Activated, not resting, platelets increase leukocyte rolling in murine skin utilizing a distinct set of adhesion molecules.

Selectin-mediated tethering and rolling initiates the multi-step process of leukocyte extravasation which is crucial for the formation of an inflammatory infiltrate. We studied the impact of platelets on this process in the skin. Using intravital microscopy, we analyzed platelet interactions with cutaneous post-capillary venules of mouse ears and observed an increase in platelet rolling if platelets were activated (41.6+/-20.2% vs. 13.1+/-8.5% rolling of resting platelets). Experiments with P-selectin deficient mice and antibodies blocking either P-selectin, GPIIb/IIIa or GPIb showed that rolling depends on platelet PSGL-1 and GPIIb/IIIa on one hand, and endothelial P-selectin on the other. Next, formation of platelet-leukocyte aggregates was demonstrated by simultaneous observation of platelets and leukocytes in vivo utilizing a newly developed two-color technique. Aggregates increased overall leukocyte rolling (leukocytes alone: 27.4+/-11.2%, leukocytes with resting platelets: 25.3+/-10.2%, leukocytes with activated platelets 38.1+/-11.8%). To investigate if activated platelets may contribute to the pathogenesis of chronic cutaneous inflammation, platelet P-selectin expression was studied in 8 patients with psoriasis. A correlation between platelet P-selectin expression and disease severity was established. In summary, we show that activated, not resting, platelets increase leukocyte rolling in murine skin. This increased rolling is due to the aggregate formation of platelets with leukocytes. We also provide evidence for a potential role of this mechanism in the pathogenesis of chronic inflammatory skin diseases.

Efomycine M, a new specific inhibitor of selectin, impairs leukocyte adhesion and alleviates cutaneous inflammation.

Specific interference with molecular mechanisms guiding tissue localization of leukocytes may be of great utility for selective immunosuppressive therapies. We have discovered and characterized efomycines, a new family of selective small-molecule inhibitors of selectin functions. Members of this family significantly inhibited leukocyte adhesion in vitro. Efomycine M, which was nontoxic and showed the most selective inhibitory effects on selectin-mediated leukocyte-endothelial adhesion in vitro, significantly diminished rolling in mouse ear venules in vivo as seen by intravital microscopy. In addition, efomycine M alleviated cutaneous inflammation in two complementary mouse models of psoriasis, one of the most common chronic inflammatory skin disorders. Molecular modeling demonstrated a spatial conformation of efomycines mimicking naturally occurring selectin ligands. Efomycine M might be efficacious in the treatment of human inflammatory disorders through a similar mechanism.