Establishment of persistent infection by parainfluenza virus type 3: Role of a syncytium inhibitor

A strain of parainfluenza virus type 3 (para 3) that had undergone a series of undiluted passages failed to produce syncytia when inoculated on to Vero cells at a high m.o.i. The strain repeatedly produced stable persistent infections. Persistently infected cells were resistant to superinfection by homologous virus, showed the presence of virus-specific antigen and shed low quantities of infectious virus into the supernatant fluid. The undiluted passage parainfluenza virus type 3 strain produced a substance that inhibited syncytium formation by homologous virus and by measles virus but appeared to have no effect on virus replication. This inhibitor had no demonstrable effect on unrelated viruses, including some that produced syncytia. It had a mol. wt. between 3500 and 14000, was acid- and heat-labile, and was inactivated by anti-para 3 serum.