Facets of Theiler’s Murine Encephalomyelitis Virus-Induced Diseases: An Update

Figure 1. Theiler’s murine encephalomyelitis virus infection as a model for multiple sclerosis, epilepsy, and myocarditis: (A) The intracerebral infection of SJL mice with TO subgroup strains of TMEV results in a biphasic disease course consisting of an acute polioencephalomyelitis followed by a chronic demyelinating leukomyelitis. (B) The intracerebral infection of C57BL/6 mice leads to early symptomatic seizures in the acute phase associated with neuronal degeneration, astrogliosis, microgliosis, and inflammation in the hippocampus. Following a latent phase, mice can develop spontaneous seizures associated with neuronal loss and astrogliosis in the hippocampus. (C) The intraperitoneal infection of C3H mice leads to an infection of the heart. The acute phase is divided into phase I characterized by viral pathology triggering innate immunity and phase II consisting of antiviral immune responses and autoimmunity inducing myocardial damage. Cardiac remodeling is a dominant feature of phase III. TMEV strains are divided into two major groups, a highly neurovirulent group called George Davis 7 (GDVII) consisting of GDVII and FA strains and a low neurovirulent group called Theiler’s original…

Figure 2. Spinal cord lesions induced by Theiler’s murine encephalomyelitis virus in SJL mice: (A) Spinal cord lesions at 14 weeks post infection consist of mononuclear meningomyelitis centered in the ventral white matter (hematoxylin eosin staining). (B) The immunohistochemistry of spinal cord lesions reveals virus protein mainly located in the ventrolateral funiculus. (C) Demyelination is illustrated by a loss of myelin basic protein staining. (D) Myelin debris is removed by activated microglia/macrophages (MAC3/CD107b). (E) Abundant T cells (CD3) infiltrate spinal cord lesions. (F) Only low numbers of B-lymphocytes (CD45R) are present in the TMEV-infected spinal cord. (G) Astrogliosis (GFAP) with numerous activated astrocytes is present in spinal cord lesions. (H) Axonal degeneration is characterized by the accumulation of non-phosphorylated neurofilaments and spheroid formation. (A–H) The inserts show higher magnifications of the areas delineated by the black rectangles. The bars represent 200 µm and 20 µm for overviews and inserts, respectively.

Figure 3. The ultrastructure of spinal cord lesions in Theiler’s murine encephalomyelitis virus-infected SJL mice: (A) A dilated myelin sheath (red asterisk) with an irregular separation of myelin lamellae (intramyelinic vacuolization) and axon loss, a degenerated axon with an accumulation of electron dense material (white asterisk) at 42 days post infection (dpi), and a normally myelinated axon (red circle) are shown. (B) Macrophages/microglia with phagocytized myelin fragments (white triangles) at 42 dpi, characteristic of myelinophagia (M = nucleus of a macrophage/microglial cell), are shown. (C) A demyelinated lesion containing remyelinated axons with thin myelin sheaths (arrows), characteristic of oligodendrocyte-mediated remyelination (O = nucleus of an oligodendrocyte), is shown at 196 dpi. (D) Schwann cell remyelination at 196 dpi, characterized by comparatively thick newly formed myelin sheaths (arrows) and a one Schwann cell per axon relationship (S = nucleus of a Schwann cell), is shown.

Figure 4. Hippocampal lesions induced by Theiler’s murine encephalomyelitis virus in C57BL/6 mice: (A) Intracerebral infection of C57BL/6 mice with the DA strain of TMEV causes hippocampal inflammation (HE) and neuronal pyknosis. (B) The TMEV antigen is mainly localized in pyramidal neurons of the CA1 and CA2 region at 7 days post infection (dpi). (C) Marked pyknosis and loss of NeuN-positive neurons are seen in the hippocampus at 7 dpi. (D) Prominent infiltration of macrophages and activation of microglia (MAC3/CD107b) are seen in the hippocampus at 7 dpi. (E) Numerous T cells (CD3) are seen in the hippocampal lesions at 7 dpi. (F) Neuronal degeneration and inflammatory changes are associated with strong astrogliosis (GFAP) at 14 dpi. (A–F) The inserts show higher magnifications of the areas delineated by the black squares. The bars represent 200 µm and 20 µm for overviews and inserts, respectively.

Figure 5. EEG recordings in TMEV-DA-infected C57BL/6 mice: TMEV-DA-infected C57BL/6 mice were continuously (24/7) video/EEG monitored for the occurrence of early (0–7 dpi) and late (>7 dpi) seizures. EEG recordings were performed via chronically implanted cortical electrodes. Early seizures were exhibited in 77% of the mice in this study, while the incidence of late spontaneous seizures was 33%. The figure illustrates (A) a normal cortical EEG recording, (B) an early generalized convulsive seizure, (C) spikes and spike clusters in a mouse with epilepsy, and (D) a late (spontaneous) generalized convulsive seizure. Based on EEG and video recordings, early and late seizures exhibited the same phenomenological characteristics. However, late epileptic seizures were much less frequent than early seizures (~1 seizure/week vs. 5.5 seizures/week; p < 0.05). Furthermore, late generalized convulsive seizures were significantly longer than such seizures in the early phase (1–7 dpi) of monitoring. In addition to generalized convulsive seizures, nonconvulsive (focal) seizures were observed both during the early and late phase following infection. The average latent phase before the onset of late…

Figure 6. The intracellular signaling pathways induced by Theiler’s murine encephalomyelitis virus: Detection of dsRNA by TLR3 and ssRNA by TLR7 in endosomes leads to the recruitment of TRIF and MyD88, which interact with TRAF3 and TRAF6 to trigger MAPK, NF-κB, and IRF signaling. This results in transcription of the pro-inflammatory cytokines IL1, IL6, and TNFα as well as antiviral IFNα/β. Viral dsRNA in the cytoplasm is recognized by MDA5, which activates IPS-1 to initiate IRF signaling via TRAF3. Moreover, MDA5 triggers NF-κB signaling via TRADD and FADD as well as caspase-8. NF-κB signaling can also be induced by PKR, which is activated by binding to cytoplasmic dsRNA. In addition, PKR phosphorylates eIF2α to inhibit mRNA and viral RNA translation and thus virus replication. Finally, caspase-8 can cleave the inhibitory N-terminus of PKR to activate this antiviral protein, whose expression is stimulated by IFNα/β. Only selected pathway members are shown due to simplification. Solid arrows indicate activation while “T” indicates inhibition.

Figure 7. A schematic overview of the cytokine- and chemokine-mediated effects during early inflammation in a Theiler’s murine encephalomyelitis virus infection: (1) TMEV-infection of neurons results in the expression of chemokines and IFN-I. (2) Virus replication in astrocytes and their stimulation by IFNγ and TNFα also causes cytokine and chemokine production. (3) The chemokines CCL2 and CX3CL1 attract monocytes and lymphocytes. (4) IFN-I activates B cells, T cells, microglia, and macrophages and promotes antigen presentation. (5) IL1 and TNFα stimulate leukocyte extravasation, whereas IL10 stabilizes the blood–brain-barrier. (6) Macrophages and microglia activated by IFNγ and chemokines upregulate MHC class II and costimulatory molecules and secrete IL6, IL12, and IL23 to induce the differentiation of T cells into Th1 and Th17 cells, which have antiviral and pro-inflammatory properties. (7) The activation of macrophages and microglia is inhibited by the anti-inflammatory cytokines IL10 and TGFβ1, which the production of can also be induced by IFN-I. (8) The virus elimination is partly dependent on the activated cytotoxic T cells, which also cause neuronal necrosis and axonal injury. (9)…