Inhibition of granulocyte cluster formation by sgp130Fc. Bone marrow cells from G-CSF^−/−/GM-CSF^−/− mice were incubated in chamber slides with NPA (conditioned medium from G-CSF^−/−/GM-CSF^−/− MEFs, at a final concentration of 30% v/v, ), Hyper-IL6 (20 ng/mL, •), or murine recombinant IL6 (20 ng/mL, □) in the presence of increasing amounts of sgp130-Fc. Four days later, slides were stained with May-Grünwald/Giemsa, and the number of granulocyte clusters was assessed by light microscopy. Means plus or minus SE of duplicate wells are plotted as percentage of the maximal activity detected in the assay (granulocyte clusters/well generated by NPA in the absence of sgp130-Fc). The mean numbers of clusters generated by each stimulus in the absence of sgp130 were 91 plus or minus 7 (Hyper-IL6), 95 plus or minus 6 (NPA), and 25 plus or minus 1 (IL6).