Mannan activates tissue native and IgE-sensitized mast cells to proinflammatory response and chemotaxis in TLR4-dependent manner.

Mast cells take part in host defense against microorganisms as they are numerous at the portal of infection, exert several essential mechanisms of pathogen destruction, and they express pattern recognition receptors. Accumulating evidence indicates that these cells are involved in the control and clearance of bacterial, viral, or parasitic infections, but much less is known about their contribution in defense against fungi. The study was aimed to establish whether mannan, which comprises an outermost layer and major structural constituent of the fungal cell wall, may directly stimulate tissue mast cells to the antifungal response. Our findings indicate that mannan activates mast cells isolated from the rat peritoneal cavity to initiate the proinflammatory response. We found that mannan stimulates mast cells to release histamine and to generate cysteinyl leukotrienes, cytokines (IFN-γ, GM-CSF, TNF), and chemokines (CCL2, CCL3). It also increased the mRNA expression of various cytokines/chemokines. We also documented that mannan strongly activates mast cells to generate reactive oxygen species and serves as a potent chemoattractant for these cells. Furthermore, we established that mannan-induced activity of mast cells is mediated via TLR4 with the involvement of the spleen tyrosine kinase molecule. Taking together, our results clearly support the idea that mast cells act as sentinel cells and crucially determine the course of the immune response during fungal infection. Additionally, presented data on IgE-coated mast cells suggest that exposure to fungal mannan could influence the severity of IgE-dependent diseases, including allergic ones. ©2020 Society for Leukocyte Biology.