ABSTRACT 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 -/- , functional S100A8/A9 -/- ) caused dysregulated Ca 2+ signatures in activated neutrophils resulting in reduced Ca 2+ availability at the formed LFA-1/F-actin clusters with defective β 2 integrin outside-in signaling during post-arrest modifications. Consequently, we observed impaired cytoskeletal rearrangement, cell polarization and spreading, as well as cell protrusion formation in Mrp14 -/- compared to WT neutrophils, making Mrp14 -/- cells more susceptible to detach under flow, thereby preventing efficient neutrophil recruitment and extravasation into inflamed tissue. One-sentence summary: intracellular S100A8/A9 is indispensable for firm leukocyte adhesion under flow

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