AIMS:To evaluate the role of nitric oxide (NO) in ocular involvement during systemic toxoplasmosis.METHODS:C57B1/6 mice were infected with Toxoplasma gondii strain ME49. The synthesis of NO was inhibited by an intraperitoneal injection of aminoguanidine every 8 hours, starting on the day of infection. Control infected mice received phosphate buffered saline vehicle alone. After 14 days, the ocular lesions were evaluated by histopathological examination. The expression of NO synthase induced in the spleen by toxoplasma infection was evaluated by immunostaining. The production of NO by the spleen cells of infected mice was measured by the colorimetric assay of Griess in the supernatant of cultures stimulated with toxoplasma antigen or concanavalin A.RESULTS:The inhibition of NO production in T gondii infected mice resulted in a marked increase in the symptoms of ocular inflammation. We observed a strong induction of NO synthase expression in the spleen of infected animals. In culture, the spleen cells from these mice produced high levels of NO in response to T gondii antigens. This elevation of NO synthesis was suppressed in the presence of aminoguanidine.CONCLUSION:This study indicates that NO plays a crucial role in the protection against T gondii infection as reflected by the severity of the ocular involvement.

Interleukin‐12 (IL‐12) is a heterodimeric cytokine that induces interferon (IFN)‐γ production and an increased generation of Th1 cells. Both IL‐12 and IL‐12 antagonists are being studied for the treatment of allergic reactions, autoimmune disease and malignancy. The goal of the present experiments was to examine the importance of IL‐12 in endotoxin‐induced ocular inflammation. The number of inflammatory cells infiltrating eyes with endotoxin‐induced uveitis (EIU) was significantly increased in animals treated with intraperitoneal anti‐IL‐12 antibody when compared to control animals, but there was no difference in infiltrating inflammatory cells in the eyes of animals treated with IL‐12 when compared to controls. In contrast, intraocular injection of IL‐12 significantly inhibited the development of endotoxin‐induced intraocular inflammation. The infiltrating inflammatory cells were reduced in the eyes of animals receiving intraocular IL‐12 when compared to controls. Cytokine analysis of the aqueous humor obtained from eyes with EIU showed increased levels of IFN‐γ and decreased levels of IL‐6 in eyes receiving intraocular IL‐12. These data show that IL‐12 has an inhibitory effect on endotoxin‐induced inflammation in the eye and suggest that IL‐12 can have an immunoregulatory function in some forms of inflammatory disease.

We studied the effect of nitric oxide (NO) production on the evolution of toxoplasmosis in C57BL/6 mice. Infection was induced by i.p. injection of Toxoplasma gondii strain ME49. NO synthesis was inhibited by treatment with aminoguanidine, a structural analogue of L-arginine. The severity of infection was evaluated by histopathologic examination of the brain. In the infected mice treated for 2 wk with aminoguanidine, we observed an increase in the number of toxoplasma tachyzoites and intracellular cysts accompanied by an exacerbated inflammation of the brain tissue compared with that in controls. When spleen cells from infected mice were stimulated in culture with toxoplasma Ag, there was a marked cytotoxic effect on cells collected during the acute stage of infection and an inhibition of proliferation of the remaining viable lymphocytes. These effects were correlated with high levels of NO and PGE2 production. The suppression of NO synthesis prevented cell death and restored the lymphocyte proliferative response as well as lymphokine production. The neutralization of IFN-gamma or TNF-alpha had no effect on NO production in the cultures of infected mouse spleen cells. Cultures in which purified macrophages and lymphocytes from infected and naive mice were mixed indicated that the production of NO was dependent on lymphocyte activation. In the later stages of infection, when the production of NO was abating, preventing PGE2 secretion with indomethacin also increased the lymphocyte proliferative response. We conclude that the opposing effects of NO in toxoplasmosis, which protects against Toxoplasma gondii and at the same time limits the immune response, probably contribute to the establishment of the characteristic chronic state of host parasite equilibrium.