Attenuation of a field Sendai virus isolate through egg-passages is associated with an impediment of viral genome replication in mouse respiratory cells

We investigated the mechanisms responsible for attenuation of mouse pathoge nicity of Sendai virus (SeV) through passages in eggs. A highly virulent cl one, E0, derived from the field SeV Hamamatsu strain, was successively pass aged in hen's eggs. Analysis of the mouse lethal dose 50% (MLD50) Of virus clones obtained from the viruses at egg-passages 1, 15, 30 and 50 demonstra ted that attenuation of E0 by egg-passage occurred due to the gradual appea rance of and replacement by virus variants possessing higher MLD50 Comparis on of viral replication in the mouse lung and mouse pathogenicity with the representative SeV clones, E0, E15c12, E30c12 and E50c19, obtained from the respective egg-passages revealed that the low pathogenicity of the egg-pas saged clones was due to poor multi-cycle viral replication in the lung. Fur thermore, MLD(50)s, of the SeV clones were found to be negatively correlate d with the replication capability in primary mouse pulmonary epithelial (MP E) cells; the egg-passaged clones with more attenuated phenotypes showed lo wer replication capability in MPE cells. In the MPE cells infected with the SeV clones at m.o.i. 10, however, viral protein and mRNA syntheses of the egg-passaged clones were enhanced or comparable to those of the parental EO clone at 1 day and 2 days post infection (p.i.) but decreased more rapidly thereafter. In contrast, viral genome synthesis of the egg-passaged clones in the cells at 2 days p.i. was several times lower than that of EO. These results strongly suggest that attenuation of a virulent: field SeV strain by egg-passage occurs due to the appearance and selection of virus variants possessing poor propagation capacity in mouse respiratory epithelial cells , which is caused primarily by an impediment of viral genome replication.