FIG. 8.

Hemolysis of chicken RBCs by the prefusion intermediate of PR8 and its inhibition by neutralizing amounts of H36 IgG and Fab. Virus (10,000 HAU/ml) was mixed with H36 IgG (67 nM) or H36 Fab (200 nM). These concentrations of antibody gave 90% PAN of the virus used here in MDCK cells. Virus was attached to RBCs at 4°C for 30 min at pH 7.5 and then treated at pH 5 and 4°C for 30 min to activate the prefusion intermediate or at pH 7.5. Virus in column 7 was treated at pH 5 and 37°C before being mixed with RBCs. PBS or H36 IgG or Fab was then added at 37°C for 60 min, and mixtures were incubated for 45 min at pH 5 and 37°C for hemolysis to occur. Released hemoglobin was determined spectrophotometrically at 520 nm. Data are the mean of three experiments, and the bars represent the standard error of the mean. Numbers in parentheses refer to each experimental procedure.