Characteristics of CR3-mediated aggregation in human eosinophils: effect of priming by platelet-activating factor.

We have used double-color fluorescence-activated cell sorter analysis to characterize the homotypic aggregation response of human eosinophils (EOs). With this method, we demonstrate for the first time that EOs are able to form stable aggregates. The aggregation response induced by the phorbol ester, phorbol myristate acetate (PMA), was low and was not primed by platelet-activating factor (PAF). In contrast, the aggregation response induced by opsonized particles was markedly enhanced after priming with PAF. Additional experiments with several blocking monoclonal antibodies indicate that the CR3 receptor present on human EOs mediates the homotypic aggregation induced by PMA and by small opsonized particles through a putative "cell-adhesion" site on CR3, which binds to its counter structure on the opposing cell. The signal that initiates this binding event is generated after PMA addition or activation of the iC3b-binding site and is not sensitive for priming by PAF. The priming by PAF of the aggregation response induced by opsonized particles is restricted to an action on the iC3b-binding site, possibly by enhancing the affinity for its ligand.