Trypsin enhancement of parainfluenza-3 virus infectivity in HEp-2 cells was demonstrated. A substantial enhancement of infectivity was observed when cell cultures were exposed to trypsin during the adsorption period or when subcultured with trypsin late in the eclipse phase. Time-course studies on virus development indicated that treatment of cells with puromycin or cycloheximide for 1 h postinfection markedly inhibited the synthesis of infectious virus. However, this inhibitory effect could be reversed by the removal of drug and subsequent subculture of trypsinized cells.

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A reproducible plaquing procedure for infectious bovine rhinotracheitis virus (IBRV) in an established bovine kidney cell line is reported. The validity of this system for quantitative analysis has been established by conventional methods.After infection at different multiplicities, one-step growth curves have shown that the eclipse period for IBRV lasts approximately 4 hours and that the infectious virus increases at a logarithmic rate for 12 to 14 hours. The virus yield with the low and high input is 30 PFU and 210 PFU per cell, respectively. Only 1 to 9% of the total virus is released at 24 hours postinfection. The data presented indicate the half-life of IBRV at 37 °C and 42 °C to be 16 and 3.5 hours, respectively. A comparison of hyperimmune bovine and rabbit sera has shown that 92% of the infective particles are neutralized within 30 minutes.

SUMMARYThe addition of either isatin-fl-thiosemicarbazone (IBT) or its N-methyl derivative effectively inhibited the multiplication of various isolates of infectious bovine rhinotracheitis virus. Treatment of infected cultures with IBT early or late in the log. phase of replication markedly suppressed the production of infectious virus, indicating that the drug inhibits virus maturation. The time course of formation of infectious virus and of synthesis of viral DNA and viral protein was determined with the inhibitors FUdR and cycloheximide, respectively. In radioisotope experiments, IBT inhibited the incorporation of label into both RNA and DNA at a fairly early stage of the virus growth cycle.

The localization of anti-IBR and anti-PI-3 activity in the serum and nasal secretory immunoglobulins following intranasal immunization of cattle with a mixed vaccine (IBR-PI-3, MLV, TCO) was studied and was found to reside in the nasal secretory IgA, serum IgM and IgG fractions. The computation of their relative virus neutralizing efficiencies from kinetic data revealed their order of neutralizing efficiencies to be IgM greater than IgA greather than IgG.