Relationship between the appearance of CPE and GFP expression.

Serial two-fold dilutions from 1∶4 to 1∶256 were mixed with an equal volume of challenge virus. In the left panel, the schematic results of two neutralization methods are shown. CPE was observed in one of the two wells at 1∶32, and the conventional neutralizing antibody titer was 1∶16 by 100% inhibition of CPE. The mean FU value of the two cell control wells was 202 and that of the 1∶32 dilution was 450, showing 1∶16 of neutralizing antibody titer. Using 1,452 serum samples, the consistency of neutralizing antibody titers was compared based on different cut-off values for GFP expression: 1.5-fold, 2.0-fold, and 2.5- fold of FU values of the cell control wells.