Bioluminescence Imaging of Toxoplasma gondii Infection in Living Mice Reveals Dramatic Differences between Strains

Abstract

We examined the in vivo growth, dissemination, and reactivation of strains of the protozoan parasite Toxoplasma gondii using a bioluminescence-based imaging system. Two T. gondii strains, one with a highly virulent disease phenotype in mice (S23) and the other with a 1,000-fold-lower virulence phenotype (S22), were engineered to stably express the light-emitting protein luciferase. One clone of each wild-type strain was isolated, and the two clones (S23-luc7 and S22-luc2) were found to express similar levels of luciferase. Mice were infected intraperitoneally with S23-luc7 (50 or 5 parasites) or S22-luc2 (500, 50, or 5 parasites), and the progress of the infections was examined noninvasively following injection of the substrate for luciferase, D-luciferin. In mice infected with 50 S23-luc7 parasites, the parasites grew exponentially within the peritoneal cavity (as measured by light emitted from luciferase-expressing parasites) during days 1 to 10 p.i., and this proliferation continued until there was severe disease. In mice infected with 500 S22-luc2 parasites, the parasites proliferated in a fashion similar to the S23-luc7 proliferation during days 1 to 6, but this was followed by a precipitous drop in the signal to levels below the limit of detection. Using this technique, we were also able to observe the process of reactivation of T. gondii in chronically infected mice. After treatment with dexamethasone, we detected reactivation of toxoplasmosis in mice infected with S23-luc7 and S22-luc2. During reactivation, growth of S23-luc7 was initially detected primarily in the head and neck area, while in S22-luc2-infected mice the parasites were detected primarily in the abdomen. This method has great potential for identifying important differences in the dissemination and growth of different T. gondii strains, especially strains with dramatically different disease outcomes.Toxoplasma gondii is an obligate intracellular parasite that is able to infect virtually any nucleated cell from a wide range of mammalian and avian species (12). In humans, Toxoplasma infections are widespread and can lead to severe disease in individuals with immature or suppressed immune systems. Consequently, toxoplasmosis became one of the major opportunistic infections of the AIDS epidemic (14). Acute infection, which is associated with the rapidly dividing form or tachyzoite, is normally controlled by innate and adaptive immune responses. Sterile immunity is not achieved, however, and instead an asymptomatic, chronic phase of infection ensues. The parasite persists by differentiating into bradyzoites, a form that expresses surface antigens that are distinct from those of tachyzoites. The bradyzoites exist within cysts that remain in the host for long periods in less immunologically active tissues, such as the central nervous system, apparently impervious to the robust immune response induced by the tachyzoite stages (6,11,22).The outcome of toxoplasmosis in the mouse model is strongly dependent on the infecting T. gondii strain. The major...

Temporal and Spatial Distribution of Toxoplasma gondii Differentiation into Bradyzoites and Tissue Cyst Formation In Vivo

“…Monitoring by BLI of mice infected with H3F33 revealed that during the first days postinfection, R-Luc photon emission was localized mainly in the abdomen. This pattern is similar to that previously observed when mice were infected with tachyzoites expressing firefly luciferase (8,10,20,36). Autopsies carried out shortly after BLI analysis revealed that the signal was localized to the adipose tissue surrounding the ab- dominal organs.…”

Toxoplasma gondiiinfection, from predation to schizophrenia: can animal behaviour help us understand human behaviour?

“…Even within T. gondii type, however, different clinical and behavioural outcomes may be predicted. Within Type II alone, variation in host cell gene expression Hill et al, 2012) and development (Diana et al, 2004), host immune and encephalitic response (Araujo and Slifer, 2003;Hill et al, 2012), parasite dissemination, reactivation and recrudescence (Saeij et al, 2005), and impact on host behaviour (Kannan et al, 2010) have all been reported between different strains. For example, whilst both Prugniaud (Pru) and ME49 have been reported to increase attraction to cat odour in mice at 2months post-infection, in at least one study this behaviour was no longer present with the ME49 strain at 7months (Kannan et al, 2010).…”

Abscisic acid controls calcium-dependent egress and development in Toxoplasma gondii

“…injection for 12 days. To monitor tissue burdens, mice were challenged with the luciferase expressing Pru-Luc strain and imaged as described previously 30 .…”