BackgroundFood‐induced anaphylaxis is a serious allergic reaction caused by Fcε‐receptor activation on mast cells (MCs). The exact mechanisms breaking oral tolerance and the effector pathways driving food allergy remain elusive. As complement is activated in food‐induced anaphylaxis, we aimed to assess the role of C5a in disease pathogenesis.MethodsOral antigen‐induced food‐induced anaphylaxis was induced in BALB/c wild‐type (wt) and C5ar1−/− mice. Readouts included diarrhea development, changes in rectal temperature, hematocrit, antigen‐specific serum IgE, MCPT‐1, and intestinal MC numbers, as well as FcεR1‐mediated MC functions including C5a receptor 1 (C5aR1) regulation. Further, histamine‐mediated hypothermia and regulation of endothelial tight junctions were determined.ResultsRepeated oral OVA challenge resulted in diarrhea, hypothermia, increased hematocrit, high OVA‐specific serum IgE, and MCPT‐1 levels in wt mice. Male C5ar1−/− mice were completely whereas female C5ar1−/− mice were partially protected from anaphylaxis development. Serum MCPT‐1 levels were reduced gender‐independent, whereas IgE levels were reduced in male but not in female C5ar1−/− mice. Mechanistically, IgE‐mediated degranulation and IL‐6 production from C5ar1−/− BMMCs of both sexes were significantly reduced. Importantly, FcεR1 cross‐linking strongly upregulated C5aR1 MC expression in vitro and in vivo. Finally, C5ar1−/− male mice were largely protected from histamine‐induced hypovolemic shock, which was associated with protection from histamine‐induced barrier dysfunction in vitro following C5aR targeting.ConclusionsOur findings identify C5aR1 activation as an important driver of IgE‐mediated food allergy through regulation of allergen‐specific IgE production, FcεR1‐mediated MC degranulation, and histamine‐driven effector functions preferentially in male mice.

Mast cells (MCs) have long been mainly regarded as effector cells in IgE-associated allergic disorders with potential immunoregulatory roles. Located close to the allergen entry sites in the skin and mucosa, MCs can capture foreign substances such as allergens, toxins, or noxious substances and are exposed to the danger signals produced by epithelial cells. MC reactivity shaped by tissue-specific factors is crucial for allergic responses ranging from local skin reactions to anaphylactic shock. Development of Th2 response leading to allergen-specific IgE production is a prerequisite for MC sensitization and induction of FcεRI-mediated MC degranulation. Up to now, IgE production has been mainly associated with proteins, whereas lipids present in plant pollen grains, mite fecal particles, insect venoms, or food have been largely overlooked regarding their immunostimulatory and immunomodulatory properties. Recent studies, however, have now demonstrated that lipids affect the sensitization process by modulating innate immune responses of epithelial cells, dendritic cells, and NK-T cells and thus crucially contribute to the outcome of sensitization. Whether and how lipids affect also MC effector functions in allergic reactions has not yet been fully clarified. Here, we discuss how lipids can affect MC responses in the context of allergic inflammation. Direct effects of immunomodulatory lipids on MC degranulation, changes in local lipid composition induced by allergens themselves and changes in lipid transport affecting MC reactivity are possible mechanisms by which the function of MC might be modulated.

Diffuse large B-cell lymphoma (DLBCL) represents the most common form of non-Hodgkin lymphoma that is still incurable in a large fraction of patients. Tetraspanin CD37 is highly expressed on mature B lymphocytes and multiple CD37-targeting therapies are under clinical development for non-Hodgkin lymphoma. However, CD37 expression is non-detectable in ~50% of DLBCL patients which correlates with inferior treatment outcome, but the underlying mechanisms for differential CD37 expression in DLBCL are still unknown. Here, we investigated the regulation of the CD37 gene in human DLBCL at the (epi-)genetic and transcriptional level. No differences were observed in DNA methylation within the CD37 promoter region between CD37-positive and CD37-negative primary DLBCL patient samples. On the contrary, CD37-negative DLBCL cells specifically lacked CD37 promoter activity, suggesting differential regulation of CD37 gene expression. Using an unbiased quantitative proteomic approach, we identified transcription factor IRF8 to be significantly higher expressed in nuclear extracts of CD37-positive as compared to CD37-negative DLBCL. Direct binding of IRF8 to the CD37 promoter region was confirmed by DNA pull-down assay combined with mass spectrometry, and targeted chromatin immunoprecipitation. Functional analysis indicated that IRF8 overexpression enhanced CD37 protein expression, while CRISPR/Cas9 knock-out of IRF8 decreased CD37 levels in DLBCL cell lines. Immunohistochemical analysis in a large cohort of primary DLBCL (n=206) revealed a significant correlation of IRF8 expression with detectable CD37 levels. Together, this study provides new insight into the molecular mechanisms underlying differential CD37 expression in human DLBCL, and reveals IRF8 as transcriptional regulator of CD37 in B-cell lymphoma.

Mast cells (MCs) are long-living immune cells highly specialized in the storage and release of different biologically active compounds and are involved in the regulation of innate and adaptive immunity. MC degranulation and replacement of MC granules are accompanied by active membrane remodelling. Tetraspanins represent an evolutionary conserved family of transmembrane proteins. By interacting with lipids and other membrane and intracellular proteins, they are involved in organisation of membrane protein complexes and act as “molecular facilitators” connecting extracellular and cytoplasmic signaling elements. MCs express different tetraspanins and MC degranulation is accompanied by changes in membrane organisation. Therefore, tetraspanins are very likely involved in the regulation of MC exocytosis and membrane reorganisation after degranulation. Antiviral response and production of exosomes are further aspects of MC function characterized by dynamic changes of membrane organization. In this review, we pay a particular attention to tetraspanin gene expression in different human and murine MC populations, discuss tetraspanin involvement in regulation of key MC signaling complexes, and analyze the potential contribution of tetraspanins to MC antiviral response and exosome production. In-depth knowledge of tetraspanin-mediated molecular mechanisms involved in different aspects of the regulation of MC response will be beneficial for patients with allergies, characterized by overwhelming MC reactions.