Enhanced survival of lung granulocytes in an animal model of asthma: evidence for a role of GM-CSF activated STAT5 signalling pathway

Background-As granulocyte/macrophage colony stimulating factor (GM-CSF) med iated delay of granulocyte apoptosis contributes to the accumulation of inf lammatory cells at the site of inflammation in many diseases, we sought to determine whether asthma is also associated with a GM-CSF dependent increas e in lung granulocyte survival. Moreover, because GM-CSF mediates its effec ts through activation of signal transducer and activator of transcription 5 (STAT5), we also investigated the potential role of STAT5 in allergic infl ammation. Methods-Blood granulocytes were recovered from six healthy and six heaves a ffected horses, a model of asthma. Lung granulocytes were obtained by bronc hoalveolar lavage (BAL) from the same horses. Granulocytes were cultured in the presence or absence of anti-GM-CSF receptor antibodies for different t imes and apoptosis was determined using the Annexin-V/propidiurn iodide det ection method. Nuclear protein extracts from cultured granulocytes were ana lysed for STAT5 binding activity by electrophoretic mobility shift assay. Results-BAL fluid granulocytes from heaves affected horses demonstrated a s ignificant delay in apoptosis compared with blood granulocytes from the sam e horses and blood and BAL fluid granulocytes from healthy horses. Converse ly, the rate of apoptosis in blood granulocytes from healthy and heaves aff ected horses was comparable. The enhanced survival of BAL fluid granulocyte s from affected horses was suppressed in the presence of antibodies directe d against GM-CSF receptors. Increased levels of active STAT5 were found in BAL fluid granulocytes from heaves affected horses and were markedly reduce d after treatment with anti-GM-CSF receptor antibodies. Conclusions-These data indicate that granulocyte survival is enhanced in th e lung of heaves affected horses and suggest a role for a GM-CSF activated STAT5 pathway in delaying apoptosis of lung granulocytes in this model of a sthma.