A circulating suppressive factor of platelet cytotoxic functions after rush immunotherapy in Hymenoptera venom...

A receptor for the Fc fragment of IgE has been described on human blood platelets. This receptor mediated the IgE-dependent stimulation of platelets by the specific allergen in Hymenoptera venom (HV) hypersensitivity. This platelet reactivity was abolished after rush desensitization. To understand the mechanism of such a down-regulation, we studied the effects of sera recovered from patients treated by HV rush desensitization on platelets of untreated HV sensitive patients. The present data describe the presence of a circulating factor able to suppress, in a dose-dependent manner, platelet stimulation by the specific allergen. This circulating factor was not allergen specific, not depleted by IgE or IgG antibody immunoadsorption, and was found at higher concentrations in the sera of patients receiving monthly high dosages of HV (200 micrograms maintenance dose). The physicochemical characterization showed that this circulating factor had a m.w. of 20,000 to 25,000, an isoelectric point of 4.2, was heat and acid stable, and sensitive to trypsin, but not to neuraminidase. These characteristics were similar to a newly described lymphokine, platelet activity suppressive lymphokine, suggesting the intervention of such a lymphokine in HV rush desensitization.