Date Published: February 18, 2010
Publisher: Public Library of Science
Author(s): Christopher R. Cotter, Marie L. Nguyen, Jacob S. Yount, Carolina B. López, John A. Blaho, Thomas M. Moran, Derya Unutmaz. http://doi.org/10.1371/journal.pone.0008684
Abstract: Molecular pathways underlying the activation of dendritic cells (DCs) in response to Herpes Simplex Virus type 1 (HSV-1) are poorly understood. Removal of the HSV virion host shut-off (vhs) protein relieves a block to DC activation observed during wild-type infection. In this study, we utilized a potent DC stimulatory HSV-1 recombinant virus lacking vhs as a tool to investigate the mechanisms involved in the activation of DCs by HSV-1. We report that the release of pro-inflammatory cytokines by conventional DC (cDC) during HSV-1 infection is triggered by both virus replication-dependent and replication-independent pathways. Interestingly, while vhs is capable of inhibiting the release of cytokines during infection of human and mouse cDCs, the secretion of cytokines by plasmacytoid DC (pDC) is not affected by vhs. These data prompted us to postulate that infection of cDCs by HSV triggers a TLR independent pathway for cDC activation that is susceptible to blockage by the vhs protein. Using cDCs isolated from mice deficient in both the TLR adaptor protein MyD88 and TLR3, we show that HSV-1 and the vhs-deleted virus can activate cDCs independently of TLR signaling. In addition, virion-associated vhs fails to block cDC activation in response to treatment with TLR agonists, but it efficiently blocked cDC activation triggered by the paramyxoviruses Sendai Virus (SeV) and Newcastle Disease Virus (NDV). This block to SeV- and NDV-induced activation of cDC resulted in elevated SeV and NDV viral gene expression indicating that infection with HSV-1 enhances the cell’s susceptibility to other pathogens through the action of vhs. Our results demonstrate for the first time that a viral protein contained in the tegument of HSV-1 can block the induction of DC activation by TLR-independent pathways of viral recognition.
Partial Text: Herpes simplex virus (HSV) is a highly efficient pathogen that infects between 60 and 80% of the US population . An important aspect of this pathogen’s success is the ability to suppress or evade the host immune response at multiple stages of the infection. Early in a primary infection, HSV-infected epithelial cells are shielded from the host immune system long enough to produce progeny virions that infect nearby sensory neurons (reviewed in ). The virus enters a latent state in these neurons in which limited gene expression occurs. Although the host immune response eventually overcomes the viral immune suppressive activities and clears the HSV infected epithelial cells, it is unable to remove latent viral genomes from the neuronal tissues. The host immune response against HSV greatly impacts the severity of primary lesions , ,  and is likely involved in the frequency of reactivation , .
Our results demonstrate that during a productive HSV-1 infection the vhs protein carried in the tegument can inhibit the early innate immune response in both human and mouse monocyte-derived cDCs. These data suggest that this protein, released into infected cells at the time of infection, inhibits the triggering of cDCs probably until other antagonistic proteins are produced. HSV is capable of productively replicating in cDCs, and its replicative capacity is improved by the presence of the vhs protein (Figure 1). This observation is presumably due to a synergism of 2 events: 1.) inefficient immediate early gene expression in the absence of vhs (Figure 1b and 1c) and 2.) elevated type I IFN found in vhs- infected cells (Figure 3b). A previous report showed that infection of cDCs with a replication-defective viral construct lacking vhs resulted in elevated levels of pro-inflammatory cytokines at late time-points post infection compared to the same viral construct containing vhs . Since we hypothesized that vhs functions early in infection we analyzed the cellular response at early times points. We confirmed that there was an enhanced release of IL-6 and TNF-α in dendritic cells in response to UV-inactivated virus lacking vhs. Interestingly, IL-12 production by infected DCs was also susceptible to vhs inhibition but was only produced in response to live virus. These data indicate that cDC activation during HSV-1 infection is triggered through both replication-dependent and replication-independent pathways. In addition, experiments using T cell co-cultures demonstrated that the vhs protein impaired the development of Th1 immunity as shown by the impairment in the expression of a number of Th1-related genes. These results demonstrated a critical role of the vhs protein in controlling the development of both innate and adaptive immunity in response to HSV-1 infection.