C15 adaptation. (A) Monolayers of HeLa-E8 and HeLa-H1, or differentiated cultures of PBE-ALI cells from three donors, grown in 12-well plates were infected with A16, A16/pR16.11, C15 or C15a virus at 4 × 108 vRNA copies per well. After 2 h (input virus to estimate binding) or 24 h (replication) incubation at 34°C, the cells were analyzed for viral RNA signals by RT-qPCR (means ± s.d., n ≥ 3). (B) C15 virus was passaged serially in HeLa-E8 cells as described in Results and each passage was evaluated 72 h p.i. for virus load (viral RNA copies per ml) by RT-qPCR (means ± s.d., n = 3). Polyclonal C15a is defined as the Passage 10 material. (C) Passage 1, Passage 5 and Passage 10 and mock-infected cells were visualized (72 h p.i.) and photographed by light microscopy. Scale bar is 100 μm.