Requirement for uracil-DNA glycosylase during the transition to late-phase cytomegalovirus DNA replication

Susceptibility of immature and mature Langerhans cell-type dendritic cells to infection and immunomodulation by human cytomegalovirus

Human cytomegalovirus (CMV) infection initiates in mucosal epithelia and disseminates via leukocytes throughout the body. Langerhans cells (LCs), the immature dendritic cells (DCs) that reside in epithelial tissues, are among the first cells to encounter virus and may play important roles in the immune response, as well as in pathogenesis as hosts for viral replication and as vehicles for dissemination. Here, we demonstrate that CD34(+) progenitor cell-derived LC-type DCs exhibit a differentiation state-dependent susceptibility to CMV infection. In contrast to the small percentage (3 to 4%) of the immature LCs that supported infection, a high percentage (48 to 74%) of mature, LC-derived DCs were susceptible to infection with endotheliotropic strains (TB40/E or VHL/E) of CMV. These cells were much less susceptible to viral strains AD169varATCC, TownevarRIT(3), and Toledo. When exposed to endotheliotropic strains, viral gene expression (IE1/IE2 and other viral gene products) and viral replication proceeded efficiently in LC-derived mature DCs (mDCs). Productive infection was associated with downmodulation of cell surface CD83, CD1a, CD80, CD86, ICAM-1, major histocompatibility complex (MHC) class I, and MHC class II on these cells. In addition, the T-cell proliferative response to allogeneic LC-derived mDCs was attenuated when CMV-infected cultures were used as stimulators. This investigation revealed important characteristics of the interaction between CMV and the LC lineage of DCs, suggesting that LC-derived mDCs are important to viral pathogenesis and immunity through their increased susceptibility to virus replication and virus-mediated immune escape.

The human cytomegalovirus

Human cytomegalovirus (HCMV), a betaherpesvirus, represents the major infectious cause of birth defects, as well as an important pathogen for immunocompromised individuals. The viral nucleocapsid containing a linear double-stranded DNA of 230 kb is surrounded by a proteinaceous tegument, which is itself enclosed by a loosely applied lipid bilayer. Expression of the HCMV genome is controlled by a cascade of transcriptional events that leads to the synthesis of three categories of viral proteins designated as immediate-early, early, and late. Clinical manifestations can be seen following primary infection, reinfection, or reactivation. About 10% of infants are infected by the age of 6 months following transmission from their mothers via the placenta, during delivery, or by breastfeeding. HCMV is a significant post-allograft pathogen and contributes to graft loss independently from graft rejection. Histopathologic examination of necropsy tissues demonstrates that the virus enters via the epithelium of the upper alimentary, respiratory, or genitourinary tracts. Hematogenous spreading is typically followed by infection of ductal epithelial cells. Infections are kept under control by the immune system. However, total HCMV clearance is rarely achieved, and the viral genome remains at selected sites in a latent state. Virological and molecular detection of HCMV, as well as serological demonstration of a specific immune response, are used for diagnosis. Treatment of HCMV infections is difficult because there are few options. The presently available drugs produced a significant clinical improvement, but suffer from poor oral bioavailability, low potency, development of resistance in clinical practice, and dose-limiting toxicities.

Differential incorporation of uracil DNA glycosylase UNG2 into HIV-1, HIV-2, and SIV(MAC) viral particles

We have previously reported that the host uracil DNA glycosylase UNG2 enzyme is incorporated into HIV-1 virions via a specific association with the viral integrase (IN) domain of Gag-Pol precursor. In this study, we investigated whether UNG2 was packaged into two phylogenetically closely related primate lentiviruses, HIV-2(ROD) and SIV(MAC239). We demonstrated by GST-pull-down and coprecipitation assays that INs from HIV-1, HIV-2(ROD), and SIV(MAC239) associated with UNG2, although the interaction of UNG2 with HIV-2(ROD) IN and SIV(MAC239) IN was less strong than with HIV-1 IN. We then showed by Western blotting that highly purified HIV-2 and SIV(MAC) viral particles did not incorporate host UNG2, contrasting with the presence of UNG2 in HIV-1 viral particles. Finally, we showed that HIV-1/SIV chimeric viruses in which residues 6 to 202 of HIV-1 IN were replaced by the SIV counterpart were impaired for packaging of UNG2, indicating that the incorporation of host UNG2 into viral particles is the hallmark of the HIV-1 strain. Moreover, we found that HIV-1/SIV IN chimeric viruses were deficient for viral propagation.

Functional role of HIV-1 virion-associated uracil DNA glycosylase 2 in the correction of G:U mispairs to G:C pairs

Human monocytes/macrophages are target cells for HIV-1 infection. As other non-dividing cells, they are characterized by low and imbalanced intracellular dNTP pool levels and an excess of dUTP. The replication of HIV-1 in this cellular context favors misincorporation of uracil residues into viral DNA because of the use of dUTP in place of dCTP. We have previously reported that the host uracil DNA glycosylase enzyme UNG2 is packaged into HIV-1 viral particles via a specific association with the integrase domain of the Gag-Pol precursor. In this study, we investigated whether virion-associated UNG2 plays a role similar to that of its cellular counterpart. We show that the L172A mutation of integrase impaired the packaging of UNG2 into viral particles. Using a primer-template DNA substrate containing G:U mispairs, we demonstrate that wild-type viral lysate has the ability to repair G:U mismatched pairs to G:C matched pairs, in contrast to UNG2-deficient viral lysate. Moreover, no correction of G:T mispairs by wild-type HIV-1 viral lysate was observed, which argues for the specificity of the repair process. We also show that UNG2 physically associates with the viral reverse transcriptase enzyme. Altogether our data indicate for the first time that a uracil repair pathway is specifically associated with HIV-1 viral particles. However, the molecular mechanism of this process remains to be characterized further.

Vaccinia virus uracil DNA glycosylase has an essential role in DNA synthesis that is independent of its glycosylase activity: catalytic site mutations reduce virulence but not virus replication in cultured cells

Previous findings that the vaccinia virus uracil DNA glycosylase is required for virus DNA replication, coupled with an inability to isolate a mutant with an active site substitution in the glycosylase gene, were surprising, as such enzymes function in DNA repair and bacterial, yeast, and mammalian null mutants are viable. To further study the role of the viral protein, we constructed recombinant vaccinia viruses with single or double mutations (D68N and H181L) in the uracil DNA glycosylase conserved catalytic site by using a complementing cell line that constitutively expresses the viral enzyme. Although these mutations abolished uracil DNA glycosylase activity, they did not prevent viral DNA replication or propagation on a variety of noncomplementing cell lines or human primary skin fibroblasts. In contrast, replication of a uracil DNA glycosylase deletion mutant occurred only in the complementing cell line. Therefore, the uracil DNA glycosylase has an essential role in DNA replication that is independent of its glycosylase activity. Nevertheless, the conservation of the catalytic site in all poxvirus orthologs suggested an important role in vivo. This idea was confirmed by the decreased virulence of catalytic-site mutants when administered by the intranasal route to mice.

Roles of uracil-DNA glycosylase and dUTPase in virus replication

Herpesviruses and poxviruses are known to encode the DNA repair enzyme uracil-DNA glycosylase (UNG), an enzyme involved in the base excision repair pathway that specifically removes the RNA base uracil from DNA, while at least one retrovirus (human immunodeficiency virus type 1) packages cellular UNG into virus particles. In these instances, UNG is implicated as being important in virus replication. However, a clear understanding of the role(s) of UNG in virus replication remains elusive. Herpesviruses, poxviruses and some retroviruses encode dUTPase, an enzyme that can minimize the misincorporation of uracil into DNA. The encoding of dUTPase by these viruses also implies their importance in virus replication. An understanding at the molecular level of how these viruses replicate in non-dividing cells should provide clues to the biological relevance of UNG and dUTPase function in virus replication.

Proteolytic processing of human cytomegalovirus glycoprotein B is dispensable for viral growth in culture

Glycoprotein B (gB) of human cytomegalovirus (HCMV), which is considered essential for the viral life cycle, is proteolytically processed during maturation. Since gB homologues of several other herpesviruses remain uncleaved, the relevance of this property of HCMV gB for viral infectivity is unclear. Here we report on the construction of a viral mutant in which the recognition site of gB for the cellular endoprotease furin was destroyed. Because mutagenesis of essential proteins may result in a lethal phenotype, a replication-deficient HCMV gB-null genome encoding enhanced green fluorescent protein was constructed, and complementation by mutant gBs was initially evaluated in transient-cotransfection assays. Cotransfection of plasmids expressing authentic gB or gB with a mutated cleavage site (gB-DeltaFur) led to the formation of green fluorescent miniplaques which were considered to result from one cycle of phenotypic complementation of the gB-null genome. To verify these results, two recombinant HCMV genomes were constructed: HCMV-BAC-DeltaMhdI, with a deletion of hydrophobic domain 1 of gB that appeared to be essential for viral growth in the cotransfection experiments, and HCMV-BACDeltaFur, in which the gB cleavage site was mutated by amino acid substitution. Consistent with the results of the cotransfection assays, only the DeltaFur mutant replicated in human fibroblasts, showing growth kinetics comparable to that of wild-type virus. gB in mutant-infected cells was uncleaved, whereas glycosylation and transport to the cell surface were not impaired. Extracellular mutant virus contained exclusively uncleaved gB, indicating that proteolytic processing of gB is dispensable for viral replication in cell culture.