Murine Cytomegalovirus Containing a Mutation at Open Reading Frame M37 Is Severely Attenuated in Growth and Virulence In Vivo

A pool of murine cytomegalovirus (MCMV) mutants was generated by using a Tn3-based transposon mutagenesis procedure. One of the mutants, RvM37, which contained the transposon sequence at open reading frame M37, was characterized both in tissue culture and in immunocompetent BALB/c and immunodeficient SCID mice. Our results provide the first direct evidence to suggest that M37 is not essential for viral replication in vitro in NIH 3T3 cells. Compared to the wild-type strain and a rescued virus that restored the M37 region, the viral mutant was severely attenuated in growth in both BALB/c and SCID mice after intraperitoneal infection. Specifically, titers of the Smith strain and rescued virus in the salivary glands, lungs, spleens, livers, and kidneys of the SCID mice at 21 days postinfection were about 5 × 10(5), 2 × 10(5), 5 × 10(4), 5 × 10(3), and 1 × 10(4) PFU/ml of organ homogenate, respectively; in contrast, titers of RvM37 in these organs were less than 10(2) PFU/ml of organ homogenate. Moreover, the virulence of the mutant virus appeared to be significantly attenuated because none of the SCID mice infected with RvM37 had died by 120 days postinfection, while all animals infected with the wild-type and rescued viruses had died by 26 days postinfection. Our results suggest that M37 probably encodes a virulence factor and is required for MCMV virulence in SCID mice and for optimal viral growth in vivo.