategies.

Extracellular adherence protein of Staphylococcus aureus suppresses disease by inhibiting T-cell recruitment in a mouse model of psoriasis

Psoriasis is a T-cell-mediated inflammatory disease. Previous studies focused on lymphocyte function-associated antigen 1 (LFA-1)-expressing T cells as a molecular target for therapeutic intervention. By contrast, information on therapeutic effects and the underlying mechanism of blocking the LFA-1 counter receptor, ICAM-1 is scarce. Here, we used the CD18 (beta2-integrin) hypomorphic (CD18hypo) mouse model of psoriasis to investigate the therapeutic role of extracellular adherence protein (Eap) of Staphylococcus aureus, which exerts antiinflammatory activities by interacting with the ICAM-1 function. We show that ICAM-1 is predominantly upregulated on endothelial cells in lesional skin of CD18hypo mice. In vitro Eap was found to disrupt cell-cell contacts between T cells and dendritic cells, and inhibit T-cell proliferation. By contrast, in vivo Eap rather blocked transmigration of T cells from vessels to inflamed skin of CD18hypo mice, but did not inhibit their proliferation and activation. Most importantly, Eap successfully suppressed the disease by blocking T-cell extravasation into the inflamed skin. Together, these data indicate that interaction between LFA-1 and ICAM-1 is causally involved in the pathogenesis of psoriasiform skin inflammation, and targeting ICAM-1 to selectively block T-cell extravasation by Eap without immune suppression may represent a potential therapeutic strategy for psoriasis.

JAK3 inhibition significantly attenuates psoriasiform skin inflammation in CD18 mutant PL/J mice

JAK3, a member of the Janus kinase family, is predominantly expressed in hemopoietic cells and binds specifically to the common gamma chain of a subfamily of cytokine receptors that includes IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Previous studies suggest that this tyrosine kinase plays key roles in mediating T cell functions, and inhibition of JAK3 has been shown to prevent graft rejection and decrease the severity of arthritis in rodent models. However, the functions of JAK3 in the development of skin immune responses and diseases such as psoriasis have not been determined. CD18 mutant PL/J mice develop spontaneous T cell-dependent psoriasiform skin disease with several similarities to human psoriasis. In this study, we treated mice with established skin disease with R348, a small molecule inhibitor of JAK3, and observed a marked attenuation of skin lesions following 6 wk of treatment. Histological analyses revealed major reductions of both epidermal and dermal lesion severity scores in R348-treated CD18-deficient PL/J mice compared with vehicle controls, which was associated with decreased CD4(+) T cell infiltration. In addition, systemic levels of IL-17, IL-22, IL-23, and TNF-alpha were significantly lower in mice receiving the compound, and T cells isolated from R348-treated mice also showed reduced phosphorylation of Stat5 after stimulation with IL-2. These findings suggest that small-molecule inhibitors of JAK3 may be useful in the treatment of inflammatory skin diseases such as psoriasis and strongly implicate JAK signaling events as important in the pathogenesis of this disease.

Key role of macrophages in the pathogenesis of CD18 hypomorphic murine model of psoriasis

Psoriasis is a chronic skin disorder of unsolved pathogenesis affecting skin in 2-3% of the general population. Research into the pathogenesis of psoriasis has profited from suitable animal models. Previously, we reported on the CD18 hypomorphic (CD18(hypo)) PL/J mouse model clinically resembling human psoriasis, which is characterized by reduced expression of the common chain of beta(2)-integrins (CD11/CD18) to only 2-16% of wild-type levels. Aside from common clinical and pathophysiological features shared with human psoriasis, the psoriasiform skin disease in CD18(hypo) PL/J mice also depends on the presence of CD4(+) T-cells. This review focuses on the role of activated macrophages in the pathogenesis of CD18(hypo) T-cell-mediated mouse model of psoriasis, and extends our understanding in unrestrained pathogenic T-cells whose activation may be crucial for the recruitment and activation of macrophages within skin. The findings in the CD18(hypo) PL/J model are discussed in the context of current literatures of human and other autoimmune disorders.

TGF-beta-dependent suppressive function of Tregs requires wild-type levels of CD18 in a mouse model of psoriasis

Dysfunctional Tregs have been identified in individuals with psoriasis. However, their role in the pathogenesis of the disease remains unclear. Here we explored the effect of diminished CD18 (beta2 integrin) expression on the function of CD4+CD25+CD127(-) Tregs using the Cd18 hypomorphic (Cd18hypo) PL/J mouse model of psoriasis that closely resembles the human disease. We found that reduced CD18 expression impaired cell-cell contact between Tregs and DCs. This led to dysfunctional Tregs, which both failed to suppress the pathogenic T cells and promoted the onset and severity of the disease. This failure was TGF-beta-dependent, as Tregs derived from Cd18hypo PL/J mice had diminished TGF-beta1 expression. Adoptive transfer of Tregs expressing wild-type levels of CD18 into affected Cd18hypo PL/J mice resulted in a substantial improvement of the psoriasiform skin disease, which did not occur upon coinjection of the cells with TGF-beta-specific neutralizing antibody. Our data indicate a primary dysfunction of Cd18hypo Tregs, allowing subsequent hyperproliferation of pathogenic T cells in the Cd18hypo PL/J mouse model of psoriasis. This study may provide a step forward in our understanding of the unique role of CD18 expression levels in avoiding autoimmunity.

A 9-centimorgan interval of chromosome 10 controls the T cell-dependent psoriasiform skin disease and arthritis in a murine psoriasis model

Psoriasis is a complex genetic disease of unresolved pathogenesis with both heritable and environmental factors contributing to onset and severity. In addition to a disfiguring skin inflammation, approximately 10-40% of psoriasis patients suffer from destructive joint involvement. Previously, we reported that the CD18 hypomorphic PL/J mouse carrying a mutation resulting in reduced expression of the common chain of beta(2) integrins (CD11/CD18) spontaneously develops a skin disease that closely resembles human psoriasis. In contrast, the same mutation on C57BL/6J background did not demonstrate this phenotype. By a genome-wide linkage analysis, two major loci were identified as contributing to the development of psoriasiform dermatitis under the condition of low CD18 expression. Using a congenic approach, we now demonstrate that the introduction of a 9-centimorgan fragment of chromosome 10 derived from the PL/J strain into the disease-resistant CD18 hypomorphic C57BL/6J was promoting the development of psoriasiform skin disease and notably also arthritis. We therefore designated this locus psoriasiform skin disease-associated locus 1 (PSD1). High numbers of CD4(+) T cells and TNF-alpha producing macrophages were detected both in inflamed skin and joints in these congenic mice, with a complete resolution upon TNF-alpha inhibitor therapy or depletion of CD4(+) T cells. For the first time, we have identified a distinct genetic element that contributes to the T cell-dependent development of both psoriasiform skin disease and associated arthritis. This congenic model will be suitable to further investigations of genetic and molecular pathways that cause psoriasiform dermatitis and arthritis, and it may also be relevant for other autoimmune diseases.

Review Paper: Preclinical Models of Psoriasis

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.

Animal models of psoriasis

Research into the pathogenesis of psoriasis has been severely hampered by the lack of a naturally occurring disorder in laboratory animals that mimics the complex phenotype and pathogenesis of the human disease. A large variety of spontaneous mutations, genetically engineered rodents, immunological reconstitution approaches, and xenotransplantation models have been used to study specific aspects of the pathophysiology of psoriasis, however. Several manipulations of resident cutaneous cell types or immigrating immunocytes appear to result in remarkably similar hyperproliferative inflammatory phenotypes in mice, thus suggesting that interfering with cutaneous homeostasis in general may ultimately result in a rather uniform reaction pattern that mirrors some features of psoriasis. Fully animal models of psoriasis have nonetheless not only shed light on the biological functions of given inflammatory mediators or other molecules but also tremendously contributed to the discussion on central pathogenic questions, such as the roles of innate and adaptive immune mechanisms, keratinocytes, and endothelial cells in psoriasis. Psoriasis research has also been greatly nourished by xenotransplantation of diseased or unaffected human skin onto immunocompromised recipients, an approach that has in many variations been used to study the role of T lymphocytes and other cells and that has been used for preclinical therapeutic studies. General approaches to generate animal models of psoriasis, features of some specific models, their value for psoriasis research, and their use for drug development are discussed in this article.

Neutrophil-selective CD18 silencing using RNA interference in vivo

Tissue-specific silencing of genes may be used for genetic engineering in mice and has possible therapeutic applications in humans. Current strategies in mice rely on Cre/loxP technology requiring the generation of multiple transgenic lines and breeding strategies. Here, we describe the selective silencing of CD18, a leukocyte-specific integrin in neutrophils using a micro RNA (miRNA) strategy that requires the generation of one transgenic line. CD18-specific miRNA hairpin driven by the myeloid specific human MRP8 promoter resulted in the generation of transgenic lines with 75% to 95% reduction in CD18 protein levels in neutrophils and monocytes. Minimal decreases in T cells and a partial diminution in macrophages were observed. Neutrophil CD18 silencing resulted in neutrophilia, splenomegaly, and significant defects in neutrophil trafficking with the degree of alterations correlating with the extent of CD18 silencing. Thus, our data demonstrate the utility of using miRNA approaches to silence genes in neutrophils, which are terminally differentiated cells with a short half-life that largely precludes their genetic manipulation in vitro. Furthermore, the mouse models provide a valuable tool to examine the contribution of CD18 on neutrophils to leukocyte adhesion deficiency type I (LAD-I), a complex inherited disorder in which reduced or absent CD18 expression in multiple leukocyte subsets leads to impaired innate and adaptive immune responses.

Complement receptor 3 blockade promotes IL-12-mediated clearance of Porphyromonas gingivalis and negates its virulence in vivo

The ability of certain pathogens to exploit innate immune function allows them to undermine immune clearance and thereby increase their persistence and capacity to cause disease. Porphyromonas gingivalis is a major pathogen in periodontal disease and is associated with increased risk of systemic conditions. We have previously shown that the fimbriae of P. gingivalis interact with complement receptor 3 (CR3; CD11b/CD18) in monocytes/macrophages, resulting in inhibition of IL-12p70 production in vitro. The in vivo biological implications of this observation were investigated in this study using a CR3 antagonist (XVA143). XVA143 was shown to block CR3 binding of P. gingivalis fimbriae and reverse IL-12p70 inhibition; specifically, CR3 blockade resulted in inhibition of ERK1/2 phosphorylation and up-regulation of IL-12 p35 and p40 mRNA expression. Importantly, mice pretreated with XVA143 elicited higher IL-12p70 and IFN-gamma levels in response to P. gingivalis i.p. infection and displayed enhanced pathogen clearance, compared with similarly infected controls. The notion that CR3 is associated with reduced IL-12p70 induction and impaired P. gingivalis clearance was confirmed using i.p. infected wild-type and CR3-deficient mice. Moreover, XVA143 dramatically attenuated the persistence and virulence of P. gingivalis in experimental mouse periodontitis, as evidenced by reduced induction of periodontal bone loss. Therefore, CR3 blockade may represent a promising immunomodulatory approach for controlling human periodontitis and possibly associated systemic diseases.

The evolution of the psoriatic lesion

Psoriasis comprises a host of abnormalities, and various aspects of the pathogenesis of psoriasis have been suggested to be of primary relevance. The aim of this review is to identity driving factors in the pathogenesis of psoriasis and to explore the dynamics of processes eventually resulting in a psoriatic lesion. In this review observations on the evolution from the symptomless skin to lesional skin in patients with psoriasis will be integrated with observations in various animal models of psoriasis.

Macrophages and neutrophils are the targets for immune suppression by glucocorticoids in contact allergy

Glucocorticoids (GCs) are widely used in the treatment of allergic skin conditions despite having numerous side effects. Here we use Cre/loxP-engineered tissue- and cell-specific and function-selective GC receptor (GR) mutant mice to identify responsive cell types and molecular mechanisms underlying the antiinflammatory activity of GCs in contact hypersensitivity (CHS). CHS was repressed by GCs only at the challenge phase, i.e., during reexposure to the hapten. Inactivation of the GR gene in keratinocytes or T cells of mutant mice did not attenuate the effects of GCs, but its ablation in macrophages and neutrophils abolished downregulation of the inflammatory response. Moreover, mice expressing a DNA binding-defective GR were also resistant to GC treatment. The persistent infiltration of macrophages and neutrophils in these mice is explained by an impaired repression of inflammatory cytokines and chemokines such as IL-1beta, monocyte chemoattractant protein-1, macrophage inflammatory protein-2, and IFN-gamma-inducible protein 10. In contrast TNF-alpha repression remained intact. Consequently, injection of recombinant proteins of these cytokines and chemokines partially reversed suppression of CHS by GCs. These studies provide evidence that in contact allergy, therapeutic action of corticosteroids is in macrophages and neutrophils and that dimerization GR is required.

Mouse models of psoriasis

Psoriasis is a T-cell-mediated chronic inflammatory skin disease believed to be of autoimmune nature that can be triggered or worsened by streptococcal throat infections. In addition to conventional chronic inflammatory changes, psoriasis is characterized by complex and striking alterations in epidermal growth and differentiation. Psoriasis is generally not observed in animals other than man, and this lack of a suitable animal model has greatly hindered research into the pathogenesis of psoriasis. Multiple transgenic, knockout, and reconstituted models of psoriasis have been developed over the past two decades. Despite their limitations, these models have demonstrated that keratinocyte hyperplasia, vascular hyperplasia, and cell-mediated immunity in the skin are closely interrelated. Xenograft models, in which involved and uninvolved psoriatic skin are transplanted onto immunodeficient mice, are the only models that come close to incorporating the complete genetic, immunologic, and phenotypic changes of the disease. They have shown conclusively that psoriasis is a T-cell-mediated disease, and have been used to elucidate novel pathogenic pathways. In this review, we describe various animal models, detail the immunologic and intracellular pathways that mediate these phenotypes and assess the utility of these models to better understand this disease.

Identification of Susceptibility Loci for Skin Disease in a Murine Psoriasis Model

Psoriasis is a frequently occurring inflammatory skin disease characterized by thickened erythematous skin that is covered with silvery scales. It is a complex genetic disease with both heritable and environmental factors contributing to onset and severity. The CD18 hypomorphic PL/J mouse reveals reduced expression of the common chain of beta(2) integrins (CD11/CD18) and spontaneously develops a skin disease that closely resembles human psoriasis. In contrast, CD18 hypomorphic C57BL/6J mice do not demonstrate this phenotype. In this study, we have performed a genome-wide scan to identify loci involved in psoriasiform dermatitis under the condition of low CD18 expression. Backcross analysis of a segregating cross between susceptible CD18 hypomorphic PL/J mice and the resistant CD18 hypomorphic C57BL/6J strain was performed. A genome-wide linkage analysis of 94 phenotypically extreme mice of the backcross was undertaken. Thereafter, a complementary analysis of the regions of interest from the genome-wide screen was done using higher marker density and further mice. We found two loci on chromosome 10 that were significantly linked to the disease and interacted in an additive fashion in its development. In addition, a locus on chromosome 6 that promoted earlier onset of the disease was identified in the most severely affected mice. For the first time, we have identified genetic regions associated with psoriasis in a mouse model resembling human psoriasis. The identification of gene regions associated with psoriasis in this mouse model might contribute to the understanding of genetic causes of psoriasis in patients and pathological mechanisms involved in development of disease.

Pathogenic role for skin macrophages in a mouse model of keratinocyte-induced psoriasis-like skin inflammation

Psoriasis is a common skin disease, the pathogenesis of which has not yet been resolved. In mice, epidermis-specific deletion of inhibitor of NF-kappaB (IkappaB) kinase 2 (IKK2) results in a skin phenotype that mimics human psoriasis in several aspects. Like psoriasis, this skin disease shows pronounced improvement when mice are treated with a TNF-neutralizing agent. We have found previously that this phenotype does not depend on the presence of alphabeta T lymphocytes. In order to evaluate contributions of other immune cell populations to the skin disease, we selectively eliminated macrophages and granulocytes from the skin of mice with epidermis-specific deletion of IKK2 (K14-Cre-IKK2fl/fl mice). Elimination of skin macrophages by subcutaneous injection of clodronate liposomes was accompanied by inhibition of granulocyte migration into the skin and resulted in a dramatic attenuation of psoriasis-like skin changes. The hyperproliferative, inflammatory skin disease in K14-Cre-IKK2fl/fl mice was a direct consequence of the presence of macrophages in the skin, as targeted deletion of CD18, which prevented accumulation of granulocytes but not macrophages, did not lead to major changes in the phenotype. Targeted deletion of the receptor for IFN-gamma revealed that the pathogenesis of the skin disease does not depend on classical IFN-gamma-mediated macrophage activation. Our results demonstrate that in mice epidermal keratinocytes can initiate a hyperproliferative, inflammatory, IFN-gamma-independent, psoriasis-like skin disease whose development requires essential contributions from skin macrophages but not from granulocytes or alphabeta T lymphocytes.

Misbehaving macrophages in the pathogenesis of psoriasis

Psoriasis is a chronic inflammatory skin disease unique to humans. In this issue of the JCI, 2 studies of very different mouse models of psoriasis both report that macrophages play a key role in inducing psoriasis-like skin disease. Psoriasis is clearly a polygenic, inherited disease of uncontrolled cutaneous inflammation. The debate that currently rages in the field is whether psoriasis is a disease of autoreactive T cells or whether it reflects an intrinsic defect within the skin--or both. However, these questions have proven difficult to dissect using molecular genetic tools. In the current studies, the authors have used 2 different animal models to address the role of macrophages in disease pathogenesis: Wang et al. use a mouse model in which inflammation is T cell dependent, whereas the model used by Stratis et al. is T cell independent (see the related articles beginning on pages 2105 and 2094, respectively). Strikingly, both groups report an important contribution by macrophages, implying that macrophages can contribute to both epithelial-based and T cell-mediated pathways of inflammation.

Activated macrophages are essential in a murine model for T cell-mediated chronic psoriasiform skin inflammation

The CD18 hypomorphic (CD18hypo) PL/J mouse model clinically resembling human psoriasis is characterized by reduced expression of the common chain of beta2 integrins (CD11/CD18) to only 2-16% of WT levels. Previously we found that this chronic psoriasiform skin inflammation also depends on the presence of CD4+ T cells. Herein we investigated the role of macrophages in this CD18hypo mouse model. Activated macrophages were significantly increased in lesional skin as well as in inflamed skin draining lymph nodes (DLNs) of affected CD18hypo mice and were identified as being an important source of TNF-alpha in vivo. Both depletion of macrophages and neutralization of TNF-alpha resulted in a significant alleviation of psoriasiform skin inflammation. As monocyte chemotactic protein 1 was enhanced in lesional skin of affected CD18hypo mice, we intradermally injected recombinant murine monocyte chemotactic protein-1 (rJE/MCP-1) alone or in combination with rTNF-alpha into the skin of healthy CD18hypo mice. Only simultaneous injection of rJE/MCP-1 and rTNF-alpha, but neither substance alone, resulted in the induction of psoriasiform skin inflammation around the injection sites with recruitment and activation of macrophages. Collectively, our data suggest that maintenance of psoriasiform skin inflammation critically depends on efficient recruitment and activation of macrophages with sufficient release of TNF-alpha.

CD18 in monogenic and polygenic inflammatory processes of the skin

The beta 2 integrin family (CD11/CD18) of leukocyte adhesion molecules plays a key role in inflammation. Absence of the common chain (CD18) leads to leukocyte adhesion deficiency-1 (LAD1) in humans. We here summarize data of two genetically defined mice models of beta 2 integrin deficiency, one with a CD18 null mutation (CD18-/-), and the other one with a hypomorphic CD18 mutation (CD18hypo). Firstly, we focus on the underlying mechanism of a severely impaired wound healing in CD18-/- mice, outlining a scenario in which a defective extravasation and phagocytosis of CD18-/- neutrophils results in delayed myofibroblast-dependent wound contraction owing to a deficient transforming growth factor-beta 1 release. Based on this, we have identified a potential therapy that fully rescued the impaired wound healing in CD18-/- mice. Secondly, we expand on a CD18hyp0 PL/J mouse model closely resembling human psoriasis. Apart from common clinical and pathophysiological features, this psoriasiform dermatitis also depends on the presence of activated CD4+ T cells. We here recapitulate the influence of a reduced CD18 gene expression on T-cell function, also with regard to CD18 gene-dose effects, and its contribution to the pathogenesis of this disease. Taken together, these unique features make this model a valuable tool for investigations into the pathogenesis of human psoriasis--including its polygenic base--and future preclinical studies.

The genetics of psoriasis and autoimmunity

Psoriasis is an inflammatory/autoimmune disease and, as with many autoimmune diseases, is associated with alleles from the major histocompatibility complex (MHC). With psoriasis and autoimmune disease, the penetrance of the MHC-associated alleles is never 100%, even for monozygotic twins. This may be because development requires additional environmental and/or genetic modifiers or requires specific T-cell receptor arrangements. Families segregating single or multilocus susceptibility alleles other than the MHC have also been reported. Overlapping genetic locations of loci for different autoimmune diseases have been known for several years and are starting to reveal common genes or genetic variants. These include genes normally involved in preventing spontaneous T-cell activation or proliferation, immune synapse formation, or cytokine production via pathways such as those mediated by NFkappaB and those involved in thymic selection. Autoimmunity may also involve dysregulation of genes or pathways regulated by the RUNX family of transcription factors. RUNX is involved in hematopoietic cell development, development of T cells in the thymus, chromatin remodeling, and gene silencing. Hence, its effect on cells of the immune system may be due to variable changes in gene expression and could account for variable body surface involvement and waxing and waning of disease.

Animal models of psoriasis

Psoriasis is a common and chronic skin disorder under active investigation around the world. Despite this, determination of its genetic basis, role of the immune system in the disease pathophysiology and development of effective therapy, have been hampered severely by the absence of any spontaneous psoriatic skin disease in animals. Furthermore, until recently, validated animal models designed to create psoriasis were unavailable to investigative skin biologists and clinical scientists. However, there is at least one animal model which has been established and validated; it uses human skin engrafted on to severe combined immunodeficient (SCID) mice. In addition, there are several other rodent models which do not involve transplantation technology that share some (but not all) features in common with psoriasis. This review will summarise these available animal models and critique their relevance with respect to illuminating the immunogenetic basis of psoriasis and their value in screening novel treatments in a preclinical setting.

Deficiency of CD11b or CD11d Results in Reduced Staphylococcal Enterotoxin-Induced T Cell Response and T Cell Phenotypic Changes

The beta(2) integrin CD11a is involved in T cell-APC interactions, but the roles of CD11b, CD11c, and CD11d in such interactions have not been examined. To evaluate the roles of each CD11/CD18 integrin in T cell-APC interactions, we tested the ability of splenocytes of CD11-knockout (KO) mice to respond to staphylococcal enterotoxins (SEs), a commonly used superantigen. The defect in T cell proliferation with SEA was more severe in splenocytes from mice deficient in CD18, CD11b, or CD11d than in CD11a-deficient splenocytes, with a normal response in CD11c-deficient splenocytes. Mixing experiments showed that the defect of both CD11b-KO and CD11d-KO splenocytes was, unexpectedly, in T cells rather than in APC. Cytometric analysis failed to detect CD11b or CD11d on resting or activated T cells or on thymocytes of wild-type adult mice, nor did Abs directed to these integrins block responses in culture, suggesting that T cells educated in CD11b-KO or CD11d-KO mice were phenotypically altered. Consistent with this hypothesis, T cells from CD11b-KO and CD11d-KO splenocytes exhibited reduced intensity of CD3 and CD28 expression and decreased ratios of CD4/CD8 cells, and CD4(+) T cells were reduced among CD11b-KO and CD11d-KO thymocytes. CD11b and CD11d were coexpressed on a subset of early wild-type fetal thymocytes. We postulate that transient thymocyte expression of both CD11b and CD11d is nonredundantly required for normal thymocyte and T cell development, leading to phenotypic changes in T cells that result in the reduced response to SE stimulation.

CD4+ T cell-associated pathophysiology critically depends on CD18 gene dose effects in a murine model of psoriasis

In a CD18 hypomorphic polygenic PL/J mouse model, the severe reduction of CD18 (beta(2) integrin) to 2-16% of wild-type levels leads to the development of a psoriasiform skin disease. In this study, we analyzed the influence of reduced CD18 gene expression on T cell function, and its contribution to the pathogenesis of this disease. Both CD4(+) and CD8(+) T cells were significantly increased in the skin of affected CD18 hypomorphic mice. But only depletion of CD4(+) T cells, and not the removal of CD8(+) T cells, resulted in a complete clearance of the psoriasiform dermatitis. This indicates a central role of CD4(+) T cells in the pathogenesis of this disorder, further supported by the detection of several Th1-like cytokines released predominantly by CD4(+) T cells. In contrast to the CD18 hypomorphic mice, CD18 null mutants of the same strain did not develop the psoriasiform dermatitis. This is in part due to a lack of T cell emigration from dermal blood vessels, as experimental allergic contact dermatitis could be induced in CD18 hypomorphic and wild-type mice, but not in CD18 null mutants. Hence, 2-16% of CD18 gene expression is obviously sufficient for T cell emigration driving the inflammatory phenotype in CD18 hypomorphic mice. Our data suggest that the pathogenic involvement of CD4(+) T cells depends on a gene dose effect with a reduced expression of the CD18 protein in PL/J mice. This murine inflammatory skin model may also have relevance for human polygenic inflammatory diseases.

Psoriasiform dermatitis susceptibility in Itgb2(tm1Bay) PL/J mice requires low-level CD18 expression and at least two additional loci for progression to severe disease

Itgb2(tm1Bay) PL/J mice express low levels of the beta(2) integrins and, unlike Itgb2(tm1Bay) C57BL/6J mice, spontaneously develop psoriasiform dermatitis with several similarities to human psoriasis. To define the genetic requirements for skin disease susceptibility we analyzed more than 500 F2 progeny from an Itgb2(tm1Bay) (PL/J x C57BL/6J) intercross. We found that 23.5% developed chronic inflammatory skin disease, although significant differences in severity were observed. Another CD18 mutation, Itgb2(tm2Bay), has now been generated that completely eliminates CD18 expression. Surprisingly, of 10 Itgb2(tm2Bay) homozygote PL/J N4 mice generated, none showed clinical or histopathological evidence of disease. However, Itgb2(tm1Bay)/Itgb2(tm2Bay) PL/J mice developed dermatitis indistinguishable from Itgb2(tm1Bay) PL/J mice. In addition, approximately half of Itgb2(tm1Bay)/Itgb2(tm2Bay) (C57BL/6J x PL/J)F1 mice were found to develop mild psoriasiform dermatitis identical to the early stages of disease seen in Itgb2(tm1Bay) PL/J mice. Collectively, these results suggest a complex inheritance pattern of psoriasiform dermatitis in this model that involves lowered, but not absent, CD18 expression and at least two additional PL/J loci for the development of severe disease. The susceptibility allele can act in either a heterozygous or homozygous state, dependent on the level of CD18 expression.

Transgenic delivery of VEGF to mouse skin leads to an inflammatory condition resembling human psoriasis

Gene therapy approaches involving vascular endothelial growth factor (VEGF) to promote therapeutic angiogenesis are under consideration for conditions ranging from ischemic heart disease to nonhealing skin ulcers. Here we make the surprising observation that the transgenic delivery of VEGF to the skin results in a profound inflammatory skin condition with many of the cellular and molecular features of psoriasis, including the characteristic vascular changes, epidermal alterations, and inflammatory infiltrates. Even longstanding psoriatic disease remains dependent on the transgenic VEGF in this model because it can be effectively reversed by the addition of VEGF Trap, a potent VEGF antagonist. Previous attempts to faithfully replicate the psoriatic phenotype through the transgenic delivery of epidermal keratinocyte growth factors or inflammatory mediators generated phenotypes with only partial resemblance to human psoriasis, leaving unanswered questions about the etiology of this disease. The ability of transgenic VEGF to induce a psoriasiform phenotype suggests a new etiology and treatment approach for this disease and further substantiates emerging concerns about possible proinflammatory adverse effects that might be associated with therapeutic attempts to deliver VEGF.