GENETIC CHANGES IN ATTENUATED POLIOVIRUS STRAINS CULTIVATED ON HUMAN INTENSIVE IN VITRO1

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Abstract
Human embryonic intestine in organ culture was found to support the multiplication of polioviruses for periods up to 4 months. The character of attenuated Type 3 strains following cultivation in this system was found to change from rct/40− to rct/40+, while Type 1 attenuated viruses remained rct/40− even after prolonged multiplication (up to 73 days in the same system). The growth rate of virulent H-24 strain was much higher than that of attenuated WM-3 virus in the intestinal explants. Similarly, an rct/40+ guanidine-re-sistant mutant of WM-3 virus, both singly and when mixed with the original rct/40− guanidine-sensitive strain, was more infectious and multiplied to higher riters in the explants. These results appear to be similar to the observed behavior of Type 3 virus strains in the human intestinal tract, and offer an explanation for the tendency to lose markers associated with attenuation.