Stefan F. Martin

Crucial role for human Toll-like receptor 4 in the development of contact allergy to nickel

Allergies to nickel (Ni2+) are the most frequent cause of contact hypersensitivity (CHS) in industrialized countries. The efficient development of CHS requires both a T lymphocyte-specific signal and a proinflammatory signal. Here we show that Ni2+ triggered an inflammatory response by directly activating human Toll-like receptor 4 (TLR4). Ni2+-induced TLR4 activation was species-specific, as mouse TLR4 could not generate this response. Studies with mutant TLR4 proteins revealed that the non-conserved histidines 456 and 458 of human TLR4 are required for activation by Ni2+ but not by the natural ligand lipopolysaccharide. Accordingly, transgenic expression of human TLR4 in TLR4-deficient mice allowed efficient sensitization to Ni2+ and elicitation of CHS. Our data implicate site-specific human TLR4 inhibition as a potential strategy for therapeutic intervention in CHS that would not affect vital immune responses.

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.

Dendritic Cell Immunization Route Determines CD8+ T Cell Trafficking to Inflamed Skin: Role for Tissue Microenvironment and Dendritic Cells in Establishment of T Cell-Homing Subsets

The effector/memory T cell pool branches in homing subsets selectively trafficking to organs such as gut or skin. Little is known about the critical factors in the generation of skin-homing CD8+ T cells, although they are crucial effectors in skin-restricted immune responses such as contact hypersensitivity and melanoma defense. In this study, we show that intracutaneous, but not i.v. injection of bone marrow-derived dendritic cells induced skin-homing CD8+ T cells with up-regulated E-selectin ligand expression and effector function in contact hypersensitivity. The skin-homing potential and E-selectin ligand expression remained stable in memory phase without further Ag contact. In contrast, i.p. injection induced T cells expressing the gut-homing integrin α4β7. Although differential expression of these adhesion molecules was strictly associated with the immunization route, the postulated skin-homing marker CCR4 was transiently up-regulated in all conditions. Interestingly, dendritic cells from different tissues effectively induced the corresponding homing markers on T cells in vitro. Our results suggest a crucial role for the tissue microenvironment and dendritic cells in the instruction of T cells for tissue-selective homing and demonstrate that Langerhans cells are specialized to target T cells to inflamed skin.

Toll-like receptor and IL-12 signaling control susceptibility to contact hypersensitivity

Allergic contact hypersensitivity (CHS) is a T cell–mediated inflammatory skin disease. Interleukin (IL)-12 is considered to be important in the generation of the allergen-specific T cell response. Loss of IL-12 function in IL-12Rβ2–deficient mice, however, did not ameliorate the allergic immune response, suggesting alternate IL-12–independent pathways in the induction of CHS. Because exposure to contact allergens always takes place in the presence of microbial skin flora, we investigated the potential role of Toll-like receptors (TLRs) in the induction of CHS. Using mice deficient in TLR4, the receptor for bacterial lipopolysaccharide (LPS), IL-12 receptor (R) β2, or both, we show that the concomitant absence of TLR4 and IL-12Rβ2, but not the absence of TLR4 or IL-12Rβ2 alone, prevented DC-mediated sensitization, generation of effector T cells, and the subsequent CHS response to 2,4,6-trinitro-1-chlorobenzene (TNCB), oxazolone, and fluorescein isothiocyanate. Introduction of the TLR4 transgene into the TLR4/IL-12Rβ2 mutant restored the CHS inducibility, showing a requirement for TLR4 in IL-12–independent CHS induction. Furthermore, the concomitant absence of TLR2 and TLR4 prevented the induction of CHS to TNCB in IL-12–competent mice. Finally, CHS was inducible in germ-free wild-type and IL-12Rβ2–deficient mice, but not in germ-free TLR4/IL-12Rβ2 double deficient mice, suggesting that the necessary TLR activation may proceed via endogenous ligands.

Dendritic cells govern induction and reprogramming of polarized tissue-selective homing receptor patterns of T cells: important roles for soluble factors and tissue microenvironments.

Tissue‐selective homing is established during naive T cell activation by the tissue microenvironment and tissue‐specific dendritic cells (DC). The factors driving induction and maintenance of T cell homing patterns are still largely unknown. Here we show that soluble factors produced during the interaction of T cells with CD11c+ DC isolated from skin‐ or small intestine‐associated tissues differentially modulate expression of the corresponding tissue‐selective homing receptors (E‐selectin ligands and α4β7 integrin/CCR9, respectively) on murine CD8+ T cells. Injection of tissue‐specific DC via different routes induces T cells with homing receptors characteristic of the corresponding local tissue microenvironment, independent of the origin of the DC. These data indicate an important role for signals delivered in trans. Moreover, DC can reprogram the homing receptor expression on T cells previously polarized in vitro for homing to skin or small intestine. Importantly, skin‐homing memory T cells stimulated directly ex vivo can also be reprogrammed by intestinal DC to a gut‐homing phenotype. Our results show that tissue‐selective homing receptor expression on effector and memory T cells is governed by inductive as well as suppressive signals from both DC and tissue microenvironments.

T cell immune responses to haptens. Structural models for allergic and autoimmune reactions

Protein-reactive chemicals, metal salts and drugs, commonly classified as immunological haptens, are major environmental noxes targeted at the immune system of vertebrates. They may not only interfere with this defense system by toxicity alone, but more often by evoking hapten-specific immune responses resulting in allergic and eventually autoimmune responses. Here, we review recent developments in the analysis of the structural basis of hapten recognition, particularly by T lymphocytes, which represent central elements in cell-mediated, as well as in IgE dependent, allergies. A break-through in this field was the finding that T cells detect haptens as structural entities, attached covalently or by complexation to self-peptides anchored in binding grooves of major histocompatibility antigens (MHC-proteins). Synthetic hapten-peptide conjugates were shown to induce hapten-specific contact sensitivity in mice, opening new routes for studying hapten-induced immune disorders.

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.

Lack of the purinergic receptor P2X7 results in resistance to contact hypersensitivity

Engagement of P2X7 on mouse dendritic cells, presumably by ATP released in response to contact allergen, is needed for IL-1β production and the sensitization phase of contact hypersensitivity.

A Potential Role for P2X7R in Allergic Airway Inflammation in Mice and Humans

P2X₇R deficiency is associated with a less severe outcome in acute and chronic inflammatory disorders. Recently, we demonstrated that extracellular adenosine triphosphate is involved in the pathogenesis of asthma by modulating the function of dendritic cells (DCs). However, the role of the purinergic receptor subtype P2X₇ is unknown. To elucidate the role of P2X₇R in allergic airway inflammation (AAI) in vitro and in vivo, P2X₇R expression was measured in lung tissue and immune cells of mice or in humans with allergic asthma. By using a specific P2X₇R-antagonist and P2X₇R-deficient animals, the role of this receptor in acute and chronic experimental asthma was explored. P2X₇R was found to be up-regulated during acute and chronic asthmatic airway inflammation in mice and humans. In vivo experiments revealed the functional relevance of this finding because selective P2X₇R inhibition or P2X₇R deficiency was associated with reduced features of acute and chronic asthma in the ovalbumin-alum or HDM model of AAI. Experiments with bone marrow chimeras emphasized that P2X₇R expression on hematopoietic cells is responsible for the proasthmatic effects of P2X₇R signaling. In the DC-driven model of AAI, P2X₇R-deficient DCs showed a reduced capacity to induce Th2 immunity in vivo. Up-regulation of P2X₇R on BAL macrophages and blood eosinophils could be observed in patients with chronic asthma. Our data suggest that targeting P2X₇R on hematopoietic cells (e.g., DCs or eosinophils) might be a new therapeutic option for the treatment of asthma.

T Cell Recognition of Haptens, a Molecular View

Our review attempts to summarize the present knowledge on how T lymphocytes recognize chemically modified autologous cells. Concerning the broad spectrum of chemically and drug-induced allergic and autoimmune diseases, the molecular mechanisms of hapten recognition by T cells are clearly of more than academic interest. The past few years revealed that in contrast to the expectations of many researchers, major histocompatibility complex (MHC)-restricted hapten-specific T cell receptors in their majority do not react to covalently modified MHC molecules, but to haptenized peptides associated with the MHC peptide-binding groove. This finding allowed the introduction of synthetic hapten-peptide conjugates, the MHC specificity of which may be predetermined by allele-specific peptide sequence motifs. Thus, it has now become feasible to selectively hapten-modify defined sets of MHC molecules on living cells, and to study their immunological properties. In that way two major types of hapten-specific T cell receptors were identified: one reacting to hapten without caring for the chemical composition of the carrier peptide, and the other contacting hapten and peptide by two apparently independent contact sites. The consequences of these findings for hapten-specific allergies and autoimmunities, but also for our molecular understanding of antigen recognition by T cells are discussed.