INTRACELLULAR AND SURFACE DISTRIBUTION OF CD9 IN HUMAN EOSINOPHILS

Expression of CD9 is a feature of both eosinophils and platelets. We h ave investigated the CD9 expression on resting and activated eosinophi ls with regard to possibly interacting platelets. Mixed leukocytes wer e obtained from the platelet-containing (PC) and platelet-depleted (PD ) peripheral blood of healthy donors. A cell membrane permeabilization technique, the FOG method, enabled us to detect the eosinophils as a separate population and permitted flow cytometric analysis of both sur face and intracellular antigens. Monoclonal antibodies against CD61 we re used to identify platelets. The CD9/CD61 ratio indicated that CD9 o n resting eosinophils originates mainly from eosinophils and not from adhered platelets. No difference in CD9 expression was obtained betwee n resting eosinophils in PC and PD blood. However, the expression of C D9 was decreased (p<0.05) on eosinophils in PMA-activated PD blood but increased (p=0.001) in PC blood, probably due to interacting platelet s since CD61 increased simultaneously. In addition, we were able to de tect an intracellularly stored pool of CD9 in eosinophils which decrea sed after activation with PMA. Together these results indicate a trans location of intracellularly stored CD9 to the cell membrane upon activ ation, probably followed by a subsequent shedding.