Mechanism of leukocyte polarity and crawling. Upon firm adhesion, neutrophils flatten and adopt a polarized shape with a cell front enriched in polymerized F-actin protrusions (red) and a rear end (or back) enriched in actomyosin contractile filaments (green). This asymmetric shape is controlled by 2 major signaling networks. One is activated at the cell front and involves high levels of PI3K-PIP3 and subsequent Rac1/2-mediated actin polymerization formation. Another is activated at the cell rear and involves phosphatase and tensin homolog (PTEN)–mediated PIP2 and RhoA-driven actomyosin contraction. In addition, Myh9 and MKL1 activity are necessary for proper regulation of F-actin and actomyosin reorganization. Cdc42 signaling, while localized at the cell front, controls the uropod via WASp, which in turn enhances CD11b clustering and RhoA signaling. Lastly, invasive protrusions, which are promoted by Src and Akt signaling but limited by Rap1b, develop at the ventral and lateral part of the leukocyte to scan for a permissive site of transmigration. During polarization, PSGL-1 accumulates at the tip of the uropod and scans for activated platelets in order to maintain neutrophil polarity and crawling properties. In ECs, CD2AP limits the ICAM-1 adhesome. pMLC, phosphorylation of the myosin light chain; ROCK, Rho-associated protein kinase.