The effect of in vitro activation and platelet interaction on the CD9 distribution and adhesion properties of human eosinophils

Objective and Design: The redistribution of CD9 in peripheral blood eosinophils was investigated with respect to the interaction with platelets during in vitro activation, and whether this interaction exerts influence on eosinophil adhesion properties. Materials and Methods: Flow cytometry was used to investigate the CD9 expression in purified eosinophils or eosinophils from different whole blood preparations, with or without platelets present. To confirm an eosinophil/ platelet interaction fluorescence microscopy was used, and to demonstrate release/shedding of CD9 molecules a biosensor technique was performed. Results: Our results show that both intracellular and surface expression of CD9 decrease upon in vitro activation in the absence of platelets, a phenomenon probably caused by release/shedding of soluble forms of CD9 and not due to intracellular degradation. Increased expression of CD9 on eosinophils, stimulated in the presence of platelets, is partly a result of interacting platelets, judged by the increase in platelet specific marker CD61. In our adhesion assay a significant increase in eosinophil adhesion properties to fibronectin was obtained when eosinophils were PMA stimulated and interacting with platelets, as compared to activated eosinophils without platelets. Conclusions: Our findings, that CD9 expression on eosinophils is dynamically regulated, support our previous suggestion that CD9 may be a useful activity marker and that platelet interaction acts on eosinophil adhesion.