Herpes simplex virus type 1 immediate-early protein Vmw110 reactivates latent herpes simplex virus type 2 in an in vitro latency system.

Reactivation of latent herpes simplex virus type 2 (HSV-2) by the immediate-early protein Vmw110 was studied by using an in vitro latency system. Adenovirus recombinants that express Vmw110 reactivated latent HSV-2. An HSV-1 mutant possessing a deletion in a carboxy-terminal region of Vmw110 reactivated latent HSV-2, whereas mutant FXE, which has a deletion in the second exon, did not. Therefore, Vmw110 alone is required to reactivate latent HSV-2 in vitro, and the region of Vmw110 defined by the deletion in FXE is important for this process.     

Psychological stress compromises CD8+ T cell control of latent herpes simplex virus type 1 infections

Recurrent HSV-1 ocular disease results from reactivation of latent virus in trigeminal ganglia, often following immunosuppression or exposure to a variety of psychological or physical stressors. HSV-specific CD8+ T cells can block HSV-1 reactivation from latency in ex vivo trigeminal ganglia cultures through production of IFN-gamma. In this study, we establish that either CD8+ T cell depletion or exposure to restraint stress permit HSV-1 to transiently escape from latency in vivo. Restraint stress caused a reduction of TG-resident HSV-specific CD8+ T cells and a functional compromise of those cells that survive. Together, these effects of stress resulted in an approximate 65% reduction of cells capable of producing IFN-gamma in response to reactivating virus. Our findings demonstrate persistent in vivo regulation of latent HSV-1 by CD8+ T cells, and strongly support the concept that stress induces HSV-1 reactivation from latency at least in part by compromising CD8+ T cell surveillance of latently infected neurons.     

Restricted expression of herpes simplex virus lytic genes during establishment of latent infection by thymidine kinase-negative mutant viruses.

Infection of cells by herpes simplex virus (HSV) can lead to either lytic, productive infection or nonlytic, latent infection. The factors influencing this infection pathway decision are largely unknown. Thymidine kinase-negative mutant viruses can establish latent infection in neurons of mouse trigeminal ganglia but do not replicate productively in these cells. We show that during the early stages of establishment of latency by these mutants, expression of viral lytic genes is drastically reduced or undetectable as assayed by in situ hybridization. Thus, establishment of latent infection by HSV can occur despite severely restricted levels of lytic gene expression. This suggests that the block to productive replication during establishment of latent infection by HSV occurs before or early during the expression of alpha genes.     

Immunization with replication-defective mutants of herpes simplex virus type 1: sites of immune intervention in pathogenesis of challenge virus infection.

Replication-defective mutants of herpes simplex virus type 1 (HSV-1) were used as a new means to immunize mice against HSV-1-mediated ocular infection and disease. The effects of the induced immune responses on pathogenesis of acute and latent infection by challenge virus were investigated after corneal inoculation of immunized mice with virulent HSV-1. A single subcutaneous injection of replication-defective mutant virus protected mice against development of encephalitis and keratitis. Replication of the challenge virus at the initial site of infection was lower in mice immunized with attenuated, wild-type parental virus (KOS1.1) or replication-defective mutant virus than in mice immunized with uninfected cell extract or UV-inactivated wild-type virus. Significantly, latent infection in the trigeminal ganglia was reduced in mice given one immunization with replication-defective mutant virus and was completely prevented by two immunizations. Acute replication in the trigeminal ganglia was also prevented in mice immunized twice with wild-type or mutant virus. The level of protection against infection and disease generated by immunization with replication-defective mutant viruses was comparable to that of infectious wild-type virus in all cases. In addition, T-cell proliferative and neutralizing antibody responses following immunization and corneal challenge were of similar strength in mice immunized with replication-defective mutant viruses or with wild-type virus. Thus, protein expression by forms of HSV-1 capable of only partially completing the replication cycle can induce an immune response in mice that efficiently decreases primary replication of virulent challenge virus, interferes with acute and latent infection of the nervous system, and inhibits the development of both keratitis and systemic neurologic disease.     

Adaptive and innate transforming growth factor beta signaling impact herpes simplex virus 1 latency and reactivation

Innate and adaptive immunity play important protective roles by combating herpes simplex virus 1 (HSV-1) infection. Transforming growth factor β (TGF-β) is a key negative cytokine regulator of both innate and adaptive immune responses. Yet, it is unknown whether TGF-β signaling in either immune compartment impacts HSV-1 replication and latency. We undertook genetic approaches to address these issues by infecting two different dominant negative TGF-β receptor type II transgenic mouse lines. These mice have specific TGF-β signaling blockades in either T cells or innate cells. Mice were ocularly infected with HSV-1 to evaluate the effects of restricted innate or adaptive TGF-β signaling during acute and latent infections. Limiting innate cell but not T cell TGF-β signaling reduced virus replication in the eyes of infected mice. On the other hand, blocking TGF-β signaling in either innate cells or T cells resulted in decreased latency in the trigeminal ganglia of infected mice. Furthermore, inhibiting TGF-β signaling in T cells reduced cell lysis and leukocyte infiltration in corneas and trigeminal ganglia during primary HSV-1 infection of mice. These findings strongly suggest that TGF-β signaling, which generally functions to dampen immune responses, results in increased HSV-1 latency.     

Corneal latency and transmission of herpes simplex virus-1

The transmission of herpes simplex virus (HSV)-1 by corneal transplantation has rarely been reported. It is believed that these cases have resulted either from reactivated virus traveling from the trigeminal ganglion to the cornea or from latent HSV-1 in the donor cornea itself. Studies of long-term viral presence in corneal tissue have sought to determine whether there is evidence of true non-neuronal latency, although there are problems in its definition. Recent studies provide new insights into neuronal latency, while similar HSV-1 gene regulation in the cornea may implicate corneal latency in pathophysiology and as a potential risk for transplant recipients. This issue has led to concerns over eye banking, which currently screens for other infectious agents but not HSV-1. Here we review the literature regarding corneal latency and the transmission of HSV-1.     

Herpes simplex virus type 1 mutant strain in1814 establishes a unique, slowly progressing infection in SCID mice.

Ocular infection of immunocompetent (BALB/c) mice with wild-type herpes simplex virus type 1 (HSV-1) 17+ may lead to acute fatal encephalitis; however, in surviving animals, a latent (nonproductive) infection of the nervous system is established. In contrast, 17+ infection invariably kills mice with severe combined immunodeficiency (SCID mice) within 2 weeks. Ocular infection of immunocompetent mice with a mutant HSV-1 strain, in1814, which does not produce a functional alpha-transinducing protein, results in no detectable viral replication in the nervous system during the time corresponding to the acute phase of infection, no mortality, and the establishment of latency. In SCID mice, however, the in1814 virus establishes a unique, slowly progressing infection. In studying the courses of in1814 infection in SCID and BALB/c mice, we found that although intact B- and/or T-lymphocytic functions were required for the control of viral replication in the nervous system, some of the infected neurons of SCID mice seemed to be able to restrict in1814 replication and harbor the virus in a latent state.     

Role for gamma interferon in control of herpes simplex virus type 1 reactivation

Observation of chronic inflammatory cells and associated high-level gamma interferon (IFN-gamma) production in ganglia during herpes simplex type 1 (HSV-1) latent infection in mice (E. M. Cantin, D. R. Hinton, J. Chen, and H. Openshaw, J. Virol. 69:4898-4905, 1995) prompted studies to determine a role of IFN-gamma in maintaining latency. Mice lacking IFN-gamma (GKO mice) or the IFN-gamma receptor (RGKO mice) were inoculated with HSV-1, and the course of the infection was compared with that in IFN-gamma-competent mice with the same genetic background (129/Sv//Ev mice). A time course study showed no significant difference in trigeminal ganglionic viral titers or the timing of establishment of latency. Spontaneous reactivation resulting in infectious virus in the ganglion did not occur during latency in any of the mice. However, 24 h after the application of hyperthermic stress to mice, HSV-1 antigens were detected in multiple neurons in the null mutant mice but in only a single neuron in the 129/Sv//Ev control mice. Mononuclear inflammatory cells clustered tightly around these reactivating neurons, and by 48 h, immunostaining was present in satellite cells as well. The incidence of hyperthermia-induced reactivation as determined by recovery of infectious virus from ganglia was significantly higher in the null mutant than in control mice: 11% in 129/Sv//Ev controls, 50% in GKO mice (P = 0.0002), and 33% in RGKO mice (P = 0.03). We concluded that IFN-gamma is not involved in the induction of reactivation but rather contributes to rapid suppression of HSV once it is reactivated.