FORMATION OF PLATELET-LEUKOCYTE CONJUGATES IN WHOLE-BLOOD

The purpose of this investigation was to obtain information on platele t-leukocyte conjugate formation in whole blood and on factors that aff ect it, We also measured platelet and leukocyte activation by quantita ting the expression of CD62P and CD11b, In both cases a now cytometric approach was used, The results show that platelet-monocyte and platel et-polymorphonuclear leukocyte (PMNL) conjugate formation is enhanced by simply stirring blood, with optimum conjugate formation occurring a fter 10 min, In the case of monocytes, conjugate formation was enhance d by adenosine diphosphate (ADP), Both monocyte and PMNL conjugate for mation was enhanced by phorbol myristate acetate (PMA), but L-formyl m ethionyl lysyl proline (FMLP) was either without effect (monocytes) or inhibitory (PMNL). EDTA also inhibited conjugate formation (implying involvement of divalent cations), as did dextran sulphate (implying in volvement of P-selectin = CD62P), Interestingly GR144053F, which acts at GpIIb-IIIa on platelets to interfere with fibrinogen binding, and a lso glycyl prolyl arginyl proline (GPRP), a peptide that interferes wi th the interaction between CD11c on leukocytes and fibrinogen, did not inhibit platelet-monocyte conjugate formation, but did inhibit the pl atelet-PMNL interaction; this indicates that GpIIb-IIIa on platelets a nd CD11c on leukocytes and fibrinogen are involved in mediating the in teraction between platelets and PMNL but not platelets and monocytes, Surprisingly arginyl glycyl aspartyl serine (RGDS) inhibited the forma tion of both types of conjugate but this may be because it also inhibi ted both platelet and leukocyte activation as measured by CD62P and CD 11b exposure and/or interfers with the binding of adhesion molecules o ther than fibrinogen, The results show that a flow cytometric procedur e can be effective in obtaining rapid information on platelet-leukocyt e conjugate formation in whole blood and on factors that are involved in its regulation, It is suggested that the technique may be applicabl e to the study of platelet-leukocyte conjugate formation in whole bloo d in disease, and also to study the effects of drugs interfering with conjugate formation.