Mechanisms of chemical-induced innate immunity in allergic contact dermatitis

Th17 cells and tissue remodeling in atopic and contact dermatitis

BACKGROUND

Eczematous skin lesions of atopic dermatitis (AD) as well as allergic and irritant contact dermatitis (ACD, ICD) are characterized by the same typical clinical signs, although due to different causes. In both AD and ACD, the presence of T helper 17 cells which play an important role in host defense, has been reported. Furthermore, IL-17 is involved in tissue repair and remodeling. This study aimed to investigate IL-17 expression in acute eczematous skin lesions and correlate it with markers of remodeling in AD, ACD, and ICD.

METHODS

Skin specimens were taken from positive patch test reactions to aeroallergens, contact allergens, and irritants at days 2, 3, and 4. Inflammatory cells as well as the expression of cytokines and extracellular matrix proteins were evaluated by immunofluorescence staining and confocal microscopy.

RESULTS

Allergic contact dermatitis and ICD were characterized by IFN-γ expression, whereas in AD lesions, IL-13 expression and high numbers of eosinophils were the prominent phenotype. Expression of IL-17, but also IL-21 and IL-22, was observed in all eczema subtypes. The number of IL-22+ T cells correlated with the number of eosinophils. Markers of remodeling such as MMP-9, procollagen-3, and tenascin C were observed in all acute eczematous lesions, while a correlation of IL-17+ T cell numbers with tenascin C-expressing cells and MMP-9+ eosinophils was apparent.

CONCLUSION

The expression of IL-17 and related cytokines, such as IL-22, was demonstrated in acute eczematous lesions independent of their pathogenesis. Our results suggest a potential role for IL-17 in remodeling of the skin.

Gene expression changes induced by skin sensitizers in the KeratinoSens™ cell line: Discriminating Nrf2-dependent and Nrf2-independent events

The KeratinoSens™ assay is an in vitro screen for the skin sensitization potential of chemicals. It is based on a luciferase reporter gene under the control of the antioxidant response element of the aldoketoreductase gene AKR1C2. The transferability, reproducibility, and predictivity of the KeratinoSens™ assay have been investigated in detail and it is currently under assessment at the European Center for Validation of Alternatives to animal testing (ECVAM). Here we investigate the sensitizer-induced gene expression in the KeratinoSens™ cell line at the mRNA level and discriminate Nrf2-dependent and Nrf2-independent events by using siRNA to better characterize this test system at the molecular level. The results show that (i) the sensitizer-induced luciferase signal in KeratinoSens™ cells is completely dependent on Nrf2. The same holds true for the luciferase induction observed for the false positive chemical Tween80, indicating that the false positive result is not due to recruitment of an alternative transcription factor. (ii) Luciferase induction parallels the induction of endogenous Nrf2-dependent genes, indicating that the luciferase signal is representative for the sensitizer-induced Nrf2-response. (iii) The induction by sensitizers of additional genetic markers related to heat shock proteins and cellular stress could be reproduced in the KeratinoSens™ cell line and they were shown to be Nrf2-independent. These results confirm that the KeratinoSens™ cell line is a rapid and adequate screening tool to assess the sensitizer-induced Nrf2-response in keratinocytes.

Synergistic effect between amoxicillin and TLR ligands on dendritic cells from amoxicillin-delayed allergic patients

Amoxicillin, a low-molecular-weight compound, is able to interact with dendritic cells inducing semi-maturation in vitro. Specific antigens and TLR ligands can synergistically interact with dendritic cells (DC), leading to complete maturation and more efficient T-cell stimulation. The aim of the study was to evaluate the synergistic effect of amoxicillin and the TLR2, 4 and 7/8 agonists (PAM, LPS and R848, respectively) in TLR expression, DC maturation and specific T-cell response in patients with delayed-type hypersensitivity (DTH) reactions to amoxicillin. Monocyte-derived DC from 15 patients with DTH to amoxicillin and 15 controls were cultured with amoxicillin in the presence or absence of TLR2, 4 and 7/8 agonists (PAM, LPS and R848, respectively). We studied TLR1-9 gene expression by RT-qPCR, and DC maturation, lymphocyte proliferation and cytokine production by flow cytometry. DC from both patients and controls expressed all TLRs except TLR9. The amoxicillin plus TLR2/4 or TLR7/8 ligands showed significant differences, mainly in patients: AX+PAM+LPS induced a decrease in TLR2 and AX+R848 in TLR2, 4, 7 and 8 mRNA levels. AX+PAM+LPS significantly increased the percentage of maturation in patients (75%) vs. controls (40%) (p=0.036) and T-cell proliferation (80.7% vs. 27.3% of cases; p=0.001). Moreover, the combinations AX+PAM+LPS and AX+R848 produced a significant increase in IL-12p70 during both DC maturation and T-cell proliferation. These results indicate that in amoxicillin-induced maculopapular exanthema, the presence of different TLR agonists could be critical for the induction of the innate and adaptive immune responses and this should be taken into account when evaluating allergic reactions to these drugs.

High frequencies of positive nickel/cobalt patch tests and high sweat nickel concentration in patients with intrinsic atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is classified into extrinsic AD with high serum IgE levels and impaired barrier, and intrinsic AD with low serum IgE levels and unimpaired barrier. Intrinsic AD has a lower frequency of FLG mutations and a higher frequency of circulating Th1 cells, implying that non-protein antigens, represented by metals, may be an exacerbation factor in intrinsic AD.

OBJECTIVE

To investigate metal allergy in intrinsic AD.

METHODS

Enrolled in this study were 86 Japanese AD patients seen in three university hospitals, consisting of 55 extrinsic and 31 intrinsic AD patients. Patch testing was performed, focusing on nickel, cobalt, and chrome, in parallel with other 14 metals. FLG mutations were analyzed in 49 patients (extrinsic, 29; intrinsic, 20). In 17 patients (extrinsic, 12; intrinsic, 5), sweat was collected from the forearms by exercise, and the concentration of nickel was fluorometrically measured.

RESULTS

Nickel, cobalt, and chrome were the major positive metals. Intrinsic AD showed significantly higher percentages of positive reactions than extrinsic AD to nickel (intrinsic 41.9% vs extrinsic 16.4%, P=0.019) and cobalt (38.7% vs 10.9%, P=0.005). There was no significant difference between FLG mutation-bearing and non-bearing patients. The concentration of nickel was higher in the sweat of intrinsic AD than extrinsic AD patients (333.8 vs 89.4ng/g, P=0.0005) and inversely correlated with serum IgE levels.

CONCLUSIONS

Nickel and cobalt allergy may be involved in intrinsic AD. Given that the metals are excreted through sweat, intrinsic AD might be exaggerated by highly metal-containing sweat.

Cells and mediators in diisocyanate-induced occupational asthma

PURPOSE OF REVIEW

Diisocyanates are the most common cause of occupational asthma in many industrialized countries, and various pathogenic mechanisms have been suggested to be involved. Occupational asthma causes airway remodeling unless diagnosed and treated within a proper time frame. However, treatment modalities are limited because of an insufficient understanding regarding underlying pathogenic mechanisms.

RECENT FINDINGS

Several immunological and nonimmunological mechanisms have been suggested, indicating that the pathogenesis of occupational asthma may be more complex than other types of asthma. Airway epithelial cells are the first to encounter diisocyanates and orchestrate various responses, such as cytokine release, oxidative stress generation, and autoantibody formation. Some evidence supports the involvement of adaptive immune responses. Additional evidence suggests that other mechanisms are involved in diisocyanate-induced occupational asthma. One such candidate mechanism is oxidative stress. Oxidative stress has been shown to trigger and aid in the development of diisocyanate-induced occupational asthma in human samples and genetic studies, and some therapeutic trials were performed based on this finding.

SUMMARY

Diisocyanate-induced occupational asthma may be caused by a complex interaction of innate and adaptive immune responses. The knowledge presented in this review may help lead to the development of new treatment modalities through an increased understanding of occupational asthma pathogenesis.

Anti-inflammatory activity of topical THC in DNFB-mediated mouse allergic contact dermatitis independent of CB1 and CB2 receptors

BACKGROUND

∆(9) -Tetrahydrocannabinol (THC), the active constituent of Cannabis sativa, exerts its biological effects in part through the G-protein-coupled CB1 and CB2 receptors, which were initially discovered in brain and spleen tissue, respectively. However, THC also has CB1/2 receptor-independent effects. Because of its immune-inhibitory potential, THC and related cannabinoids are being considered for the treatment of inflammatory skin diseases. Here we investigated the mechanism of the anti-inflammatory activity of THC and the role of CB1 and CB2 receptors.

METHODS

We evaluated the impact of topically applied THC on DNFB-mediated allergic contact dermatitis in wild-type and CB1/2 receptor-deficient mice. We performed immunohistochemical analyses for infiltrating immune cells and studied the influence of THC on the interaction between T cells, keratinocytes and myeloid immune cells in vitro.

RESULTS

Topical THC application effectively decreased contact allergic ear swelling and myeloid immune cell infiltration not only in wild-type but also in CB1/2 receptor-deficient mice. We found that THC (1) inhibited the production of IFNγ by T cells, (2) decreased the production of CCL2 and of IFNγ-induced CCL8 and CXL10 by epidermal keratinocytes and (3) thereby limited the recruitment of myeloid immune cells in vitro in a CB1/2 receptor-independent manner.

CONCLUSIONS

Topically applied THC can effectively attenuate contact allergic inflammation by decreasing keratinocyte-derived pro-inflammatory mediators that orchestrate myeloid immune cell infiltration independent of CB1/2 receptors. This has important implications for the future development of strategies to harness cannabinoids for the treatment of inflammatory skin diseases.

Immunology of atopic eczema: overcoming the Th1/Th2 paradigm

Atopic eczema (AE) is a challenge for modern medicine, because it is prevalent, severely affects quality of life of patients and their families, and causes high socioeconomic costs. The pathogenesis of AE is complex. While initial studies suggested a Th2 deviation as primary reason for the disease, numerous studies addressed a genetically predetermined impaired epidermal barrier as leading cause in a subgroup of patients. Recently, immune changes beyond the initial Th2 concept were defined in AE, with a role for specialized dendritic cells as well as newly identified T helper cell subsets such as Th17 and Th22 cells. Furthermore, trigger factors are expanded beyond classical Th2 allergens such as pollen or house dust mites to microbial products as well as self-antigens. This review pieces together our current understanding of immune as well as barrier abnormalities into the pathogenesis mosaic of AE.

Epicutaneous immunization with protein antigen induces antigen-non-specific suppression of CD8 T cell mediated contact sensitivity

BACKGROUND

Allergic contact dermatitis (ACD) resulting from exposure to low molecular weight chemicals is a common clinical condition in industrialized countries and can be mediated by either Th1 or Tc1 lymphocytes. The animal model of contact sensitivity (CS) is commonly used to study ACD in mice and helps to test new therapeutics. We have previously shown that epicutaneous (EC) immunization with TNP-Ig prior to hapten sensitization inhibits Th1-mediated CS and observed that the suppression is mediated by TCRαβ(+) CD4(+) CD8(+) cells and is TGF-β dependent. More recently we have shown that EC immunization with DNP-BSA induces TCRαβ(+) CD4(+) CD25(+) FoxP3(+) T regulatory (Treg) cells that suppress Tc1-mediated CS.

METHODS

Animal model of contact sensitivity was used to study skin-induced suppression.

RESULTS

Current work employing Tc1-mediated CS shows that skin-induced suppression is dose-dependent and declines with time. Experiments with the four non-cross-reacting antigens 2,4-dinitrophenylated bovine serum albumin (DNP-BSA), ovalbumin (OVA), myelin basic protein (MBP) and immunoglobulins conjugated with oxazolone (OX-Ig) employing models of active suppression, "transfer in" and "transfer out" protocols showed that EC immunization with any tested protein antigen inhibits CS response suggesting lack of antigen-specificity of the investigated phenomenon.

CONCLUSION

The ease of EC generation of antigen-non-specific regulatory cells may have important implications for designing therapeutic schemes aimed at modulating immune responses.

Phosphodiesterase 4 inhibition in the treatment of psoriasis, psoriatic arthritis and other chronic inflammatory diseases

Agents which increase intracellular cyclic adenosine monophosphate (cAMP) may have an antagonistic effect on pro-inflammatory molecule production so that inhibitors of the cAMP degrading phosphodiesterases have been identified as promising drugs in chronic inflammatory disorders. Although many such inhibitors have been developed, their introduction in the clinic has been hampered by their narrow therapeutic window with side effects such as nausea and emesis occurring at sub-therapeutic levels. The latest generation of inhibitors selective for phosphodiesterase 4 (PDE4), such as apremilast and roflumilast, seems to have an improved therapeutic index. While roflumilast has been approved for the treatment of exacerbated chronic obstructive pulmonary disease (COPD), apremilast shows promising activity in dermatological and rheumatological conditions. Studies in psoriasis and psoriatic arthritis have demonstrated clinical activity of apremilast. Efficacy in psoriasis is probably equivalent to methotrexate but less than that of monoclonal antibody inhibitors of tumour necrosis factor (TNFi). Similarly, in psoriatic arthritis efficacy is less than that of TNF inhibitors. PDE4 inhibitors hold the promise to broaden the portfolio of anti-inflammatory therapeutic approaches in a range of chronic inflammatory diseases which may include granulomatous skin diseases, some subtypes of chronic eczema and probably cutaneous lupus erythematosus. In this review, the authors highlight the mode of action of PDE4 inhibitors on skin and joint inflammatory responses and discuss their future role in clinical practice. Current developments in the field including the development of topical applications and the development of PDE4 inhibitors which specifically target the subform PDE4B will be discussed.

Allergic skin inflammation induced by chemical sensitizers is controlled by the transcription factor Nrf2

Allergic contact dermatitis (ACD) is induced by low-molecular weight electrophilic chemicals and metal ions. Chemical contact sensitizers trigger reactive oxygen species production and provoke electrophilic stress, leading to the accumulation of the transcription factor nuclear-related factor 2 (Nrf2) in innate immune cell types. The objective of this work was to identify the role of Nrf2 in the regulation of ACD. We used the local lymph node assay (LLNA) and the mouse ear swelling test (MEST) to study the role of Nrf2 in both the sensitization and elicitation phase in nrf2 knockout (nrf2(-/-)) and wild-type (nrf2(+/+)) mice. Five chemicals were used: two compounds known to react with cysteine residues, 2,4-dinitrochlorobenzene (DNCB) and cinnamaldehyde (CinA); one sensitizer known to exhibit mixed reactivity to cysteine and lysine residues, isophorone diisocyanate; and one reacting specifically with lysine residues, trimellitic anhydride and croton oil, a well-known irritant. In the MEST assay, DNCB (1 and 2%) induced a significant increase in ear thickness in nrf2(-/-) compared with nrf2(+/+) mice, suggesting a role for Nrf2 in the control of the inflammatory process. When DNCB was used at 0.25 and 0.5% or when mice were treated with CinA, inflammation was found only in nrf2(-/-) mice. In the LLNA, all chemical sensitizers induced an increase of lymphocyte proliferation in nrf2(-/-) compared with nrf2(+/+) mice for the same chemical concentration. These results reveal an important role for Nrf2 in controlling ACD and lymphocyte proliferation in response to sensitizers.

Idiosyncratic adverse drug reactions: current concepts

Idiosyncratic drug reactions are a significant cause of morbidity and mortality for patients; they also markedly increase the uncertainty of drug development. The major targets are skin, liver, and bone marrow. Clinical characteristics suggest that IDRs are immune mediated, and there is substantive evidence that most, but not all, IDRs are caused by chemically reactive species. However, rigorous mechanistic studies are very difficult to perform, especially in the absence of valid animal models. Models to explain how drugs or reactive metabolites interact with the MHC/T-cell receptor complex include the hapten and P-I models, and most recently it was found that abacavir can interact reversibly with MHC to alter the endogenous peptides that are presented to T cells. The discovery of HLA molecules as important risk factors for some IDRs has also significantly contributed to our understanding of these adverse reactions, but it is not yet clear what fraction of IDRs have a strong HLA dependence. In addition, with the exception of abacavir, most patients who have the HLA that confers a higher IDR risk with a specific drug will not have an IDR when treated with that drug. Interindividual differences in T-cell receptors and other factors also presumably play a role in determining which patients will have an IDR. The immune response represents a delicate balance, and immune tolerance may be the dominant response to a drug that can cause IDRs.

Role of oxidative stress in chemical allergens induced skin cells activation

Allergic contact dermatitis (ACD) is an important occupational and environmental disease caused by topical exposure to chemical allergens. It describes the adverse effects that may results when exposure to a chemical elicits a T cell-mediated inflammatory skin disease. The ability of contact sensitizers to induce the oxidative stress pathway in keratinocytes and dendritic cells has been confirmed by several authors. Reactive oxygen species (ROS) can serve as essential second messengers mediating cellular responses resulting in immune cells activation. Oxidative stress may be the starter point, as it leads to the activation of transcription factors and signaling pathways, including NF-kB and p38 MAPK, which leads to the release of cytokines and chemokines. ROS are also involved in the activation of the NLRP3/NALP3 inflammasome, which is required to direct the proteolytic maturation of inflammatory cytokines such as IL-1β and IL-18, which are all integral to the process of dendritic cells mobilization, migration and functional maturation. Moreover, emerging evidence correlates ROS to changes in the constitution of the extracellular microenvironment found to facilitate ACD. The purpose of this review is to provide both conceptual and technical frameworks on the role of oxidative stress in chemical allergy.

Nickel and human health

This review focuses on the impact of nickel on human health. In particular, the dual nature of nickel as an essential as well as toxic element in nature is described, and the main forms of nickel that can come in contact with living systems from natural sources and anthropogenic activities are discussed. Concomitantly, the main routes of nickel uptake and transport in humans are covered, and the potential dangers that nickel exposure can represent for health are described. In particular, the insurgence of nickel-derived allergies, nickel-induced carcinogenesis as well as infectious diseases caused by human pathogens that rely on nickel-based enzymes to colonize the host are reviewed at different levels, from their macroscopic aspects on human health to the molecular mechanisms underlying these points. Finally, the importance of nickel as a beneficial element for human health, especially being essential for microorganisms that colonize the human guts, is examined.

Induction of contact hypersensitivity in the mouse model

Contact hypersensitivity (CHS) in the mouse model is a standard method to assess delayed type hypersensitivity (DTH) responses in the skin induced by low molecular weight chemicals that in humans cause contact dermatitis. These responses are clinically important and present as eczematous skin reactions. Here, this chapter describes the standard protocol for T cell-mediated CHS and a variation thereof, which allows to address more specific questions regarding immunologic pathomechanisms.

Therapeutic potential of B and T lymphocyte attenuator expressed on CD8+ T cells for contact hypersensitivity

In the past decade, mechanisms underlying allergic contact dermatitis have been intensively investigated by using contact hypersensitivity (CHS) models in mice. However, the regulatory mechanisms, which could be applicable for the treatment of allergic contact dermatitis, are still largely unknown. To determine the roles of B and T lymphocyte attenuator (BTLA), a CD28 family coinhibitory receptor, in hapten-induced CHS, BTLA-deficient (BTLA(-/-)) mice and littermate wild-type (WT) mice were subjected to DNFB-induced CHS, severe combined immunodeficient (SCID) mice were injected with CD4(+) T cells, and CD8(+) T cells from either WT mice or BTLA(-/-) mice were subjected to CHS. BTLA(-/-) mice showed enhanced DNFB-induced CHS and proliferation and IFN-γ production of CD8(+) T cells as compared with WT mice. SCID mice injected with WT CD4(+) T cells and BTLA(-/-) CD8(+) T cells exhibited more severe CHS as compared with those injected with WT CD4(+) T cells and WT CD8(+) T cells. On the other hand, SCID mice injected with BTLA(-/-) CD4(+) T cells and WT CD8(+) T cells exhibited similar CHS to those injected with WT CD4(+) T cells and WT CD8(+) T cells. Finally, to evaluate the therapeutic potential of an agonistic agent for BTLA on CHS, the effects of an agonistic anti-BTLA antibody (6A6) on CHS were examined. In vivo injection of 6A6 suppressed DNFB-induced CHS and IFN-γ production of CD8(+) T cells. Taken together, these results suggest that stimulation of BTLA with agonistic agents has therapeutic potential in CHS.

Metal allergens nickel and cobalt facilitate TLR4 homodimerization independently of MD2

Development of contact allergy requires cooperation of adaptive and innate immunity. Ni(2+) stimulates innate immunity via TLR4/MD2, the bacterial LPS receptor. This likely involves receptor dimerization, but direct proof is pending and it is unclear if related haptens share this mechanism. We reveal Co(2+) as second metal stimulating TLR4 and confirm necessity of H456/H458 therein. Experiments with a new TLR4 dimerization mutant established dimerization as a mechanism of metal- and LPS-induced TLR4 activation. Yet, in interaction studies only LPS- but not metal-induced dimerization required MD2. Consistently, soluble TLR4 expressed without MD2 inhibited metal- but not LPS-induced responses, opening new therapeutic perspectives.

Contact dermatitis: from pathomechanisms to immunotoxicology

Contact allergens are small reactive chemicals. They cause allergic contact dermatitis (ACD) by activating the innate and adaptive immune system. Contact allergens are very peculiar because of their built-in autoadjuvanticity that allows them to trigger sterile inflammation following skin penetration. The innate inflammatory response involves the triggering of pattern recognition receptors either by direct chemical interaction with such receptors or by induction of endogenous activators. I discuss here the recent findings regarding prevalence and predisposition, the identification of innate immune and stress response mechanisms relevant for sensitization and the orchestration of the innate and adaptive immune response to contact allergens. Despite still significant gaps of knowledge, recent advances in our understanding of the immunopathogenesis of ACD can now be used for the development of causative treatment strategies and of in vitro alternatives to animal testing for the identification of contact allergens in immunotoxicology.

Allergic contact dermatitis: xenoinflammation of the skin

Many xenobiotic chemicals cause sterile inflammation. This xenoinflammation is often induced by protein reactive contact allergens resulting in allergic contact dermatitis (ACD). Recent findings reveal that these chemicals mimick infection by triggering innate immune responses via pattern recognition receptors (PRRs) and endogenous danger signals. The emerging cellular responses in ACD are mediated by various innate effector cells. Here, an important role for mast cells has now been recognized. Eventually, chemical specific T cells such as CD8+ and CD4+ Tc1/Th1 as well as Tc17/Th17 cells are activated. Langerhans cells may serve a tolerogenic function. The mechanisms of tolerance induction by ultraviolet irradiation or by very low doses of contact allergen are now understood in much greater detail.

Update of immune events in the murine contact hypersensitivity model: toward the understanding of allergic contact dermatitis

Allergic contact dermatitis (ACD) is one of the most common skin diseases, consisting of sensitization and elicitation phases. With the advancement of technology and the discovery of new types of immune cells, our knowledge of the immunological mechanisms of contact hypersensitivity (CHS) as a murine model of ACD has expanded significantly in the past decade. For example, by introducing regulatory T cells, CD4(+) T-helper 17 cells, and Langerin-positive dermal dendritic cells, the initiation and termination mechanism of CHS has been revealed. In addition, the role of mast cells in CHS, long a matter of debate, has become apparent by developing conditional mast cell-deficient mice. Moreover, the role of the innate immunity system, such as that of Toll-like receptor signaling, has made a breakthrough in this field. In this review, we will integrate the recent advancement of immunological mechanisms of both the sensitization and elicitation phases of CHS into the classic view, and we will discuss updated mechanisms on its development and future directions.

Contact sensitizers induce skin inflammation via ROS production and hyaluronic acid degradation

BACKGROUND

Allergic contact dermatitis (ACD) represents a severe health problem with increasing worldwide prevalence. It is a T cell-mediated skin disease induced by protein-reactive organic and inorganic chemicals. A key feature of contact allergens is their ability to trigger an innate immune response that leads to skin inflammation. Previous evidence from the mouse contact hypersensitivity (CHS) model suggests a role for endogenous activators of innate immune signaling. Here, we analyzed the role of contact sensitizer induced ROS production and concomitant changes in hyaluronic acid metabolism on CHS responses.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed in vitro and in vivo ROS production using fluorescent ROS detection reagents. HA fragmentation was determined by gel electrophoresis. The influence of blocking ROS production and HA degradation by antioxidants, hyaluronidase-inhibitor or p38 MAPK inhibitor was analyzed in the murine CHS model. Here, we demonstrate that organic contact sensitizers induce production of reactive oxygen species (ROS) and a concomitant breakdown of the extracellular matrix (ECM) component hyaluronic acid (HA) to pro-inflammatory low molecular weight fragments in the skin. Importantly, inhibition of either ROS-mediated or enzymatic HA breakdown prevents sensitization as well as elicitation of CHS.

CONCLUSIONS/SIGNIFICANCE

These data identify an indirect mechanism of contact sensitizer induced innate inflammatory signaling involving the breakdown of the ECM and generation of endogenous danger signals. Our findings suggest a beneficial role for anti-oxidants and hyaluronidase inhibitors in prevention and treatment of ACD.

Surveillance of contact allergies: methods and results of the Information Network of Departments of Dermatology (IVDK)

Contact allergy (CA) surveillance networks provide information to a multitude of stakeholders, which is indispensable for evidence-based decision-making in the field of prevention. Methods and results of the German surveillance system on CA are reviewed and discussed with reference to other systems. The German network structure comprises 56 departments of dermatology and includes all patients who are patch-tested for suspected CA. Data analysis considers the results of patch testing and further pertinent information for each patient. Following aspects are addressed: (i) the description of the clinical population, (ii) evaluation of patch test reactions, (iii) relationship between patch test results and population characteristics. Trend analyses on chromate (decreasing), epoxy resin (increasing) and nickel (heterogeneous) served as examples for surveillance system analyses, with the identification of sentinel events, as well as proof of success or failure of prevention. In addition, external data sources can be used such as sales data of patch test preparations to estimate frequencies of sensitization on a population level. National prescription data of drugs and statistics of labelling of preservatives on cosmetics can be included, the latter two approaches allowing for risk estimates conferred by specific allergens.

Regulation of allergic responses to chemicals and drugs: possible roles of epigenetic mechanisms

There is increasing evidence that epigenetic regulation of gene expression plays a pivotal role in the orchestration of immune and allergic responses. Such regulatory mechanisms have potentially important implications for the acquisition of sensitization to chemical and drug allergens; and in determining the vigor, characteristics, and longevity of allergic responses. Importantly, the discovery of long-lasting epigenetic alterations in specific immunoregulatory genes provides a mechanistic basis for immune cell memory, and thereby the potential of chemical allergens to influence the subsequent orientation of the adaptive immune system. In this article, we consider the implications of epigenetic mechanisms for the development of sensitization to chemical and drug allergens and the form that allergic reactions will take.

Dendritic cells and the assessment in vitro of skin sensitizing potential

It is now well established that dendritic cells (DC) play pivotal roles in the initiation and orchestration of adaptive immune responses, including cutaneous immune responses to chemical allergens that drive the acquisition of skin sensitization. It is not unexpected, therefore, that a large number, and wide variety, of proposed approaches for the identification of skin sensitizing chemicals in vitro are based upon the use of cultured DC or DC-like cells. The use of DC in this context is legitimate. However, with our rapidly increasing understanding of the diversity of cutaneous DC with respect to both phenotype and function, it is timely now to review briefly the potential limitations and interpretive difficulties that are associated with the use of DC-based assays. Among the important considerations are the fact that chemical-induced changes in the characteristics and function of cultured DC will not necessarily reflect accurately the events that that support the development of skin sensitization in vivo. In addition, most DC-based assays are predicated on a view that cutaneous DC have as their primary function the initiation of adaptive immune responses. However, it is now appreciated that cutaneous DC, and in particular epidermal Langerhans cells (LC), may also play important immunoregulatory roles that serve to limit and contain skin immune responses. Notwithstanding these considerations there is reason to believe that at least some in vitro DC-based assays are of value, and indeed some are currently the subject of a formal validation process. However, it is appropriate that such assays are configured and interpreted carefully, and with an appreciation of the complexity of DC biology.

The contact sensitizer diphenylcyclopropenone has adjuvant properties in mice and potential application in epicutaneous immunotherapy

BACKGROUND

Epicutaneous vaccination has gained increasing interest during the past decade as it offers a safe, needle-free, and patient-friendly alternative to invasive vaccine administrations. Recently, the safety and early efficacy of epicutaneous immunotherapy were also demonstrated in patients with hay fever, as an alternative to conventional subcutaneous allergen-specific immunotherapy (SCIT). One major challenge to epicutaneous vaccination is the barrier function of the stratum corneum, which must be overcome either by abrasive methods or by hydration. Such barrier function of the stratum corneum also hampers the use of common adjuvants used to enhance the efficacy of vaccination.

METHODS

In a mouse model of allergy, we tested the adjuvant potential of diphenylcyclopropenone (DCP), a strong contact sensitizer, which is currently used for the treatment of a T cell-mediated hair loss disease (alopezia areata).

RESULTS

Diphenylcyclopropenone enhanced antigen-specific IgG2a antibody responses as well as IL-10 cytokine production after epicutaneous immunization with ovalbumin (OVA). Epicutaneous allergen-specific immunotherapy (EPIT) with OVA and DCP also protected sensitized mice from anaphylaxis and asthma. The protective effect was more robust than that of conventional SCIT, which did not significantly alleviate the symptoms of allergy in the murine models of anaphylaxis and asthma.

CONCLUSIONS

This preclinical study confirmed previous clinical data that have demonstrated the potential of the skin as a target for allergen immunotherapy. The study also suggests that epicutaneous immunization or immunotherapy can be improved when an appropriate adjuvant such as DCP is used.

Langerhans cells require MyD88-dependent signals for Candida albicans response but not for contact hypersensitivity or migration

Langerhans cells (LC) are a subset of skin-resident dendritic cells (DC) that reside in the epidermis as immature DC, where they acquire Ag. A key step in the life cycle of LC is their activation into mature DC in response to various stimuli, including epicutaneous sensitization with hapten and skin infection with Candida albicans. Mature LC migrate to the skin-draining LN, where they present Ag to CD4 T cells and modulate the adaptive immune response. LC migration is thought to require the direct action of IL-1β and IL-18 on LC. In addition, TLR ligands are present in C. albicans, and hapten sensitization produces endogenous TLR ligands. Both could contribute to LC activation. We generated Langerin-Cre MyD88(fl) mice in which LC are insensitive to IL-1 family members and most TLR ligands. LC migration in the steady state, after hapten sensitization and postinfection with C. albicans, was unaffected. Contact hypersensitivity in Langerin-Cre MyD88(fl) mice was similarly unaffected. Interestingly, in response to C. albicans infection, these mice displayed reduced proliferation of Ag-specific CD4 T cells and defective Th17 subset differentiation. Surface expression of costimulatory molecules was intact on LC, but expression of IL-1β, IL-6, and IL-23 was reduced. Thus, sensitivity to MyD88-dependent signals is not required for LC migration, but is required for the full activation and function of LC in the setting of fungal infection.

The multitasking organ: recent insights into skin immune function

The skin provides the first line defense of the human body against injury and infection. By integrating recent findings in cutaneous immunology with fundamental concepts of skin biology, we portray the skin as a multitasking organ ensuring body homeostasis. Crosstalk between the skin and its microbial environment is also highlighted as influencing the response to injury, infection, and autoimmunity. The importance of the skin immune network is emphasized by the identification of several skin-resident cell subsets, each with its unique functions. Lessons learned from targeted therapy in inflammatory skin conditions, such as psoriasis, provide further insights into skin immune function. Finally, we look at the skin as an interacting network of immune signaling pathways exemplified by the development of a disease interactome for psoriasis.

Early immune events in the induction of allergic contact dermatitis

The skin is a barrier site that is exposed to a wide variety of potential pathogens. As in other organs, pathogens that invade the skin are recognized by pattern-recognition receptors (PRRs). Recently, it has been recognized that PRRs are also engaged by chemical contact allergens and, in susceptible individuals, this elicits an inappropriate immune response that results in allergic contact dermatitis. In this Review, we focus on how contact allergens promote inflammation by activating the innate immune system. We also examine how innate immune cells in the skin, including mast cells and dendritic cells, cooperate with each other and with T cells and keratinocytes to initiate and drive early responses to contact allergens.

Contact sensitizers induce skin inflammation via ROS production and hyaluronic acid degradation

BACKGROUND

Allergic contact dermatitis (ACD) represents a severe health problem with increasing worldwide prevalence. It is a T cell-mediated skin disease induced by protein-reactive organic and inorganic chemicals. A key feature of contact allergens is their ability to trigger an innate immune response that leads to skin inflammation. Previous evidence from the mouse contact hypersensitivity (CHS) model suggests a role for endogenous activators of innate immune signaling. Here, we analyzed the role of contact sensitizer induced ROS production and concomitant changes in hyaluronic acid metabolism on CHS responses.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed in vitro and in vivo ROS production using fluorescent ROS detection reagents. HA fragmentation was determined by gel electrophoresis. The influence of blocking ROS production and HA degradation by antioxidants, hyaluronidase-inhibitor or p38 MAPK inhibitor was analyzed in the murine CHS model. Here, we demonstrate that organic contact sensitizers induce production of reactive oxygen species (ROS) and a concomitant breakdown of the extracellular matrix (ECM) component hyaluronic acid (HA) to pro-inflammatory low molecular weight fragments in the skin. Importantly, inhibition of either ROS-mediated or enzymatic HA breakdown prevents sensitization as well as elicitation of CHS.

CONCLUSIONS/SIGNIFICANCE

These data identify an indirect mechanism of contact sensitizer induced innate inflammatory signaling involving the breakdown of the ECM and generation of endogenous danger signals. Our findings suggest a beneficial role for anti-oxidants and hyaluronidase inhibitors in prevention and treatment of ACD.

Allergic contact dermatitis: a commentary on the relationship between T lymphocytes and skin sensitising potency

T lymphocytes mediate skin sensitisation and allergic contact dermatitis. Not unexpectedly, therefore, there is considerable interest in the use of T lymphocyte-based assays as alternative strategies for the identification of skin sensitising chemicals. However, in addition to accurate identification of hazards the development of effective risk assessments requires that information is available about the relative skin sensitising potency of contact allergens. The purpose of this article is to consider the relationships that exist between the characteristics of T lymphocyte responses to contact allergens and the effectiveness/potency of sensitisation. We propose that there are 3 aspects of T lymphocyte responses that have the potential to impact on the potency of sensitisation. These are: (a) the magnitude of response, and in particular the vigour and duration of proliferation and the clonal expansion of allergen-reactive T lymphocytes, (b) the quality of response, including the balance achieved between effector and regulatory cells, and (c) the breadth of response and the clonal diversity of T lymphocyte responses. A case is made that there may be opportunities to exploit an understanding of T lymphocyte responses to contact allergens to develop novel paradigms for predicting skin sensitising potency and new approaches to risk assessment.

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.