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Journal of Virology, September 1999, p. 7678-7693, Vol. 73, No. 9
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

In Vivo Replication, Latency, and Immunogenicity of Murine Cytomegalovirus Mutants with Deletions in the M83 and M84 Genes, the Putative Homologs of Human Cytomegalovirus pp65 (UL83)

Christopher S. Morello,1 Lee D. Cranmer,2,dagger and Deborah H. Spector2,3,*

Departments of Pathology1 and Biology2 and Center For Molecular Genetics,3 University of California, San Diego, La Jolla, California 92093-0366

Received 16 February 1999/Accepted 7 June 1999

We previously identified two open reading frames (ORFs) of murine cytomegalovirus (MCMV), M83 and M84, which are putative homologs of the human cytomegalovirus (HCMV) UL83 tegument phosphoprotein pp65 (L. D. Cranmer, C. L. Clark, C. S. Morello, H. E. Farrell, W. D. Rawlinson, and D. H. Spector, J. Virol. 70:7929-7939, 1996). In this report, we show that unlike the M83 gene product, the M84 protein is expressed at early times in the infection and cannot be detected in the virion. To elucidate the functional differences between the two pp65 homologs in acute and latent MCMV infections, we constructed two MCMV K181 mutants in which either the M83 or M84 ORF was deleted. The resultant viruses, designated Delta M83 and Delta M84, respectively, were found to replicate in NIH 3T3 cells with kinetics identical to those of the parent strain. Western blot analysis demonstrated that except for the absence of M83 or M84 protein expression in the respective mutants, no global perturbations of protein expression were detected. When Delta M83 and Delta M84 were inoculated intraperitoneally (i.p.) into BALB/c mice, both viruses showed similar attenuated growth in the spleen, liver, and kidney. However, only Delta M83 was severely growth restricted in the salivary glands, a phenotype that was abolished upon restoration of the M83 ORF. Delta M83's growth was similarly restricted in the salivary glands of the resistant C3H/HeN or highly sensitive 129/J strain, as well as in the lungs of all three strains following intranasal inoculation. Using a nested-PCR assay, we found that both Delta M83 and Delta M84 established latency in BALB/c mice, with slightly decreased levels of Delta M83 and Delta M84 genomic DNAs, relative to K181, observed in the salivary glands and lungs. Immunization of BALB/c mice with 105 PFU of K181, Delta M83, or Delta M84 i.p. provided similar levels of protection against lethal challenge. Although immunization with 200 PFU of Delta M83 also provided complete protection, this dose allowed both the immunizing and challenge viruses to establish latency in the spleen. Our results show that the two MCMV pp65 homologs differ in their expression kinetics, virion association, and influence on viral tropism and/or dissemination.

* Corresponding author. Mailing address: Department of Biology 0366, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0366. Phone: (858) 534-9737. Fax: (858) 534-6083. E-mail: dspector{at}ucsd.edu.

dagger Present address: Department of Internal Medicine, The Mayo Clinic, Rochester, MN 55902.

Journal of Virology, September 1999, p. 7678-7693, Vol. 73, No. 9
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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