Contribution of IL6 to the generation of granulocytes in vitro. (A) MEFs derived from individual G-CSF^−/−/GM-CSF^−/−/IL6^+/+or G-CSF^−/−/GM-CSF^−/−/IL6^−/− embryos were exposed to control medium (□), LPS (0.05 μg/mL, ■), or LPS (0.05 μg/mL) plus IL6 (50 ng/mL, ▨) for 48 hours. The resultant CM were tested for NPA in a standard bone marrow assay as described in “NPA assay.” Conditioned media were produced from individual embryos in 3 separate litters for each genotype; the data for embryos with identical genotype have been pooled and are presented as mean plus or minus SD. (B) Conditioned medium from G-CSF^−/−/GM-CSF^−/− immortalized embryonic fibroblast (■) or recombinant IL6 (50 ng/mL, ○) were preincubated with increasing concentrations of a neutralizing antibody to murine IL6 before their assay on bone marrow cells in liquid culture. Data are presented as percentage of the activity in the absence of added antibody. Error bars represent SE.