vIL-6–mediated signaling in mouse splenic B cells can only be inhibited in the absence of endogenous IL-6. Western blot for STAT3 tyrosine-705 phosphorylation (P-STAT3), total STAT3, and HPRT1 (loading control) from primary B cells isolated from normal (wt) or IL-6–deficient mice before treatment with or without mIL-6 (5 ng/mL), vIL-6 (500 ng/mL), or the trans-signaling inhibitor sgp130Fc (5 μg/mL) as indicated, for 18 hours. Ratio of P-STAT3 to total STAT3 to HPRT1 band intensities is given. *Nonspecific bands.