Two clones obtained from Urabe AM9 mumps virus vaccine differ in their replicative efficiency in neuroblastoma cells

A high rate of post-vaccinal aseptic meningitis for Urabe AM9 mumps virus strain is well documented. This strain is composed of two virus variants differing at the nt 1081 (A/G) region in the hemagglutinin-neuraminidase (HN) gene. An association of HN-A1081 variant with neurovirulence has been proposed. In order to test for neurotropism we isolated the HN-A1081 and HN-G1081 virus variants from Urabe AM9 mumps virus vaccine. Sequential passages were performed in monkey kidney Vero cells and human neuroblastoma SH-SY5Y cells. Viral replication was determined by conventional and real-time RT-PCR. The results show that clone HN-A1081 can replicate efficiently in both cell types. However, a defective replication of clone HN-G1081, lacking its genetic marker, was observed after the third passage in neuroblastoma cells. Kinetics assays showed that clone HN-A1081 replicates faster than clone HN-G1081. Viral clones were also inoculated into the brains of newborn rats. Clone HN-A1081 replicated 14 times, while clone HN-G1081 merely duplicated its level over the initial inoculum. These results suggest that there is a selective replication of HN-A1081 mumps virus variants in cells of nervous origin.