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Mast cells generate and release histamine during anaphylactic reactions, and there is pharmacological evidence that histamine regulates this process via specific receptors. Therefore, we examined human leukemic (HMC-1) and normal skin mast cells for the expression of all four currently known histamine receptors. Both cell types expressed H2 and H4 receptors at mRNA and protein levels, whereas H3 receptor specific mRNA and receptor protein was undetectable. Similarly, immunohistochemistry of cutaneous tissue showed an absence of H3 receptor in these cells. Despite transcription of mRNA, H1 receptor protein was only moderately expressed in HMC-1 cells and was virtually absent in skin mast cells. Furthermore, only H1, H2, and H4 receptors were detectable by Western blot analysis of HMC-1 cells. Radiolabeled histamine binding was strongly inhibited only by H2 (ranitidine)- and H3/H4 (FUB 108)-specific antagonists. Histamine-induced increase of cAMP was inhibited by the H2 receptor antagonist famotidine, whereas induction of IP3 was not observed, making signaling via the H1 receptor unlikely. These data show that human mast cells constitutively express primarily H2 and H4 receptors and that H2 receptors are functionally linked to cellular processes. They provide new insights into the mechanisms that govern auto- and paracrine histamine-induced mast cell functions.

Summary Psoriasis is a chronic inflammatory skin disease in which epidermal hyperplasia results from the release of cytokines by infiltrating type 1 T cells. Up- regulation of endogenous interleukin-10 controls type 1 skin responses in animal models; however, interleukin-10 production is low in psoriatic lesions. Consistent with an important role of interleukin-10 in psoriasis, we and colleagues have recently demonstrated clinical efficacy of subcutaneous administration of recombinant interleukin-10 to affected patients. Here, we studied the effects of interleukin-10 on disease-related inflammatory pathways. Patients were treated with recombinant interleukin-10 over 6 wk in an open-label phase II clinical trial. Tissue was obtained before and after therapy and examined by histology/immunohistochemistry, in situ hybridization, and quantitative real-time reverse transcription–polymerase chain reaction. Ten of 14 patients showed a marked reduction of the clinical disease activity. The clinical response was associated with a significant decrease of cutaneous T cell infiltration and the lesional expression of type 1 cytokines interferon-γ and tumor necrosis factor-α. Interleukin-10 inhibited the epidermal interleukin-8 pathway by downregulating the expression of interleukin-8, its receptor CXCR2, and its inducer interleukin-17, and partially reversed the aberrant keratinocyte maturation defining psoriatic epidermal pathology. Remarkably, there was evidence that genetic factors are involved in the response to interleukin-10 as individual variations in the downregulation of tumor necrosis factor-α were related to the presence of polymorphisms in the tumor necrosis factor-α promoter. These data suggest that excessive production of type 1 cytokines in human skin disease can be counter-regulated by the administration of recombinant interleukin-10. Genotypic analysis may help to identify patients that will preferentially respond to interleukin-10 therapy. [ABSTRACT FROM AUTHOR]

Since mast cells and basophils are thought to play a central role in several types of cutaneous inflammatory and allergic reactions and since interleukin-6 (IL-6) is an important mediator in these processes, we have studied the ability of the human mast cell line HMC-1, the human basophilic cell line KU812 and human skin mast cells to produce IL-6. All three cell types proved to be potent sources of this cytokine after appropriate stimulation. Transcription of IL-6 mRNA was first detectable 2 h after stimulation with the ester phorbol myristate acetate (PMA) and the calcium ionophore A23187 in both cell lines, as evidenced by semiquantitative reverse transcriptase polymerase chain reaction analysis. Whereas resting cells did not produce IL-6 protein, PMA/A23187- stimulated cells released immunoreactive and biologically active IL-6, as demonstrated and quantitated by enzyme-linked immunosorbent assay and by the use of TEPC 1033 cells an IL-6-dependent murine plasmacytoma cell line. Stimulated KU812 cells secreted sevenfold more IL-6 (up to 15 ng/ml) than HMC-1 cells (up to 2.4 ng/ml). Immunoblotting of HMC-1- and KU812 cell-derived IL-6 revealed several IL-6 forms in the molecular weight range of 2l to 30 kDa. Immunoelectron microscopic studies of human skin biopsies provided evidence that unstimulated mast cells do not contain preformed IL-6 but accumulate IL-6 in cytoplasmic and extruded granules after IgE-dependent stimulation. These findings suggest that IL-6 secreted by human mast cells and basophils potentially contributes to allergic, other immunologically mediated and nonspecific inflammatory responses. [ABSTRACT FROM AUTHOR]