Reversible Nerve Damage and Corneal Pathology in Murine Herpes Simplex Stromal Keratitis

Abstract

Herpes simplex virus type 1 (HSV-1) shedding from sensory neurons can trigger recurrent bouts of herpes stromal keratitis (HSK), an inflammatory response that leads to progressive corneal scarring and blindness. A mouse model of HSK is often used to delineate immunopathogenic mechanisms and bears many of the characteristics of human disease, but it tends to be more chronic and severe than human HSK. Loss of blink reflex (BR) in human HSK is common and due to a dramatic retraction of corneal sensory nerve termini in the epithelium and the nerve plexus at the epithelial/stromal interface. However, the relationship between loss of BR due to nerve damage and corneal pathology associated with HSK remains largely unexplored. Here, we show a similar retraction of corneal nerves in mice with HSK. Indeed, we show that much of the HSK-associated corneal inflammation in mice is actually attributable to damage to the corneal nerves and accompanying loss of BR and can be prevented or ameliorated by tarsorrhaphy (suturing eyelids closed), a clinical procedure commonly used to prevent corneal exposure and desiccation. In addition, we show that HSK-associated nerve retraction, loss of BR, and severe pathology all are reversible and regulated by CD4 ؉ T cells. Thus, defining immunopathogenic mechanisms of HSK in the mouse model will necessitate distinguishing mechanisms associated with the immunopathologic response to the virus from those associated with loss of corneal sensation. Based on our findings, investigation of a possible contribution of nerve damage and BR loss to human HSK also appears warranted. IMPORTANCEHSK in humans is a potentially blinding disease characterized by recurrent inflammation and progressive scarring triggered by viral release from corneal nerves. Corneal nerve damage is a known component of HSK, but the causes and consequences of HSK-associated nerve damage remain obscure. We show that desiccation of the corneal surface due to nerve damage and associated loss of BR severely exacerbates and prolongs inflammation-induced pathology in mice. Preventing corneal desiccation results in a milder and more transient HSK with variable scarring that mirrors HSK seen in most humans. We further show that nerve damage is reversible and regulated by CD4 ؉ T cells. Thus, we provide a mouse model that more closely resembles typical human HSK and suggest nerve damage is an important but largely overlooked factor in human disease.

A Highly Efficacious Herpes Simplex Virus 1 Vaccine Blocks Viral Pathogenesis and Prevents Corneal Immunopathology via Humoral Immunity

“…Following HSV-1 infection in mice, pathological corneal edema and neovascularization ensue commensurate with epithelial thinning within geographic corneal lesions (1,2,44). Similar to corneal neovascularization, hypoesthesia is a hallmark of herpes stromal keratitis in mice and humans and results from denervation of the epitheliumassociated subbasal mechanosensory nerve fibers (40,45,46). It was of interest to determine whether vaccinated mice could be phenotypically categorized based on quantifiable measurements of tissue pathology.…”

IL-6 Contributes to Corneal Nerve Degeneration after Herpes Simplex Virus Type I Infection

“…The idea that immune components play a role in the preservation of corneal nerves 45 or influence the long-term persistence of their defects after herpetic infection 22 has been recently suggested. Our study offers novel mechanistic insights that link the elicited immune response to HSV-1 infection, and not local viral replication, as the explanation for corneal nerve regression after infection.…”

“…16,22,31 The anti-inflammatory effects of glucocorticoids, such as DEX, are largely considered to be exerted through cross talk with proinflammatory signaling pathways, including NF-kB. 37,38 The analysis of two NF-kBedriven cytokine genes, associated with the immunosuppressive action of DEX, IL-6, and IL-1b, showed a strong trend toward up-regulation at 2 and 8 days pi ( Figure 3A).…”

“…We previously reported that HSV-1 infection of the mouse cornea causes nerve regression during the acute phase of infection, followed by an abnormal process of regeneration. 23 Although CD4 þ T cells, a set of inflammatory cells infiltrating the cornea, have been suggested to influence the long-term persistence of corneal nerve defects after HSV-1 infection, 22 whether the initial events of nerve regression after infection are elicited by inflammatory cells/ mediators, or by other means, such as viral infection of the cornea, resident cell response, or signaling within the trigeminal ganglia, remains a debatable question. To test the hypothesis that the elicited immune response to HSV-1 infection is responsible for the regression of corneal nerves during acute infection, we analyzed the effect of the antiinflammatory drug dexamethasone sodium phosphate (DEX) 24e26 in the corneal nerve network structure and function, and uncovered potential cells and factors as part of the mechanism of HSV-1einduced NTK.…”

Impact of Type I Interferon on the Safety and Immunogenicity of an Experimental Live-Attenuated Herpes Simplex Virus 1 Vaccine in Mice

“…These include corneal scarring, neovascularization, and sensation loss, all of which are recapitulated in the animal model of ocular infection (11,45,46,51,70). Phylogenetic differences among virulence determinants in different HSV-1 strains are associated with various degrees of corneal neovascularization (71).…”