Viral models of multiple sclerosis: neurodegeneration and demyelination in mice infected with Theiler's virus

Abstract

Multiple sclerosis (MS) is a complex inflammatory disease of unknown etiology that affects the central nervous system (CNS) white matter, and for which no effective cure exists. Indeed, whether the primary event in MS pathology affects myelin or axons of the CNS remains unclear. Animal models are necessary to identify the immunopathological mechanisms involved in MS and to develop novel therapeutic and reparative approaches. Specifically, viral models of chronic demyelination and axonal damage have been used to study the contribution of viruses in human MS, and they have led to important breakthroughs in our understanding of MS pathology. The Theiler's murine encephalomyelitis virus (TMEV) model is one of the most commonly used MS models, although other viral models are also used, including neurotropic strains of mouse hepatitis virus (MHV) that induce chronic inflammatory demyelination with similar histological features to those observed in MS. This review will discuss the immunopathological mechanisms involved in TMEV-induced demyelinating disease (TMEV-IDD). The TMEV model reproduces a chronic progressive disease due to the persistence of the virus for the entire lifespan in susceptible mice. The evolution and significance of the axonal damage and neuroinflammation, the importance of epitope spread from viral to myelin epitopes, the presence of abortive remyelination and the existence of a brain pathology in addition to the classical spinal cord demyelination, are some of the findings that will be discussed in the context of this TMEV-IDD model. Despite their limitations, viral models remain an important tool to study the etiology of MS, and to understand the clinical and pathological variability associated with this disease.

Fig. 1

Scheme of molecular mimicry and epitope spreading models of viral induced autoimmunity. In black (continuous line): after an acute viral infection, macrophages or APC process viral peptides and present antigen to virus epitope-specific CD4 T cells in the periphery (1a) or in the CNS (1b). Activated peripheral cells cross the blood–brain barrier, releasing proinflammatory cytokines and chemokines (2) that activate and recruit monocytes and macrophages, which cause tissue damage also by bystander activation allow self-antigens release (3). The CNS persistence of virus and the presence of self-antigens in the inflammatory context cause the self-peptides processing (4). The similarity between some self-peptides (galactocerebroside, GALC) and viral peptides (VP1) causes a cross-reaction (molecular mimicry) (5). In red (dashed line): in the epitope spreading model, after a persistent viral infection (1), APCs process the antigen (2) and present it to virus epitope-specific CD4 T cells (either in the nervous tissue or in peripheral tissue, in which case the T cell would have to cross the blood–brain barrier) (3). This activation causes a release of proinflammatory cytokines and chemokines, recruiting and activating monocytes and macrophages (4), which initiate the self-tissue destruction (5). A prolonged inflammation and processing self-antigens (6) induced activation of self-epitope-specific CD4 T cells and virus-specific CD4 T cells (7), causing an immune-mediated disease prolonged in time.

Fig. 2

Pathogenic mechanisms involved in demyelination in the TMEV-IDD model. Cells of the immune system potentially involved in demyelination. APCs can take up antigens from TMEV or from self-tissue (myelin or oligodendrocyte proteins). Antigens are processed and presented to T cells via the TCR in the context of MHC class I or class II. Activated cytolytic T cells (MHC class I) induce damage by direct lysis of the target. Th cells (activated by MHC class II) release inflammatory cytokines that also activate monocytes/macrophages and microglial cells. B cells recognize surface antigen and when facilitated by T cells (Th2), secrete antibodies specific for foreign and self-epitopes.

Fig. 3

Models proposed to explain the axonal degeneration and demyelination in the TMEV model. In the Outside-In model, the lesion develops from the outside (myelin) and extends inwards (to the axon). Myelin and/or oligodendrocytes represent the primary targets for injury and after the primary destruction of myelin axonal damage is a secondary consequence. In the Inside-Out model, lesions extend from the axon to myelin. The primary target in this scenario is the axon or its cell body, the neuron, and the primary axon injury drives secondary demyelination.

Fig. 4

Microglial activation in the motor cortex of TMEV-infected susceptible SJL/J mice. (A) Representative sections (frozen sections obtained by criostate) of the motor cortex showing microglia staining by Iba-1 in sham mice and in the acute (19 dpi), early preclinical (35 dpi) and chronic phases (180 dpi) of TMEV-IDD. Scale bars 100 μm. (B) Scheme showing the time course of Iba-1 expression throughout the disease course in susceptible SJL/J infected mice.

Fig. 5

Axonal density and myelin expression in the motor cortex of TMEV-infected susceptible SJL/J mice. (A) Representative sections (frozen sections obtained by criostate) of the motor cortex showing axonal density (pan neurofilament: pan NF staining) and CNPase expression (Scale bars: 200 μm) in sham animals and TMEV-infected mice during the chronic phase (180 dpi). (B) Scheme showing the time course of pan NF and CNPase expression throughout the disease course in susceptible SJL/J infected mice.

Fig. 6

Microglial activation in the brainstem of TMEV-infected susceptible SJL/J mice. (A) Representative brainstem sections (frozen sections obtained by criostate) showing microglia stained with Iba-1 in sham mice and in the acute (19 dpi), early preclinical (35 dpi) and chronic phases (180 dpi) in TMEV-IDD mice. Scale bars: 100 μm. (B) Scheme showing the time course of Iba-1 expression throughout the disease course in susceptible SJL/J infected mice.

Fig. 7

Axon density and myelin expression in the brainstem of TMEV-infected susceptible mice. (A) Representative brain stem sections (frozen sections obtained by criostate) showing axon density (pan neurofilament: pan NF staining) and CNPase expression (Scale bar: 200 μm) in sham animals and in TMEV-infected mice during the chronic phase (180 dpi). (B) Scheme showing the time course of Pan NF and CNPase expression throughout the disease course in susceptible SJL/J infected mice.