T. J. Hill

Herpes simplex virus latency

The epidemiology of herpes simplex virus (HSV) infections in man has a number of unusual features (see Whitley, this volume). In particular, the recurrent lesions (herpes labialis, herpes genitalis, and herpes keratitis) often appear at the same peripheral site, the recurrences are often precipitated by particular stimuli, e. g., fever, excess exposure to sunlight, etcetera, and serum antibodies to the virus remain constant throughout life irrespective of the frequency of recurrent disease. Moreover, during intervals of clinical normality it has proved difficult to isolate virus from peripheral tissues such as the skin where lesions are prone to occur (see Section II.B). Such observations led to the concept that recurrent herpetic disease arises from an endogenous, lifelong, “hidden” or latent infection rather than from frequent exogenous reinfection with the virus (Burnet and Williams, 1939). This “natural history” of the virus infection poses a number of fascinating questions: What tissue or tissues harbor the latent infection? How is the latent infection established and maintained? What mechanisms underlie the reactivation of latency and the production of recurrent disease? Such questions have intrigued virologists since the early part of this century but definitive answers only began to appear during the last decade. In particular, the experimental confirmation of the role of the nervous system in latent infections with HSV has prompted a number of recent excellent reviews (Stevens and Cook, 1973a,b; Docherty and Chopan, 1974; Baringer, 1975; Stevens, 1975, 1978, 1980; Klein, 1976; Longson, 1978; Wildy et al., 1982).

Trauma to the skin causes recurrence of herpes simplex in the mouse.

Mild trauma was induced in the skin of mice latently infected with herpes simplex virus type I by stripping the originally infected ear with cellophane tape. Recurrent herpes simplex developed at this site 2 to 5 days later. It was detected clinically by the development of erythema and vesicles and by the appearance of virus in the skin. On any one occasion about 30% of mice showed reactivated disease and increasing the severity of trauma did not increase this proportion. However the majority of animals developed reactivated disease on some occasions when stripping was repeated at monthly intervals. The results are discussed in relation to the skin trigger theory of reactivation of herpes simplex.

Cytokine production in the nervous system of mice during acute and latent infection with herpes simplex virus type 1.

Immunocytochemistry on serial paraffin sections was used to monitor the production dynamics of cytokines (IL-2, IL-4, IL-6, IL-10, IFN-gamma and TNF-alpha) and viral antigens in the trigeminal ganglion (TG) and the central side of the dorsal root entry zone (DRE) of mice, following infection of the cornea with herpes simplex virus type 1. In normal TG, scattered satellite cells were TNF-alpha+ and in the DRE, TNF-alpha+ and/or low numbers of IL-6+ cells were detected. On day 3 after infection, foci of TG neurons with viral antigens were surrounded by large numbers of TNF-alpha+ and/or IL-6+ cells and low numbers of IFN-gamma+ cells. IL-2+ and/or IL-4+ cells appeared later, when viral antigens had almost cleared. In the TG, the most striking changes occurred with TNF-alpha, with respect to its source (satellite cells, Schwann cells and infiltrating cells) and the extent and long duration of its production. TNF-alpha was the predominant cytokine throughout acute and latent infection and even by day 30, numbers of satellite cells expressing this cytokine were three times higher than those in normal ganglia. Moreover, in the DRE, TNF-alpha was the only cytokine detected during virus clearance and again, its production continued, along with that of IL-6, on days 20 to 30, in both infiltrating cells and astrocytes. Thus, cytokines, particularly TNF-alpha and perhaps IL-6, from infiltrating cells and resident glial cells may have a role both in virus clearance and in normal homeostatic mechanisms in the nervous system such as repair and protection of neurons from damage.

Reactivation of Herpes Simplex Virus Infection by Ultraviolet Light and Possible Involvement of Prostaglandins

Herpes simplex infection in the mouse ear was used to investigate whether various treatments would reactivate the disease. Immunosuppressive drugs failed to induce clinical signs of reactivation but irradiation of the skin of the originally infected ear with ultraviolet light or injection of prostaglandin E2 or PBSA into this site, caused reactivation of infection. This was detected by the appearance of infectious virus in the skin 2 to 3 days after these treatments. The results are discussed in relation to the mechanism of herpes reactivation in man.

Reactivation of latent infection and induction of recurrent herpetic eye disease in mice.

During primary ocular infection of mice with herpes simplex virus type 1 (HSV-1) strain McKrae, dendritic corneal ulcers developed and many eyes became permanently damaged. When primary infection had subsided, latent infection was detected in the three parts of the trigeminal ganglion and in the superior cervical ganglion. Such latently infected mice were treated with cyclophosphamide, dexamethasone and u.v. irradiation, or cyclophosphamide and dexamethasone alone. After treatment with immunosuppressive drugs and u.v. irradiation infectious virus was isolated from the ophthalmic part of the trigeminal ganglion, and in eyelids and eyewashings; recurrent herpetic eye disease was seen but only in eyes undamaged by primary infection. After treatment with cyclophosphamide and dexamethasone alone there was a lower incidence of virus isolated from eyewashings and no recurrent disease was seen. There was a good correlation between the pattern and distribution of recurrent lesions and the distribution of cells stained due to the presence of virus antigens.

Recurrent herpes simplex in the mouse: inflammation in the skin and activation of virus in the ganglia following peripheral stimulation.

The originally infected ear of mice latently infected in the cervical ganglia with herpes simplex virus (HSV) was treated with one of five stimuli: stripping with cellophane tape, irradiation with u.v. light, or the application of xylene, dimethyl sulphoxide (DMSO) or retinoic acid. Each of these stimuli induced the appearance of infectious virus in the ganglia 1 to 5 days later, most frequently after 1 to 3 days. Virus was also isolated from the treated ears, most frequently 3 to 5 days after stimulation. In a proportion of mice treated with cellophane tape stripping, xylene, retinoic acid or DMSO, clinical recurrent disease was observed, although in the case of DMSO this proportion was low. Some of the physiological changes induced in the skin by the five stimuli were studied. Treatment with DMSO, cellophane tape stripping or xylene induced almost immediate inflammation in the skin as judged by extravasation of Evans blue dye. Studies with inhibitors suggested that this was mediated by a neurogenic factor together with histamine or 5-hydroxytryptamine, or both of these. In addition, with the exception of mice treated with DMSO, the levels of prostaglandins of the E and F classes in the skin of the ear were elevated 1 to 3 days after treatment. These results are discussed with reference to the mechanisms by which recurrent herpetic disease develops.

Spread of virus and distribution of latent infection following ocular herpes simplex in the non-immune and immune mouse.

In both non-immune and immune mice infected with herpes simplex virus the incidence of latent infection of the trigeminal ganglion was related to the severity of ocular virus infection. During primary infection, virus was shown to travel via the ophthalmic part of the ganglion to reach the brainstem, from where centrifugal spread resulted in latent infection of neurones in the trigeminal ganglion which did not serve the site of inoculation. Primary infection also resulted in latent infection of the superior cervical ganglion. Shedding of virus occurred rarely in the tears of animals which had recovered from primary disease. In immune mice, spread of virus resulted in a much lower incidence of latent infection and that occurred only in ophthalmic neurones.

Prostaglandins Enhance Spread of Herpes Simplex Virus in Cell Cultures

Stimuli such as u.v. light or trauma which induce recurrence of herpes simplex may act by affecting virus replication in the skin. Such stimuli release pharmacologically active agents in the skin, including prostaglandins (PGs) such as PGE2. These agents, and other compounds which alter levels of adenosine cyclic monophosphate (cyclic AMP), were tested for their effect on the replication of herpes simplex virus (HSV) in Vero cells. Prostanglandin E2 (PGE2) and prostaglandin F2alpha both increase the size of HSV plaques; PGE2 also increases the yield of virus inoculated at low m.o.i. Moreover, inhibitors of prostanglandin synthesis decrease plaque size and inhibit the growth of virus inoculated at low m.o.i.; such inhibition can be partially overcome by adding PGE2. Analysis of the results suggest that prostaglandins can enhance cell-to-cell spread of HSV, but that cyclic AMP is probably not involved in this effect.

Pathogenesis of zosteriform spread of herpes simplex virus in the mouse.

Zosteriform spread of herpes simplex virus (HSV) infection occurs after primary inoculation of the skin of both outbred and inbred mice. With HSV type 1 strain SC16 few outbred animals died if they were inoculated when 8 weeks old whereas up to 50% of animals died if inoculated when 4 weeks old. However, at either age, zosteriform spread of infection occurred in almost all animals as it did when 4-week-old outbred animals were inoculated with the avirulent strain KOS. Thus, control of zosteriform spread must act by different mechanisms from those controlling the encephalitis which leads to death. During replication in the epidermis virus enters axons and could first be found in sensory ganglia 2 days after inoculation of the skin. Thereafter, it was found in the nerve roots and in skin within the same dermatome but remote from the site of inoculation. When sensory nerves to this latter area were cut during the 4 days after primary inoculation lesions developing as a result of zosteriform spread were either completely inhibited or, with later section, decreased in incidence. Mortality was not affected by such nerve section. Latent infection must be established in neurons serving areas of skin remote from the inoculation site since with HSV-1 strain SC16, recrudescent lesions on the pinna could be induced by stripping the skin of the ear when the original inoculation had been in the skin of the neck. Such recrudescent disease was not demonstrated in animals infected with HSV-1 strain KOS even though this virus efficiently established latent infection in sensory ganglia.

Isolation of Herpes Simplex Virus from the Skin of Clinically Normal Mice During Latent Infection

Herpes simplex virus (HSV) was isolated from the ears of clinically normal, latently infected mice by culturing the skin in vitro. The results are discussed with reference to current theories of HSV latency.