Actin polymerization is required for leukocyte resistance to fluid flow–induced detachment and deformation. (a and b) Accumulation assays performed with U937 cells infused at 2 dynes/cm² into a parallel plate flow chamber. After 2 min, the number of adherent cells per high power field was determined (n = 8 per treatment group; two independent experiments (IEs); *, P < 0.001 between indicated groups or relative to the vehicle-treated group). (c) Flow chamber detachment assays with U937 cells or human peripheral blood monocytes pretreated with vehicle, LatB, or CytoD. Leukocytes suspended in an Mn²⁺-containing buffer were adhered to a VCAM-1–coated surface, and the number of cells remaining adherent after progressively increasing fluid flow was determined (n = 10 per treatment group; three IEs; *, P < 0.001 relative to the vehicle-treated group). (d) Phase-contrast video microscopy images of representative vehicle or CytoD-pretreated U937 cells adhered to VCAM-1 under static conditions and then exposed to fluid flow (2 dynes/cm²). The time relative to the initial image is indicated. Arrowheads point to elongated structures formed upstream of the CytoD-pretreated cell body. (e and f) Representative SEM images of leukocytes adhered and exposed to fluid flow (2 and then 4 dynes/cm², each for 60 s) and perfusion fixed in situ (three IEs). Arrowheads point to elongated structures upstream of the cell body and thin long structures without associated cell bodies. (d–f) Arrows indicate the direction of fluid flow. Error bars show means ± SD.