A) Dose dependent fusion activity of HRSV F derived. 293T cells were transfected with either pFR-Luc alone (■), pBD-NFκB alone (▲), co-transfected with pFR-Luc and pBD-NFκB (▼), or co-transfected with pHRSVFOptA2 and pBD-NFκB and mixed 24 hours after transfection in various amounts with cells that had been transfected with pFR-Luc alone (◆). Luciferase activity was measured 24 hours post mixing as described in methods and is reported as relative light units. B) Fusion activity of HRSV F derived from subgroups A and B. 293T cells co-transfected with pBD-NFκB and either pHRSVFOptA2, pHRSVFOptB18537, or vector only (NFκB only). Cells were mixed 24 hours later with a separate population of 293T cells transfected with pFR-Luc, and luciferase activity was measured 24 hours post mixing as described in methods. Luciferase activity is reported as relative light units.