Influenza virus: a novel method to assess viral and neutralizing antibody titers in vitro.

The present report describes novel in vitro assays to determine influenza virus titers and virus neutralizing antibody levels. For determination of viral titers, serial dilutions of influenza virus were incubated with MDCK-cells and cultured for 48 h under a methylcellulose overlay in 24 well plates. Cells were fixed, permeabilized and stained with a monoclonal antibody specific for hemagglutitin (HA) and a peroxidase labelled second stage antibody. The sensitivity of the assay was 100-1000 times greater than a conventional hemagglutination test using fresh chicken blood. For determination of influenza virus neutralizing activity, viral samples were incubated with serial dilutions of antibody and residual viral activity was assessed in 96 well plates by the same procedure as described above. This assay made it possible to distinguish between IgM and IgG antibody titers and was about 5-10 fold more sensitive than a classical hemagglutination inhibition assay using fresh chicken blood.