Stefan Kraft

The high-affinity IgE receptor (FcεRI) blocks apoptosis in normal human monocytes

Monocytes have a limited life span, and their homeostasis is regulated by apoptosis in vivo. When cultured in the absence of appropriate exogenous stimuli, they undergo apoptosis, but under the influence of survival signals, these cells differentiate into macrophages or dendritic cells. Here we show that ligation of the high-affinity IgE receptor (FcepsilonRI) on human monocytes from nonatopic individuals markedly reduces apoptosis induced by serum deprivation or by CD95/Fas ligation. Aggregation of FcepsilonRI reduces its own expression but fails to modulate CD95/Fas expression. In contrast, FcepsilonRI ligation enhances the expression of the antiapoptotic molecules Bcl-2 and Bcl-xL, but not Mcl-1, in monocytes. Incubation of unstimulated cells with culture supernatants of FcepsilonRI-activated monocytes prolongs their life span, whereas CD95/Fas expression remains unaffected. The incidence of apoptosis is restored considerably when the supernatant is depleted of TNF-alpha, whereas elimination of IL-1beta, GM-CSF, or IL-12 has no effect. These results indicate that FcepsilonRI mediates signals preventing monocyte apoptosis directly by increasing the levels of Bcl-2 and Bcl-xL, and indirectly by means of TNF-alpha in an autocrine and paracrine fashion. This process may contribute to the establishment of chronic allergic disorders such as atopic dermatitis.

Atopic dermatitis: pathogenetic mechanisms

Atopic dermatitis (AD) is a chronic inflammatory skin disease with increasing incidence and socio‐economical relevance. The diagnosis is made on clinical grounds and different diagnostic criteria sets have been established. The majority of all AD cases is associated with a sensitization to environmental allergens and increased serum IgE (so‐called extrinsic AD), but about 10‐30% of all cases suffer from the so‐called intrinsic AD, which obviously lacks any link to the classical atopic diathesis. The genetic background of AD has been investigated by target gene approach by different groups with mostly contradictory results for each of the genes under study. An imbalance in the spectrum of Th1/Th2 responses, a disturbed prostaglandin metabolism, intrinsic defects in keratinocyte function, delayed eosinophil apoptosis, IgE‐mediated facilitated antigen presentation by epidermal dendritic cells, a two phase model of the inflammatory response and staphylococcal superantigen effects are among the currently studied pathogenetical aspects of extrinsic AD, which are reviewed in this paper.

The central role of FcepsilonRI in allergy.

The high‐affinity receptor for immunoglobulin E (IgE), FcεRI, plays a central role in the initiation and control of atopic allergic inflammation. On mast cells and basophils, the function of the receptor is well known and constitutes cellular degranulation and the release of various mediators. FcεRI on antigen‐presenting cells (APCs), however, does not lead to degranulation of preformed granula, but has different functions: signal transduction pathways like the activation of NF‐κB are initiated to induce inflammatory cytokine gene expression. In addition, FcεRI on APCs acts as an allergen‐focusing structure and can efficiently amplify allergen presentation in an IgE‐dependent manner. Recently, we and others have gained new insight into the regulation and function of FcεRI on APCs, which has shed new light on the modulating effects of the immune system in atopic inflammation.

IL-4 induces the intracellular expression of the α chain of the high-affinity receptor for IgE in in vitro–generated dendritic cells

BACKGROUNDnRecent findings have shown that the surface expression of the high-affinity receptor for IgE (FcepsilonRI) on human CD1a(+) Langerhans cells (LC) and related dendritic cells (DC) in the skin, despite a constant intracellular expression of its alpha chain (FcepsilonRIalpha), is highly up-regulated in atopic dermatitis. Moreover, this surface expression correlates with the IgE serum level, strongly suggesting yet-to-be-defined common signals in the regulation of FcepsilonRI display on LC/DC and IgE synthesis.nnnOBJECTIVESnIn this study we examined the influence of different cytokines on the expression of FcepsilonRI on in vitro-generated CD1a(+) LC/DC.nnnMETHODSnCD34(+) precursor cells were isolated from cord blood with use of high-gradient magnetic cell sorting, cultured with GM-CSF, TNF-alpha, IL-4, or IFN-gamma, and surface and cytoplasmic staining for flow cytometry were performed.nnnRESULTSnIL-4 strongly enhanced the generation of CD1a(+) LC/DC and also up-regulated the expression of the skin-homing structures E-cadherin and cutaneous lymphocyte antigen. In contrast, IFN-gamma was found to suppress the E-cadherin expression and to be a strong antagonist of IL-4 by inhibiting the production of CD1a(+) cells. Most important, IL-4 induced the cytoplasmic expression of FcepsilonRIalpha in CD1a(+) LC/DC but not its surface expression. This up-regulation was antagonized by IFN-gamma.nnnCONCLUSIONnIL-4 is not only a key cytokine in the regulation of IgE but also induces the expression of its receptor binding chain as well as up-regulation of skin homing molecules on LC/DC. Expression of these structures during generation of LC/DC reflects the in vivo situation encountered in LC.

Unexpected Functions of FcεRI on Antigen-Presenting Cells

In contrast to mast cells and basophils, the high affinity IgE receptor (FcΕRI) on antigen-presenting cells (APC) shows structural and functional differences. It consists only of a minimal structure of one α- and two γ-chains and enables APC to efficiently take up and present antigen in IgE-mediated delayed-type hypersensitivity reactions that are thought to play a pivotal role in atopic diseases. However, recent studies of FcΕRI signal transduction and function on APC suggest additional mechanisms by which FcΕRI engagement on APC could affect inflammatory reactions. FcΕRI ligation is able to induce major signaling events like protein tyrosine kinase activation including p72syk leading to PLC-γ1 phosphorylation and consecutive calcium influx. Late signaling events like the activation of transcription factors such as NF-ĸB provide a link to the release of proinflammatory cytokines and chemokines, Th-polarizing factors such as IL-12 and the induction of antiapoptotic factors. FcΕRI-mediated IL-10 production in monocytes could also influence their differentiation. Since there are hints that in vivo a functional FcΕRI signaling pathway only exists in individuals from an atopic background, we suggest that these unexpected mechanisms may have an effect on inflammatory reactions in atopic diseases.

FcεRI-Mediated Activation of Transcription Factors in Antigen-Presenting Cells

Professional antigen-presenting cells (APC) such as monocytes and dendritic cells (DC) bearing high-affinity IgE receptors (FcΕRI) efficiently present IgE-bound antigens to T cells. FcΕRI expression is upregulated on APC from atopic donors, especially in inflamed tissues. These data suggest a pathophysiological concept of an IgE-mediated delayed-type hypersensitivity reaction in atopic diseases. However, FcΕRI ligation also leads to the synthesis of proinflammatory cytokines and other molecules involved in inflammatory reactions. The investigation of transcription factors mediating these effects has only recently commenced. In general, members of the NF-ĸB family are known to regulate APC function and differentiation, with the RelB subunit being especially important in DC generation. In addition, Ikaros and PU.1 have also been shown to be essential factors for DC differentiation, whereas Oct-2 is upregulated by differentiation towards macrophages. Recently, FcΕRI has been demonstrated to induce NF-ĸB activation via IĸB-α serine phosphorylation and degradation in monocytes and DC. Inhibitors of NF-ĸB activation such as N-acetylcysteine or N-tosyl-L-phenylalanine chloromethyl ketone can suppress FcΕRI-induced TNF-α and MCP-1 release. Interestingly, in human epidermal Langerhans’ cells (LC), NF-ĸB activation can only be observed when large amounts of FcΕRI are present. In addition, the composition of NF-ĸB complexes differs between monocytes, monocyte-derived DC and LC, suggesting a cell type-specific regulation. Moreover, the transcription factor NFAT is induced upon FcΕRI ligation in human APC. The elucidation of further transcription factors involved in FcΕRI signaling in APC should contribute to the employment of new inhibition strategies for the treatment of atopic and other inflammatory diseases.

Limited reliability of E-cadherin as a specific marker for in vitro-generated Langerhans cells.

Langerhans cells (LC) are a unique population of dendritic cells (DC) found in the epidermis where they can be identified by the expression of CD1a, E-cadherin and cytoplasmic Birbeck granules (BG) as their hallmark. Over the past years many techniques have been described to generate LC in vitro from either monocytes or CD34+ hematopoietic cell progenitors. Antibodies against Lag and Langerin (two epitopes associated with BG) and E-cadherin (a Ca2+-dependent homophilic adhesion molecule) have been used to detect in vitro-generated LC. In this study we investigated whether the expression of E-cadherin on in vitro-generated CD1a+ from either CD34+ cells or monocytes is able to discriminate LC from other DC. Our results demonstrate that E-cadherin alone is not a reliable marker to specifically identify in vitro-generated LC.

EcɛRI Expressing Dendritic Cells: The Missing Link in the Pathophysiology of Atopic Dermatitis ?

Introduction Atopic dermatitis (AD) is a chronic inflammatory skin disease clinically and histologically highly similar to allergic contact dermatitis. Recently, it has been proposed to subdivide AD into 2 distinct forms: the extrinsic form (occuring in the context of sensitization toward environmental allergens) and the intrinsic form (occuring in the absence of any typical atopicalbackground) (1). While the pathophysiology of the intrinsic form remains almost elusive, tremendous progress has been made in the understanding of the extrinsic form. Thus, since IgE plays a major role in other atopic diseases, such as asthma and rhinitis, it is assumed that, in this extrinsic form, IgE also mediates the specificity of the inflammatory conditions in the skin.