both routes are possible. Gagliuso and coworkers63 offer the observation that most ocular lesions in patients with AIDS are unassociated with a preexisting retinochoroidal scar and would therefore suggest acquired disease. This may be, but it is possible that cysts in the retina may remain dormant and become activated only in the immunosuppressed state. It is also possible that small lesions indicative of previous disease are simply engulfed in the large retinal necrotic lesion present in the patient with AIDS.

The patient who has undergone iatrogenic immunosuppression has a high risk of reactivation of ocular toxoplasmosis. As with patients with AIDS, if the immunosuppression cannot be reversed serious consequences may ensue, as reported by Yeo and colleagues.64 Singer and coworkers65 and Blanc-Jouvan and associates66 have reported cases of ocular toxoplasmic retinochoroiditis occurring after liver transplantation. In the patient of Singer and coworkers, because of the difficulty of diagnosing the disorder, the eye was ultimately enucleated. These authors emphasize the fulminant nature of the disease, suggestive far more of the type of ocular disease seen in patients with AIDS.

We conclude with the interesting observation that immune recovery uveitis may be driven by Toxoplasma antigens and not only by CMV, as it is generally thought. Sendi and colleagues67 reported the case of a 34-year-old HIVpositive man with a CD4 count of 11, who, after being placed on HAART therapy developed a uveitis after an increase in his CD4 count. Aqueous PCR for CMV was negative but positive for Toxoplasma, and his disease abated only after periocular steroid injections.

In summary, the diagnosis of ocular toxoplasmosis is primarily clinical. The typical lesions as described constitute the most important factor in our decision making. The one additional supportive test we believe to be very important is a positive toxoplasmosis titer at any dilution (see below). When the presentation is highly unusual, the diagnosis rests on a combination of multiple factors. In young children, infection with lymphochoriomeningitis virus, found in rodent feces, urine, and saliva, has been reported to mimic the ocular lesions associated with toxoplasmosis.68 In these patients toxoplasmosis could be confused with the ocular histoplasmosis syndrome, although vitreal cells are not present in the latter entity and the peripapillary changes are rarely evident in toxoplasmosis. The deep retinal presentation of toxoplasmosis may be confused with the white-dot syndromes, such as an unusual case of acute posterior multifocal placoid pigment epitheliopathy (see Chapter 29). In the immunocompromised host a single toxoplasmosis lesion early on may be confused with CMV retinitis. One possible way to help discriminate between the two entities is the use of fluorescein angiography. In an active toxoplasmosis lesion the central area will block fluorescein early and stain late, because the central part is where the greatest inflammatory response is taking place. This is in contrast to CMV retinitis, in which the central area will be atrophic and more readily hyperfluorescent early in the angiogram (Phuc LeHoang, MD, personal communication, 1989). Indocyanine green angiography will show that the lesion extends beyond the visible area, with hypofluorescent foci at all phases.69 A very good review of ocular disease can be found in Dr Gary Holland’s Jackson Memorial lecture.70,71

Histopathology and immune factors

Figure 14-13.  Photomicrograph showing toxoplasmic cysts in retina. Cysts are larger circular structures, located mainly in or close to the nerve fiber layer. (Courtesy of D. Cogan, MD.)

Histopathology and immune factors

In ocular toxoplasmosis, cysts and tachyzoites can be found in the retina (Fig. 14-13). The Toxoplasma organism most frequently is seen in the superficial portions of the retina. The lesion induced is necrotic, destroying the architecture of the retina. In many cases the underlying structures are destroyed as well, so that the disease at this point can certainly be classified as a chorioretinitis. Clinically, this destruction will permit the examiner to see underlying sclera quite clearly. Dutton and coworkers72,73 were able to study these alterations more carefully using a murine model of congenital toxoplasmic retinochoroiditis. They noted that the inflammation ranged from a low-grade mononuclear infiltrate to total destruction of the outer retina, the retinal pigment epithelium, and choroid. Of great interest was the fact that photoreceptor outer segments were phagocytosed by macrophages, whereas the Toxoplasma cysts did not appear to be the center of the inflammatory attack. Roberts and colleagues74 characterized histologically the eyes from 10 fetuses and two infants with congenital toxoplasmosis. Retinitis was present in 10 of 18 eyes, necrosis in four of 18, retinal pigment epithelial changes in 12 of 18, choroidal inflammation in 15 of 18, and optic neuritis in five of eight fetal eyes. Parasites were found on immunohistologic examination in 10 of 18 eyes. It appeared to the authors that the inflammatory response mounted by the host accounted for part of the damage seen.

Because of the selective photoreceptor destruction, one can speculate that autoimmune mechanisms may be important in the tissue destruction seen (see discussion of autoimmunity in Chapter 1). Indeed, when we had the opportunity to evaluate this issue, we found that in 16 of 40 patients (40%) with ocular toxoplasmosis an in vitro proliferative response to the retinal S-antigen was seen.75 This finding could indicate that an autoimmune component to the inflammatory disease is initiated, with destruction of the retina by a parasite and subsequent sensitization to the uveitogenic antigen. We also found that proliferative responses

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Part 4 • Infectious Uveitic Conditions

Chapter 14 Ocular Toxoplasmosis

to the p22 antigen approached those seen to crude toxoplasmosis antigen, whereas the response to the p30 membrane antigen was considerably less striking. Others have shown that Toxoplasma antigen stimulates CD25+ helper cells.76

Immune response

The host’s immune response is exceptionally important in the ultimate expression of toxoplasmosis. In animal models, resistance, as measured by survival after challenge with the organism, has been reported to be regulated by at least five genes,77 one of which is within the region of the H-2 antigen (the equivalent of the major histocompatibility complex [MHC] in humans).78 Brown and McLeod79 have found that class I MHC genes, as well as the CD8+ fraction of T cells, determine the cyst number in Toxoplasma infection. Jamieson et al.80 reported polymorphism associations at the COL2A1 encoding type II collagen associated only with those who had ocular disease. Others have looked at the effect of cytokines on the multiplication of the Toxoplasma organism. Interferon (IFN)-γ, as well as tumor necrosis factor (TNF)-α and transforming growth factor-β all appear to play a role in inhibiting multiplication.81, 82 Gazzinelli and colleagues83 reported that a reactivation of T. gondii, at least in the experimental mouse model, is due to a downregulation of IFN-g and TNF-a, which leads to reduced macrophage (with a decrease in inducible nitric oxide synthase and macrophage activation gene 1) and glial activation, a release of parasite growth, and tissue damage. Shen and associates84 found the presence of IFN in the eyes of Toxoplasma-infected mice, but that inflammatorily induced apoptosis was caused by several factors, not only Fas/FasL interactions. Beaman and colleagues85 reported that interleukin (IL)-6 enhanced intracellular reproduction of T. gondii and actually reversed the effect of IFN-g-mediated killing, which contradicts the finding of Lyons and colleagues86 who stated that IL-6 knockout mice in a chronic toxoplasmosis model had more severe disease and an increased parasite burden. This finding is particularly important in light of the fact that the retinal pigment epithelium produces large amounts of IL-6. An analysis of ocular fluids from uveitic eyes (including those with from patients with toxoplasmosis) demonstrated the presence of various cytokines, including IL-6, IFN, and IL-10. Recently, Zamora and coworkers showed that the Toxoplasma organism invades human retinal endothelial cells more efficiently than they do human dermal endothelial cells.87 In another study, Feron and colleagues88 characterized 10 T-cell specimens from the vitreous of patients with toxoplasmosis. Although the cell lines were initially generated by mitogenic stimulation, they were all CD4+ and appeared reactive to Toxoplasma antigens and not to any retinal antigens. The majority had a Th2 profile. It has also been suggested that CD8+ T cells directed against the Toxoplasma organism appear during the acute phase of the disease, whereas CD4+ parasite-specific T cells appear with chronicity of the disease.89 Denkers and colleagues90 reported that the Toxoplasma organism possesses a superantigen that expands murine Vβ5- expressing cells, most of which were CD8+. It may be that this superantigen-driven expansion of predominantly IFN-g- secreting CD8+ cells is partly why the early immune response is seen. Curiel and coworkers91 reported the cloning of human CD3+, CD4+ T cells that lyzed autologous target cells

which had been pulsed with Toxoplasma antigen or infected with live tachyzoites. These results suggest that specific immunotherapy through the development of vaccine may be possible. Patients with acute toxoplasmosis have increased serum levels of CXCL8, part of the chemokines that control leukocyte infiltration and can even modulate angiogenesis.92

Although antibodies are usually readily made, it is the cellular component of the immune system that must be intact for a resolution of the disease process. However, antibody production may play an important role in establishing a state of premunition (immunity from infection) in Toxoplasma infection.93 In an attempt to evaluate why newborn infants seem to have difficulty in fighting the Toxoplasma infection, Wilson and Haas94 evaluated the cellular defenses against T. gondii in newborns. They noted that newborn and adult macrophages killed the organism equally well, but supernatants from cord blood-derived concanavalin A- stimulated mononuclear cells activated macrophages less effectively than supernatants produced from adult blood cells. This difference appeared to lie in the CD4+ fraction of T cells. The cord blood appeared to produce fewer lymphokines capable of activating macrophages, including IFN-g. The authors did not believe that enhanced generation of reactive oxygen intermediates was important in explaining the differences between the adult and newborn responses, although recent notions would suggest that nitrous oxide and its effects in macrophages may indeed play a very important role. Roberts and associates,95 in a murine model, demonstrated that inhibition of nitric oxide by administration of lω-nitro-l-arginine methyl ester made the disease worse. Of interest is the fact the organism replicates in the macrophage, and this reduced killing in the newborn may then lead to greater susceptibility.

Inflammatory response

What then is the cause of the focal, retinal inflammatory response? So far the data still support the notion that it is the release of actively proliferating tachyzoites, often from a long-dormant cyst. We know now of many immune components that can stimulate this tachyzoite–bradyzoite interconversion (Fig. 14-14) However, immune studies with patients with ocular toxoplasmosis suggest that other factors are involved. Wyler and coworkers96 noted that lymphocytes from patients with ocular toxoplasmosis demonstrated in vitro responses not only to Toxoplasma antigens but also to a crude retinal preparation. As already mentioned, our patients with toxoplasmosis have demonstrated in vitro cellular responses to the retinal S-antigen, a purified antigen from the photoreceptor region, and the site of the most intense inflammatory response in Dutton and colleagues’ murine model for toxoplasmosis.73 Abrahams and Gregerson97 detected circulating antibodies to various retinal antigens in patients with toxoplasmosis. Whittle and colleagues,98 using indirect immunofluorescent techniques over normal human cadaver retina, determined the presence of human antiretinal antibodies in patients with ocular toxoplasmosis. Of the sera from 36 toxoplasmosis patients, 94% demonstrated photoreceptor layer reactivity. However, only 27 of these patients had anti-S-antigen antibodies as determined by an enzyme-linked immunosorbent assay (ELISA).