Cytosolic S100A8/A9 promotes Ca²⁺supply at LFA-1 adhesion clusters during neutrophil recruitment

S100A8/A9 is an endogenous alarmin secreted by myeloid cells during many acute and chronic inflammatory disorders. Despite increasing evidence of the proinflammatory effects of extracellular S100A8/A9, little is known about its intracellular function. Here, we show that cytosolic S100A8/A9 is indispensable for neutrophil post-arrest modifications during outside-in signaling under flow conditions in vitro and neutrophil recruitment in vivo, independent of its extracellular functions. Mechanistically, genetic deletion of S100A9 in mice (Mrp14^−/−, functionalS100a8/a9^−/−) caused dysregulated Ca²⁺signatures in activated neutrophils resulting in reduced Ca²⁺availability at the formed LFA-1/F-actin clusters with defective β₂integrin outside-in signaling during post-arrest modifications. Consequently, we observed impaired cytoskeletal rearrangement, cell polarization and spreading, as well as cell protrusion formation inMrp14^−/−compared to WT neutrophils, makingMrp14^−/−cells more susceptible to detach under flow, thereby preventing efficient neutrophil recruitment and extravasation into inflamed tissue.