Isolation and preliminary characterization of temperature-sensitive mutants of mouse cytomegalovirus of differing virulence for 1-week-old mice.

To study the pathogenicity of murine cytomegalovirus (MCMV) and to identify virulence determinants, we have isolated and phenotypically characterized a set of temperature-sensitive mutants. One mutant, PP269/38, was avirulent for 1-week-old BALB/c mice and restricted in its plaque formation and replication at 39 degrees C. Mutants PP242/68 and PP268/38 were 100-fold less virulent than salivary gland-grown virus (SGV), even after two passages in the salivary glands of 1-week-old mice. The former mutant was unable to replicate or form plaques at 39 degrees C whereas the latter replicated poorly at 39 degrees C but not at all at 40 degrees C although it was able to form plaques at 40 degrees C with a reduced plaque size. PP31/15 exhibited a 40-fold reduction in virulence compared to SGV after two passages in vivo and was unable to form plaques or to replicate at 40 degrees C; at 39 degrees C it was able to, with reduced efficiency. The remaining two mutants, PP99/3 and PP392/31, were 10-fold less virulent than SGV and were restricted at 40 degrees C. The six mutants have been classified into at least four complementation groups. These mutants may be useful for studying various aspects of MCMV pathogenicity.