Protective function and differentiation cues of brain-resident CD8+ T cells during surveillance of latent Toxoplasma gondii infection.

Chronic Toxoplasma gondii infection induces brain-resident CD8+ T cells (bTr), but the protective functions and differentiation cues of these cells remain undefined. Here, we used a mouse model of latent infection by T. gondii leading to effective CD8+ T cell-mediated parasite control. Thanks to antibody depletion approaches, we found that peripheral circulating CD8+ T cells are dispensable for brain parasite control during chronic stage, indicating that CD8+ bTr are able to prevent brain parasite reactivation. We observed that the retention markers CD69, CD49a, and CD103 are sequentially acquired by brain parasite-specific CD8+ T cells throughout infection and that a majority of CD69/CD49a/CD103 triple-positive (TP) CD8+ T cells also express Hobit, a transcription factor associated with tissue residency. This TP subset develops in a CD4+ T cell-dependent manner and is associated with effective parasite control during chronic stage. Conditional invalidation of Transporter associated with Antigen Processing (TAP)-mediated major histocompatibility complex (MHC) class I presentation showed that presentation of parasite antigens by glutamatergic neurons and microglia regulates the differentiation of CD8+ bTr into TP cells. Single-cell transcriptomic analyses revealed that resistance to encephalitis is associated with the expansion of stem-like subsets of CD8+ bTr. In summary, parasite-specific brain-resident CD8+ T cells are a functionally heterogeneous compartment which autonomously ensure parasite control during T. gondii latent infection and which differentiation is shaped by neuronal and microglial MHC I presentation. A more detailed understanding of local T cell-mediated immune surveillance of this common parasite is needed for harnessing brain-resident CD8+ T cells in order to enhance control of chronic brain infections.

The apicoplast is important for the viability and persistence of Toxoplasma gondii bradyzoites.

Toxoplasma gondii is responsible for toxoplasmosis, a disease that can be serious when contracted during pregnancy, but can also be a threat for immunocompromised individuals. Acute infection is associated with the tachyzoite form that spreads rapidly within the host. However, under stress conditions, some parasites can differentiate into cyst-forming bradyzoites, residing mainly in the central nervous system, retina and muscle. Because this latent form of the parasite is resistant to all currently available treatments, and is central to persistence and transmission of the parasite, specific therapeutic strategies targeting this developmental stage need to be found. T. gondii contains a plastid of endosymbiotic origin called the apicoplast, which is an appealing drug target because it is essential for tachyzoite viability and contains several key metabolic pathways that are largely absent from the mammalian host. Its function in bradyzoites, however, is unknown. Our objective was thus to study the contribution of the apicoplast to the viability and persistence of bradyzoites during chronic toxoplasmosis. We have used complementary strategies based on stage-specific promoters to generate conditional bradyzoite mutants of essential apicoplast genes. Our results show that specifically targeting the apicoplast in both in vitro or in vivo-differentiated bradyzoites leads to a loss of long-term bradyzoite viability, highlighting the importance of this organelle for this developmental stage. This validates the apicoplast as a potential area to look for therapeutic targets in bradyzoites, with the aim to interfere with this currently incurable parasite stage.

A Virus Hosted in Malaria-Infected Blood Protects against T Cell-Mediated Inflammatory Diseases by Impairing DC Function in a Type I IFN-Dependent Manner.

Coinfections shape immunity and influence the development of inflammatory diseases, resulting in detrimental or beneficial outcome. Coinfections with concurrent Plasmodium species can alter malaria clinical evolution, and malaria infection itself can modulate autoimmune reactions. Yet, the underlying mechanisms remain ill defined. Here, we demonstrate that the protective effects of some rodent malaria strains on T cell-mediated inflammatory pathologies are due to an RNA virus cohosted in malaria-parasitized blood. We show that live and extracts of blood parasitized by Plasmodium berghei K173 or Plasmodium yoelii 17X YM, protect against P. berghei ANKA-induced experimental cerebral malaria (ECM) and myelin oligodendrocyte glycoprotein (MOG)/complete Freund's adjuvant (CFA)-induced experimental autoimmune encephalomyelitis (EAE), and that protection is associated with a strong type I interferon (IFN-I) signature. We detected the presence of the RNA virus lactate dehydrogenase-elevating virus (LDV) in the protective Plasmodium stabilates and we established that LDV infection alone was necessary and sufficient to recapitulate the protective effects on ECM and EAE. In ECM, protection resulted from an IFN-I-mediated reduction in the abundance of splenic conventional dendritic cell and impairment of their ability to produce interleukin (IL)-12p70, leading to a decrease in pathogenic CD4+ Th1 responses. In EAE, LDV infection induced IFN-I-mediated abrogation of IL-23, thereby preventing the differentiation of granulocyte-macrophage colony-stimulating factor (GM-CSF)-producing encephalitogenic CD4+ T cells. Our work identifies a virus cohosted in several Plasmodium stabilates across the community and deciphers its major consequences on the host immune system. More generally, our data emphasize the importance of considering contemporaneous infections for the understanding of malaria-associated and autoimmune diseases.IMPORTANCE Any infection modifies the host immune status, potentially ameliorating or aggravating the pathophysiology of a simultaneous inflammatory condition. In the course of investigating how malaria infection modulates the severity of contemporaneous inflammatory diseases, we identified a nonpathogenic mouse virus in stabilates of two widely used rodent parasite lines: Plasmodium berghei K173 and Plasmodium yoelii 17X YM. We established that the protective effects of these Plasmodium lines on cerebral malaria and multiple sclerosis are exclusively due to this virus. The virus induces a massive type I interferon (IFN-I) response and causes quantitative and qualitative defects in the ability of dendritic cells to promote pathogenic T cell responses. Beyond revealing a possible confounding factor in rodent malaria models, our work uncovers some bases by which a seemingly innocuous viral (co)infection profoundly changes the immunopathophysiology of inflammatory diseases.

Robust Control of a Brain-Persisting Parasite through MHC I Presentation by Infected Neurons.

Control of CNS pathogens by CD8 T cells is key to avoid fatal neuroinflammation. Yet, the modalities of MHC I presentation in the brain are poorly understood. Here, we analyze the antigen presentation mechanisms underlying CD8 T cell-mediated control of the Toxoplasma gondii parasite in the CNS. We show that MHC I presentation of an efficiently processed model antigen (GRA6-OVA), even when not expressed in the bradyzoite stage, reduces cyst burden and dampens encephalitis in C57BL/6 mice. Antigen presentation assays with infected primary neurons reveal a correlation between lower MHC I presentation of tachyzoite antigens by neurons and poor parasite control in vivo. Using conditional MHC I-deficient mice, we find that neuronal MHC I presentation is required for robust restriction of T. gondii in the CNS during chronic phase, showing the importance of MHC I presentation by CNS neurons in the control of a prevalent brain pathogen.

MHC I presentation of Toxoplasma gondii immunodominant antigen does not require Sec22b and is regulated by antigen orientation at the vacuole membrane.

The intracellular Toxoplasma gondii parasite replicates within a parasitophorous vacuole (PV). T. gondii secretes proteins that remain soluble in the PV space, are inserted into PV membranes or are exported beyond the PV boundary. In addition to supporting T. gondii growth, these proteins can be processed and presented by MHC I for CD8+ T-cell recognition. Yet it is unclear whether membrane binding influences the processing pathways employed and if topology of membrane antigens impacts their MHC I presentation. Here we report that the MHC I pathways of soluble and membrane-bound antigens differ in their requirement for host ER recruitment. In contrast to the soluble SAG1-OVA model antigen, we find that presentation of the membrane-bound GRA6 is independent from the SNARE Sec22b, a key molecule for transfer of host endoplasmic reticulum components onto the PV. Using parasites modified to secrete a transmembrane antigen with opposite orientations, we further show that MHC I presentation is highly favored when the C-terminal epitope is exposed to the host cell cytosol, which corresponds to GRA6 natural orientation. Our data suggest that the biochemical properties of antigens released by intracellular pathogens critically guide their processing pathway and are valuable parameters to consider for vaccination strategies.

Reorganization of supramammillary-hippocampal pathways in the rat pilocarpine model of temporal lobe epilepsy: evidence for axon terminal sprouting.

In mesial temporal lobe epilepsy (MTLE), spontaneous seizures likely originate from a multi-structural epileptogenic zone, including several regions of the limbic system connected to the hippocampal formation. In this study, we investigate the structural connectivity between the supramammillary nucleus (SuM) and the dentate gyrus (DG) in the model of MTLE induced by pilocarpine in the rat. This hypothalamic nucleus, which provides major extracortical projections to the hippocampal formation, plays a key role in the regulation of several hippocampus-dependent activities, including theta rhythms, memory function and emotional behavior, such as stress and anxiety, functions that are known to be altered in MTLE. Our findings demonstrate a marked reorganization of DG afferents originating from the SuM in pilocarpine-treated rats. This reorganization, which starts during the latent period, is massive when animals become epileptic and continue to evolve during epilepsy. It is characterized by an aberrant distribution and an increased number of axon terminals from neurons of both lateral and medial regions of the SuM, invading the entire inner molecular layer of the DG. This reorganization, which reflects an axon terminal sprouting from SuM neurons, could contribute to trigger spontaneous seizures within an altered hippocampal intrinsic circuitry.

Adenosine receptor antagonists including caffeine alter fetal brain development in mice.

Consumption of certain substances during pregnancy can interfere with brain development, leading to deleterious long-term neurological and cognitive impairments in offspring. To test whether modulators of adenosine receptors affect neural development, we exposed mouse dams to a subtype-selective adenosine type 2A receptor (A2AR) antagonist or to caffeine, a naturally occurring adenosine receptor antagonist, during pregnancy and lactation. We observed delayed migration and insertion of γ-aminobutyric acid (GABA) neurons into the hippocampal circuitry during the first postnatal week in offspring of dams treated with the A2AR antagonist or caffeine. This was associated with increased neuronal network excitability and increased susceptibility to seizures in response to a seizure-inducing agent. Adult offspring of mouse dams exposed to A2AR antagonists during pregnancy and lactation displayed loss of hippocampal GABA neurons and some cognitive deficits. These results demonstrate that exposure to A2AR antagonists including caffeine during pregnancy and lactation in rodents may have adverse effects on the neural development of their offspring.