Mast cells trap and cannibalize swarming neutrophils for metabolic independence

Neutrophils are activated and attracted to sites of anaphylaxis by degranulating tissue resident mast cells. After they facilitate inflammation and an anti-microbial immune response. However, the dynamics of neutrophils and direct interactions with mast cells are poorly investigated yet. By approaching 2-photon intravital microscopy and live cell imaging, we show that degranulating mast cells induce neutrophil swarming by releasing leukotriene B4 (LTB4). Attracted neutrophils interact with mast cells and surprisingly penetrate inside mast cells where they are stably trapped inside mast cell granules. We call this novel form of cell-in-cell structure “Mast Cell Intracellular Trap” (MIT). Trapped neutrophils and mast cells are both viable in the first 12 hours after MIT formation. However, neutrophils start to dye after 12hrs and translatome analysis of mast cells revealed improved mast cell metabolism and recovery. Therefore, neutrophils allow a faster and more efficient mast cell re-stimulation. Furthermore, mast cells get more resistant to extracellular nutrient limitations. Thus, beside their inflammatory role in anaphylaxis, neutrophils support mast cell recovery by forming a cell in cell structure. This might affect chronic inflammation and recurrent infections.