Yoshinori Yamanishi

Background: Patients with atopic dermatitis (AD) often show the infiltration of basophils in the affected skin. Because basophils represent only a minor fraction among cellular infiltrates in the skin lesion, the functional significance of skin‐infiltrating basophils in AD pathogenesis remains ill‐defined. In this study, we aimed to clarify the role of basophils and their effector molecules triggering skin inflammation in oxazolone (OX)‐induced murine model of AD.Methods: A panel of mouse strains were sensitized and repeatedly challenged with topical applications of OX to induce AD‐like skin inflammation. Both local and systemic Th2 immune responses were analyzed.Results: Basophils progressively accumulated in the skin lesion but barely in draining lymph nodes (LNs). When basophils were depleted during the elicitation phase, skin inflammation was ameliorated while Th2 cell differentiation in draining LNs remained intact. The expression of IL‐4 was highly upregulated in the affected skin, and basophils turned out to be the major producers of IL‐4 among cellular infiltrates, suggesting the involvement of basophil‐derived IL‐4 in the Th2 skin inflammation. Indeed, basophil‐specific IL‐4‐deficient mice displayed attenuated skin inflammation with a marked reduction of IL‐4 in the skin lesion, even though cutaneous basophil infiltration and serum levels of IgE remained intact.Conclusions: Skin‐infiltrating basophils promoted OX‐induced AD‐like skin inflammation through their local production of IL‐4, rather than the induction of Th2 cell differentiation in draining LNs. This study suggests that the selective targeting of basophils could be a beneficial strategy in the treatment of a certain type of AD.

Basophils have nonredundant roles in various immune responses that require Ca²⁺ influx. Here, we examined the role of two Ca²⁺ sensors, stromal interaction molecule 1 and 2 (STIM1 and STIM2), in basophil activation. We found that loss of STIM1, but not STIM2, impaired basophil IL-4 production after stimulation with immunoglobulin E (IgE)–containing immune complexes. In contrast, when basophils were stimulated with IL-3, loss of STIM2, but not STIM1, reduced basophil IL-4 production. This difference in STIM proteins was associated with distinct time courses of Ca²⁺ influx and transcription of the Il4 gene that were elicited by each stimulus. Similarly, basophil-specific STIM1 expression was required for IgE-driven chronic allergic inflammation in vivo, whereas STIM2 was required for IL-4 production after combined IL-3 and IL-33 treatment in mice. These data indicate that STIM1 and STIM2 have differential roles in the production of IL-4, which are stimulus dependent. Furthermore, these results illustrate the vital role of STIM2 in basophils, which is often considered to be less important than STIM1.

Zymosan is a glucan that is a component of the yeast cell wall. Here, we determined the mechanisms underlying the zymosan-induced accumulation of neutrophils in mice. Loss of the receptor CD300b reduced the number of neutrophils recruited to dorsal air pouches in response to zymosan, but not in response to lipopolysaccharide (LPS), a bacterial membrane component recognized by Toll-like receptor 4 (TLR4). An inhibitor of nitric oxide (NO) synthesis reduced the number of neutrophils in the zymosan-treated air pouches of wild-type mice to an amount comparable to that in CD300b^−/− mice. Treatment with clodronate liposomes decreased the number of NO-producing, CD300b⁺ inflammatory dendritic cells (DCs) in wild-type mice, thus decreasing NO production and neutrophil recruitment. Similarly, CD300b deficiency decreased the NO-dependent recruitment of neutrophils to zymosan-treated joint cavities, thus ameliorating subsequent arthritis. We identified phytosphingosine, a lipid component of zymosan, as a potential ligand of CD300b. Phytosphingosine stimulated NO production in inflammatory DCs and promoted neutrophil recruitment in a CD300b-dependent manner. Together, these results suggest that the phytosphingosine-CD300b interaction promotes zymosan-dependent neutrophil accumulation by inducing NO production by inflammatory DCs and that CD300b may contribute to antifungal immunity.

Hapten is a small molecule that is not immunogenic on its own but can stimulate the production of antibodies at the sensitization phase when conjugated to carrier proteins. The hapten then reacts specifically with the antibodies generated against it to elicit an immune or allergic response at the challenge phase. Here we compared various carrier proteins conjugated with the same hapten in their ability to induce hapten-specific IgE-mediated allergic responses in vitro and in vivo, and characterized the nature of carrier proteins that determines the magnitude of response at the challenge phase of allergic reactions. Hapten TNP-conjugated ovalbumin (TNP-OVA) and bovine serum albumin (TNP-BSA) elicited TNP-specific, mast cell-dependent, immediate-type allergic reactions at a comparable level in mice that had been passively sensitized with TNP-specific IgE. In contrast, TNP-OVA but not TNP-BSA efficiently induced a basophil-dependent, IgE-mediated chronic allergic inflammation (IgE-CAI), even though both proteins could stimulate basophils in vitro at a comparable level. By comparing different carrier proteins and structurally modifying them, we found that the formation of large aggregates is crucial for TNP-conjugated carrier proteins to efficiently elicit IgE-CAI, regardless of the type of proteins. Thus, aggregation status of carrier proteins appears to determine the magnitude of allergic response at the challenge phase of hapten-specific IgE-CAI. Our findings suggest that the allergenicity of substances is a matter of importance not only at the sensitization but also at the challenge phase in a certain type of allergy including a basophil-mediated allergic inflammation.

Summary: Basophils, the least common granulocytes, represent only ~0.5% of peripheral blood leukocytes. Because of the small number and some similarity with mast cells, the functional significance of basophils remained questionable for a long time. Recent studies using newly‐developed analytical tools have revealed crucial and non‐redundant roles for basophils in various immune responses, particularly Th2 immunity including allergy and protective immunity against parasitic infections. In this review, we discuss the mechanisms how basophils mediate Th2 immune responses and the nature of basophil‐derived factors involved in them. Activated basophils release serine proteases, mouse mast cell protease 8 (mMCP‐8), and mMCP‐11, that are preferentially expressed by basophils rather than mast cells in spite of their names. These proteases elicit microvascular hyperpermeability and leukocyte infiltration in affected tissues, leading to inflammation. Basophil‐derived IL‐4 also contributes to eosinophil infiltration while it acts on tissue‐infiltrating inflammatory monocytes to promote their differentiation into M2 macrophages that in turn dampen inflammation. Although basophils produce little or no MHC class II (MHC‐II) proteins, they can acquire peptide‐MHC‐II complexes from dendritic cells via trogocytosis and present them together with IL‐4 to naive CD4 T cells, leading to Th2 cell differentiation. Thus, basophils contribute to Th2 immunity at various levels.

Key Points: Mice deficient for basophil tryptase mMCP-11 showed ameliorated IgE-mediated allergic inflammation with reduced leukocyte infiltration. This is the first demonstration that the basophil-derived protease plays a crucial role in allergic inflammation.

Recent studies demonstrated that basophils play crucial and non-redundant roles in the immune system, in spite of the fact that they are the rarest granulocytes and represent less than 1 % of peripheral blood leukocytes. In response to various stimuli, basophils release effector molecules stored in their cytoplasmic granules, including chemical mediators and proteases, and also secrete cytokines and chemokines. In this review, we will focus on the physiological and pathological roles of basophil-derived IL-4. Basophils can readily produce large quantities of IL-4 and are therefore the important source of IL-4. Basophil-derived IL-4 has been shown to regulate other immune cells, including T cells, B cells, group 2 innate lymphoid cells, monocytes, and macrophages. It also acts on non-hematopoietic cells such as fibroblasts and endothelial cells. Those cells stimulated with basophil-derived IL-4 contribute to the positive or negative regulation of a variety of immune responses in health and disease, including protection against parasitic and bacterial infections, allergy, and autoimmune diseases. Thus, basophil-derived IL-4 plays versatile roles in immunity.