Shear stress-induced apoptosis of adherent neutrophils: A mechanism for persistence of cardiovascular device infections

The mechanisms underlying problematic cardiovascular device-associated infe ctions are not understood. Because the outcome of the acute response to inf ection is largely dependent on the function of neutrophils, the persistence of these infections suggests that neutrophil function may be compromised b ecause of cellular responses to shear stress. A rotating disk system was us ed to generate physiologically relevant shear stress levels (0-18 dynes/ cm (2); 1 dyne = 10 mu N) at the surface of a polyetherurethane urea film. We demonstrate that shear stress diminishes phagocytic ability in neutrophils adherent to a cardiovascular device material, and causes morphological and biochemical alterations that are consistent with those described for apopto sis. Complete neutrophil apoptosis occurred at shear stress levels above 6 dynes/cm(2) after only 1 h. Morphologically, these cells displayed irrevers ible cytoplasmic and nuclear condensation while maintaining intact membrane s. Analysis of neutrophil area and filamentous actin content demonstrated c oncomitant decreases in both cell area and actin content with increasing le vels of shear stress. Neutrophil phagocytosis of adherent bacteria decrease d with increasing shear stress. Biochemical alterations included membrane p hosphatidylserine exposure and DNA fragmentation, as evaluated by in situ a nnexin V and terminal deoxynucleotidyltransferase-mediated dUTP end labelin g (TUNEL) assays, respectively. The potency of the shear-stress effect was emphasized by comparative inductive studies with adherent neutrophils under static conditions, The combination of tumor necrosis factor-or and cyclohe ximide was ineffective in inducing >21% apoptosis after 3 h. These findings suggest a mechanism through which shear stress plays an important role in the development of bacterial infections at the sites of cardiovascular devi ce implantation.