Role of interferon in persistent infection of macrophages with herpes simplex virus

Host factor SAMHD1 restricts DNA viruses in non-dividing myeloid cells

SAMHD1 is a newly identified anti-HIV host factor that has a dNTP triphosphohydrolase activity and depletes intracellular dNTP pools in non-dividing myeloid cells. Since DNA viruses utilize cellular dNTPs, we investigated whether SAMHD1 limits the replication of DNA viruses in non-dividing myeloid target cells. Indeed, two double stranded DNA viruses, vaccinia and herpes simplex virus type 1, are subject to SAMHD1 restriction in non-dividing target cells in a dNTP dependent manner. Using a thymidine kinase deficient strain of vaccinia virus, we demonstrate a greater restriction of viral replication in non-dividing cells expressing SAMHD1. Therefore, this study suggests that SAMHD1 is a potential innate anti-viral player that suppresses the replication of a wide range of DNA viruses, as well as retroviruses, which infect non-dividing myeloid cells.

Various viral pathogens such as HIV-1, herpes simplex virus (HSV) and vaccinia virus infect terminally-differentiated/non-dividing macrophages during the course of viral pathogenesis. Unlike dividing cells, non-dividing cells lack chromosomal DNA replication, do not enter the cell cycle, and harbor very low levels of cellular dNTPs, which are substrates of viral DNA polymerases. A series of recent studies revealed that the host protein SAMHD1 is dNTP triphosphohydrolase, which contributes to the poor dNTP abundance in non-dividing myeloid cells, and restricts proviral DNA synthesis of HIV-1 and other lentiviruses in macrophages, dendritic cells, and resting T cells. In this report, we demonstrate that SAMHD1 also controls the replication of large dsDNA viruses: vaccinia virus and HSV-1, in primary human monocyte-derived macrophages. SAMHD1 suppresses the replication of these DNA viruses to an even greater extent in the absence of viral genes that are involved in dNTP metabolism such as thymidine kinase. Therefore, this study supports that dsDNA viruses evolved to express enzymes necessary to increase the levels of dNTPs as a mechanism to overcome the restriction induced by SAMHD1 in myeloid cells.

Macrophages and cytokines in the early defence against herpes simplex virus

Herpes simplex virus (HSV) type 1 and 2 are old viruses, with a history of evolution shared with humans. Thus, it is generally well-adapted viruses, infecting many of us without doing much harm, and with the capacity to hide in our neurons for life. In rare situations, however, the primary infection becomes generalized or involves the brain. Normally, the primary HSV infection is asymptomatic, and a crucial element in the early restriction of virus replication and thus avoidance of symptoms from the infection is the concerted action of different arms of the innate immune response. An early and light struggle inhibiting some HSV replication will spare the host from the real war against huge amounts of virus later in infection. As far as such a war will jeopardize the life of the host, it will be in both interests, including the virus, to settle the conflict amicably. Some important weapons of the unspecific defence and the early strikes and beginning battle during the first days of a HSV infection are discussed in this review. Generally, macrophages are orchestrating a multitude of anti-herpetic actions during the first hours of the attack. In a first wave of responses, cytokines, primarily type I interferons (IFN) and tumour necrosis factor are produced and exert a direct antiviral effect and activate the macrophages themselves. In the next wave, interleukin (IL)-12 together with the above and other cytokines induce production of IFN-gamma in mainly NK cells. Many positive feed-back mechanisms and synergistic interactions intensify these systems and give rise to heavy antiviral weapons such as reactive oxygen species and nitric oxide. This results in the generation of an alliance against the viral enemy. However, these heavy weapons have to be controlled to avoid too much harm to the host. By IL-4 and others, these reactions are hampered, but they are still allowed in foci of HSV replication, thus focusing the activity to only relevant sites. So, no hero does it alone. Rather, an alliance of cytokines, macrophages and other cells seems to play a central role. Implications of this for future treatment modalities are shortly considered.

Re-evaluating natural resistance to herpes simplex virus type 1

It is often stated that individuals of a species can differ significantly in their innate resistance to infection with herpes simplex virus type 1 (HSV-1). Three decades ago Lopez reported that C57BL/6 mice could survive a 5,000-fold-higher inoculum of HSV-1 given intraperitoneally than mice of the A or BALB/c strain (Nature 258:152-153, 1975). Susceptible strains of mice died of encephalitis-like symptoms, suggesting that viral spread to the central nervous system was the cause of death. Although Lopez's study documented that C57BL/6 mice were resistant to the development of HSV-1 encephalitis and mortality, the resistance of C57BL/6 mice to other steps of the HSV-1 infection process was not assessed. The results of the present study extend these observations to clarify the difference between resistance to (i) HSV-1 pathogenesis, (ii) HSV-1 replication, (iii) HSV-1 spread, and (iv) the establishment of latent HSV-1 infection. Although C57BL/6 mice are more resistant to HSV-1 pathogenesis than BALB/c mice, the results of the present study establish that HSV-1 enters, replicates, spreads, and establishes latent infections with virtually identical efficiencies in C57BL/6 and BALB/c mice. These observations raise questions about the validity of the inference that differences in natural resistance are relevant in explaining what differentiates humans with recurrent herpetic disease from the vast majority of asymptomatic carriers of HSV-1 and HSV-2.

Characteristics of a macrophage culture persistently infected with herpes simplex virus type 1

BACKGROUND

Persistence of herpes simplex type 1 (HSV-1) has been reported in sensory neurons, corneal epithelium, and lymphocytes, although other cell types such as macrophages should also be considered as hosts for HSV-1 persistence. Here we report the establishment and characterization of HSV-1 persistence in an immortalized murine macrophage-like cell line (P388D1).

METHODS

The persistently HSV-1 infected culture (P388D1per) was obtained from surviving P388D1 macrophages infected with HSV-1 MP strain at multiplicity of 0.001. P388D1per was characterized by [corrected] extracellular production of viruses, cells expressing viral antigens, and cells releasing infectious viruses. Viral plaque size and cytophatic effect were determined in viruses (HSVA and HSVB) obtained from two different P388D1per passages. Host and viral proteins were detected in P388D1per and in P388D1 cells infected with HSV-1 by metabolic [35S]-methionine labeling assays.

RESULTS

P388D1per culture was characterized [corrected] by cyclic production of infectious viruses from non-detectable to 10(6) TCID50/mL, [corrected] from 1.0 to 15.0% cells expressing viral antigens and macrophages released infectious viruses from 0.008 to 12.5%. Differences in viral plaque size and cytopathic effect morphology between HSVA, HSVB and HSV-1 were observed. Similar patterns of viral proteins were observed in P388D1per and in P388D1 infected with HSV-1. Nonetheless, the characteristic interference effect of HSV-1 on host protein synthesis was not observed in P388D1per culture.

CONCLUSIONS

An HSV-1 persistently infected immortalized macrophage culture was established and characterized. Virus produced during persistence showed phenotypic alterations with respect to the original virus. P388D1per cell protein synthesis was not affected by the presence of HSV-1.

Spontaneous shift of an HSV type 1 productive, persistently infected macrophage-like culture to a non-productive culture

A herpes simplex virus type 1 (HSV1) productive, persistently infected murine macrophage-like culture was obtained by infection at low multiplicity (0.001). Persistent cultures were characterized by spontaneously ceasing to produce infectious viruses after initial HSV1 production. A similar pattern of virus production was observed in cultures obtained by reinfection of the persistent macrophages with the original virus. Nonetheless, in persistent and reinfected cultures (17/18) which did not produce infectious viruses, viral proteins were detected by immunoblot, although their molecular weight, number and pattern differed among the cultures.

Growth and cytopathogenicity of herpes simplex virus in a macrophage cell line, RAW264: A good indicator of intraperitoneal pathogenicity

Macrophages are known to play a critical role in host resistance to herpes simplex virus (HSV). In this study, we investigate the interaction between various HSV strains with different virulence and a murine macrophage cell line, RAW264. Highly attenuated strains replicated poorly in RAW264 cells and were cleared from the cultures. For the eleven viruses tested, there was good correlation between intraperitoneal pathogenicity for adult mice and replication in RAW264 cells. It was also shown that interferon alpha/beta was involved in restricted replication of some strains.

Transcriptional and translational analyses of the UL2 gene of equine herpesvirus 1: a homolog of UL55 of herpes simplex virus type 1 that is maintained in the genome of defective interfering particles

Defective interfering particles (DIPs) of equine herpesvirus 1 (EHV-1; Kentucky A strain) mediate persistent infection. DNA sequences at the L terminus, which contain the UL2 gene (homolog of UL55 of herpes simplex virus type 1 and open reading frame 3 of varicella-zoster virus) of standard EHV-1, have been shown to be highly conserved in all clones of the EHV-1 DIP genome. The UL2 mRNA was characterized by S1 nuclease analyses, which mapped the 5' and 3' termini of the 0.9-kb early UL2 mRNA to approximately 26 and 16 nucleotides downstream of a TTTAAA box and polyadenylation signal, respectively. The UL2 open reading frame, present within both the EHV-1 standard and DIP genomes, was inserted into the transcription expression vector pGEM-3Z to yield constructs pGEML2 and pDIL2, respectively. After in vitro transcription and translation, both constructs yielded a comigrating 23-kDa protein, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyclonal antiserum was raised against the UL2 protein by injecting rabbits with a TrpE/UL2 fusion protein expressed from plasmid pATH23L2 in Escherichia coli. The UL2-specific antiserum reacted in Western immunoblot and immunoprecipitation analyses with a 23-kDa polypeptide synthesized in cells infected with standard EHV-1 or DIP-enriched virus. These data also indicated that the UL2 polypeptide was more abundant in DIP-infected cells than in standard EHV-1-infected cells. Results from time course and pulse-chase analyses suggested that the UL2 polypeptide has a rapid turnover rate in DIP-infected cells.

A major role of macrophage activation by interferon-gamma during mouse hepatitis virus type 3 infection. II. Age-dependent resistance

In contrast to adult mice, young A/J mice, developed an acute hepatitis following infection with Mouse Hepatitis virus type 3. 100% of the young animals died 4 to 5 days after the infection and high levels of virus were found in the liver and peritoneal exudate. Very low levels of IFN-gamma were found in the serum and peritoneal exudate of infected young mice. This was in contrast to the levels observed in adult mice. Spleen cells and macrophage cultures from young A/J mice, again in contrast to adult A/J mice, were shown to be unable to synthesize IFN-gamma and IFN-alpha/beta respectively. Macrophages from either young or adult A/J mice were able to be activated with exogenous recombinant IFN-gamma or IFN-alpha/beta, enabling both sets of cells to restrict MHV3 replication. The results indicate that the ability of the immune system to synthesize IFN-gamma and IFN-alpha/beta may play a major role in the age-dependent resistance of A/J mice to MHV3.

Stimulation of macrophages by endotoxin results in the reactivation of a persistent herpes simplex virus infection

Previous work has shown that splenic macrophages derived from herpes simplex virus (HSV)-resistant C57BL/6 mice undergo a persistent HSV infection which is characterized by the continuous release of infectious virus particles from a small subpopulation of infected cells. Treatment of persistently infected macrophages for 2 weeks with lipopolysaccharide (LPS) resulted in an increase of HSV yield and in virus-induced cytopathic effects. HSV was also reactivated by treatment of macrophage cultures with lipid A or tumour necrosis factor (TNF). Like macrophages of C57BL/6 origin, cells from LPS-hyporesponsive C3H/HeJ mice could be persistently infected with HSV. These cells were resistant to LPS-induced virus reactivation. The results show that macrophages derived from C57BL/6 mice are rendered susceptible to lytic HSV infection by treatment with LPS or TNF. Thus, these substances may interfere with persistent HSV infection which can be established due to genetically controlled properties of the host.

In vitro development of bone-marrow-derived macrophages. Influence of mouse genotype on response to colony-stimulating factors and autocrine interferon induction

Responsiveness to granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage CSF (M-CSF) of bone marrow cells derived from different mouse strains was investigated. There were great variations in proliferation between different strains of inbred mice. Bone marrow cells from mouse strains with a high rate of proliferation in response to GM-CSF also had a high proliferating capacity to M-CSF. The response to either CSF did not correlate with a certain H-2 haplotype. GM-CSF induced consistently higher proliferation than M-CSF. Proliferation in response to M-CSF, but not to GM-CSF, could be enhanced by the addition of antibodies against interferon (IFN). IFN is the only known inducer of (2'-5') oligoadenylate (oligo (A] synthetase. This enzyme was induced in macrophages grown in the presence of M-CSF, but not in GM-CSF promoted cells. Enzyme induction was completely abrogated by simultaneous treatment with anti-IFN alpha/beta. Infection of macrophages with herpes simplex virus type 1 (HSV) and vesicular stomatitis virus (VSV) revealed that GM-CSF-promoted cells were highly susceptible to lytic infection by these viruses. In contrast, virus titres in M-CSF-cultured cells were 100-fold lower. We conclude that, contrary to M-CSF, GM-CSF does not induce autocrine IFN during haematopoiesis. As judged from data with BALB/c mice, the sensitivity to the anti-proliferative effect of the autocrine IFN may be a factor which influences M-CSF-promoted proliferation.

Molecular localization of abortive infection of resident peritoneal macrophages by herpes simplex virus type 1

Mononuclear phagocytes exhibit different patterns of intrinsic resistance to herpes simplex virus type 1 (HSV-1) that are related to the heterogeneity of macrophage populations and may reflect the particular differentiation or maturation state of the macrophages. In this study, we characterized the molecular basis for the block in HSV-1 replication in resident peritoneal macrophages from B6C3F1 mice. Infected resident peritoneal macrophages were analyzed for the presence of virus-specific mRNA by Northern (RNA) blotting and in situ hybridization and for proteins by immunofluorescence. The data were compared with those obtained in HSV-1-infected permissive Vero cells. The immediate-early genes ICP4, ICP0, ICP22, and ICP27 were transcribed in resident peritoneal macrophages, as was the early gene tk. Virus-specific mRNA for the major DNA-binding protein ICP8 was barely detectable, and that for another early gene, the viral DNA polymerase, was not detected. In addition, transcripts for the delayed-early gene glycoprotein D and the true late gene glycoprotein C (gC) were not detectable in resident peritoneal macrophages. In situ hybridization and immunofluorescence studies confirmed that transcripts and proteins for the immediate-early and some early HSV-1 genes were present. These data also established that 14% of the resident peritoneal macrophages were positive for RNA and polypeptide specific for the immediate-early gene ICP4 and that 7 to 11% were positive for RNA or polypeptides specific for the early genes tk and ICP8. The fact that only a few cells expressed viral products emphasizes the heterogeneity that exists even in this relatively homogeneous resident peritoneal macrophage population. Consistent with the Northern blot analysis, no RNA specific for the early DNA polymerase gene or the late gC gene was detected by in situ hybridization nor could the polypeptide for the gC gene be seen by immunofluorescence. Thus, while early transcriptional events were initiated in some resident peritoneal macrophages, there was a block in replication localized at the level of expression of the early to delayed-early viral genes.

Resistance of human blood monocytes to infection with herpes simplex virus

Human blood monocytes isolated by centrifugal elutriation were resistant to infection with herpes simplex virus type 1 (HSV). In vitro cultivation for several days resulted in a stepwise increase of virus yield. Similar amounts of virus absorbed to fresh and cultured monocytes. No viral DNA was associated with the nuclear fraction of freshly isolated monocytes early after infection indicating that early steps of virus infection were already inhibited in resistant cells. This argues against HSV induced interferon (IFN) being a major mediator of resistance. Culturing the cells for some hours was sufficient to overcome the early block. However, as revealed by virus yield assays, monocytes after 1 day of cultivation were not as susceptible as cells after 6 days of cultivation. Viral sequences could be demonstrated in the nuclei of freshly isolated monocytes after treatment with the fusion-promoting agent polyethylene glycol. Under these conditions no DNA replication occurred, indicating that overcoming the block of viral DNA entry into the nucleus was not sufficient to break resistance of the cells. Thus, the results show that several mechanisms are involved in the primary resistance of human blood monocytes to HSV.

Restriction of HIV-1 replication in promonocytic cells: a role for IFN-alpha

A comparative study of the replication kinetics of human immunodeficiency virus type 1 (HIV-1) was performed in the promonocytic U937 cells and in the T lymphoblastoid H9 cells. If a productive HIV-1 infection of both cell types could be established, the time which elapses before most of the cells could express viral proteins is always proportionally longer for U937 cells than for H9 cells. Indeed, when U937 cells are infected with HIV-1, this nonproductive phase is followed by a lag phase during which the percentage of virus-producing cells is slowly increasing when compared to H9 cells. The restriction of HIV-1 replication in U937 cells might be consecutive to the lower adsorption of viral particles to these cells, even though the same percentage of U937 and H9 cells was expressing the CD4 molecule. Furthermore, we demonstrate that HIV-1 replication in U937 cells is mainly restricted by endogenous IFN-alpha. Indeed, addition of anti-IFN-alpha antibodies at the time of infection, during the nonproductive phase of the viral replication cycle, or during the lag phase leads to an earlier expression of viral proteins and/or to a rapid increase in the percentage of virus-producing cells. Likewise, the treatment of cultures of HIV-1 chronically infected U937 cells with the same antibodies induces an increased production of viral particles. Thus, IFN-alpha appears to be involved in the persistence of HIV-1 in the monocytes/macrophages of infected individuals.

Interferon treatment inhibits onset of herpes simplex virus immediate-early transcription

Pretreatment of primary cultures of splenic mouse macrophages with murine IFN-alpha/beta leads to a stable inhibition of herpes simplex virus type 1. Analysis of viral DNA, RNA, and protein synthesis identifies expression of "immediate-early" genes as a major target of IFN-mediated inhibition. Determination of viral DNA in the nuclei early after infection, i.e., before onset of DNA replication, suggests that virus uptake, transport to the nucleus, and DNA stability are not decreased in IFN-pretreated macrophages. Nuclear runoff transcription analysis shows a significant reduction of immediate-early transcription rates following IFN treatment. End-specific probes for the ICP4 gene locate the inhibition to the onset of transcription. Northern blot analysis reveals a decrease in ICP4 transcripts in accordance with the observed inhibition of transcription. The observed inhibition of early gene transcription may be a consequence of decreased immediate-early gene expression.