Mast cell and macrophage chemokines CXCL1/CXCL2 control the early stage of neutrophil recruitment during tissue inflammation

Neutrophil recruitment is an important early step in controlling tissue infections or injury. Here, we report that this influx depends on both tissue-resident mast cells and macrophages. Mice with mast cell deficiency recruit reduced numbers of neutrophils in the first few hours of intraperitoneal lipopolysaccharide (LPS) stimulation. Conversely, in mice with clodronate-ablated macrophages, neutrophils extravasate, but have limited ability to reach the peritoneal fluid. Tissue macrophages synthesize neutrophil chemoattractants CXCL1/CXCL2 (CXC chemokine ligands 1/2) in response to LPS. Mast cells also produce these chemokines of which a proportion are preformed in granules. Release of the granules and new CXCL1/CXCL2 synthesis is Toll-like receptor 4-dependent. Both in vivo studies with blocking monoclonal antibodies and in vitro chemotaxis experiments show the neutrophil response to mast cells and macrophages to be CXCL1/CXCL2-dependent. The data are in keeping with the model that mast cells, optimally positioned in close proximity to the vasculature, initiate an early phase of neutrophil recruitment by releasing the chemoattractants CXCL1/CXCL2. Having arrived within the stimulated tissue, neutrophils penetrate further in a macrophage-dependent manner. Therefore, we demonstrate a positive role for mast cells in tissue inflammation and define how this comes about with contribution from a second tissue cell, the macrophage.