Recent insights into the immunopathogenesis of psoriasis provide new therapeutic opportunities

New ENU-induced semidominant mutation, Ali18, causes inflammatory arthritis, dermatitis, and osteoporosis in the mouse

Inflammation is a complex cellular and humoral response against trauma and infection, and its presence leads to destruction of tissue in humans. The mechanisms that initiate inflammatory diseases remain largely unknown because of complex interactions between multiple genetic and environmental factors during pathogenesis. Animal models for human diseases offer dissection of complex pathogenesis by inbred genetic backgrounds and controlled circumstances. In this article we report a chemically induced new mutation, Ali18 (Abnormal limb), as a mouse model for inflammatory arthritis and dermatitis. Ali18/+ mice exhibit rubor and swelling of footpads in hindlimbs in adults. In Ali18/Ali18 mice, the digits in forelimbs and hindlimbs and tails were necrotic and/or deformed by severe swelling. Histologic analysis revealed infiltration of mixed populations of inflammatory cells into bone marrow, peripheral joints, and skin in the affected areas of Ali18/Ali18 mice. In addition, generalized osteoporosis-like phenotypes were confirmed by dual energy X-ray absorptiometry (DXA), microcomputed tomography (muCT), and peripheral quantitative computed tomography (pQCT) in homozygous animals. Whereas the Ali18 mutation was mapped to a single locus, the phenotype presentation was altered by complex modifier effects from other inbred genetic backgrounds. Detailed analysis of the Ali18 phenotype and identification of the mutation and its modifier genes may provide molecular insights into the complex nature of inflammatory diseases and the relationship between inflammation and bone metabolism.

Corticotropin releasing hormone and the skin

Cotricotropin-releasing hormone (CRH) and related peptides are produced in skin that is dependent on species and anatomical location. Local peptide production is regulated by ultraviolet radiation (UVR), glucocorticoids and phase of the hair cycle. The skin also expresses the corresponding receptors (CRH-R1 and CRH-R2), with CRH-R1 being the major receptor in humans. CRH-R1 is expressed in epidermal and dermal compartments, and CRH-R2 predominantly in dermal structures. The gene coding for CRH-R1 generates multiple isoforms through a process modulated by UVR, cyclic adenosine monophosphate (cAMP) and phorbol 12-myristate 13-acetate. The phenotypic effects of CRH in human skin cells are largely mediated by CRH-R1alpha through increases in concentrations of cAMP, inositol triphosphate (IP3), or Ca2+ with subsequent activation of protein kinases A (PKA) and C (PKC) dependent pathways. CRH also modulates the activity of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-kappaB), activator protein 1 (AP-1) and cAMP responsive element binding protein (CREB). The cellular functions affected by CRH depend on cell type and nutritional status and include modulation of differentiation program(s), proliferation, viability and immune activity. The accumulated evidence indicates that cutaneous CRH is also a component of a local structure organized similarly to the hypothalamo-pituitary-adrenal axis.

Dysregulation of T-cell function in the elderly : scientific basis and clinical implications

The function of the immune system is to maintain body integrity by defending against infections, cancers, autoimmune diseases and inflammation-related chronic diseases. The immune response is known to become defective with aging, leading to decreased longevity and appearance of age-related disease. The most important changes occur in T-cell immunity, and are manifested particularly as altered clonal expansion of cells of limited antigen specificity. The causes of these alterations are multifactorial, and include thymic involution, T-cell subset changes and signal transduction alterations. The clinical consequences of these changes are not well defined, except for their extremely important negative impact on defence against infections, especially by new pathogens, and decreased responses to vaccination. Considering the public health consequences of decreased immune competence in old age, strategies for immune response modulation are desirable to decrease the health burden for the elderly and improve their quality of life.

Pattern VEP alterations in psoriatic patients may indicate a sub clinic optic neuritis

We examined 44 subject (Group A) of both sexes (27 males and 17 females) aging between 16 and 80 (average: 45+/-16.6), divided into age bands, affected by mild-medium psoriasis with PASI (psoriasis area and severity index) between 1.2 and 48.6 (average: 11.2+/-9.7) without any other disease and we performed pattern transient VEP (Visual Evoked Potential) at the frequencies usually used in clinical experience (73', 36', 18' check size). For a good statistic comparison we choose 55 healthy subjects (group B) divided into age bands on which we performed the same test. Comparison of VEP parameters between psoriatic and healthy subjects, showed in group A 10 normal (22.7%) and 34 pathological (77.3%). In the latter group there are 16 subjects who show only a P100 reduced amplitude (36.3%), 3 with only increased latency (6.8%), 15 with alterations of both values (34%). The achieved data show that more than 3/4 of group A subjects have VEP alterations as index of the presence of a sub clinic optic neuritis with a probably toxic autoimmune origin due to the action of TNFalpha, of IgG, of ECP or of other cytokines (IL6, IL7, etc) that are increased in the blood of this patients. The electro physiologic monitoring of optic nerve seems to represent a good routine test to evaluate the global conditions of psoriatic patients.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.

The long-term efficacy and safety of new biological therapies for psoriasis

Long-term therapy is often required for psoriasis. This article reviews the most recent long-term clinical data for biological agents that have been approved or for which late-stage development data have been released for the treatment of patients with moderate to severe plaque psoriasis. Efficacy data are available for up to five 12-week courses of alefacept (approximately 60 weeks of therapy), 36 months (144 weeks) of continuous efalizumab, 48 weeks of continuous etanercept, and 50 weeks of bimonthly infliximab. Data sources include publications, product labeling, and posters presented at recent international scientific meetings. Alefacept appears to continue to be efficacious over multiple treatment courses for some responsive patients. The efficacy of efalizumab achieved during the first 12-24 weeks of therapy appears to be maintained or improved through at least 60 weeks of continuous treatment. The efficacy of etanercept appears to be maintained through at least 48 weeks of continuous treatment. Infliximab demonstrates a high response rate soon after initiation, which appears to be maintained through 24 weeks but declines modestly with therapy out to 50 weeks. After 48 weeks, approximately 60% of efalizumab-treated and 45% of etanercept-treated patients remaining on therapy achieved > or =75% improvement from baseline in Psoriasis Area and Severity Index, as did 70.5% of infliximab patients who did not miss more than two infusions. Safety data suggest that these agents may be used for long-term administration. Long-term data from psoriasis trials continue to accumulate. Recent data suggest that biological therapies have efficacy and safety profiles suitable for the long-term treatment of patients with moderate to severe psoriasis.

Connexin 26 regulates epidermal barrier and wound remodeling and promotes psoriasiform response

Inflammatory skin disorders result in significant epidermal changes, including keratinocyte hyperproliferation, incomplete differentiation, and impaired barrier. Here we test whether, conversely, an impaired epidermal barrier can promote an inflammatory response. Mice lacking the transcription factor Kruppel-like factor 4 (Klf4) have a severe defect in epidermal barrier acquisition. Transcription profiling of Klf4(-/-) newborn skin revealed similar changes in gene expression to involved psoriatic plaques, including a significant upregulation of the gap junction protein connexin 26 (Cx26). Ectopic expression of Cx26 from the epidermis-specific involucrin (INV) promoter (INV-Cx26) demonstrated that downregulation of Cx26 is required for barrier acquisition during development. In juvenile and adult mice, persistent Cx26 expression kept wounded epidermis in a hyperproliferative state, blocked the transition to remodeling, and led to an infiltration of immune cells. Mechanistically, ectopic expression of Cx26 in keratinocytes resulted in increased ATP release, which delayed epidermal barrier recovery and promoted an inflammatory response in resident immune cells. These results provide a molecular link between barrier acquisition in utero and epidermal remodeling after wounding. More generally, these studies suggest that the most effective treatments for inflammatory skin disorders might concomitantly suppress the immune response and enhance epidermal differentiation to restore the barrier.

Impaired Langerhans cell migration in psoriasis

We have examined whether psoriasis is associated with systemic effects on epidermal Langerhans cell (LC) function and, specifically, the migration of LCs from the skin. Compared with normal skin, the frequency and morphology of epidermal LCs in uninvolved skin from patients with psoriasis was normal. However, mobilization of these cells in response to stimuli that normally induce migration (chemical allergen, tumor necrosis factor α [TNF-α], and interleukin-1β [IL-1β]) was largely absent, despite the fact that treatment with TNF-α and IL-1β was associated with comparable inflammatory reactions in patients and controls. The failure of LC migration from uninvolved skin was not attributable to altered expression of receptors for IL-1β or TNF-α that are required for mobilization, nor was there an association with induced cutaneous cytokine expression. Although a role for altered dynamics of LC migration/turnover has not been formally excluded, these data reveal a very consistent decrement of LC function in psoriasis that may play a decisive role in disease pathogenesis.

Role of LIM kinases in normal and psoriatic human epidermis

We present evidence that LIM kinases can control cell adhesion and compaction in human epidermis. LIMK2 is expressed in the epidermal basal layer and signals downstream of the GTPase Rac1 to promote extracellular matrix adhesion and inhibit terminal differentiation. Conversely, LIMK1 is expressed in the upper granular layers and phosphorylates and inhibits cofilin. Expression of LIMK1 is lost in psoriatic lesions and other skin disorders characterized by lack of cell compaction in the differentiating cell layers. In psoriatic lesions down-regulation of LIMK1 correlates with up-regulation of Myc. Expression of constitutively active cofilin or Myc in reconstituted human epidermis blocks cell compaction. Overexpression of LIMK1 leads to down-regulation of Myc, whereas inhibition of Rho kinase, an upstream activator of LIMK1, stimulates Myc expression. Inhibition of Myc by LIMK1 is via inhibition of Stat3 phosphorylation, because constitutively active cofilin or inhibition of Rho kinase results in Stat3 phosphorylation and increased Myc levels, whereas dominant negative Stat3 abolishes the effect. In conclusion, we have uncovered a novel antagonistic relationship between the LIMK1/phosphocofilin and Myc/Stat3 pathways in the differentiating layers of human epidermis and propose that down-regulation of LIMK1 contributes to one of the pathological features of psoriatic epidermal lesions.

SHP-1 promoter 2 methylation in normal epithelial tissues and demethylation in psoriasis

SHP-1 promoter hypermethylation has been studied in hematopoietic cells and observed only in various types of lymphoma and leukemia. This study reports a contrasting situation in normal epithelial tissues and an association with skin pathogenesis, particularly in psoriasis. We investigated several cell lines, five of them were epithelial and six were hematopoietic, white blood cells from normal, healthy donors, and normal microdissected epithelium of kidney, liver, breast, cervix, lung, prostate, bladder, and skin. Interestingly, promoter 2 hypermethylation was apparent in all epithelial cell lines and tissues. However, distinctive degrees of demethylation were noted in some skin samples. The methylation patterns of each cell line corresponded to their mRNA isoforms, in that isoforms I and II could not be detected with either promoter 1 or 2 hypermethylation, respectively. We further explored whether an enhanced degree of demethylation could be observed in various dermatopathology lesions. While the promoter 2 methylation levels of squamous cell cancers, eczemas, and normal skins were not different, a significant degree of demethylation can be observed in psoriasis (p<0.005). In addition, psoriasis displays a higher level of SHP-1 isoform II than normal skin (p<0.05). In conclusion, this study discovered an unprecedented role of SHP-1 methylation in tissue-specific expression and its alteration in a nonmalignant human disease besides the transcription inhibition in leukemia and lymphoma. Furthermore, the promoter demethylation may play an important role in skin pathogenesis by enhancing SHP-1 isoform II transcription in psoriatic skin lesions.

Increase in TNF-alpha and inducible nitric oxide synthase-expressing dendritic cells in psoriasis and reduction with efalizumab (anti-CD11a)

We find that CD11c(+) cells with many markers of dendritic cells (DCs) are a major cell type in the skin lesions of psoriasis. These CD11c(+) cells, which are evident in both epidermis and dermis, are the sites for the expression of two mediators of inflammation, inducible nitric oxide synthase (iNOS) and TNF-alpha in diseased skin. These cells express HLA-DR, CD40, and CD86, lack the Langerin and CD14 markers of Langerhans cells and monocytes, respectively, and to a significant extent express the DC maturation markers DC-LAMP and CD83. Treatment of psoriasis with efalizumab (anti-CD11a, Raptiva) strongly reduces infiltration by these DCs in patients responding to this agent. Disease activity after therapy was more related to DC infiltrates and iNOS mRNA levels than T cell infiltrates, and CD11c(+) cells responded more quickly to therapy than epidermal keratinocytes. Our results suggest that a type of DC, which resembles murine "Tip-DCs" that can accumulate during infection, has proinflammatory effects in psoriasis through nitric oxide and TNF-alpha production, and can be an important target for suppressive therapies.

Peptidoglycan recognition proteins Pglyrp3 and Pglyrp4 are encoded from the epidermal differentiation complex and are candidate genes for the Psors4 locus on chromosome 1q21

Psoriasis is a common inflammatory skin disease caused by genetic and environmental factors, including bacterial and viral infections. Since the skin is in constant contact with commensal and pathogenic microorganisms, we examined well-supported psoriasis genetic linkage intervals to identify genes encoding innate immune pattern recognition proteins that may play a role in pathogenesis. Two peptidoglycan recognition proteins, Pglyrp3 and Pglyrp4, are localized to the Psors4 locus on chromosome 1q21 in a gene cluster known as the epidermal differentiation complex (EDC). We show that these genes are expressed in the skin as well as in germinal centers in the tonsil. We tested 13 SNPs in or near these genes for association with psoriasis in two independent patient collections: a family-based patient set comprised of 375 individuals from 101 families, and a case-control patient collection of 282 patients with moderate to severe psoriasis and 192 healthy controls. In the family-based analysis, several SNPs in the Pglyrp3-Pglyrp4 locus show association with psoriasis (0.01 < P < 0.05). Multiple-SNP haplotypes incorporating Pglyrp3 and Pglyrp4 SNPs also show significant association in the transmission disequilibrium test (TDT; P < 0.01). In the case-control test, none of the SNPs that we tested show association with psoriasis when analyzed in single-SNP or haplotype-based tests. The discordance between the TDT and case-control results suggests that the two populations are significantly different in disease etiology, that the polymorphism responsible for the Psors4 linkage is elsewhere in the Pglyrp locus, or that the causative Psors4 polymorphism is in a location near but not in the Pglyrp locus. These data are consistent with previous reports of association of psoriasis with genes on 1q21, and suggest a role for Pglyrps in skin biology.

Evidence for susceptibility determinant(s) to psoriasis vulgaris in or near PTPN22 in German patients

INTRODUCTION

Variant R620W of protein tyrosine phosphatase non-receptor type 22 (PTPN22) has consistently been reported as a susceptibility factor for several autoimmune diseases. We investigated its role in susceptibility to psoriasis, the relevance of possibly other disease-causing variants, and interdependency of the major risk factor for psoriasis at PSORS1.

METHODS

R620W was tested in a case-control study initially with 375 German patients and then with an enlarged sample of an additional 418 patients. Analyses were extended to linkage disequilibrium (LD) based haplotypes. Potential interaction between risk haplotypes of PTPN22 and the PSORS1 associated risk allele was tested by regression analysis. PTPN22 coding sequence was determined in 20 patients carrying the risk haplotype. Association and regression analysis were also performed in the extended case-control study.

RESULTS

R620W was not associated in either case-control study, while significant association (corrected for multiple testing) with one haplotype (C-4) of the LD block encompassing PTPN22 as well with another haplotype (B-3) within an adjacent telomeric LD block was detected. No evidence for interaction between risk haplotype C-4 and the PSORS1 associated risk allele was found. Sequencing excluded other coding variants within PTPN22 as a basis for association findings. Analysis of the extended study group confirmed association for haplotypes B-3 and C-4 and independence of risk haplotypes C-4 and PSORS1.

DISCUSSION

We exclude a major role of *620W in German psoriasis patients but suggest that other susceptibility determinant(s) within non-coding regions of PTPN22 or its proximity might exist acting independently of the major PSORS1 risk factor.

Plasmacytoid predendritic cells initiate psoriasis through interferon-alpha production

Psoriasis is one of the most common T cell–mediated autoimmune diseases in humans. Although a role for the innate immune system in driving the autoimmune T cell cascade has been proposed, its nature remains elusive. We show that plasmacytoid predendritic cells (PDCs), the natural interferon (IFN)-α–producing cells, infiltrate the skin of psoriatic patients and become activated to produce IFN-α early during disease formation. In a xenograft model of human psoriasis, we demonstrate that blocking IFN-α signaling or inhibiting the ability of PDCs to produce IFN-α prevented the T cell–dependent development of psoriasis. Furthermore, IFN-α reconstitution experiments demonstrated that PDC-derived IFN-α is essential to drive the development of psoriasis in vivo. These findings uncover a novel innate immune pathway for triggering a common human autoimmune disease and suggest that PDCs and PDC-derived IFN-α represent potential early targets for the treatment of psoriasis.