Impaired Langerhans cell migration in psoriasis is due to an altered keratinocyte phenotype induced by interleukin-17.

BACKGROUND: Psoriasis is a common skin condition driven by increased expression of interleukin (IL)-17. Langerhans cells (LCs) are epidermal dendritic cells that regulate cutaneous immune responses. Within the uninvolved skin of patients with psoriasis, LCs display impaired migration from the epidermis. OBJECTIVES: To investigate the role of keratinocytes (KCs) in the regulation of LC function and the response of KCs to IL-17. METHODS: KCs were cultured from the uninvolved skin of patients with psoriasis and healthy individuals with or without IL-17 treatment and the conditioned medium examined for its ability to alter LC function in an ex vivo human skin explant model. Furthermore, we examined the effect of IL-17 on LC mobilization in psoriasis by neutralizing IL-17 in the same skin explant model. RESULTS: Conditioned medium from psoriasis KCs inhibited LC migration in healthy skin. Moreover, conditioned medium from healthy KCs treated with IL-17 also inhibited healthy LC migration. Finally, neutralizing IL-17 in psoriasis skin resulted in enhanced LC migration. CONCLUSIONS: Collectively, these data suggest that an altered KC secretome, driven by increased expression of IL-17, is responsible for impaired LC migration in the uninvolved skin of patients with psoriasis.

A Review on the Potential Role of Basement Membrane Laminin in the Pathogenesis of Psoriasis.

We have previously reviewed alterations to basement membrane laminin in psoriasis and how disruption of this layer could lead to at least some of the pathological changes observed. We here postulate that basement membrane laminin is the key antigen in driving psoriasis, inducing a T cell-mediated autoimmune response. For laminin to be considered as the key autoantigen in psoriasis, it would be reasonable to expect the following to be demonstrable: (1) that autoantigens are present in psoriatic inflammation; (2) that basement membrane laminin is perturbed in involved and uninvolved skin, and that some of the pathological changes associated with psoriasis could be predicted as a sequel to this; (3) that disruption of the basement membrane is among the earliest events in the evolution of psoriatic lesions; (4) that as streptococcal pharyngitis is the most clearly defined event to trigger or exacerbate psoriasis, then a T cell-mediated autoimmune response to laminin should be anticipated as a potential sequelae to streptococcal pharyngitis; (5) that T cells in psoriasis can be shown to react to peptides with homology to laminin; (6) that HLACw6, as the most closely related gene associated with psoriasis and which is involved in antigen expression, should be preferentially expressed within lesional psoriasis towards the basement membrane, together with other proximal associated immune activity; and (7) that there is some association between antilaminin pemphigoid, a humorally mediated autoimmune disease to skin basement membrane laminin, and psoriasis. We here review the data relevant to each of these requirements.

An in vitro human skin test for assessing sensitization potential.

Sensitization to chemicals resulting in an allergy is an important health issue. The current gold-standard method for identification and characterization of skin-sensitizing chemicals was the mouse local lymph node assay (LLNA). However, for a number of reasons there has been an increasing imperative to develop alternative approaches to hazard identification that do not require the use of animals. Here we describe a human in-vitro skin explant test for identification of sensitization hazards and the assessment of relative skin sensitizing potency. This method measures histological damage in human skin as a readout of the immune response induced by the test material. Using this approach we have measured responses to 44 chemicals including skin sensitizers, pre/pro-haptens, respiratory sensitizers, non-sensitizing chemicals (including skin-irritants) and previously misclassified compounds. Based on comparisons with the LLNA, the skin explant test gave 95% specificity, 95% sensitivity, 95% concordance with a correlation coefficient of 0.9. The same specificity and sensitivity were achieved for comparison of results with published human sensitization data with a correlation coefficient of 0.91. The test also successfully identified nickel sulphate as a human skin sensitizer, which was misclassified as negative in the LLNA. In addition, sensitizers and non-sensitizers identified as positive or negative by the skin explant test have induced high/low T cell proliferation and IFNγ production, respectively. Collectively, the data suggests the human in-vitro skin explant test could provide the basis for a novel approach for characterization of the sensitizing activity as a first step in the risk assessment process.

T helper cell 2 immune skewing in pregnancy/early life: chemical exposure and the development of atopic disease and allergy.

During the last 50 years there has been a significant increase in Western societies of atopic disease and associated allergy. The balance between functional subpopulations of T helper cells (Th) determines the quality of the immune response provoked by antigen. One such subpopulation - Th2 cells - is associated with the production of IgE antibody and atopic allergy, whereas, Th1 cells antagonize IgE responses and the development of allergic disease. In seeking to provide a mechanistic basis for this increased prevalence of allergic disease, one proposal has been the 'hygiene hypothesis', which argues that in Westernized societies reduced exposure during early childhood to pathogenic microorganisms favours the development of atopic allergy. Pregnancy is normally associated with Th2 skewing, which persists for some months in the neonate before Th1/Th2 realignment occurs. In this review, we consider the immunophysiology of Th2 immune skewing during pregnancy. In particular, we explore the possibility that altered and increased patterns of exposure to certain chemicals have served to accentuate this normal Th2 skewing and therefore further promote the persistence of a Th2 bias in neonates. Furthermore, we propose that the more marked Th2 skewing observed in first pregnancy may, at least in part, explain the higher prevalence of atopic disease and allergy in the first born.

Setting Occupational Exposure Limits for Chemical Allergens--Understanding the Challenges.

Chemical allergens represent a significant health burden in the workplace. Exposures to such chemicals can cause the onset of a diverse group of adverse health effects triggered by immune-mediated responses. Common responses associated with workplace exposures to low molecular weight (LMW) chemical allergens range from allergic contact dermatitis to life-threatening cases of asthma. Establishing occupational exposure limits (OELs) for chemical allergens presents numerous difficulties for occupational hygiene professionals. Few OELs have been developed for LMW allergens because of the unique biological mechanisms that govern the immune-mediated responses. The purpose of this article is to explore the primary challenges confronting the establishment of OELs for LMW allergens. Specific topics include: (1) understanding the biology of LMW chemical allergies as it applies to setting OELs; (2) selecting the appropriate immune-mediated response (i.e., sensitization versus elicitation); (3) characterizing the dose (concentration)-response relationship of immune-mediated responses; (4) determining the impact of temporal exposure patterns (i.e., cumulative versus acute exposures); and (5) understanding the role of individual susceptibility and exposure route. Additional information is presented on the importance of using alternative exposure recommendations and risk management practices, including medical surveillance, to aid in protecting workers from exposures to LMW allergens when OELs cannot be established.

The multiple factors affecting the association between atopic dermatitis and contact sensitization.

Atopic dermatitis and allergic contact dermatitis are both common skin diseases having an immune pathogenesis. There has been considerable interest about their inter-relationships with regard to altered susceptibility. Recent investigations have shed new light on this important question, and in this article, we explore whether there is evidence that atopic dermatitis affects the risk of contact sensitization and allergic contact dermatitis. The use of topical products to treat xerotic and inflamed skin in atopic dermatitis often results in a higher prevalence of sensitization to, for example, fragrances and other ingredients in emollients. Moreover, the prevalence of metal allergy seems to be increased, probably due to compromised chelation of the metals in the stratum corneum of patients with atopic dermatitis. However, conversely, the T-helper cell 2 bias that characterizes immune responses in atopic dermatitis appears to lower the risk of contact sensitization compared to healthy controls. Based on these observations, we conclude that multiple factors affect the association between atopic dermatitis and contact sensitization, and that these need to be appreciated in the clinical management of atopic dermatitis patients.

The hapten-atopy hypothesis III: the potential role of airborne chemicals.

One explanation for the large increase in the prevalence of atopic disease in developed countries during the last 50 years is the 'hygiene hypothesis'. This proposes that a reduced exposure to pathogenic microorganisms at a key period(s) during development results in the maintenance or acquisition of an atopic phenotype. Alternatively, or additionally, we have postulated that increased exposure to chemicals generally, and to irritant/haptenic chemicals in particular, during critical windows of maternal pregnancy/early life have also contributed to changes in the prevalence of atopic disease. Having previously reviewed the potential roles of oral and cutaneous exposure to chemicals on the subsequent diagnosis of atopic disease, we here consider possible evidence of a role for exposure to airborne chemicals as a contributory factor in acquired susceptibility to atopic allergy. After controlling for known confounders, five specific maternal occupations during pregnancy have been implicated as being associated with subsequent atopic disease in the offspring. Each of these occupations is characterized by high and persistent exposure to airborne chemicals. High-level exposure to volatile organic compounds in the domestic environment, either during pregnancy or in early life, is also associated with development of childhood atopic disease. Similarly, sustained exposure to airborne chlorinated chemicals from swimming pools during childhood has been associated with the development of atopic allergy. A possible immunological basis for these associations is that exposure to certain airborne chemicals, even at low levels, can result in the delivery of 'danger' signals that, in turn, bias the immune response towards the selective induction or maintenance of preferential T helper 2-type immune responses consistent with the acquisition of allergic sensitization.

Guttate psoriasis is associated with an intermediate phenotype of impaired Langerhans cell migration.

BACKGROUND: An episode of guttate psoriasis can be an isolated event, can recur as guttate episodes, or develop into chronic plaque psoriasis (CPP). A previous study revealed that early-onset (before age 40 years) CPP is associated with inhibition of epidermal Langerhans cell (LC) migration. OBJECTIVES: To determine whether guttate psoriasis is also associated with abnormal LC mobilization. METHODS: Three groups of patients were recruited: current guttate episode (n = 5); guttate psoriasis progressed to CPP (n = 6); and resolved guttate psoriasis (n = 2). Biopsies were taken from uninvolved skin and LC migration was measured ex vivo using an epidermal explant model. RESULTS: Patients with a current episode of guttate psoriasis displayed epidermal LC migration, although the extent was significantly lower than in skin from healthy controls (P < 0·05). In contrast, in those patients in whom guttate psoriasis developed into CPP there was no mobilization of LC. Finally, in patients in whom guttate psoriasis had resolved, LC migration was normal. CONCLUSIONS: We have shown that guttate psoriasis is associated with an abnormality of LC mobilization, but a less marked inhibition compared with that seen in CPP. In resolved guttate psoriasis LC function returns to normal. These data provide further evidence that the pathogenesis of psoriasis is characterized by significant changes in epidermal LC function.

Acne, quorum sensing and danger.

Propionibacterium acnes is a ubiquitous skin commensal bacterium, which is normally well tolerated by the immune system in healthy human skin. However, there is increasing evidence to suggest a pivotal role for P. acnes in the inflammatory process underlying the acne pathogenesis. With its features of inflammation and pustulation, acne vulgaris resembles the skin's normal reaction to bacterial pathogens. P. acnes flourishes when sebum production increases in the follicles. Bacteria may undergo behavioural changes based on the surrounding bacterial population, a process called quorum sensing (QS). Evidence from in vitro studies suggests that QS enables P. acnes to upregulate its hydrolysis of sebum triglycerides by its bacterial lipases, secreting free fatty acids (FFAs) such as oleic, palmitic and lauric acids. These FFAs act as danger-associated molecular patterns (DAMPs), and activate Toll-like receptor (TLR)2 and TLR4, leading to selective T-helper (Th)-driven immunity, with subsequent expression of Th1/Th17-associated inflammatory cytokines. To our knowledge, there is currently no explanation as to what determines the shift of recognition by the immune system of P. acnes from being symbiotic to pathogenic. We present a novel hypothesis based on the essence of QS and DAMPs. P. acnes sends no or only 'safety' signals when present in 'controlled' quantities under commensal conditions, but becomes pathogenic and sends 'danger' signals via QS in the form of excess FFA production, which stimulates TLR2 and TLR4 as the bacterial population flourishes.

Integrating asthma hazard characterization methods for consumer products.

Despite extensive study, definitive conclusions regarding the relationship between asthma and consumer products remain elusive. Uncertainties reflect the multi-faceted nature of asthma (i.e., contributions of immunologic and non-immunologic mechanisms). Many substances used in consumer products are associated with occupational asthma or asthma-like syndromes. However, risk assessment methods do not adequately predict the potential for consumer product exposures to trigger asthma and related syndromes under lower-level end-user conditions. A decision tree system is required to characterize asthma and respiratory-related hazards associated with consumer products. A system can be built to incorporate the best features of existing guidance, frameworks, and models using a weight-of-evidence (WoE) approach. With this goal in mind, we have evaluated chemical hazard characterization methods for asthma and asthma-like responses. Despite the wealth of information available, current hazard characterization methods do not definitively identify whether a particular ingredient will cause or exacerbate asthma, asthma-like responses, or sensitization of the respiratory tract at lower levels associated with consumer product use. Effective use of hierarchical lines of evidence relies on consideration of the relevance and potency of assays, organization of assays by mode of action, and better assay validation. It is anticipated that the analysis of existing methods will support the development of a refined WoE approach.

Why does allergic contact dermatitis exist?

The skin immune system's propensity to produce allergic contact dermatitis (ACD) to harmless chemicals, while otherwise being an efficient defence system, represents a dermatological paradox. We postulate that a major role in signalling in ACD is played by Toll-like receptor (TLR)2 and TLR4, and arises from their activation by extracellular danger-associated molecular patterns (DAMPs). Ligand activation of TLR4/2 results in the expression of interleukins (ILs) IL-1ß, IL-6, IL-12, IL-18 and IL-23, tumour necrosis factor-α and interferon-α. These cytokines promote acquisition of sensitization, and facilitate elicitation of contact allergy via multiple mechanisms, including the recruitment of CD4+ Th1 and Th17 cells. As Th1 cells secrete large amounts of DAMPs, a DAMP immune circuit (positive-feedback loop) is created. This is an important driver of skin sensitization and skin inflammation. Pathogenic extracellular bacteria, but not commensal bacteria, produce pathogen-associated molecular pattern molecules, which stimulate the expression of Th1- and Th17-promoting cytokines via TLR2 and TLR4. This also induces an immune circuit. The ability of the skin immune system to activate host defence mechanisms and to distinguish between pathogenic bacteria and commensals provides an explanation for why skin sensitization and ACD develop, as they are processes that rely on the same biological pathways. These pathways may also shed light on the pathogenesis of chronic pustular inflammatory dermatoses (e.g. acne vulgaris). The existence of safety signals from commensal bacteria, which prevent initiation of these pathways, may provide opportunities for novel therapeutic approaches to the treatment of inflammatory skin diseases.

Allergic contact dermatitis: epidemiology, molecular mechanisms, in vitro methods and regulatory aspects. Current knowledge assembled at an international workshop at BfR, Germany.

Contact allergies are complex diseases, and one of the important challenges for public health and immunology. The German 'Federal Institute for Risk Assessment' hosted an 'International Workshop on Contact Dermatitis'. The scope of the workshop was to discuss new discoveries and developments in the field of contact dermatitis. This included the epidemiology and molecular biology of contact allergy, as well as the development of new in vitro methods. Furthermore, it considered regulatory aspects aiming to reduce exposure to contact sensitisers. An estimated 15-20% of the general population suffers from contact allergy. Workplace exposure, age, sex, use of consumer products and genetic predispositions were identified as the most important risk factors. Research highlights included: advances in understanding of immune responses to contact sensitisers, the importance of autoxidation or enzyme-mediated oxidation for the activation of chemicals, the mechanisms through which hapten-protein conjugates are formed and the development of novel in vitro strategies for the identification of skin-sensitising chemicals. Dendritic cell cultures and structure-activity relationships are being developed to identify potential contact allergens. However, the local lymph node assay (LLNA) presently remains the validated method of choice for hazard identification and characterisation. At the workshop the use of the LLNA for regulatory purposes and for quantitative risk assessment was also discussed.

Inter-relationships between different classes of chemical allergens.

Although allergic sensitization of the respiratory tract induced by chemicals is not as common as skin sensitization, it is nevertheless an important occupational health issue. Respiratory allergy to chemicals, characterized typically by rhinitis and asthma, is associated with considerable morbidity and with related socioeconomic costs. Several experimental approaches have been proposed for the prospective identification of chemical respiratory allergens, but none of these has yet been validated formally. In the absence of a widely accepted method for respiratory allergen identification, it is appropriate and relevant to explore their relationship with skin-sensitizing chemicals. A series of chemicals known to cause immune-mediated respiratory allergy in humans has been examined. The majority of the respiratory allergens tested were found to elicit positive responses in one or more standard tests used for the identification of skin-sensitizing potential (guinea pig maximization test, the Buehler test and/or the local lymph node assay). We suggest that this observation might form the basis of a potentially useful paradigm for initial characterization of the respiratory-sensitizing potential of chemicals. Specifically, chemicals that fail to elicit positive responses in accepted skin-sensitization test methods might also be regarded as lacking the inherent potential to cause allergic sensitization of the respiratory tract.

Functional rather than immunoreactive levels of IgG4 correlate closely with clinical response to grass pollen immunotherapy.

BACKGROUND: Induction of allergen-specific IgG(4) antibodies is the most consistent immunological finding in immunotherapy trials. However, quantitative assessments of IgG(4) antibodies have not proven beneficial in evaluating clinical changes during or after immunotherapy. In the current study, we investigated the relationship between clinical outcome and allergen-specific IgG(4) titres or functional antibody responses following immunotherapy. We hypothesized that functional assays of serum IgG-associated inhibitory activity such as inhibition of IgE-allergen interactions (IgE-blocking factor) and inhibition of CD23-dependent IgE-facilitated allergen binding (IgE-FAB) correlate more closely with clinical outcome and may be biomarkers of clinical response. METHODS: In an 8-month dose-response randomized double-blind placebo-controlled study, 221 polysensitized subjects with severe seasonal rhinitis received Alutard SQ, Phleum pratense 100,000 SQ-U, 10,000 SQ-U or placebo injections. Serum specimens were collected before treatment, after up-dosing, during the peak season and at the end of the study. Allergen-specific IgG(4) titres and IgG-associated inhibitory activity were evaluated. RESULTS: A time- and dose-dependent increase in serum inhibitory activity for both the IgE-blocking factor and IgE-FAB was observed, which paralleled increases in grass pollen-specific IgG(4) antibodies. A modest but significant inverse relationship was demonstrated between postimmunotherapy serum inhibitory activity and combined symptom-rescue medication scores (IgE-FAB: r = -0.25, P = 0.0002; IgE-blocking factor: r = -0.28, P < 0.0001), whereas this was not observed for immunoreactive IgG(4) levels (r = -0.11, P = 0.12). CONCLUSIONS: Functional assays of inhibitory IgG(4) and IgE-blocking factor may be more useful surrogates of clinical response than IgG(4). Whether these antibody effects may serve as predictive biomarkers of clinical efficacy in individual patients requires further investigation.

Concepts in psoriasis: psoriasis and the extracellular matrix.

Post-streptococcal guttate psoriasis only sometimes progresses to the persistent plaque psoriasis. We have previously proposed that psoriasis is driven by multiple extradomain A+ fibronectin (EDA+ FN) 'feedback loops'. The psoriasis-specific loop, the 'keratinocyte loop', depends upon expression by keratinocytes of α5ß1 integrin. In normal skin, α5ß1 expression is minimal or absent in resting epithelial cells, where basal cells are anchored to the laminin component of the basement membrane (via integrins α6ß4 and α3ß1). However, when the laminin layer is disrupted, due to wounding for instance, α5ß1 is then strongly expressed by basal cells as a means for anchoring. At uninvolved skin sites in patients with psoriasis the laminin layer within the basement membrane is disrupted, with a consequential increase in the expression by basal keratinocytes of EDA+ FN and α5ß1. We are postulating that in guttate psoriasis streptococcal lytic enzymes, and in particular streptokinase, may - via plasminogen activation - initiate a disruption of the laminin layer in the basement membrane. It is also possible that the expression by proliferating keratinocytes of plasminogen-activating factor might result in additional laminin digestion. The suggestion is that progression of guttate psoriasis to full-blown plaque psoriasis, or to its spontaneous resolution, is governed largely by the triggering of humoral immune responses. Thus, if streptococcal lytic enzymes are neutralized by host antibodies in advance of irreversible and lasting damage to the basement membrane of the dermal papillae, then guttate psoriasis will resolve spontaneously. In contrast, if permanent damage is induced prior to effective neutralization of the relevant bacterial toxins, then keratinocytes will become chronically destabilized. The consequence will be to liberate keratinocytes to proliferate in response to various stimuli, and to continue to elaborate plasminogen activation enzymes; the combined effect being, in concert with immunological priming, to precipitate and sustain plaque psoriasis.

No impairment of monocyte-derived Langerhans cell phenotype or function in early-onset psoriasis.

BACKGROUND: Migration of epidermal Langerhans cells (LCs) in response to the cytokines interleukin (IL)-1ß and tumour necrosis factor (TNF)-α is impaired in uninvolved skin of patients with early-onset psoriasis. AIM: To investigate whether this impairment is a reflection of a systemic defect in dendritic cells (DCs), using an established model of monocyte-derived LC-like cells (mLCs). METHODS: CD14+ monocytes isolated from both patients with psoriasis and healthy control volunteers were cultured in a cytokine cocktail for 5 days to promote their differentiation into mLCs, then stimulated for 24 h with TNF-α, IL-1ß (both 100 ng/mL) or medium alone. Cellular surface protein expression was quantified by flow cytometry, and the ability of cells to migrate to media supplemented with C-C motif ligand (CCL)19 was assessed using a Transwell migration assay. The cytokine and chemokine content of supernatants was analysed by cytokine array. RESULTS: CD14+ cells acquired an LC-like phenotype with high expression of CD1a and major histocompatibility complex (MHC) class II. There were no differences in the expression of activation markers or in the secretion of cytokines by mLCs isolated from patients with psoriasis and those isolated from healthy controls. Moreover, mLCs isolated from both groups displayed comparable ability to migrate in vitro. CONCLUSIONS: These data suggest that the failure of LCs to migrate in response to stimulation in patients with psoriasis is not attributable to a systemic defect in DC function, but is rather a reflection of local changes in the epidermal microenvironment.

The Hapten-Atopy hypothesis II: the 'cutaneous hapten paradox'.

One explanation for the striking increase in atopic disease in developed countries over the last 50 years has been the 'Hygiene Hypothesis'; a reduced exposure to pathogenic microorganisms. We have postulated previously that oral and cutaneous exposure to chemicals generally and to haptens in particular, may have also contributed to the increased prevalence of atopic disease; the 'Hapten-Atopy Hypothesis'. The purpose here is to extend further that hypothesis by consideration of the impact interplay between the innate and adaptive immune systems may have on the development of atopic allergy. It is clear that experimental cutaneous exposure to hapten can generate immune responses of different types with regard to T-helper (Th) cell phenotype. Allergic contact dermatitis is frequently associated with a selective Th1 (and Tc1)-driven inflammation, whereas atopic dermatitis is characterized by preferential Th2 cell responses. We postulate here that initial innate immune responses to chemical haptens result in the promotion of Th1 cell responses secondary to stimulation of Toll-like receptor. However, we argue also that under conditions where there is prolonged skin exposure to hapten there will be a shift of Th cell phenotype to selective Th2-type responses. The significance of such interactions is the possibility that repeated low-level skin exposure to certain types of hapten may result in the creation of an immunological environment in which the development of Th2 immune responses to third party antigens is favoured. The hypothesis is advanced that the nature and conditions of skin exposure to common haptens may impact on the quality of cutaneous immune responses such that in some circumstances the development atopic disease is favoured.

The effect of ageing on phenotype and function of monocyte-derived Langerhans cells.

BACKGROUND: With increasing age the immune system shows functional decline. In the skin this is associated with an increased incidence of epidermal malignancies and infections. Epidermal Langerhans cells (LCs) act as sentinels of the immune system, recognizing, processing and presenting antigen and inducing T-cell responses. Previous investigations have demonstrated a reduction in the number of epidermal LCs in elderly subjects. Moreover, the ability of LCs to migrate in response to tumour necrosis factor (TNF)-α, but not interleukin (IL)-1ß, is significantly impaired in the elderly. OBJECTIVES: To characterize further the changes in LC function that are associated with increasing chronological age, we have evaluated age-related changes in the response of monocyte-derived LCs (mLCs) to IL-1ß and TNF-α. METHODS: The phenotype and function of mLCs were compared in six young (≤ 30 years) and six aged (≥ 70 years) healthy individuals using a combination of flow cytometry, cytokine and chemokine array, and a Transwell migration assay. RESULTS: Monocytes from aged individuals were able to differentiate into LCs. There were no significant differences in expression of activation markers, or in baseline or inducible cytokine secretion, by mLCs derived from aged or young subjects. Furthermore, migration in response to a chemokine ligand, CCL19, was equivalent in both age groups. CONCLUSIONS: These data demonstrate that changes in LC function in the elderly are not associated with changes in systemic dendritic cell phenotype and function. Conditioning of LCs in situ by the epidermal microenvironment is likely to be more important.