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Background The function of skin mast cells has been well documented in IgE-mediated allergic reactions, whereas other mast cell functions are poorly defined. This study aimed at identifying novel mast cell proteins by proteome analysis of primary human skin mast cells. Methods The proteome of skin mast cells was compared to other cell types and analyzed using bioinformatics. The expression and function of two proteins hitherto not described in skin mast cells was investigated in isolated mast cells as well as in mast cells in situ. Results Within the mast cell proteome, we identified 49 highly expressed proteins previously not described in mast cells; 21 of these proteins were found to be selectively expressed in mast cells. Two proteins, the neural cell adhesion molecule L1 and dipeptidyl peptidase 4, were further studied. L1 was found to be highly expressed in mast cells in normal, psoriasis, and mastocytosis skin. Dipeptidyl peptidase 4 was found to be expressed in mast cells in normal, psoriasis, and mastocytosis skin as well as in bone marrow mast cells in patients with systemic mastocytosis. In normal skin, mast cells were identified as a major source of dipeptidyl peptidase 4 and we also found that skin mast cells and fibroblasts secrete an active form of this enzyme. Conclusions In a systematic proteomics approach we identified two novel mast cell proteins potentially relevant to skin homeostasis: neural cell adhesion molecule L1 and dipeptidyl peptidase 4.

In normal skin, tightly connected keratinocytes in the stratum granulosum and terminally differentiated keratinocytes in the stratum corneum build an efficient barrier that inhibits extensive water loss while simultaneously preventing the entry of microbial pathogens and allergens into the skin (1). Dysregulation of keratinocyte differentiation together with pathologic changes in this natural barrier function is commonly associated with skin diseases such as atopic dermatitis, psoriasis, and ichthyosis vulgaris (2–4). Recently, loss-of-function mutations in the filaggrin gene have been identified (5, 6). These mutations not only impair the normal formation of the skin barrier, but also decrease the production of endogenous moisturizing molecules that lead to reduced stratum corneum hydration (4, 7). However, besides genetic defects of filaggrin production, other factors play a role in the pathogenesis of atopic dermatitis and the associated skin barrier defects as indicated by several observations: (i) The majority of individuals with atopic dermatitis do not exhibit null mutations in the filaggrin gene (8), (ii) individuals with atopic dermatitis without mutations in the filaggrin gene may also develop severe skin barrier defects (9), and (iii) a significant fraction of individuals carrying double-allele loss-of-function mutations in the filaggrin gene do not develop atopic dermatitis (4, 10). In line with this concept, it was shown recently that the pro-inflammatory cytokines interleukin-4 (IL-4), IL-31, and TNF-α compromise barrier function (11) and modulate the expression of differentiation-associated proteins in keratinocytes (12–14). Mast cells are present in normal skin, and increased numbers of mast cells are regularly observed in the skin of patients with atopic dermatitis even before the onset of inflammation (4, 15). Mast cells release a number of important signaling molecules, among which histamine has particularly potent pro-inflammatory activities (16). After mast cell degranulation, histamine concentrations within the tissue can rise to 10–1000 μM (17), and increased histamine levels have been reported for lesional and nonlesional skin of patients with atopic dermatitis (18). A role of endogenous histamine in the modulation of keratinocyte maturation has been suggested previously based on the observation that antihistamines have a beneficial effect on skin barrier recovery after tape striping in normal mouse skin (19). In the present study, we address the impact of histamine on the differentiation of human keratinocytes in different in vitro systems, among them a three-dimensional organotypic human skin model. This skin model has been shown previously to resemble native human skin, especially with regard to skin development and keratinocyte differentiation (20, 21). Our findings show that histamine prevents the expression of late differentiation antigens in keratinocytes and strongly decreases the expression of tight junction and desmosomal proteins, leading to the formation of a defective skin barrier.

To cite this article: Gschwandtner M, Rossbach K, Dijkstra D, Baumer W, Kietzmann M, Stark H, Werfel T, Gutzmer R. Murine and human Langerhans cells express a functional histamine H4 receptor: modulation of cell migration and function. Allergy 2010; 65: 840–849. Abstract Background: Histamine is an important mediator of allergic reactions, and recent studies indicated that the function of different types of antigen presenting cells (APC) can be modulated by histamine, in particular via the newly described histamine H4 receptor (H4R). Therefore, we investigated possible interactions of histamine via the H4R on Langerhans cells (LC), which represent the professional APC in the skin and therefore have an important role in the initiation and maintenance of allergic skin diseases. Methods: The expression of the H4R was evaluated by real-time PCR, flow cytometry and immunofluorescence staining. The function of the H4R was determined by intracellular flow cytometric measurement of chemokine production and LC migration assays. Results: Here, we show H4R expression on in vitro generated monocyte-derived LC (mRNA and protein) and on primary LC from murine and human skin samples (protein). The immunofluorescence staining in murine and human skin samples clearly proved that LC express the H4R in situ. Stimulation with histamine or a H4R agonist downregulated the chemokine (C-C motif) ligand 2 (CCL2) in human monocyte-derived LC and primary LC. Prestimulation with a selective H4R antagonist abolished this effect. Moreover, migration of LC from the epidermis was increased after H4R agonist stimulation in ex vivo migration assays using human epidermis and murine in vivo assays. Conclusion: Our findings show that LC express a functional H4R and point towards a possible pathogenic relevance of the H4R in inflammatory and allergic diseases.