Development and Serial Cell-Free Passage of a Highly Potent Strain of Mouse Leukemia Virus.

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Abstract
After it was found that filtered extracts prepared from tissues of leukemic Ak or C58 mice may induce leukemia following inoculation into newborn mice of a susceptible strain (Gross,L.Proc.Soc.Exptl. Biol. and Med.l951,v76,27;1954,v86,734) it became apparent that the results of individual experiments may vary to a considerable degree depending on the particular extract used. Some extracts were inactive; other extracts were highly potent. A particularly active extract, originally prepared from spontaneous Ak leukemia, was passed through several consecutive cell-free inoculations of newborn C3H hosts, resulting eventually in a strain of leukemic virus of high potency. This agent induced acute generalized leukemia within 2.5-3.5 months in 40% - 100% of the inoculated newborn mice (Bittner substrata). Adult C3H mice were, with few exceptions, resistant to inoculation of the agent: newborn mice had to be inoculated, usually less than 12 hours old, preferably less than 6 hours old, never more than 16 hours old. The leukemic agent could be preserved at -70[degree]C in sealed ampoules with no apparent loss of infectivity for period of times thus far tested from 46 days to 5.5 months. This highly potent leukemic agent strain was designated "mouse leukemia virus passage A", and is the most potent of those thus far developed in our laboratory.