Poxvirus-induced alteration of arachidonate metabolism

Stereoselective ozonolysis of TMS-substituted allylic alcohol derivatives and synthesis of 14R,15S- and 14S,15S-diHETE

Ozonolysis of TMS-substituted olefins produces α-carbonyl TMS peroxides without cleavage of the C[double bond, length as m-dash]C bond. Herein, stereochemistry in the ozonolysis was studied using silyl derivatives of (E)- and (Z)-(1-TMS)alk-1-en-3-ols. The (E)-isomers afforded the anti-3-siloxy-2-(TMS-oxy)aldehydes as the major stereoisomer (anti/syn = 3-9 : 1) after reductive work-up with Ph₃P. In contrast, Z-olefins selectively gave the syn isomers with syn/anti ratios of 4-19 : 1. Facial selection was speculated based on the Cieplak effect. This ozonolysis was successfully applied for the synthesis of 14R,15S- and 14S,15S-diHETEs (anti and syn isomers, respectively) in enantioenriched forms.

A Randomized Placebo Controlled Trial of Ibuprofen for Respiratory Syncytial Virus Infection in a Bovine Model

Background

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and hospital admission in infants. An analogous disease occurs in cattle and costs US agriculture a billion dollars a year. RSV causes much of its morbidity indirectly via adverse effects of the host response to the virus. RSV is accompanied by elevated prostaglandin E₂ (PGE₂) which is followed by neutrophil led inflammation in the lung. Ibuprofen is a prototypical non-steroidal anti-inflammatory drug that decreases PGE₂ levels by inhibiting cyclooxygenase.

Hypotheses

We hypothesized that treatment of RSV with ibuprofen would decrease PGE₂ levels, modulate the immune response, decrease clinical illness, and decrease the histopathological lung changes in a bovine model of RSV. We further hypothesized that viral replication would be unaffected.

Methods

We performed a randomized placebo controlled trial of ibuprofen in 16 outbred Holstein calves that we infected with RSV. We measured clinical scores, cyclooxygenase, lipoxygenase and endocannabinoid products in plasma and mediastinal lymph nodes and interleukin (Il)-4, Il-13, Il-17 and interferon-γ in mediastinal lymph nodes. RSV shedding was measured daily and nasal Il-6, Il-8 and Il-17 every other day. The calves were necropsied on Day 10 post inoculation and histology performed.

Results

One calf in the ibuprofen group required euthanasia on Day 8 of infection for respiratory distress. Clinical scores (p<0.01) and weight gain (p = 0.08) seemed better in the ibuprofen group. Ibuprofen decreased cyclooxygenase, lipoxygenase, and cytochrome P450 products, and increased monoacylglycerols in lung lymph nodes. Ibuprofen modulated the immune response as measured by narrowed range of observed Il-13, Il-17 and IFN-γ gene expression in mediastinal lymph nodes. Lung histology was not different between groups, and viral shedding was increased in calves randomized to ibuprofen.

Conclusions

Ibuprofen decreased PGE₂, modulated the immune response, and improved clinical outcomes. However lung histopathology was not affected and viral shedding was increased.

Evolutionary aspects of lipoxygenases and genetic diversity of human leukotriene signaling

Leukotrienes are pro-inflammatory lipid mediators, which are biosynthesized via the lipoxygenase pathway of the arachidonic acid cascade. Lipoxygenases form a family of lipid peroxidizing enzymes and human lipoxygenase isoforms have been implicated in the pathogenesis of inflammatory, hyperproliferative (cancer) and neurodegenerative diseases. Lipoxygenases are not restricted to humans but also occur in a large number of pro- and eucaryotic organisms. Lipoxygenase-like sequences have been identified in the three domains of life (bacteria, archaea, eucarya) but because of lacking functional data the occurrence of catalytically active lipoxygenases in archaea still remains an open question. Although the physiological and/or pathophysiological functions of various lipoxygenase isoforms have been studied throughout the last three decades there is no unifying concept for the biological importance of these enzymes. In this review we are summarizing the current knowledge on the distribution of lipoxygenases in living single and multicellular organisms with particular emphasis to higher vertebrates and will also focus on the genetic diversity of enzymes and receptors involved in human leukotriene signaling.

Orthopoxvirus genes that mediate disease virulence and host tropism

In the course of evolution, viruses have developed various molecular mechanisms to evade the defense reactions of the host organism. When understanding the mechanisms used by viruses to overcome manifold defense systems of the animal organism, represented by molecular factors and cells of the immune system, we would not only comprehend better but also discover new patterns of organization and function of these most important reactions directed against infectious agents. Here, study of the orthopoxviruses pathogenic for humans, such as variola (smallpox), monkeypox, cowpox, and vaccinia viruses, may be most important. Analysis of the experimental data, presented in this paper, allows to infer that variola virus and other orthopoxviruses possess an unexampled set of genes whose protein products efficiently modulate the manifold defense mechanisms of the host organisms compared with the viruses from other families.

Production of prostaglandin E₂ in response to infection with modified vaccinia Ankara virus

Prostaglandin E₂ (PGE₂) is an arachidonic acid (AA)-derived signaling molecule that can influence host immune responses to infection or vaccination. In this study, we investigated PGE₂ production in vitro by cells infected with the poxvirus vaccine strain, modified vaccinia Ankara virus (MVA). Human THP-1 cells, murine bone marrow-derived dendritic cells, and murine C3HA fibroblasts all accumulated PGE₂ to high levels in culture supernatants upon infection with MVA. We also demonstrated that MVA induced the release of AA from infected cells, and this was, most unusually, independent of host cytosolic phospholipase A₂ activity. The accumulation of AA and PGE₂ was dependent on viral gene expression, but independent of canonical NF-κB signaling via p65/RelA. The production of PGE₂ required host cyclooxygenase-2 (COX-2) activity, and COX-2 protein accumulated during MVA infection. The results of this study provide insight into a novel aspect of MVA biology that may affect the efficacy of MVA-based vaccines.

vLIP, a viral lipase homologue, is a virulence factor of Marek's disease virus

The genome of Marek's disease virus (MDV) has been predicted to encode a secreted glycoprotein, vLIP, which bears significant homology to the alpha/beta hydrolase fold of pancreatic lipases. Here it is demonstrated that MDV vLIP mRNA is produced via splicing and that vLIP is a late gene, due to its sensitivity to inhibition of DNA replication. While vLIP was found to conserve several residues essential to hydrolase activity, an unfavorable asparagine substitution is present at the lipase catalytic triad acid position. Consistent with structural predictions, purified recombinant vLIP did not show detectable activity on traditional phospholipid or triacylglyceride substrates. Two different vLIP mutant viruses, one bearing a 173-amino-acid deletion in the lipase homologous domain, the other having an alanine point mutant at the serine nucleophile position, caused a significantly lower incidence of Marek's disease in chickens and resulted in enhanced survival relative to two independently produced vLIP revertants or parental virus. These data provide the first evidence that vLIP enhances the replication and pathogenic potential of MDV. Furthermore, while vLIP may not serve as a traditional lipase enzyme, the data indicate that the serine nucleophile position is nonetheless essential in vivo for the viral functions of vLIP. Therefore, it is suggested that this particular example of lipase homology may represent the repurposing of an alpha/beta hydrolase fold toward a nonenzymatic role, possibly in lipid bonding.

Inhibition of fowlpox virus by an aqueous acetone extract from galls of Guiera senegalensis J. F. Gmel (Combretaceae)

An aqueous acetone extract from the galls of Guiera senegalensis was screened for in vitro antiviral activity against fowlpox virus (FPV). Cytopathic effect (CPE) inhibition and plaque inhibition assays were used to show presence of antiviral effects against FPV, whilst cytotoxicity assays established the relative safety of the extract for cells in vitro. Phytochemical analyses revealed the presence of phenolic compounds including flavonoids, tannins and anthocyanins as well as steroids and alkaloids. Thin-layer chromatographical (TLC) analysis also revealed the presence of quercitrin, quercetin, kaempferol, apigenin, rutin, gallic acid as well as unknown flavonoids and unknown phenolic acids. The antiviral effect of the extract was partially attributed to phenolic components including flavonoids.

Cyclooxygenase-1 and -2 are required for production of infectious pseudorabies virus

We have recently shown that cyclooxygenase-2 (COX-2) transcription is markedly induced after herpes simplex virus type 1 and pseudorabies virus (PRV) infections of rat embryonic fibroblast (REF) cells (N. Ray and L. W. Enquist, J. Virol. 78:3489-3501, 2004). For this study, we investigated the role of cyclooxygenase induction in the replication and growth of PRV. We demonstrate here a concordant increase in COX-2 mRNA and protein levels after the infection of REF cells. Inhibitors blocking the activity of cyclooxygenases caused a dramatic reduction in PRV growth. Viral growth could be restored if prostaglandin E(2), the final product of COX-2 activity, was added simultaneously with the COX inhibitors. Immediate-early protein IE180, major capsid protein VP5, and glycoprotein expression were slightly reduced in the presence of COX-2 inhibitors, but expression of the early protein EP0 was not affected by COX inhibition. Viral DNA replication was marginally reduced in the presence of a COX-1/2 inhibitor, but there was no defect in viral DNA cleavage. Electron microscopy analysis revealed an increased number of unusual empty capsid structures in the nuclei of cells infected with PRV in the presence of a COX-1/2 inhibitor. These capsid structures shared some characteristics with procapsids but had a novel appearance by negative staining. Our data establish a role for COX-1 and COX-2 in facilitating the efficient growth and replication of PRV in primary cells.

Inhibition of cyclooxygenase activity reduces rotavirus infection at a postbinding step

Elevated levels of prostaglandins (PGs), products of cyclooxygenases (COXs), are found in the plasma and stool of rotavirus-infected children. We sought to determine the role of COXs, PGs, and the signal transduction pathways involved in rotavirus infection to elucidate possible new targets for antiviral therapy. Human intestinal Caco-2 cells were infected with human rotavirus Wa or simian rotavirus SA-11. COX-2 mRNA expression and secreted PGE2 levels were determined at different time points postinfection, and the effect of COX inhibitors on rotavirus infection was studied by an immunofluorescence assay (IFA). To reveal the signal transduction pathways involved, the effect of MEK, protein kinase A (PKA), p38 mitogen-activated protein kinase (MAPK), and NF-kappaB inhibitors on rotavirus infection was analyzed. In infected Caco-2 cells, increased COX-2 mRNA expression and secreted PGE2 levels were detected. Indomethacin (inhibiting both COX-1 and COX-2) and specific COX-1 and COX-2 inhibitors reduced rotavirus infection by 85 and 50%, respectively, as measured by an IFA. Indomethacin reduced virus infection at a postbinding step early in the infection cycle, inhibiting virus protein synthesis. Indomethacin did not seem to affect viral RNA synthesis. Inhibitors of MEK, PKA, p38 MAPK, and NF-kappaB decreased rotavirus infection by at least 40%. PGE2 counteracted the effect of the COX and PKA inhibitors but not of the MEK, p38 MAPK, and NF-kappaB inhibitors. Conclusively, COXs and PGE2 are important mediators of rotavirus infection at a postbinding step. The ERK1/2 pathway mediated by PKA is involved in COX induction by rotavirus infection. MAPK and NF-kappaB pathways are involved in rotavirus infection but in a PGE2-independent manner. This report offers new perspectives in the search for therapeutic agents in treatment of severe rotavirus-mediated diarrhea in children.

The role and regulation of COX-2 during viral infection

Prostaglandins are lipid mediators, generated by cyclooxygenase (COX), that have been shown to participate in the regulation of virus replication and the modulation of inflammatory responses following infection. A number of studies support a role for PGE2 in the modulation of virus replication and virulence in a cell type and virus selective manner. Virus infection also stimulates the expression of a number of proinflammatory gene products, including COX-2, inducible nitric oxide synthase (iNOS) as well as proinflammatory cytokines. This review will focus on the mechanisms by which proinflammatory prostaglandin production regulates virus replication and virulence. In addition, the signaling pathways that are activated during a virus infection, and that regulate proinflammatory gene expression in macrophages will be reviewed. Specific attention will be placed on the ability of virus infection to activate multiple signaling cascades (such as PKR, MAPK, iPLA2, NF-kappaB) and how these pathways are integrated in the regulation of individual target gene expression.

The genomic sequence of ectromelia virus, the causative agent of mousepox

Ectromelia virus is the causative agent of mousepox, an acute exanthematous disease of mouse colonies in Europe, Japan, China, and the U.S. The Moscow, Hampstead, and NIH79 strains are the most thoroughly studied with the Moscow strain being the most infectious and virulent for the mouse. In the late 1940s mousepox was proposed as a model for the study of the pathogenesis of smallpox and generalized vaccinia in humans. Studies in the last five decades from a succession of investigators have resulted in a detailed description of the virologic and pathologic disease course in genetically susceptible and resistant inbred and out-bred mice. We report the DNA sequence of the left-hand end, the predicted right-hand terminal repeat, and central regions of the genome of the Moscow strain of ectromelia virus (approximately 177,500 bp), which together with the previously sequenced right-hand end, yields a genome of 209,771 bp. We identified 175 potential genes specifying proteins of between 53 and 1924 amino acids, and 29 regions containing sequences related to genes predicted in other poxviruses, but unlikely to encode for functional proteins in ectromelia virus. The translated protein sequences were compared with the protein database for structure/function relationships, and these analyses were used to investigate poxvirus evolution and to attempt to explain at the cellular and molecular level the well-characterized features of the ectromelia virus natural life cycle.

COX-2 induction during murine gammaherpesvirus 68 infection leads to enhancement of viral gene expression

The murine gammaherpesvirus 68 (MHV-68 or gammaHV-68) model provides many advantages for studying virus-host interactions involved in gammaherpesvirus replication, including the role of cellular responses to infection. We examined the effects of cellular cyclooxygenase-2 (COX-2) and its by-product prostaglandin E(2) (PGE(2)) on MHV-68 gene expression and protein production following de novo infection of cultured cells. Western blot analyses revealed an induction of COX-2 protein in MHV-68-infected cells but not in cells infected with UV-irradiated MHV-68. Luciferase reporter assays demonstrated activation of the COX-2 promoter during MHV-68 replication. Two nonsteroidal anti-inflammatory drugs, a COX-2-specific inhibitor (NS-398) and a COX-1-COX-2 inhibitor (indomethacin), substantially reduced MHV-68 protein production in infected cells. Inhibition of viral protein expression and virion production by NS-398 was reversed in the presence of exogenous PGE(2). Global gene expression analysis using an MHV-68 DNA array showed that PGE(2) increased production of multiple viral gene products, and NS-398 inhibited production of many of the same genes. These studies suggest that COX-2 activity and PGE(2) production may play significant roles during MHV-68 de novo infection.

Analysis of host response modifier ORFs of ectromelia virus, the causative agent of mousepox

From the right-hand end of the ectromelia virus (strain Moscow) genome, 32318 bps have been sequenced, and characterized to include a total of 18 open reading frames (ORFs) and six regions which apparently no longer code for functional proteins. At least six of the ORFs appear to be involved in blocking the inflammatory/immune host response to infection, and therefore probably contribute significantly to the virulence of this virus in its natural host, the mouse. One of these genes encoded an isolog of the poxvirus chemokine binding protein, and was shown to be the most abundant protein secreted from ectromelia virus infected cells. Two regions were found to have significant similarity to poxvirus genes encoding tumor necrosis factor (TNF) binding proteins. Both are distinct from cytokine response modifier (crm)B and crmC but only one is predicted to encode a functional TNF binding protein. A novel similarity between the C-terminal domain of poxvirus TNF binding proteins and several other poxvirus proteins is also presented. The results are discussed in the context of ectromelia virus pathogenesis of mice.

The genomic sequence analysis of the left and right species-specific terminal region of a cowpox virus strain reveals unique sequences and a cluster of intact ORFs for immunomodulatory and host range proteins

Sequencing and computer analysis of the left (52,283 bp) and right (49,649 bp) variable DNA regions of the cowpox virus strain GRI-90 (CPV-GRI) has revealed 51 and 37 potential open reading frames (ORFs), respectively. Comparison of the structure-function organization of these DNA regions of CPV-GRI with those previously published for corresponding regions of genomes of vaccinia virus, strains Copenhagen (VAC-COP) and Western Reserve (VAC-WR); and variola major virus, strains India-1967 (VAR-IND), Bangladesh-1975 (VAR-BSH); and alastrim variola minor virus, strain Garcia-1966 (VAR-GAR), was performed. Within the left terminal region under study, an extended DNA sequence (14,171 bp), unique to CPV, has been found. Within the right region of the CPV-GRI genome two segments, which are unique to CPV DNA (1579 and 3585 bp) have been found. Numerous differences have been revealed in the genetic structure of CPV-GRI DNA regions, homologous to fragments of the genomes of the above-mentioned orthopoxvirus strains. A cluster of ORFs with structural similarity ot immunomodulatory and host range function of other poxviruses have also been detected. A comparison of the sequences of ORF B, crmA, crmB, crmC, IMP, and CHO hr genes of CPV Brighton strain (CPV-BRI) with the corresponding genes in strain GRI-90 have revealed an identity at the amino acid level ranging from 82 to 96% between the two strains. The findings are significant in light of the recent demonstration of CPV as an important poxvirus model system to probe the precise in vivo role(s) of the unique virally encoded immunomodulatory proteins. Also, the presence of a complete and intact repertoire of immunomodulatory proteins, ring canal proteins family, and host range genes indicates that CPV may have been the most ancient of all studied orthopoxviruses.

Complete sequence and genomic analysis of murine gammaherpesvirus 68

Murine gammaherpesvirus 68 (gammaHV68) infects mice, thus providing a tractable small-animal model for analysis of the acute and chronic pathogenesis of gammaherpesviruses. To facilitate molecular analysis of gammaHV68 pathogenesis, we have sequenced the gammaHV68 genome. The genome contains 118,237 bp of unique sequence flanked by multiple copies of a 1,213-bp terminal repeat. The GC content of the unique portion of the genome is 46%, while the GC content of the terminal repeat is 78%. The unique portion of the genome is estimated to encode at least 80 genes and is largely colinear with the genomes of Kaposi's sarcoma herpesvirus (KSHV; also known as human herpesvirus 8), herpesvirus saimiri (HVS), and Epstein-Barr virus (EBV). We detected 63 open reading frames (ORFs) homologous to HVS and KSHV ORFs and used the HVS/KSHV numbering system to designate these ORFs. gammaHV68 shares with HVS and KSHV ORFs homologous to a complement regulatory protein (ORF 4), a D-type cyclin (ORF 72), and a G-protein-coupled receptor with close homology to the interleukin-8 receptor (ORF 74). One ORF (K3) was identified in gammaHV68 as homologous to both ORFs K3 and K5 of KSHV and contains a domain found in a bovine herpesvirus 4 major immediate-early protein. We also detected 16 methionine-initiated ORFs predicted to encode proteins at least 100 amino acids in length that are unique to gammaHV68 (ORFs M1 to 14). ORF M1 has striking homology to poxvirus serpins, while ORF M11 encodes a potential homolog of Bcl-2-like molecules encoded by other gammaherpesviruses (gene 16 of HVS and KSHV and the BHRF1 gene of EBV). In addition, clustered at the left end of the unique region are eight sequences with significant homology to bacterial tRNAs. The unique region of the genome contains two internal repeats: a 40-bp repeat located between bp 26778 and 28191 in the genome and a 100-bp repeat located between bp 98981 and 101170. Analysis of the gammaHV68, HVS, EBV, and KSHV genomes demonstrated that each of these viruses have large colinear gene blocks interspersed by regions containing virus-specific ORFs. Interestingly, genes associated with EBV cell tropism, latency, and transformation are all contained within these regions encoding virus-specific genes. This finding suggests that pathogenesis-associated genes of gammaherpesviruses, including gammaHV68, may be contained in similarly positioned genome regions. The availability of the gammaHV68 genomic sequence will facilitate analysis of critical issues in gammaherpesvirus biology via integration of molecular and pathogenetic studies in a small-animal model.

Attenuation of B5R mutants of rabbitpox virus in vivo is related to impaired growth and not an enhanced host inflammatory response

The rabbitpox virus (RPV) B5R protein, synthesized late in infection, is found as a 45-kDa membrane-associated protein of the envelope of infectious extracellular enveloped virus (EEV) and as a 38-kDa protein secreted from the cell by a process independent of morphogenesis. The protein is not found associated with intracellular mature virus (IMV). Deletion of the gene attenuates the virus (RPV delta B5R) in animals (mice and rabbits), has relatively little effect on formation of IMV, prevents EEV formation in some but not all cells, and leads to a reduced host range. Analysis of the sequence of the protein suggests relatedness to factor H of the complement cascade. Collectively, these observations suggest that attenuation of the virus in vivo could be linked to an inhibition of the inflammatory response, a deficiency in growth, or both. In this report we have analyzed the behavior of RPV delta B5R in infected mice and rabbits and conclude that attenuation of the mutant virus likely results from simple failure to grow within the infected animal and that the inflammatory response probably contributes little to the observed attenuation.

Functional organization of variola major and vaccinia virus genomes

Comparison of the genomic organization of variola and vaccinia viruses has been carried out. Molecular factors of virulence of these viruses is the focus of this review. Possible roles of the genes of soluble cytokine receptors, complement control proteins, factors of virus replication, and dissemination in vivo for variola virus pathogenesis are discussed. The existence of "buffer" genes in the vaccinia virus genome is proposed.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

Multigenic evasion of inflammation by poxviruses

Analyses of different cowpox virus (Brighton Red strain [CPV-BR]) mutants indicate that there is a minimum of three genes encoded by CPV-BR that are nonessential for virus replication in tissue culture but are involved in inhibiting the generation of an inflammatory response in the chicken embryo chorioallantoic membrane (CAM) model. The CPV-BR-encoded anti-inflammatory genes include the gene encoding the 38-kDa protein (also called 38K, crmA, SPI-2, or VV-WR-ORF-B13R), a tumor necrosis factor receptor homolog, and an unidentified gene that maps to the right end of the CPV genome. The kinetics of triggering of an inflammatory response at the site of virus infection as well as the magnitude of the response is dependent on the virus-encoded inhibitor that is deleted. Virus yields recovered from pocks decreased in proportion to the magnitude of the inflammatory response. The deletion of these identified inhibitors of inflammation was associated with attenuation of the mutant viruses in mice. These data confirm the existence of multiple poxvirus-encoded host defense modifiers whose function is to block the generation of an inflammatory response at the site of virus infection, which allows enhanced virus replication and potentially facilitates virus transmission.

Importance of interferons in recovery from mousepox

Gamma interferon is shown to be critical in recovery of C57BL/6 mice from mousepox. Anti-gamma interferon treatment of mice infected in the footpad with ectromelia virus resulted in enhanced spread to and efficient virus replication in the spleen, lungs, ovaries, and, especially, liver. All treated, infected mice died within a mean of 7 days, 2.5 days earlier than mice with severe combined immunodeficiency that were given a comparable infection. On the other hand, alpha interferon appeared not to have a major role in controlling virus replication in tissues examined, and beta interferon was important for virus clearance in the liver and ovaries but not the spleen. Either anti-alpha, beta interferon or anti-beta interferon antibody therapy resulted in only 25% mortality. Infected control mice survived but showed persistence of ectromelia virus at the site of infection (the footpad) and transient presence of the virus in the spleen, liver, lungs, and ovaries and in the fibroreticular but not lymphoid cells of the draining popliteal lymph node. Depletion of gamma interferon but not alpha and/or beta interferon resulted in a significant reduction in the numbers of splenic T (especially gamma delta-TCR+), B, and Mac-1+ cells, although the proportion of Mac-1+ cells in the spleen increased compared with control values. Depletion of alpha, beta, or gamma interferons did not severely affect the generation of virus-specific cytotoxic T-lymphocyte responses or natural killer cell cytolytic activity. This study, in which a natural virus disease model was used, underscores the crucial importance of gamma interferon in virus clearance at all stages of infection and in all tissues tested except the primary site of infection, where virus clearance appears to be delayed.