Suitability of MDCK cells grown in a serum-free medium for influenza virus production.

MDCK cells have been adapted to grow in a serum-free environment using Ultra-MDCK medium (BioWHITTAKER). The growth of adapted U-MDCK cells was maintained for over a year without any reduction in growth rate or modification of cell karyotype; cells were scaled up to spinner culture using several microcarriers. The cells were shown to be a very good host for influenza A and B virus replication in both the presence and absence of trypsin in the infection medium. Trypsin-independent viruses replicated to high titres (10(7)-10(8) TCID50/ml) in U-MDCK cells, after selection through serial passages without trypsin. This virus progeny exhibited uncleaved and antigenically modified haemagglutinin compared with standard viruses grown with trypsin. Finally, large amounts of influenza A and B viruses were produced in U-MDCK cells grown on microcarriers under rod-stirred conditions using selected trypsin-independent variants.