Biology subjects

Merritt, E. F.

Publications and source records attributed to Merritt, E. F..

5 recordsLinked to original sources

bioRxiv2024

The latent stage of Toxoplasma gondii is targeted by the immune response and host protective

Latency is a microbial strategy for persistence. For Toxoplasma gondii the ability of the bradyzoite stage to form long-lived cysts is critical for transmission, while their presence in neurons is considered important for immune evasion. Development of a mathematical model highlighted that immune pressure on bradyzoites should contribute to dynamics of cyst formation and reactivation. Experimental data demonstrated that a cyst-derived antigen was recognized by CD8+ T cells and that IFN-{gamma} signaling in neurons contributes to cyst control. In addition, modeling and the use of a parasite strain unable to form bradyzoites revealed that this stage was not required for long-term persistence, but the absence of cyst formation resulted in increased tachyzoite replication in the CNS with associated tissue damage and mortality. Thus, the latent form of T. gondii is under immune pressure, mitigates infection-induced damage, and promotes survival of host and parasite.

microbiology↗

bioRxiv2024

Toxoplasma type II effector GRA15 has limited influence in vivo

Toxoplasma gondii is an intracellular parasite that establishes a long-term infection in the brain of many warm-blooded hosts, including humans and rodents. Like all obligate intracellular microbes, Toxoplasma uses many effector proteins to manipulate the host cell to ensure parasite survival. While some of these effector proteins are universal to all Toxoplasma strains, some are polymorphic between Toxoplasma strains. One such polymorphic effector is GRA15. The gra15 allele carried by type II strains activates host NF-{kappa}B signaling, leading to the release of cytokines such as IL-12, TNF, and IL-1{beta} from immune cells infected with type II parasites. Prior work also suggested that GRA15 promotes early host control of parasites in vivo, but the effect of GRA15 on parasite persistence in the brain and the peripheral immune response has not been well defined. For this reason, we sought to address this gap by generating a new II{Delta}gra15 strain and comparing outcomes at 3 weeks post infection between WT and II{Delta}gra15 infected mice. We found that the brain parasite burden and the number of macrophages/microglia and T cells in the brain did not differ between WT and II{Delta}gra15 infected mice. In addition, while II{Delta}gra15 infected mice had a lower number and frequency of splenic M1-like macrophages and frequency of PD-1+ CTLA-4+ CD4+ T cells and NK cells compared to WT infected mice, the IFN-{gamma}+ CD4 and CD8 T cell populations were equivalent. In summary, our results suggest that in vivo GRA15 may have a subtle effect on the peripheral immune response, but this effect is not strong enough to alter brain parasite burden or parenchymal immune cell number at 3 weeks post infection.

microbiology↗

bioRxiv2024

Chronic Toxoplasma gondii infection curtails the cytotoxic potential of acute T cell responses to West Nile virus in the brain.

Toxoplasma gondii (T. gondii), a common brain-tropic parasite, chronically infects the central nervous system (CNS) of up to a third of the worlds population. Constant immune surveillance interrupts cyst reactivation within the CNS and dramatically alters the immune landscape of the brain. West Nile virus (WNV) is a mosquito-borne infection with a clinical spectrum ranging from asymptomatic to mild flu-like symptoms to severe neuroinvasive disease. In a cohort of WNV infected people, we discovered a positive correlation between WNV disease severity and T. gondii seropositivity. In a mouse model pairing chronic T. gondii with acute WNV infection, we found an increased susceptibility of mice to WNV, with reduced granzyme B expression in WNV-specific T cells and increased regulatory T cell (Treg) numbers in the brain, but not the periphery. This demonstrates that the T. gondii-infected tissue microenvironment impairs immune defense against other brain infections by blunting local T cell responses.

immunology↗

bioRxiv2021

IFN-γ stimulated murine and human neurons mount anti-parasitic defenses against the intracellular parasite Toxoplasma gondii

Dogma holds that Toxoplasma gondii persists in neurons because neurons cannot clear intracellular parasites, even with IFN-{gamma} stimulation. As several recent studies questioned this idea, we used primary murine neuronal cultures from wild-type and transgenic mice in combination with IFN-{gamma} stimulation and parental and transgenic parasites to reassess IFN-{gamma} dependent neuronal clearance of intracellular parasites. We found that neurons respond to IFN-{gamma} and that a subset of neurons clear intracellular parasites via immunity regulated GTPases. Whole neuron reconstructions from mice infected with parasites that trigger neuron GFP expression only after full invasion revealed that [~]40% of these T. gondii-invaded neurons no longer harbor parasites. Finally, IFN-{gamma} stimulated human stem cell derived neurons showed a [~] 50% decrease in parasite infection rate when compared to unstimulated cultures. This work highlights the capability of human and murine neurons to mount cytokine-dependent anti-T. gondii defense mechanisms in vitro and in vivo.

neuroscience↗

bioRxiv2020

Transcriptional profiling reveals T cells cluster around neurons injected with Toxoplasma gondii proteins

Toxoplasma gondiis tropism for and persistence in the CNS underlies the symptomatic disease Toxoplasma causes in humans. Our recent work has shown that neurons are the primary CNS cell with which Toxoplasma interacts and infects in vivo. This predilection for neurons suggests that Toxoplasmas persistence in the CNS depends specifically upon parasite manipulation of the host neurons. Yet, most work on Toxoplasma-host cell interactions has been done in vitro and in non-neuronal cells. We address this gap by utilizing our Toxoplasma-Cre system that allows permanent marking and tracking of neurons injected with parasite effector proteins in vivo. Using laser capture microdissection (LCM) and RNA-seq, we isolated and transcriptionally profiled Toxoplasma-injected neurons (TINs), Bystander neurons (nearby non-Toxoplasma injected neurons), and neurons from uninfected mice (controls). These profiles show that TINs transcriptomes significantly differ from the transcriptomes of Bystander and control neurons and that much of this difference is driven by increased levels of transcripts from immune cells, especially CD8+ T cells and monocytes. These data suggest that when we used LCM to isolate neurons from infected mice, we also picked up fragments of CD8+ T cells and monocytes clustering in extreme proximity around TINs and, to a lesser extent, Bystander neurons. In addition, we found that Toxoplasma transcripts were primarily found in the TINs transcriptome, not in the Bystander transcriptome. Collectively, these data suggest that, contrary to common perception, neurons that directly interact with or harbor parasites can be recognized by CD8+ T cells.

bioinformatics↗