Role of mast cell-derived mediators during the resolution of inflammation
Authors:   , Friedel Joschua 
Published: February 2025

Background: Mast cells are leukocytes that are primarily known for their pro-inflammatory properties such as IgE-mediated allergic reactions. They possess a versatile arsenal of biologically active mediators that can be released in response to a specific stimulus over a wide temporal range. In addition to their pro-inflammatory functions, mast cells also have the potential to suppress an immune response and thereby promote resolution of inflammation. Here, our working group has already demonstrated that mast cells are located in an anti-inflammatory microenvironment during the Toll-like receptor 2-mediated inflammation and also produce mediators such as interleukin (IL)-4, prostaglandin (PG)E2, interferon (IFN)-β, which can beneficially influence the resolution of inflammation. However, the exact processes and mechanisms of mast cells, in particular interactions in vivo with other immune cells to promote the resolution of inflammation, have so far been poorly investigated.
Methods: We used the high-content imaging technology multi-epitope ligand cartography (MELC) to investigate local inflammation during the course of the zymosan-activated inflammatory process in mast cell-deficient mice. This form of high-content imaging allows us to determine the localization and phenotype of immune cell populations by simultaneously visualizing a high number of fluorescence-labeled antibodies on the same tissue section. Mast cell-deficient animals were pre-stimulated with IL-4 prior to zymosan-linked initiation of the Toll-like receptor 2-mediated inflammatory process to observe possible differences of immune cell phenotypes. Furthermore, MELC experiments with eosinophil-depleted mice were re-analyzed to determine the specificity of mast cell-produced IL-4. For further characterization of the different activation states of dendritic cells, isolated primary cells of paw tissue were treated overnight with the lactone antibiotic Brefeldin A. The blockage of the vesicle transport trapped synthesized cytokines and allowed to determine cytokine patterns, produced by dendritic cells using Fluorescence Activated Cell Sorting analyses.
Results: We demonstrated that the distance between the zymosan-containing core inflammatory region and the surrounding anti-inflammatory region, defined by M2-like macrophages, was increased in mast cell-deficient mice. Furthermore, absence of mast cells caused a dendritic cell phenotype, which was characterized by the absence of the key marker CD86 and an increased production of proinflammatory IL-12/23p40. Likewise, the non-activated dendritic cells in mast cell-deficient mice were located in greater distant to the pro-inflammatory core region than the activated dendritic cells in the control mice. Administration of IL-4 restored the maturation of dendritic cells in mast cell-deficient mice as well as their cytokine expression profile. However, the localization of dendritic cells was only partially restored by IL-4 stimulation. By further evaluating MELC-analysis eosinophil-depleted mice, we were able to show that the significant IL-4 effects were mast cell-specific.
Conclusion: This thesis shows that mast cells, despite their relatively small number, can influence the activation of dendritic cells through the production and release of IL-4 in vivo. Furthermore, it could be shown that the absence of mast cells significantly influences the regional immune cell architecture during TLR-2-mediated inflammation and thus possibly also impacts the course of the inflammatory process. Overall, these results support previously shown notions that mast cells could decisively influence the resolution of inflammation through the selective release of mediators. Based on these results, pro-resolving processes of mast cells should be investigated more extensively, which may prove to be quite complex due to a temporally mediated and concentration-dependent combination of biologically active mediators.