Role of the adhesion molecule CD99 in platelet–neutrophil interactions

The interaction of platelets with neutrophils plays an important role in inflammation and thrombosis and is coordinated by multiple adhesive interactions. The adhesion molecule CD99 is a key mediator of neutrophil migration across the endothelium but whether it is involved in platelet–neutrophil adhesive interactions has not previously been addressed. We found that platelet CD99 is predominantly localized on the cell surface and is not shed following platelet activation. Blocking of either platelet or neutrophil CD99 significantly diminished neutrophil migration across surface‐adherent activated platelets in a quantitatively equivalent manner. In contrast, the blocking of CD99 affected neither neutrophil adhesion to surface‐adherent activated platelets nor formation of circulating platelet–neutrophil conjugates. Thus, homophilic CD99 interaction mediates neutrophil transplatelet migration but is not involved or is redundant in neutrophil adhesion to surface‐adherent or circulating platelets.