Herpes simplex virus (HSV) infection in humans has been one of the most frequently studied infectious diseases during the last century. HSV-1 remains a significant ocular pathogen, despite the existence of highly specific antiviral agents. HSV-1 corneal infection presents considerable challenges for both ophthalmologists and virologists, not only because herpetic keratitis may cause severe corneal scarring that is a major cause of acquired blindness in industrialized countries, but also because herpetic keratitis is a complicated complex of live viral infection and host immune and inflammatory responses the exact pathogeneses of which are not well understood.
A Unique Clinical Entity
It has been generally accepted that herpetic epithelial keratitis results directly from viral replication in epithelial cells, whereas stromal keratitis is thought to be due, at least in part, to host immune response to HSV antigens within the stroma. Another clinical entity, in which the target of inflammation is thought to be the endothelium, has been recognized and recently classified as corneal endotheliitis. 1,2
Not uncommonly, ophthalmologists see patients in whom corneal stromal edema develops without stromal infiltrates or neovascularization. The characteristic ocular findings in these patients are keratic precipitates (KP), overlying stromal and epithelial edema, and mild to moderate iritis. Visualizing the KP may be difficult in some patients who present with extensive stromal edema, but these usually become evident as the stromal edema resolves. Many clinicians may diagnose this clinical entity as stromal keratitis; however, it may more properly be considered an inflammatory reaction of the endothelium, with only secondary stromal and epithelial edema. In review articles by Liesegang 1 and Holland and Schwartz, 2 HSV endotheliitis was well defined as an independent clinical entity that can be further separated into three forms on the basis of the distribution of the KP and the configuration of the overlying stromal and epithelial edema. These three forms are disciform, diffuse, and linear endotheliitis. 1,2
Since Khodadoust and Attarzadeh 3 reported a “presumed autoimmune corneal endotheliopathy,” in which they assumed that the disease resulted from an autoimmune reaction in the corneal endothelium, several dozen patients have been described in the literature as having corneal endotheliitis that has been identified by a variety of names, including keratitis linearis migrans,4progressive herpetic corneal endotheliitis,5herpetic endothelial keratitis,6idiopathic corneal endotheliopathy,7 and sporadic diffuse corneal endotheliitis.8 Although the exact pathogenesis of each of these diseases is not known, certain observations support the concept that the primary target of inflammation in each is endothelium. Evidence includes the facts that KP are always present under the areas of edematous cornea and that the only stromal finding is edema, stromal infiltration and neovascularization being not notably present.
Although many clinicians once believed that immunological reaction to HSV antigen within the stroma or endothelium was the underlying mechanism for corneal endotheliitis, accumulating evidence indicates that corneal endotheliitis very probably is of viral origin. The role of an infectious virus as the cause of endothelial infection is supported by the detection of HSV-1 antigen, DNA, or viral particles in corneal endothelial cells. 4,5,9–13 It is supported also by the finding that endothelial lesions can resolve rapidly only with the addition of topically applied and systemic antiviral treatment. 5,13–15
Corneal Endotheliitis Rabbit Model
Important concerns regarding corneal endotheliitis include the reasons that, if herpetic infection exists in the eye in this disorder (as suggested by a large number of reports), 4–15 the inflammation in the anterior chamber is so mild and the pathogenic region is restricted to the endothelium. We have reasoned that the answer may lie in a unique immune response that occurs when antigens enter the anterior chamber. Anterior chamber–associated immune deviation (ACAID), first described by Streilein and associates, 16–20 offers a potential explanation for this. Delayed-type hypersensitivity is a powerful inducer of immunogenic inflammation in which interferon-y is a prominent effector cytokine. In ACAID, the cellular immune response is impaired, whereas the humoral immune response is intact or enhanced. We hypothesize that the existence of ACAID due to viral antigens may permit HSV-1 to invade and damage corneal endothelial cells while avoiding the antiviral effects of T cells.
We carried out experiments in a rabbit model in the setting of induced ACAID. 21 HSV-1 (McKrae) was inactivated by ultraviolet irradiation to obtain viral antigen. The anterior chamber of one rabbit eye was inoculated with viral antigen for ACAID induction. One week later, the contralateral eye in experimental animals was intracamerally infected with a certain dose of live virus. Infected eyes then were examined by slit-lamp microscopy. The results showed that 14 days after live virus infection, the infected eyes in rabbits primed with viral antigen through the anterior chamber (contralateral eyes) 1 week before infection developed a distinctive pattern of infection that very much resembled clinical corneal endotheliitis in humans.
Immunohistochemical staining revealed that HSV-1 antigen was detected only in the destroyed endothelial cell layer. Inflammatory cells, including lymphocytes, plasma cells, and polymorphonuclear cells, were found to infiltrate the region where endothelial cells were destroyed (evidenced by electron microscopy). The inflammatory cell infiltration of the endothelial layer might reflect the KP, and the edema that manifested in the infected eyes could reflect the destruction or dysfunction of the endothelial cell layer.
In marked contrast, control rabbits that had preliminarily been primed with viral antigen before infection through routes other than the anterior chamber (e.g., via subconjunctival injection) did not develop corneal endotheliitis. Also, our separate experiments confirmed the induction of ACAID (induction of virus-specific antibody and impaired delayed-type hypersensitivity response) in rabbits intracamerally primed with HSV-1 antigen.
ACAID in Corneal Endotheliitis
The rationale for ACAID as the underlying mechanism in herpetic corneal endotheliitis are as follows: It is well-known that HSV-1 can be transported in retrograde fashion to neuronal ganglia, such as the trigeminal ganglion, after primary infection of mucocutaneous tissues. Within the trigeminal ganglion, the virus usually establishes a long-term, latent infection that can become intermittently active. 22,23 A low dose of virus may be spontaneously shed in the eye. It is likely that HSV-1 particles can be released into the anterior chamber through the trigeminal nerves that innervate the trabecular meshwork, the iris, and the ciliary body. Some of the viral particles can be captured by indigenous antigen-presenting cells, which, in turn, generate an immunogenic signal that induces virus-specific ACAID. We believe that anterior chamber priming with viral antigen in our rabbit model produced ACAID by a similar mechanism.
The amount of viral shedding during any particular reactivation may vary considerably. We suspect that when the dose of released virus is at or near a certain dose (10 4 pfu), which is similarly produced in the rabbit model by intracameral infection of the infectious HSV-1, the reactivating virus may travel into the immunosuppressive microenvironment of the anterior chamber and cause herpetic corneal endotheliitis. In pilot experiments, attempts had been made to infect the rabbits with 100-fold lower or higher doses of HSV-1, but in neither case did we observe typical clinical manifestations of corneal endotheliitis. 21 It is likely that there is a threshold amount of infectious virus that creates this disease and that the balance of ACAID and host effector responses plays the critical role.
In ACAID, virus-specific delayed-type hypersensitivity is suppressed and virus-specific cytotoxic T-cell responses are blunted, but serum anti-HSV antibodies are present or enhanced. 16–20 This spectrum of immune effectors makes it possible for HSV-1 to avoid lysis by class I–restricted T cells. In our rabbit model, we found that high titers of neutralizing antibodies were present in the eyes exhibiting corneal endotheliitis, indicating that these antibodies may effectively neutralize the extracellular virus, thus preventing the spread of virus into other cells and tissues within the anterior chamber. The rather minimal inflammation observed in the anterior segments of the eyes with corneal endotheliitis supports this postulation.
Our separate experiments showed that corneal endotheliitis did not develop in rabbits primed subconjunctivally with viral antigen. Moreover, intrastromal inoculation of HSV-1 antigen followed by same-eye intracameral HSV-1 infection could promote stromal keratitis and keratouveitis but not corneal endotheliitis. These findings suggest that the susceptibility of the cornea to the burden of keratouveitis or endotheliitis during HSV-1 infection probably is dictated by the unique features of the immune response elicited by antigens presented first to the immune system through the eye. 24
Herpetic corneal endotheliitis is being increasingly seen in ophthalmological clinics. Although topical and systemic administration of acyclovir has been shown to be effective against this disease, bullous keratopathy is sometimes the final outcome because of a profound loss of corneal endothelial function through repeated exacerbations. This distinct disease may threaten vision especially when caused by an acyclovir-resistant strain. This kind of shift in infection to a deeper ocular site may be an evolutionary response of HSV to evade the antiviral effect of topical acyclovir. 14
The animal model approach has revealed that HSV-1 infection of the endothelium could be at least one cause of corneal endotheliitis. ACAID may play the pivotal role in corneal herpetic endotheliitis. It is our belief that the strain of HSV-1, viral dose, and subtle changes in the immune status of the host may conspire to create the variety of manifestations of corneal endotheliitis observed in clinics. Furthermore, the possibility exists that elucidating the underlying mechanisms for other intriguing ocular diseases such as Posner-Schlossman syndrome and Fuchs' uveitic syndrome may shed light on and reveal a link to the unique immune privilege of the microenvironment of the eye.
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