Eosinophil granulocyte proliferation induced by an intermediate factor generated in the pleural cavity of rats injected with platelet-activating factor-acether.

In previous research, we have observed that intrathoracic administration of platelet-activating factor-acether (PAF) promoted a delayed eosinophilia in the pleural cavity of rats that lasted for at least 96 h. We investigated the ability of pleural washings from rats previously injected with PAF (1 micrograms/cavity) to stimulate in vitro murine hematopoietic eosinophil proliferation. We observed that pleural fluid sustained eosinophil proliferation but not differentiation, under conditions in which PAF itself had no effect. The phenomenon lasted for 3 days and was maximal on the 1st day of culture. Treatment with neutralizing antibodies against interleukin (IL)-5, granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-3, alone or in combination, did not modify the eosinophil proliferation induced by PAF pleural fluid, suggesting that these cytokines may not be involved in the studied phenomenon. We conclude that the rat pleural fluid obtained 6 h after PAF administration induces eosinophil proliferation in vitro by a mechanism probably independent of IL-5, GM-CSF or IL-3.