Platelet Aggregation in Flowing Blood at a Site of Injury to an Endothelial Cell Monolayer: Quantitation and Real-Time Imaging with the TAB Monoclonal Antibody

Abstract

Epifluorescence videomicroscopy permits real-time imaging of platelet adhesion/aggregation to a defined microinjury of a monolayer of endothelial cells exposed to flowing blood. The fluorescent label is the TAB murine monoclonal antibody directed against human platelet GP IIB, together with a fluorescein-conjugated goat F(ab')2 against murine immunoglobulin. The combination assures specificity for platelet membranes, yet leaves platelet function intact. TAB is first added to gently mixed, citrated human blood; the second antibody is added 1 hour after the first, mixing continuing for a second hour. Bovine aortic endothelial cell monolayers (ECMs), grown on rectangular cover glasses precoated with microfibrillar collagen, comprise one wall of a flow chamber mounted on a vertical microscope stage. A loop of 6-0 sterile suture is drawn across the ECM in order to create microinjuries of width 70 +/- 15 microns (mean +/- SD) oriented in a direction transverse to flow. Platelet adhesion/aggregation is virtually absent on intact and confluent regions of the monolayer. On micro-injury sites and at shear rates of 60 to 1,080 sec-1, however, computer-enhanced images obtained by means of videomicroscopy show arrival and adherence of single platelets resulting in the formation of platelet aggregates elongated in the flow direction. When the monolayers are pretreated with 1.0 mmol/L lysine acetylsalicylate, the mean aggregate thickness increases (2P less than .05) to 260 +/- 58% (mean +/- SE, N = 6) of control, aggregates are regularly shed downstream, and the surface area of the injury site covered by platelets is augmented (2P less than .05) from 14.8 +/- 3.9% to 49.2 +/- 4.7% (mean +/- SE, N = 6). Donor ingestion of aspirin, on the other hand, leads to an increase (2P less than .01) in percent surface coverage to 42.7 +/- 8.5 without a concomitant increase in mean aggregate thickness. In parallel with the above, outflow levels of serum thromboxane and prostacyclin are measured by radioimmunoassays (RIAs) for thromboxane B2 and 6-Keto-PGF1 alpha, respectively. Thromboxane B2 is increased (2P less than .01) by monolayer pretreatment with lysine acetylsalicylate from 5.08 +/- 1.47 to 9.35 +/- 2.42, but decreased (2P less than .05) after oral aspirin to 1.21 +/- 0.38 ng/mL (mean +/- SE, N = 6). Levels of 6-Keto-PGF1 alpha were reduced (2P less than .05) by monolayer pretreatment from 0.48 +/- 0.046 to 0.36 +/- 0.016 ng/mL. Platelet adhesion/aggregation at a site of injury to an endothelial cell monolayer, therefore, can be imaged in flowing blood in real time using a monoclonal antibody approach.(ABSTRACT TRUNCATED AT 400 WORDS)