HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Expression, activation, and function of integrin M 2 (Mac-1) on neutrophil-derived microparticles

Leukocyte-derived microparticles (MPs) are markers of cardiovascular diseases and contribute to pathogenesis by their interaction with various cell types. The presence and activation state of a multifunctional leukocyte receptor, integrin M 2 (CD11b/18), on MPs derived from human neutrophils (PMNs) were examined. M 2 expression was significantly enhanced on MPs derived from stimulated compared with resting PMNs. Furthermore, M 2 on MPs from stimulated but not resting PMNs was in an activated conformation because it was capable of binding activation-specific monoclonal antibodies (CBRM1/5 and mAb24) and soluble fibrinogen. MPs expressing active M 2 interacted with and were potent activators of resting platelets as assessed by induction of P-selectin expression and activation of IIb 3. With the use of function-blocking antibodies and MPs obtained from M / -deficient mice, we found that engagement of GPIb on platelets by M 2 on MPs plays a pivotal role in MP binding. Platelet activation by MPs occurs by a pathway dependent on Akt phosphorylation. PSGL-1/P-selectin interaction also is involved in the conjugation of MPs to platelets, and the combination of blocking reagents to both M 2/GPIb and to PSGL-1/P-selectin completely abrogates MP-induced platelet activation. Thus, cooperation of these 2 receptor/ counterreceptor systems regulates the prothrombotic properties of PMN-derived MPs. (Blood. 2008;112:2327-2335)