FIG. 1.

Binding of TGEV and the mutants HAD3 and m10 to ST cells (upper panel) and LLC-PK1 cells (lower panel) in microtiter plates. After unbound virus was washed away, cells were fixed and bound virus was detected by an enzyme-linked immunoassay. The designation NA (TGEV-NA, HAD3-NA, m10-NA) indicates virus that has been treated with neuraminidase to inactivate competitive inhibitors that may accumulate on the virion surface during the course of infection. Cells were either mock treated (MES buffer) or pretreated with neuraminidase from V. cholerae (VCNA) to release sialic acids from the cell surface. ΔE indicates the difference between the extinction value of the cells incubated with virus and antibodies and the extinction value of the cells incubated only with antibodies. The amount of added virions was determined by protein analysis as described in Materials and Methods.