Murine Cytomegalovirus Infection Inhibits Tumor Necrosis Factor Alpha Responses in Primary Macrophages

102

111

Interstitial pneumonia is a major clinical manifestation of primary or recurrent cytomegalovirus (CMV) infection in immunocompromised recipients of a bone marrow transplant. In a murine model, lungs were identified as a prominent site of CMV latency and recurrence. Pulmonary latency of murine CMV is characterized by high viral genome burden and a low incidence of variegated immediate-early (IE) gene expression, reflecting a sporadic activity of the major IE promoters (MIEPs) and enhancer. The enhancer-flanking promoters MIEP1/3 and MIEP2 are switched on and off during latency in a ratio of ϳ2:1. MIEP1/3 latencyassociated activity generates the IE1 transcript of the ie1/3 transcription unit but not the alternative splicing product IE3 that encodes the essential transactivator of early gene expression. Splicing thus appeared to be an important checkpoint for maintenance of latency. In accordance with previous work of others, we show here that signaling by the proinflammatory cytokine tumor necrosis factor alpha (TNF-␣) activates IE1/3 transcription in vivo. As an addition to current knowledge, Poisson distribution analysis revealed an increased incidence of IE1/3 transcriptional events as well as a higher amount of transcripts per event. Notably, TNF-␣ promoted the splicing to IE3 transcripts, but transcription did not proceed to the M55/gB early gene. Moreover, the activated transcriptional state induced by TNF-␣ did not predispose latently infected mice to a higher incidence of virus recurrence after hematoablative treatment. In conclusion, TNF-␣ is an important inductor of IE gene transcriptional reactivation, whereas early genes downstream in the viral replicative cycle appear to be the rate-limiting checkpoint(s) for virus recurrence.

mentioning

confidence: 70%

Role for Tumor Necrosis Factor Alpha in Murine Cytomegalovirus Transcriptional Reactivation in Latently Infected Lungs

Simon

Seckert

Dreis

et al. 2005

102

111

“…Popkin and Virgin have shown previously that MCMV inhibits TNF-␣-induced NF-B activation in infected macrophages and have correlated this effect with the down-regulation of TNF receptor 1 (TNFR1) from the cell surface (35). Receptor down-regulation may render MCMV-infected cells unresponsive to TNF-␣.…”

Section: Discussionmentioning

confidence: 99%

Murine Cytomegalovirus m38.5 Protein Inhibits Bax-Mediated Cell Death

Jurak

Schumacher

Šimić

et al. 2008

Many viruses encode proteins that inhibit the induction of programmed cell death at the mitochondrial checkpoint. Murine cytomegalovirus (MCMV) encodes the m38.5 protein, which localizes to mitochondria and protects human HeLa cells and fibroblasts from apoptosis triggered by proteasome inhibitors but not from Fas-induced apoptosis. However, the ability of this protein to suppress the apoptosis of murine cells and its role during MCMV infection have not been investigated previously. Here we show that m38.5 is expressed at early time points during MCMV infection. Cells infected with MCMVs lacking m38.5 showed increased sensitivity to cell death induced by staurosporine, MG132, or the viral infection itself compared to the sensitivity of cells infected with wild-type MCMV. This defect was eliminated when an m38.5 or Bcl-X L gene was inserted into the genome of a deletion mutant. Using fibroblasts deficient in the proapoptotic Bcl-2 family proteins Bak and/or Bax, we further demonstrated that m38.5 protected from Bax-but not Bak-mediated apoptosis and interacted with Bax in infected cells. These results consolidate the role of m38.5 as a viral mitochondrion-localized inhibitor of apoptosis and its functional similarity to the human cytomegalovirus UL37x1 gene product. Although the m38.5 gene is not homologous to the UL37x1 gene at the sequence level, m38.5 is conserved among rodent cytomegaloviruses. Moreover, the fact that MCMV-infected cells are protected from both Bak-and Bax-mediated cell death suggests that MCMV possesses an additional, as-yet-unidentified mechanism to block Bak-mediated apoptosis.

“…To initiate transcriptional programs leading to productive infection, genes must be reactivated by cytokines or the viral activation of transcription factors like NF-B (14,61,71). The mouse cytomegalovirus (MCMV) model supports a role for TNF-␣ in reactivation from latency (29,58), although cells productively infected with MCMV display downregulated levels of TNFR1 (50). Four NF-B target sites are found in the core and enhancer region of the HCMV major immediate-early promoter (MIEP) stimulating, for example, genes upon TNF-␣ exposure (52).…”

Section: Discussionmentioning

confidence: 99%

The Cytomegaloviral Protein pUL138 Acts as Potentiator of Tumor Necrosis Factor (TNF) Receptor 1 Surface Density To Enhance ULb′-Encoded Modulation of TNF-α Signaling

Trilling

Hengel

2011

Human cytomegalovirus is a ubiquitous herpesvirus that establishes lifelong latent infection. Changes in immune homeostasis induce the reactivation of lytic infection, which is mostly inapparent in healthy individuals but often causes overt disease in immunocompromised hosts. Based on discrepant tumor necrosis factor receptor 1 surface disposition between human cytomegalovirus AD169 variants differing in the ULb region, we identified the latency-associated gene product pUL138, which also is expressed during productive infection, as a selective potentiator of tumor necrosis factor receptor 1, one of the key receptors of innate immunity. Ectopically expressed pUL138 coprecipitated with tumor necrosis factor receptor 1, extended the protein half-life, and enhanced its signaling responses, thus leading to tumor necrosis factor receptor 1 hyperresponsiveness. Conversely, the targeted deletion of UL138 from the human cytomegaloviral genome strongly reduced tumor necrosis factor receptor 1 surface densities of infected cells. Remarkably, the comparison of UL138 deficiency to ULb deficiency revealed the presence of further positive modulators of tumor necrosis factor alpha signal transduction encoded within the human cytomegalovirus ULb region, identifying this region as a hub for multilayered tumor necrosis factor alpha signaling regulation.