Platelet augmentation of IgE-dependent histamine release from human basophils and mast cells

A factor(s) from human platelets enhances IgE-mediated histamine release from human basophils and mast cells. This effect is directly related to the platelet number; at physiological platelet/leukocyte ratios (40:1), the enhancement was 66 ±11 %. Platelet stimulation by thrombin more than doubled the enhancement, to 172 ± 10% at 40:1. Mast cell release was also enhanced by platelets although the magnitude was more limited (86 ± 13% at 40:1 with thrombin). Direct basophil/platelet contact was unnecessary in that platelet supernat- ants were fully active; a direct platelet factor/basophil interaction is suggested, however, by the fact that basophils purified 100-fold with respect to other leukocytes were enhanced by the platelet factors. The appearance of platelet-enhancing activity is associated with the release of an alpha-granule marker (PF₄) rather than with products of arachidonic acid metabolism (thromboxane B₂). The platelet factor(s) responsible for these effects are not dialyzable, are heat stable and do not appear to be identical to PF₄ or platelet-derived growth factor (PDGF). Since anti-IgE-stimulated basophils cause PF₄ release and this correlates with the release of enhancing factor, we suggest that a pro-inflammatory feed forward relationship exists. Together with our previous data showing that platelets are activated in vivo during antigen challenge of allergic asthmatic subjects, these results suggest that platelets may be important in modulating IgE-mediated allergic reactions in man.