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IgG antibodies and eosinophils kill schistosomula of Schistosoma mansoni in vitro, and there is now evidence to suggest that the main factor that contributes to the expression of purified IgG effector function is the degree of activation of the donor's eosinophils. This study was designed to identify serum-derived activating factors in sera from individuals infected with S. mansoni. Such activating factors may be responsible for enhancing eosinophil cytotoxicity against schistosomula. Serum-borne mediators were prepared by fractionation of sera from infected individuals by gel filtration high-performance liquid chromatography. The eosinophil-stimulating activity of these mediators was assayed by a new method which depends on the increased expression of the CR3 alpha chain (CD11b) on the surface of activated eosinophils. Sera from infected individuals exhibited different levels of eosinophil activation, and activation appeared to be due to several serum factors, including interleukin 5. In conclusion, our results suggest that eosinophil-activating factors present in infection sera may not only be responsible for enhancing eosinophil cytotoxicity but also be necessary for its expression.

Monocytes from moderately eosinophilic individuals secrete material that enhances the cytotoxic activity of eosinophils against antibody-coated schistosomula of Schistosoma mansoni. This material is not a single substance, but can be fractionated into several active components of different size and different charge. Gel filtration of mononuclear cell supernatants separated the eosinophil-activating activity into a major component of molecular mass of 40 kDa and a minor component of molecular mass of less than 10 kDa. The major component exhibited further heterogeneity on fractionation by high performance liquid chromatography. The bulk of the eosinophil-activating activity could be separated from both colony-stimulating factor (CSF) alpha activity and from tumor necrosis factor (TNF) activity. However, human recombinant CSF alpha (GM-CSF), human recombinant TNF and rabbit tumor necrosis serum all had eosinophil-activating activity when tested against schistosomula. Eosinophils were not activated by interleukin 1, interleukin 2, interferon-alpha, lipopolysaccharide or phorbol myristate acetate.

A protein (eosinophil-activating factor, EAF), which enhances the capacity of human peripheral blood eosinophils to kill antibody-coated schistosomula of Schistosoma mansoni, has been partially purified from supernatants of cultured peripheral blood mononuclear cells by sequential chromatography on Sephacryl S200 and DEAE-cellulose. This protein is acidic with a molecular mass on gel filtration of 40 +/- 7 kDa. It not only enhances the activity of eosinophils against schistosomula but also increases their ability to lyse antibody-coated, herpes simplex virus-infected Chang liver cells. It enhances the production of superoxide and hydrogen peroxide by eosinophils that occurs both spontaneously and in response to opsonized zymosan. However, increased respiratory burst activity does not appear to be responsible for the enhancement of eosinophil-mediated killing of schistosomula, since a comparable or greater increase in hydrogen peroxide production is induced by column fractions that have little or no effect on schistosomulum killing. EAF enhances eosinophil degranulation, both spontaneously and after incubation with opsonized zymosan. Enhanced degranulation is associated with release of eosinophil peroxidase and eosinophil cationic protein. These findings suggest that EAF enhances the capacity of eosinophils to kill parasites by increasing the extent of eosinophil degranulation and the amount of toxic granule proteins that are secreted.