Philipp R. Esser

Mechanisms of chemical-induced innate immunity in allergic contact dermatitis

To cite this article:  Martin SF, Esser PR, Weber FC, Jakob T, Freudenberg MA, Schmidt M, Goebeler M. Invited review for the journal allergy mechanisms of chemical‐induced innate immunity in allergic contact dermatitis. Allergy 2011; 66: 1152–1163.

Metal allergens nickel and cobalt facilitate TLR4 homodimerization independently of MD2

Development of contact allergy requires cooperation of adaptive and innate immunity. Ni2+ 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 Co2+ 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.

UVB-induced DNA damage, generation of reactive oxygen species, and inflammation are effectively attenuated by the flavonoid luteolin in vitro and in vivo

Ultraviolet (UV) radiation induces DNA damage, oxidative stress, and inflammatory processes in human keratinocytes resulting in skin inflammation, photoaging, and photocarcinogenesis. The flavonoid luteolin is one of the most potent antioxidative plant polyphenols. We investigated the UV protective and antioxidant properties of luteolin in human keratinocytes in vitro, ex vivo, and in vivo. Spectrophotometric measurements revealed extinction maxima of luteolin in the UVB and UVA range. UV transmission below 370 nm was <10%. In human skin, luteolin effectively reduced the formation of UVB-induced cyclobutane pyrimidine dimers. The free radical scavenging activity of luteolin was assessed in various cell-free and cell-based assays. In the cell-free DPPH assay the half-maximal effective concentration (EC₅₀) of luteolin (12 μg/ml) was comparable to those of Trolox (25 μg/ml) and N-acetylcysteine (32 μg/ml). In contrast, in the H₂DCFDA assay performed with UVB-irradiated keratinocytes, luteolin (EC₅₀ 3 μg/ml) was much more effective compared to Trolox (EC₅₀ 12 μg/ml) and N-acetylcysteine (EC₅₀ 847 μg/ml). Luteolin also inhibited both UVB-induced skin erythema and the upregulation of cyclooxygenase-2 and prostaglandin E₂ production in human skin via interference with the MAPK pathway. These data suggest that luteolin may protect human skin from UVB-induced damage by a combination of UV-absorbing, DNA-protective, antioxidant, and anti-inflammatory properties.

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.

Kindlin-1 and -2 Have Overlapping Functions in Epithelial Cells: Implications for Phenotype Modification

Kindlins are a novel family of intracellular adaptor proteins in integrin-containing focal adhesions. Kindlin-1 and -2 are expressed in the skin, but whether and how they cooperate in adult epithelial cells have remained elusive. We uncovered the overlapping roles of kindlin-1 and -2 in maintaining epithelial integrity and show that the phenotype of kindlin-1-deficient cells can be modulated by regulating kindlin-2 gene expression and vice versa. The experimental evidence is provided by use of human keratinocyte cell lines that express both kindlins, just kindlin-1 or kindlin-2, or none of them. Double deficiency of kindlin-1 and -2 had significant negative effects on focal adhesion formation and actin cytoskeleton organization, cell adhesion, survival, directional migration, and activation of β(1) integrin, whereas deficiency of one kindlin only showed variable perturbation of these functions. Cell motility and formation of cell-cell contacts were particularly affected by lack of kindlin-2. These results predict that kindlin-1 and -2 can functionally compensate for each other, at least in part. The high physiologic and pathologic significance of the compensation was emphasized by the discovery of environmental regulation of kindlin-2 expression. UV-B irradiation induced loss of kindlin-2 in keratinocytes. This first example of environmental regulation of kindlin expression has implications for phenotype modulation in Kindler syndrome, a skin disorder caused by kindlin-1 deficiency.

Role of Kindlin-2 in Fibroblast Functions: Implications for Wound Healing

Kindlins are a protein family that regulates cell-matrix interactions. Although kindlin-2 is known to be essential for the early developmental processes, its role in the homeostasis of adult tissues has remained elusive. In this study, we used new domain-specific antibodies and small interfering RNA technology to uncover physiological functions of kindlin-2 in human dermal fibroblasts in vitro and in adult skin in situ. In primary dermal fibroblasts, kindlin-2 is essential for the integrin clustering and activation, and it regulates cell adhesion and directed migration by guiding formation and maturation of focal adhesions (FAs) and organization of the cytoskeleton. These functions are linked to the transmission of mechanical cues between cells and their microenvironment, typically in processes wherein fibroblasts differentiate to myofibroblasts under mechanical tension. In concert, kindlin-2 was shown to be required for the stabilization and maturation of FAs and stress fibers in activated fibroblasts and myofibroblasts. These findings implicated that in physiological wound closure and tissue repair, the prediction was validated by strong upregulation of kindlin-2 in contractile myofibroblasts during middle stages of wound healing in human skin. Taken together, the data reveal a physiological role for kindlin-2 in skin fibroblasts under normal steady-state conditions and during tissue regeneration.

Correlation of contact sensitizer potency with T cell frequency and TCR repertoire diversity.

Allergic contact dermatitis is a T cell-mediated skin disease. Many hundreds of organic chemicals and some metal ions are contact sensitizers. They induce an innate inflammatory immune response in the skin that results in the priming of contact sensitizer-specific T cells by dendritic cells in the draining lymph nodes. The factors that determine the strength of this T cell response and thereby define the potency of a contact sensitizer are largely unknown. This chapter highlights different variables such as precursor frequency of antigen-specific T cells, possible bystander activation, and T cell receptor diversity or avidity of the TCR/peptide-MHC interactions, which might impact the quality and strength of T cell responses to contact sensitizers. In addition, different methods available to determine both the frequency of antigen-specific T cells and T cell receptor repertoires are discussed. Identification of the factors determining potency may allow for the development of suitable in vitro assays for potency assessment of contact sensitizers.

Loss of Proteostasis Is a Pathomechanism in Cockayne Syndrome

Retarded growth and neurodegeneration are hallmarks of the premature aging disease Cockayne syndrome (CS). Cockayne syndrome proteins take part in the key step of ribosomal biogenesis, transcription of RNA polymerase I. Here, we identify a mechanism originating from a disturbed RNA polymerase I transcription that impacts translational fidelity of the ribosomes and consequently produces misfolded proteins. In cells from CS patients, the misfolded proteins are oxidized by the elevated reactive oxygen species (ROS) and provoke an unfolded protein response that represses RNA polymerase I transcription. This pathomechanism can be disrupted by the addition of pharmacological chaperones, suggesting a treatment strategy for CS. Additionally, this loss of proteostasis was not observed in mouse models of CS.