EFFECT OF TRYPSIN ON REPRODUCTION OF TYPE 4 PARAINFLUENZA VIRUS IN VERO CELL CULTURES UNDER FLUID OVERLAY

Prototype strain, M-25, of parainfluenza type 4 (PI-4) virus was grown to high titers and passaged serially in Vero (Cercopithecus kidney cell line) cell cultures under fluid overlay containing low concentrations of trypsin. Infectivity titers in both primary cercopithecus kidney cell cultures (VK) and in Vero cell cultures under ordinary conditions (Vro) did not change essentially through passages in the trypsin-added Vero cell cultures (Vrt) from those of the original VK-grown virus. The 14th Vrt-passage virus was not transferable in Vro beyond two subpassages. Consequently, occurrence of adaptation to Vro during passages in Vrt can be excluded. Identification of Vrt-grown virus as PI-4 was demonstrated by the cross neutralization and hemagglutination-inhibition tests performed among paramyxoviruses and by several biological and physico-chemical tests. Other proteolytic enzymes, pancreatin and pronase, gave similar results as did trypsin, but hyaluronidase and lysozyme were ineffective. It was found that animal sera were inhibitory to the trypsin-effect mentioned above. Therefore, the Vrt system must be serum-free for obtaining reproducible results.