Mast cells (MCs) accumulate in chronic inflammatory sites; however, it is not clear which adhesion molecules are involved in this process. Recently, the expression of Notch ligands was reported to be upregulated in inflammatory sites. Although Notch receptors are known as signaling molecules that can activate integrins, their contributions to the adhesion of MCs have not been studied. In this study, we demonstrated that mouse MCs efficiently adhered to stromal cells forced to express a Notch ligand, Delta-like 1 (Dll1). Surprisingly, the adhesion was a consequence of direct cell–cell interaction between MCs and Dll1-expressing stromal cells rather than activation of downstream effectors of Notch receptor(s)-Dll1. The adhesion of MCs to Dll1-expressing stromal cells remained even when the cell metabolism was arrested. The recognition was blocked only by inhibition of Notch receptor(s)–Dll1 interaction by addition of soluble DLL1, or mAbs against Dll1 or Notch2. Taken together, these results indicate that Notch receptor(s) and Dll1 directly promote the adhesion of MCs to stromal cells by acting as adhesion molecules. This appreciation that Notch receptor–ligand interactions have an adhesion function will provide an important clue to molecular basis of accumulation of MCs to inflammatory sites.

Notch family members are generally recognized as signaling molecules that control various cellular responses in metazoan organisms. Early fly studies and our mammalian studies demonstrated that Notch family members are also cell adhesion molecules; however, information on the physiological roles of this function and its origin is limited. In this review, we discuss the potential present and ancestral roles of Notch-mediated cell adhesion in order to explore its origin and the initial roles of Notch family members dating back to metazoan evolution. We hypothesize that Notch family members may have initially emerged as cell adhesion molecules in order to mediate multicellularity in the last common ancestor of metazoan organisms.

Notch family members were first identified as cell adhesion molecules by cell aggregation assays in Drosophila studies. However, they are generally recognized as signaling molecules, and it was unclear if their adhesion function was restricted to Drosophila. We previously demonstrated that a mouse Notch ligand, Delta-like 1 (Dll1) functioned as a cell adhesion molecule. We here investigated whether this adhesion function was conserved in the diversified mammalian Notch ligands consisted of two families, Delta-like (Dll1, Dll3 and Dll4) and Jagged (Jag1 and Jag2). The forced expression of mouse Dll1, Dll4, Jag1, and Jag2, but not Dll3, on stromal cells induced the rapid and enhanced adhesion of cultured mast cells (MCs). This was attributed to the binding of Notch1 and Notch2 on MCs to each Notch ligand on the stromal cells themselves, and not the activation of Notch signaling. Notch receptor-ligand binding strongly supported the tethering of MCs to stromal cells, the first step of cell adhesion. However, the Jag2-mediated adhesion of MCs was weaker and unlike other ligands appeared to require additional factor(s) in addition to the receptor-ligand binding. Taken together, these results demonstrated that the function of cell adhesion was conserved in mammalian as well as Drosophila Notch family members. Since Notch receptor-ligand interaction plays important roles in a broad spectrum of biological processes ranging from embryogenesis to disorders, our finding will provide a new perspective on these issues from the aspect of cell adhesion.

Allergic conditions result in the increase of immunoglobulin (Ig)E-producing plasma cells (IgE-PCs); however, it is unclear how IgE production is qualitatively controlled. In this study, we found that IgE-PCs in spleen of immunized mice formed homotypic cell aggregates. By employing IgE-producing hybridomas (IgE-hybridomas) as a model of IgE-PCs, we showed that these cells formed aggregates in the presence of specific antigens (Ags). The formation of the Ag-induced cell aggregation involved secreted IgE and Fcγ receptor (FcγR)II/FcγRIII, but not FcεRs. Ag-induced cell aggregation plus lipopolysaccharide signaling resulted in an enhancement of IgE production in aggregated IgE-hybridomas. Furthermore, the administration of anti-FcγRII/FcγRIII antagonistic monoclonal antibody to immunized mice tended to reduce the splenic IgE-PC aggregation as well as the serum IgE levels. Taken together, our results suggested that Ag-IgE complexes induced IgE-PCs aggregation via FcγRII/FcγRIII, leading to the enhancement of IgE production. These findings suggest the presence of a novel mechanism for regulation of IgE production.

Under homeostatic conditions, skin DCs migrate to regional LNs transporting self-antigens (self-Ags). The transport of self-Ags is considered to be critical for maintaining peripheral tolerance. Although the chemokine receptor CCR7 potently induces the migration of skin DCs to regional LNs, Ccr7^−/− (Ccr7-KO) mice do not show skin auto-immune diseases. To resolve this inconsistency, we examined Ccr7-KO epidermis- or dermis-hyperpigmented transgenic (Tg) mice, in which the transport of skin self-Ags is traceable by melanin granules (MGs). Under CCR7-deficient conditions, the transport of epidermal MGs to regional LNs was impaired at 7 weeks of age. However, epidermal MGs could be transported when they had accumulated in the dermis. Ccr7-KO-dermis-pigmented Tg mice confirmed the presence of CCR7-independent transport from the dermis. Compared with WT-dermis-pigmented Tg mice, the amount of transported melanin and number of MG-laden CD11c⁺ cells were both approximately 40% of the WT levels, while the number of MG-laden CD205⁺ or CD207⁺ cells decreased to about 10% in skin regional LNs of Ccr7-KO-dermis-pigmented Tg mice. Cell sorting highlighted the involvement of CD11c⁺ cells in the CCR7-independent transport. Here, we show that CCR7-independent transport of skin self-Ags occurs in the dermis. This system might contribute to the continuous transport of self-Ags, and maintain peripheral tolerance.