Monocytes downregulate the early stage of collagen‐induced platelet activation by a PECAM‐1‐dependent mechanism

Background: Blood vessel damage results in exposure of the subendothelial matrix, to which platelets adhere. Monocytes are recruited and activated at the site of injury. Objectives: Here we studied the effect of monocytes on platelet activation induced by exposure to fibrillar collagen. Methods: Washed platelets and isolated monocytes (100/1) were coincubated with type I collagen in static adhesion conditions or in suspension. Platelet activation was assessed by measuring RANTES production and α‐granule secretion. Platelet adherence on immobilized collagen was analyzed by fluorescence confocal microscopy. Cell–cell contacts were prevented by incubating platelets and monocytes in transwell coculture dishes. Experiments were also performed in the presence of soluble recombinant platelet endothelial cell adhesion molecule‐1 (PECAM‐1) or of antibodies to PECAM‐1. Results: Unexpectedly, unstimulated monocytes limited the initial phase of platelet activation by fibrillar collagen. In adhesion conditions, monocytes reduced the secretion by platelets of the inflammatory chemokine RANTES and of β‐thromboglobulin and the formation of platelet aggregates. The inhibitory effect of monocytes on platelet activation required direct cell–cell contacts between platelets and monocytes. Monocytes also inhibited collagen‐induced platelet activation in suspension conditions as assessed by the reduction of P‐selectin exposure and RANTES secretion. A recovery of platelet responses was observed in the presence of soluble PECAM‐1 and of PECAM‐1.3 Fab, indicating that PECAM‐1 is involved in monocyte‐triggered downregulation of platelet reactivity. Conclusions: Our data provide the first evidence that unstimulated monocytes limit the initial phase of platelet activation by collagen via a mechanism that is, at least in part, PECAM‐1‐dependent.