Persistent influenza C virus possesses distinct functional properties due to a modified HEF glycoprotein

A model of long term viral persistence has been established by selecting a spontaneous mutant strain of influenza C/Ann Arbor/1/50 virus in a permanent carrier culture of MDCK cells. Infectivity and cell tropism are mainly determined by the multifunctional viral membrane glycoprotein (HEF). HEF analysis was aimed at identifying a putative correlation between sequence and function, i.e. receptor binding, enzymatic activity, antigenicity and rate of infection. The current experimental picture is summarized by the following findings: (i) C/Ann Arbor/1/50 persistent virus carries a modified receptor-binding sequence, (ii) receptor-binding activity is altered, as indicated by a higher efficiency in recognizing low amounts of the receptor determinant N-acetyl-9-O-acetylneuraminic acid, (iii) direct attachment to cell surfaces differs from that of wild-type virus, as measured by slower kinetics of viral elution, (iv) receptor-destroying enzymatic activity is diminished, (v) characteristic features of virion surface morphology are altered or unstable, (vi) persistent-type HEF epitopes are distinguishable by monoclonal antibodies from wild-type and (vii) viral infectivity is intensified for cells bearing a low number of receptors. The sum of these changes highlights a structurally and functionally modified HEF glycoprotein that allows long term viral persistence. In order to clarify which of the described points are required for the persistent viral phenotype, a working concept is presented.