Characterization of a strain of murine cytomegalovirus which fails to grow in the salivary glands of mice.

Characterization of a tissue culture-adapted strain of murine cytomegalovirus (MCMV), the Vancouver strain, which demonstrated altered tissue tropism in mice was undertaken to help understand the mechanism of pathogenesis of cytomegaloviruses. The Vancouver strain grew to a limited extent in the spleen but failed to grow in the salivary glands of inoculated mice. This mutation probably arose during multiple in vitro passaging of the parental Smith strain. The Vancouver strain replicated more quickly and produced a greater yield of virus per cycle than the Smith strain in vitro, resulting in a larger plaque size. In addition to these phenotypic differences, the Vancouver strain was found to have a 9.4 kb deletion spanning the XbaI I/L junction of the parental Smith strain (0.960 to 0.995 map units), and a 0.9 kb insertion which mapped to the EcoRI K fragment (0.37 to 0.47 map units). Analysis of virus-induced proteins at various times post-infection identified only one major change in Vancouver strain-infected cells, the absence of a 42K protein found in Smith-infected cells at early and late times.

MHC class I ubiquitination by a viral PHD/LAP finger protein.

The murine gamma-herpesvirus-68 K3 (MK3) is a PHD/LAP finger protein that downregulates major histocompatibility complex (MHC) class I expression. In transfected cell lines, MK3 was expressed in the endoplasmic reticulum (ER) membrane, where it bound the cytoplasmic tail of newly synthesized H-2D(b) glycoproteins and targeted them for degradation. Proteasome inhibitors blocked the degradation and led to an accumulation of ubiquitinated H-2D(b). Because this retained its native conformation, ubiquitination preceded any denaturation or dislocation to the cytosol. The PHD/LAP finger of MK3 was not required for H-2D(b) binding but was essential for its ubiquitination and degradation. Thus, gamma-herpesviruses have adapted the cellular PHD/LAP motif to immune evasion, apparently for the catalysis of MHC class I ubiquitination.

Peripheral blood mononuclear phagocytes mediate dissemination of murine cytomegalovirus.

Cytomegalovirus is transmitted with blood and organs from seropositive individuals, although the particular leukocyte population harboring latent or persistent virus remains poorly characterized. Murine cytomegalovirus, tagged with the Escherichia coli lacZ gene, was used to identify cells in which virus replicates during acute infection of immunocompetent mice. Recombinant murine cytomegaloviruses, RM461, RM460, and RM427, were constructed to express beta-galactosidase under control of the human cytomegalovirus ie1/ie2 promoter/enhancer. The lacZ gene was inserted between the ie2 and sgg1 genes in RM461 and RM460, disrupting a 0.85-kb late transcript that was found to be dispensable for replication in cultured cells as well as for infection of mice. In BALB/c mice, lacZ-tagged and wild-type viruses exhibited a similar 50% lethal dose and all had the capacity to latently infect the spleen. Peripheral blood mononuclear phagocytes were the major infected leukocyte cell type, as demonstrated by the ability of infected cells to adhere to glass and to phagocytize latex beads; however, these cells did not exhibit typical monocyte markers. Plaque assay for virus and 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining of frozen sections of organs from infected mice revealed that the major target organs included the spleen, adrenal glands, liver, and salivary glands, although tissues as diverse as brown fat and lungs were also involved. Individual blue-staining cells were readily identified in all infected tissues. These studies identified a mononuclear phagocyte, possibly a macrophage or dendritic cell precursor, as the vehicle of virus dissemination during acute infection, and demonstrate the utility of using lacZ-tagged murine cytomegalovirus for tropism, pathogenesis, and latency studies.