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Influenza virus can be propagated in vitro in the presence of trypsin without inactivating virion-bound IL-2. Influenza virus (A/Udorn/72) was harvested from NA-chIL-2-bearing MDCK or control MDCK cells in the presence of variable amounts of TPCK-trypsin. Virus was purified by gradient sedimentation, inactivated at 56°C for 20 min, and adjusted to 200 HAU/well. Samples were tested in quadruplicate for chIL-2 bioactivity.
