Β-1,3-glucan, but Not Β-1,3/1,6-glucan, Exacerbates Experimental Food Allergy, While Both Increase IgA Induction.

Dietary fiber has gained interest as a prebiotic to treat allergy.1 However, the immunomodulatory function of β-glucan has been debated.2, 3 In the present study, we found evidence that (i) β-1,3-glucan may exacerbate the development of food allergy, but also act as a mucosal adjuvant increasing IgA, and (ii) β-1,3/1,6-glucan also possesses an adjuvant effect, but to a lesser extent than β-1,3-glucan, using a murine model (Figure S1A). Linear β-1,3-glucan backbones with β-1,6-linked side chains (β-1,3/1,6-glucan) and without side chains (β-1,3-glucan) are isolated from yeast and fungi, and are ligands binding TLR2 and Dectin-1.2 Surprisingly, β-1,3-glucan diet, but not β-1,3/1,6-glucan diet, promoted the development of allergic diarrhea and the drop in body temperature associated with anaphylaxis in a food allergic model (Figure 1A; Figure S1B). Neither β-glucan diet enhanced the serum levels of allergen-specific IgE, but tended to reduce IgG2a (Figure 1B; Figure S1C). Notably, β-1,3-glucan diet, and β-1,3/1,6-glucan diet to a lesser extent, increased total and allergen-specific IgA, that is, mucosal antibodies, in feces of the mice (Figure 1C; Figure S1D). β-1,3-glucan induced higher IL-1β and IL-6 secretion from bone marrow-derived dendritic cells than β-1,3/1,6-glucan, suggesting that β-1,3-glucan to have higher adjuvant effect (Figure S2A). In vitro allergen stimulation assay showed that the mesenteric lymph node (MLN) cells and splenocytes from β-1,3-glucan group enhanced production of Th2 cytokine IL-4 and Th17 cytokine IL-17A, whereas those from β-1,3/1,6-glucan group enhanced production of Th1 cytokine IFN-γ in MLNs (Figure 1D; Figure S2B). The levels of IL-10 production and the frequency of Treg in the tissues were comparable between the β-glucan and control groups (Figure S2B). Histological analysis of the jejunum showed that β-1,3 glucan diet thickens the muscular layer (Figure 1E; Figure S2C). Furthermore, the numbers of mast cells and eosinophils in the submucosa increased in allergen-sensitised and challenged β-1,3-glucan group (Figure 2A; Figures S2D and S3A). Ketotifen, a mast cell stabilizer, abolished the effect of β-1,3-glucan diet on the development of allergic diarrhea (Figure 2B). The concentrations of IL-4 and IL-5, but not IL-33, a cytokine activating group 2 innate lymphoid cells, were the highest in intestinal homogenates of β-1,3-glucan group (Figure 2C). These results suggest that β-1,3-glucan promotes type 2 immunity in mucosal tissues because of its adjuvant effect, increases intestinal mast cells and eosinophils, and thereby exacerbates the development of allergic features. IL-6, which is mainly derived from dendritic cells and macrophages, promotes IgA class-switching in B-cells.4 We observed that β-1,3-glucan diet increased the frequency of CD11c+, or F4/80+ cells expressing TLR2, a receptor of β-glucans, and the levels of IL-6 production and in the MLNs and spleens (Figure S3B). Such effects of β-1,3-glucan may contribute to the enhanced IgA production. β-1,3-glucan and β-1,3/1,6-glucan diets, but not allergen sensitization, altered gut microbiome in mice (Figures S3C, and S4A,B). β-1,3-glucan diet increased Lachnospirales, Lachnospiraceae, and [Eubacterium] ventriosum, producers of immunomodulatory short chain fatty acids (SCFA) (Figure 2D).5 However, the levels of the majority of SCFA, acetate, propionate and butylate, were comparable among the tested groups (Figure 2E; Figure S4C). The results suggest that the microbiome altered by β-1,3-glucan diet is not directly related to the enhanced allergic features. In the formula for human equivalent dose, 5% β-glucan diet in mice (25 g) is equivalent to approximately 32 g/day of the glucan in human (60 kg), which is higher than the average fiber intake from 16 to 29 g/day in adults.6 Collectively, our study indicates that food allergic features might be more severe upon intake of high doses of β-1,3-glucan, but also potential of β-1,3-glucan and β-1,3/1,6-glucan as mucosal adjuvants with IgA-boosting effects. Association of β-glucans with adjuvant effects and gut microbiome has to be investigated furthermore. Chaoqi He and Yunhui Liu performed animal experiments and immune assays. Chaoqi He, Shunsuke Nishio, and Tsukasa Matsuda performed gut microbiome analysis. Ting-Yu Cheng supported FACS analysis. Yingnan Guo verified data of β-1,3-glucan diet. Tomonori Nochi supported IgA measurement and interpretation of histology data. Hannah Rainer, Krause Maren, and Stefan Schülke performed BMDC experiments. Together with Masako Toda, Chaoqi He, Stefan Vieths, Stefan Schülke, Stephan Scheurer, and Tsukasa Matsuda interpretated data. Chaoqi He and Masako Toda wrote the paper. Masako Toda designed and supervised the study. All authors reviewed the manuscript. The authors thanks Professor Ikeda Ikuo for advice on SCFA measurement and Kota Torii for support of animal experiments. Sample preparation for histological analysis was supported by Biomedical Research Core of Tohoku University Graduate School of Medicine. Funding sources are indicated in supplemental file. Statement of COI is in supplemental file. The data that support the findings of this study are available from the corresponding author upon reasonable request. Data S1. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.