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

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Abstract
As granulocyte/macrophage colony stimulating factor (GM-CSF) mediated delay of granulocyte apoptosis contributes to the accumulation of inflammatory cells at the site of inflammation in many diseases, we sought to determine whether asthma is also associated with a GM-CSF dependent increase in lung granulocyte survival. Moreover, because GM-CSF mediates its effects through activation of signal transducer and activator of transcription 5 (STAT5), we also investigated the potential role of STAT5 in allergic inflammation. Blood granulocytes were recovered from six healthy and six heaves affected horses, a model of asthma. Lung granulocytes were obtained by bronchoalveolar lavage (BAL) from the same horses. Granulocytes were cultured in the presence or absence of anti-GM-CSF receptor antibodies for different times and apoptosis was determined using the Annexin-V/propidium iodide detection method. Nuclear protein extracts from cultured granulocytes were analysed for STAT5 binding activity by electrophoretic mobility shift assay. BAL fluid granulocytes from heaves affected horses demonstrated a significant delay in apoptosis compared with blood granulocytes from the same horses and blood and BAL fluid granulocytes from healthy horses. Conversely, the rate of apoptosis in blood granulocytes from healthy and heaves affected horses was comparable. The enhanced survival of BAL fluid granulocytes from affected horses was suppressed in the presence of antibodies directed against GM-CSF receptors. Increased levels of active STAT5 were found in BAL fluid granulocytes from heaves affected horses and were markedly reduced after treatment with anti-GM-CSF receptor antibodies. These data indicate that granulocyte survival is enhanced in the 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 asthma.