Figure 4.

Neutralization of different CSFV strains by pig anti-CSFV sera. Virus-specific neutralizing antibodies were cross-examined by neutralization assay with antisera collected from pigs as indicated in Figure 2 legend. Two-fold serial dilutions of the different heat-inactivated sera were mixed with equal volumes of 100 TCID₅₀ of the viruses, incubated at 37°C for 1 h and subsequently transferred to confluent monolayers of ST cells in 96-well plates. The starting dilution of each serum was 1:50. At 72 hours post-infection, the cells were fixed and stained for the presence of E2 glycoprotein by immunofluorescence assay. Neutralization index (NI) is the log₁₀ of the antibody dilution factor (reciprocal of dilution) when 50% of the wells are protected from infection. Since the starting dilution factor was 50, the NI value of 1.7 is the detection threshold of our neutralization assay. Neutralization indices below 1.7 are indicated as "†".