FIG 3.

In vitro replication of H9N2 and H9N1 variants. Replication of H9N2 and H9N1 viruses in mammalian MDCK cells (A, C) and avian-origin DF1 cells (B, D) at 37°C. Naturally occurring amino acids previously found in H9 HA are shown as squares; others are shown as triangles. Confluent monolayers of MDCK or DF1 cells were infected with viruses at an MOI of 0.01, and supernatant was collected at 0, 6, 12, 24, 48, and 72 hpi. The virus in supernatant collected from MDCK cells was quantified by determination of the TCID₅₀ using the Reed and Muench method (62). The plotted data represent means ± standard errors. WT, wild type.