The plasma concentration of soluble adhesion receptors is increased under pathological circumstances, but their function remains enigmatic. Soluble P-selectin (sP-sel) is shed from activated platelets and endothelial cells. Mice genetically engineered to express P-selectin without the cytoplasmic tail (⌬CT) constitutively show a 3-to 4-fold increase of sP-sel in plasma. We observed that the ⌬CT mice formed fibrin very readily. In an ex vivo perfusion chamber, there was more fibrin deposited at the site of platelet thrombus formation than in wild type (WT), whereas no fibrin deposits were detected using P-selectin-deficient blood during the same interval. Similarly, in vivo, the hemorrhage produced by local Shwartzman reaction was smaller in the ⌬CT mice than in WT. In contrast, we previously showed hemorrhage to be more prominent in P-selectin knock-out mice. Infusion of mouse P-sel-Ig chimera produced the same protective effect in WT mice as seen in the ⌬CT mice, indicating that the effect was due to increased levels of sP-sel. Mice infused with P-sel-Ig showed significantly more fibrin deposited on the luminal face of the injured vessels than control mice. Plasma from ⌬CT mice or mice infused with P-sel-Ig contained higher concentration of pro-coagulant microparticles and clotted one minute faster than WT. This pro-coagulant phenotype of ⌬CT mice could be reversed by a 4-day treatment with PSGL-Ig, a P-selectin inhibitor. We propose that sP-sel should no longer be considered only as a marker of inflammation or platelet activation, but also as a direct inducer of pro-coagulant activity associated with vascular and thrombotic diseases. P-selectin is a member of the selectin family localized in the membranes of ␣-granules of platelets and the Weibel-Palade bodies of endothelial cells (1). A soluble form of P-selectin can be found in the plasma as a circulating protein (2). In vivo, two main physiological roles are attributed to the integral membrane form of P-selectin. First, in inflammation, P-selectin is redistributed onto the surface of activated endothelial cells where it mediates the rolling of leukocytes (3). Second, in thrombosis, P-selectin expressed on activated platelets present in a thrombus supports the recruitment of leukocytes (4). Soluble P-selectin (sP-sel) of healthy individuals has been suggested to originate from the alternatively spliced form found in endothelial cells and platelets (5). Alternatively, elevated levels of sP-sel may reflect platelet activation (6) because P-selectin is proteolytically shed from the plasma membrane in vivo shortly after activation (7,8). Therefore, plasma levels of sP-sel have been considered a useful tool to predict thrombotic consumptive platelet disorders (9-12), but they can also reflect endothelial cell activation (13,14). Although the circulating form of P-selectin is potentially active because only the lectin and epidermal growth factor (EGF) domains are required to bind its receptor, P-selectin glycoprotein ligand-1 (PSGL-1) (15), the biological role of ...

P-selectin is an adhesion receptor for leukocytes expressed by activated platelets and endothelial cells. To assess a possible role of P-selectin in platelet clearance, we adapted an in vivo biotinylation technique in mice. Wild-type and P-selectin–deficient mice were infused with N-hydroxysuccinimido biotin. The survival of biotinylated platelets was followed by flow cytometry after labeling with fluorescent streptavidin. Both wild-type and P-selectin–deficient platelets presented identical life spans of about 4.7 days, suggesting that P-selectin does not play a role in platelet turnover. When biotinylated platelets were isolated, activated with thrombin, and reinjected into mice, the rate of platelet clearance was unchanged. In contrast, storage of platelets at 4°C caused a significant reduction in their life span in vivo but again no significant differences were observed between the two genotypes. The infused thrombin-activated platelets rapidly lost their surface P-selectin in circulation, and this loss was accompanied by the simultaneous appearance of a 100-kD P-selectin fragment in the plasma. This observation suggests that the platelet membrane P-selectin was shed by cleavage. In conclusion, this study shows that P-selectin, despite its binding to leukocytes, does not mediate platelet clearance. However, the generation of a soluble form of P-selectin on platelet activation may have biological implications in modulating leukocyte recruitment or thrombus growth.

P-selectin is an adhesion receptor for leukocytes expressed on activated platelets and endothelial cells. The cytoplasmic domain of P-selectin was shown in vitro to contain signals required for both the sorting of this protein into storage granules and its internalization from the plasma membrane. To evaluate in vivo the role of the regulated secretion of P-selectin, we have generated a mouse that expresses P-selectin lacking the cytoplasmic domain (ΔCT mice). The deletion did not affect the sorting of P-selectin into α-granules of platelets but severely compromised the storage of P-selectin in endothelial cells. Unstored P-selectin was proteolytically shed from the plasma membrane, resulting in increased levels of soluble P-selectin in the plasma. The ΔCT–P-selectin appeared capable of mediating cell adhesion as it supported leukocyte rolling in the mutant mice. However, a secretagogue failed to upregulate leukocyte rolling in the ΔCT mice, indicating an absence of a releasable storage pool of P-selectin in the endothelium. Furthermore, the neutrophil influx into the inflamed peritoneum was only 30% of the wild-type level 2 h after stimulation. Our results suggest that different sorting mechanisms for P-selectin are used in platelets and endothelial cells and that the storage pool of P-selectin in endothelial cells is functionally important during early stages of inflammation.