Purification of influenza virions

This chapter describes basic workflows for concentrating and for purifying influenza virions. It presents several ways in which ultracentrifugation can be used to concentrate influenza virions from the growth media of infected cells, with notes on the different degrees of purity that can be expected when using different approaches. These approaches are also suitable for purifying influenza virions from the allantoic fluid of embryonated chicken eggs. As a small quantity of cell-derived microvesicles is invariably co-purified with virions, optional steps are included to increase the stringency of purification by enriching material with viral receptor binding and cleaving activity. As well as methods that will concentrate the approximately spherical virions produced by many laboratory-adapted influenza strains, a density gradient protocol is presented which can be used to separate the virions of filamentous influenza strains based on their morphology. In order to monitor the enrichment of different virion morphologies, a simple protocol for measuring the length of filamentous virions by indirect immunofluorescence and automated image analysis is also given. Influenza virions purified in the ways described here can be used in a variety of downstream protocols in virology, biochemistry and immunology.