Allergic asthma is characterized by pulmonary infiltration and accumulation of eosinophils, which is enhanced by granulocyte-macrophage colony-stimulating factor (GM-CSF). T cells, fibroblasts, and eosinophils themselves produce GM-CSF, suggesting it functions in the lung microenvironment as a survival factor. However, the amounts and the mechanism by which GM-CSF supports eosinophil survival remain poorly understood. We have previously reported that human peripheral blood eosinophils (PBEo) can be transfected with GM-CSF mRNA using particle-mediated gene transfer (PMGT). Using this technology, GM-CSF mRNA was introduced into resting PBEo, and GM-CSF production and cell survival were assessed. GM-CSF protein was undetectable (< 1 pg/ml) in the supernatant but present intracellularly at very low levels. Unexpectedly, the in vitro survival of transfected PBEo was 4-fold greater than that of controls. Neutralizing anti-GM-CSF but not anti-interleukin-5 (anti-IL-5) antibody added up to 24 h after transfection abolished enhanced survival, demonstrating that the continuous presence of GM-CSF was required. Conditioned medium prepared from transfected PBEo prolonged the survival of naive cells. Comparable survival activity was mimicked by a single dose of 100-500 pg/ml or multiple administrations of 0.1 pg/ml recombinant human GM-CSF (rHuGM-CSF). Survival was completely inhibited by a Jak2 inhibitor, suggesting that GM-CSF-mediated survival involved signaling through the Jak-Stat pathway. Thus, autocrine production of low levels of GM-CSF by a minority of PBEo can block apoptosis of the entire culture by a minute but sustained GM-CSF release.