Abstract 229: Shear-Induced Mac-1 Cleavage Influences Neutrophil-Platelet Adhesion
INTRODUCTION. Under physiologic (i.e., non-inflamed) conditions, fluid flow-derived shear stresses serve to minimize the activity of neutrophils in the blood to ensure their efficient transit through the microcirculation. In addition to preventing pseudopod formation, shear exposure has also been shown to cleave CD18 integrins on neutrophils. In fact, CD18 integrins, particularly LFA-1 and Mac-1, play a dominant role in neutrophil adhesion to other cells such as platelets in the vascular flow field. We, therefore, tested the hypothesis that shear-induced CD18 cleavage impacts neutrophil-platelet binding.
METHODS. Mixed populations of leukocytes were exposed to constant shear fields of 2.5, 5, and 10 dyn/cm2 in a cone-plate viscometer for up to 10 min to characterize CD18 cleavage based on integrin subtypes. We also examined the role of CD18 cleavage in leukocyte-platelet binding using E64, a protease inhibitor previously shown to block CD18 proteolysis by shear.
RESULTS. Shear-induced CD18 cleavage was leukocyte-specific and, beyond threshold levels, was independent of flow exposure time and magnitude. Furthermore, for neutrophils and monocytes that express LFA-1 and Mac-1 on their surfaces, shear stress elicited cleavage of Mac-1, but not LFA-1. In contrast, shear cleaved LFA-1 on lymphocytes, cells that do not exhibit detectable surface levels of Mac-1. Moreover, neutrophils, but neither monocytes nor lymphocytes, pretreated with E64 and exposed to shear exhibited significantly (p < 0.05) increased platelet binding and CD18 surface levels, compared to untreated cells.
DISCUSSION. Shear-induced cleavage of CD18 is thus a complex process related to the differential expression of CD18 subtypes by different leukocyte subsets. Future work is needed to elucidate the underlying mechanism(s) involved. However, results of the present study further support that fluid shear stress mechanotransduction serves to negatively regulate the activation of neutrophils, the gatekeepers of the acute inflammatory processes. In light of the negative effects of leukocyte-platelet interactions on microvascular function, the results establish neutrophil responses to shear stress as critical mediators of blood homeostasis under physiologic conditions.
- © 2012 by American Heart Association, Inc.