GM-CSF differentially regulates eosinophil and neutrophil adhesive interactions with vascular endothelium in vivo

Allergic airway inflammation is characterized by elaboration of cytokines and chemokines leading to recruitment of inflammatory leukocytes, predominantly eosinophils, to the airways. Granulocyte macrophage colony stimulating factor (GM-CSF) is generated in the lungs of human subjects with asthma in response to allergen challenge and is necessary for the development of allergen-induced bronchial eosinophilia in mice. The effect of GM-CSF on human eosinophil and neutrophil interactions with the vascular endothelium under conditions of blood flow was investigated in post-capillary venules of the rabbit mesentery by intravital microscopy.While GM-CSF significantly reduced the rolling fraction of neutrophils in vivo and induced consistent shedding of neutrophil L-selectin in vitro, its effect on eosinophil rolling was variable. Eosinophils from 57% of the donors demonstrated inhibition of rolling, while eosinophils from the remaining 43% of donors demonstrated no inhibition or increased rolling. The variable effect of GM-CSF on inhibition of eosinophil rolling was associated with variable shedding of L-selectin in vitro. In contrast to the differential effect of GM-CSF on neutrophils versus eosinophils, stimulation with phorbol myristate acetate demonstrated a similar degree of inhibition of rolling and L-selectin shedding by neutrophils and eosinophils suggesting that there was no defect in L-selectin shedding in the eosinophil donors who did not respond to GM-CSF. Overall, these studies demonstrate that GM-CSF consistently inhibits interaction of neutrophils with endothelium in vivo, whereas its effect on eosinophil-endothelial interactions is variable. GM-CSF may thus be one factor accounting for the varying percentage of eosinophils and neutrophils recruited to sites of allergic inflammation in different individuals.