An Analysis of the Process of Adaptation of Influenza Virus B of Recent Human Origin to the Mouse Lung

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Abstract
The Rob strain of influenza B virus was used. "Pure" clones of virus were isolated from bronchial washings at succeeding passages by the limiting infective dilution technique in chick embryos, and were studied for various in vitro and in vivo properties. From the heterogeneous population of particles found in the early course of mouse passage a virus was isolated which exhibited all of the tested properties of the adapted virus, including (a) the production of extensive pulmonary consolidation, high mortality of mice, and multiplication of virus on serial mouse passage; (b) the production of a more rapid growth rate; and (c) a decrease of enzymic activity on sheep mucin and mouse lung inhibitors. The adaptation process appears to be a selection of mutants found in the unadapted heterogeneous virus population.