The platelet physiology subcommittee meeting in Jerusalem was divided into two parts which ran approximately for two hours (part I) and two hours and 20 minutes (part II). The meeting was attended by over 200 scientists.

Part I: Methods, standards and important practical issues resulting from platelet-leukocyte interactions. The first speaker reviewed the role of P-selectin, expressed by activated platelets, in inducing tissue factor (TF) expression by monocytes. This induction could be observed, albeit at a slightly reduced level, with purified P-selectin as well as the protein when expressed by CHO cells. A cofactor is thought, by some groups, to be present with P-selectin on platelets to induce higher levels of TF expression. Platelet chemokines, as well as some of their receptors, were then reviewed by the second speaker who showed the importance of chemokines (CXC, CC, SC) at sites of aggregation, injury, inflammation and neovascularization. A third speaker described an assay to evaluate the effect, on platelet rich plasma (PRP), of supernatants obtained from platelet-neutrophil interactions.

Results show that neutrophils inhibit platelets recruitment and 5 HT release. Such an assay may be used in characterizing cell-cell interactions and detect new aspects of pre- thrombotic states. However, observations from this assay may not always parallel those seen in whole blood. Platelet-neutrophil interactions were also investigated by another speaker through the use of flow cytometry techniques. Such cell-cell interactions were observed to be greatly reduced by monoclonal antibodies directed against P-selectin and CD18 but not other adhesion molecules present on platelets or leukocytes. The fifth author observed that large number of platelets (85%) adhere to extracellular matrix proteins (ECM) (at a shear rate of 1600 s-1) compared to platelets (10-20%) adhering to HUVEC in a perfusion chamber. Neutrophils in such a set-up massively bound to adhering platelets. In such a system the authors quantitated rolling and adherence of leukocytes. The round table discussion showed that one of the components, possibly implicated with P-selectin in tissue factor expression by monocytes, was 12 HETE. The possibility of platelet GPIIb-IIIa, fibrinogen or fibrin being implicated, as well as CD11/CD18, in platelet-leukocyte interactions was discussed. The session showed the growing importance of platelet adhesion molecules, cytokines and metabolites in mediating platelet-leukocyte interactions. A large number of techniques are being set up to investigate such cell-cell interactions.

Part II: Cytoskeleton interaction in platelets. The first speaker reviewed platelet adhesion molecules and cytoskeleton interactions. Assay systems to study such molecules and cytoskeleton were described. Other speakers presented, using flow cytometry and/or electron microscopy, the effect of thrombin and other agonists on GPIb-IX movement. Techniques used involved the use of a panel of monoclonal antibodies directed against different components of GPIb-IX-V, polyclonal antibodies to different domains of GPIb alpha as well as the use of permeabilized platelets. Differences in results were observed between two different groups with one observing a down regulation and the other seeing no differences in GPIb-IX expression. Conceivably, differences could come as a result of assay conditions (such as calcium concentrations), type of polyclonal or monoclonal antibodies used, concentration of fixative. Standardization of techniques used in the different laboratories and exchange of results could clearly help in resolving key issues.

The platelet physiology subcommittee is strongly interested in establishing an Internet network to allow members to select topics for future meetings, set up a forum to discuss issues of interest, allow the exchange of probes, antibodies and techniques. Subcommittee members strongly recommend that the ISTH support an effort to establish an ISTH "home page" on the World Wide Web (WWW).