sIL-6R upregulates IL-6 expression during IAV infection. (a–c) A549 cells (a) and human PBMCs (b) were incubated with recombinant human IL-6Rα protein (40 ng/ml) for the indicated times before the cell culture supernatants were collected. The levels of secreted IL-6 protein were detected by ELISA. (c) MRC-5 cells were incubated with IL-6Rα at the indicated doses, and the levels of IL-6 mRNA (top graphs) and secreted protein (bottom graphs) were determined by real-time RT-PCR and ELISA, respectively. (d) A549 cells were transfected with pCMV-sIL-6R for the indicated times before the levels of IL-6 mRNA (top graphs) and secreted protein (bottom graphs) were determined by real-time RT-PCR and ELISA, respectively. (e) A549 cells were transfected with shRNA-sIL-6R or shRNA-control for 24 or 48 h and infected with IAV (MOI=1) for 6 h. The levels of IL-6 mRNA (top graphs) and secreted protein (bottom graphs) were determined by real-time RT-PCR and ELISA, respectively. Data shown are mean±s.e.; n=3. **P<0.01; *P<0.05; ns, not significant; N.D., not detected. IAV, influenza A virus; MOI, multiplicity of infection; PBMC, peripheral blood mononuclear cell; sIL-6R, soluble interleukin-6 receptor.