Reply to Ruprecht and Mayer: Unearthing genomic fossils in the pathogenesis of multiple sclerosis

Transgenic expression of the HERV-W envelope protein leads to polarized glial cell populations and a neurodegenerative environment

The human endogenous retrovirus type W (HERV-W) has been identified and repeatedly confirmed as human-specific pathogenic entity affecting many cell types in multiple sclerosis (MS). Our recent contributions revealed the encoded envelope (ENV) protein to disturb myelin repair by interfering with oligodendroglial precursor differentiation and by polarizing microglial cells toward an axon-damage phenotype. Indirect proof of ENV's antiregenerative and degenerative activities has been gathered recently in clinical trials using a neutralizing anti-ENV therapeutic antibody. Yet direct proof of its mode of action can only be presented here based on transgenic ENV expression in mice. Upon demyelination, we observed myelin repair deficits, neurotoxic microglia and astroglia, and increased axon degeneration. Experimental autoimmune encephalomyelitis activity progressed faster in mutant mice equally accompanied by activated glial cells. This study therefore provides direct evidence on HERV-W ENV's contribution to the overall negative impact of this activated viral entity in MS.

The role of human endogenous retroviruses (HERVs) in Multiple Sclerosis and the plausible interplay between HERVs, Epstein-Barr virus infection, and vitamin D

Multiple Sclerosis (MS) is one of the chronic inflammatory diseases with neurological disability in the central nervous system (CNS). Although the exact cause of MS is still largely unknown, both genetic and environmental factors are thought to play a role in disease risk. Human Endogenous Retroviruses (HERVs) are endogenous viral elements of the human genome whose expression is associated with MS. HERVs are normally silenced or expressed at low levels, although their expression is higher in MS than in the healthy population. Several studies highlighted the plausible interaction between HERVs and other MS risk factors, including viral infection like Epstein-Barr viruses and vitamin D deficiency which may lead to high expression of HERVs in these patients. Understanding how HERVs act in this scenario can improve our understanding towards MS etiology and may lead to the development of antiretroviral therapies in these patients. Here in this review, we try to examine the different HERVs expression implicated in MS and their association with EBV infection and vitamin D status.

Human Endogenous Retrovirus Type W Envelope from Multiple Sclerosis Demyelinating Lesions Shows Unique Solubility and Antigenic Characteristics

In multiple sclerosis (MS), human endogenous retrovirus W family (HERV-W) envelope protein, pHERV-W ENV, limits remyelination and induces microglia-mediated neurodegeneration. To better understand its role, we examined the soluble pHERV-W antigen from MS brain lesions detected by specific antibodies. Physico-chemical and antigenic characteristics confirmed differences between pHERV-W ENV and syncytin-1. pHERV-W ENV monomers and trimers remained associated with membranes, while hexamers self-assembled from monomers into a soluble macrostructure involving sulfatides in MS brain. Extracellular hexamers are stabilized by internal hydrophobic bonds and external hydrophilic moieties. HERV-W studies in MS also suggest that this diffusible antigen may correspond to a previously described high-molecular-weight neurotoxic factor secreted by MS B-cells and thus represents a major agonist in MS pathogenesis. Adapted methods are now needed to identify encoding HERV provirus(es) in affected cells DNA. The properties and origin of MS brain pHERV-W ENV soluble antigen will allow a better understanding of the role of HERVs in MS pathogenesis. The present results anyhow pave the way to an accurate detection of the different forms of pHERV-W ENV antigen with appropriate conditions that remained unseen until now.

Endogenous Retroviruses in Nervous System Disorders

Human endogenous retroviruses (HERV) have been implicated in the pathogenesis of several nervous system disorders including multiple sclerosis and amyotrophic lateral sclerosis. The toxicity of HERV-derived RNAs and proteins for neuronal cells has been demonstrated. The involvement of HERV in the pathogenesis of currently incurable diseases might offer new treatment strategies based on the inhibition of HERV activities by small molecules or therapeutic antibodies.

Myeloid-derived suppressor cells support remyelination in a murine model of multiple sclerosis by promoting oligodendrocyte precursor cell survival, proliferation, and differentiation

The most frequent variant of multiple sclerosis (MS) is the relapsing–remitting form, characterized by symptomatic phases followed by periods of total/partial recovery. Hence, it is possible that these patients can benefit from endogenous agents that control the inflammatory process and favor spontaneous remyelination. In this context, there is increasing interest in the role of myeloid‐derived suppressor cells (MDSCs) during the clinical course of experimental autoimmune encephalomyelitis (EAE). MDSCs speed up infiltrated T‐cell anergy and apoptosis. In different animal models of MS, a milder disease course is related to higher presence/density of MDSCs in the periphery, and smaller demyelinated lesions in the central nervous system (CNS). These observations lead us to wonder whether MDSCs might not only exert an anti‐inflammatory effect but might also have direct influence on oligodendrocyte precursor cells (OPCs) and remyelination. In the present work, we reveal for the first time the relationship between OPCs and MDSCs in EAE, relationship that is guided by the distance from the inflammatory core. We describe the effects of MDSCs on survival, proliferation, as well as potent promoters of OPC differentiation toward mature phenotypes. We show for the first time that osteopontin is remarkably present in the analyzed secretome of MDSCs. The ablation of this cue from MDSCs‐secretome demonstrates that osteopontin is the main MDSC effector on these oligodendroglial cells. These data highlight a crucial pathogenic interaction between innate immunity and the CNS, opening ways to develop MDSC‐ and/or osteopontin‐based therapies to promote effective myelin preservation and repair in MS patients.

Meeting report: "Human endogenous retroviruses: HERVs or transposable elements in autoimmune, chronic inflammatory and degenerative diseases or cancer", Lyon, France, november 5th and 6th 2019 - an MS scientist's digest

The Third International Workshop on Human Endogenous Retroviruses and disease (www.hervanddisease.com), addressing HERVs or transposable elements in autoimmune, chronic inflammatory and degenerative diseases or cancer, in Lyon, France on November 5-6th 2019, once again gathered an international group of basic and clinical scientists investigating the involvement of human endogenous retroviruses (HERVs) in human diseases.