Diesel exhaust particles impair platelet response to collagen and are associated with GPIbα shedding

Objective: Air pollution with fine particulates (PM₁₀ and PM_2.5) is associated with an increased incidence of cardiovascular events. The proposed mechanisms include indirect proinflammatory and procoagulant reactions involving activation of pulmonary macrophages, endothelial cells and the TNF/TF pathway, or direct procoagulant effects. Our laboratory has observed a reduction of the platelet responsiveness to collagen after exposure to diesel exhaust particles (DEP). Hypothesis: DEP directly interfere with platelet-collagen interactions by selectively inducing the shedding of platelet signaling receptors via metalloproteinases, which would represent a novel mechanism for DEP action on platelets. Methods: Citrated blood from healthy volunteers was exposed to highly standardized DEP at concentrations of 0.1, 2.5 and 5.0 μg/ml or ultrafine carbon black (ufCB, 0.1 μg/ml) and the plasmatic and platelet response was analysed. The closure times with the PFA-100 device and the platelet aggregation in response to a variety of agonists were monitored. Interleukins (IL)-1β and IL-8 levels were determined by ELISA and soluble P-selectin by the Luminex bead assay. Thrombin activity was measured as the endogenous thrombin potential (ETP) by fluorescence spectrometry. Soluble GPVI and GPIbα (glycocalicin) ectodomain fragments were measured by ELISA. ADAMTS13 activity was determined by a FRETS based assay and plasmin activity with Spectrozyme PL. Results: Aggregation assays where platelets were treated with low dose DEP or ultrafine carbon black (ufCB) revealed a significantly increased response to low doses of collagen (p< 0.05, n= 5). At higher doses, however, collagen induced aggregation was suppressed by DEP treatment: at 2.5 μg/ml, the inhibition was 34 ± 12% (p< 0.01, n= 10). Aggregations with cross-linked collagen related peptide (CRPxl), convulxin and with the monoclonal antibody 9O12.2 (all known to specifically bind to and activate GPVI) were also diminished. Ristocetin, arachidonic acid and ADP responses were normal at all DEP concentrations. No cleavage of GPVI ectodomain was detected (soluble GPVI 27.8 ± 3 vs. 28 ± 4 μg/ml mean ± SEM, n= 10); however increased plasma glycocalicin (GPIbα ectodomain) was detected upon diesel exposure (2.58 ± 0.11 vs. 2.28 ± 0.03 μg/ml p< 0.01, n= 10). ADAMTS13 and plasmin activity remained unaffected by DEP under the conditions tested. Platelets were not activated by either DEP or ufCB as soluble P-selectin was insensitive to these. Conclusions: DEP specifically and directly interferes with platelet-collagen interactions. The functional consequences are biphasic and include enhance platelet aggregation at lower DEP concentrations and inhibition at a higher dose. Our data indicate that this interaction does not involve P-selectin or GPVI shedding. It is however associated with an increase in GPIb cleavage.