1. Defining neuron-Toxoplasma interactions in vivo
To enable our studying of the Toxoplasma-CNS interaction at the level of the single infected cell in vivo, we pioneered a system in which host cell expression of green fluorescent protein (GFP) is triggered by injection of a specific parasite protein (Koshy et al Nat Methods 2010, Koshy et al PLoS Pathogen 2012). Using this system, we have been able to show that Toxoplasma primarily interacts with neurons throughout CNS infection, which contradicts the current dogma that all CNS cells interact equally with invading parasites (Cabral, Tuladhar, et al, accepted PLoS Pathogen). From this work, we are now actively pursuing the following areas:
a. What is the effect of Toxoplasma interaction on the injected (GFP+) neuron, compared to the bystander neuron (GFP-)?
b. Why does Toxoplasma only interact with neurons?
c. Why does Toxoplasma infect specific regions of the CNS and how does this affect functions served by those CNS regions?
2. Determining how different Toxoplasma strains cause different CNS immune responses.
In congenital infection, reactivation disease in the immunocompromised, and acute infection in the immunocompetent, disease severity can range from mild (fever) to severe (death). Recent data suggest that the genotype of the infecting strain may play a role in influencing these clinical outcomes, a hypothesis further supported by the burgeoning work that polymorphic effector proteins can trigger strain-specific effects on infected host cells. Thus, we hypothesize that these differences in clinical outcomes may be driven, in part, by strain-specific differences in global CNS immune responses. To test this hypothesis, we have established mechanisms for analyzing and quantifying the brain immune response to two of the canonical encysting Toxoplasma strains (type II and type III). We have found, contrary to what has been previously predicted, the type III strain appears to generate a more pro-inflammatory response than the type II strain. From this work, we are now actively pursuing the following areas:
a. Are these strain-specific neuroinflammatory changes driven by differences in parasite dissemination?
b. Are these neuroinflammatory differences confined to the CNS or reflective of the systemic immune response?
c. What are the strain-specific parasite proteins and genes that drive these immune response differences?
3. Identifying the mechanisms underlying Toxoplasma’s neuroprotective effect.
Toxoplasma’s ability to asymptomatically infect the CNS of up to 1/3 of the world population as well as the CNS of other mammalian hosts suggest that Toxoplasma and the mammalian CNS have co-evolved to tolerate each other. While this tolerance clearly benefits the parasite, the parasite-induced changes might also benefit the host. Support for such an idea comes from two recent mouse studies in which chronic toxoplasmosis was associated with better outcomes in both a stroke and Alzheimer’s disease (AD) mouse model. Given that our lab studies the CNS-Toxoplasma interface, our long term is to define the cellular and molecular mechanisms that underlie this Toxoplasma-associated neuroprotection. We are actively working on the following areas:
a. How does Toxoplasma prevent β-amyloid deposition in a human amyloid precursor protein mouse model?
b. Do these Toxoplasma-neuroprotective effects also protect against the cognitive decline found in normal aging?