Nonstructural Protein L* Species Specificity Supports a Mouse Origin for Vilyuisk Human Encephalitis Virus

Abstract

Vilyuisk human encephalitis virus (VHEV) is a picornavirus related to Theiler's murine encephalomyelitis virus (TMEV). VHEV was isolated from human material passaged in mice. Whether this VHEV is of human or mouse origin is therefore unclear. We took advantage of the species-specific activity of the nonstructural L* protein of theiloviruses to track the origin of TMEV isolates. TMEV L* inhibits RNase L, the effector enzyme of the interferon pathway. By using coimmunoprecipitation and functional RNase L assays, the species specificity of RNase L antagonism was tested for L* from mouse (DA) and rat (RTV-1) TMEV strains as well as for VHEV. Coimmunoprecipitation and functional assay data confirmed the species specificity of L* activity and showed that L* from rat strain RTV-1 inhibited rat but not mouse or human RNase L. Next, we showed that the VHEV L* protein was phylogenetically related to L* of mouse viruses and that it failed to inhibit human RNase L but readily antagonized mouse RNase L, unambiguously showing the mouse origin of VHEV.IMPORTANCE Defining the natural host of a virus can be a thorny issue, especially when the virus was isolated only once or when the isolation story is complex. The species Theilovirus includes Theiler's murine encephalomyelitis virus (TMEV), infecting mice and rats, and Saffold virus (SAFV), infecting humans. One TMEV strain, Vilyuisk human encephalitis virus (VHEV), however, was isolated from mice that were inoculated with cerebrospinal fluid of a patient presenting with chronic encephalitis. It is therefore unclear whether VHEV was derived from the human sample or from the inoculated mouse. The L* protein encoded by TMEV inhibits RNase L, a cellular enzyme involved in innate immunity, in a species-specific manner. Using binding and functional assays, we show that this species specificity even allows discrimination between TMEV strains of mouse and of rat origins. The VHEV L* protein clearly inhibited mouse but not human RNase L, indicating that this virus originates from mice. KEYWORDS Theiler's murine encephalomyelitis virus, Vilyuisk human encephalitis virus, L* protein, mitochondria, RNase L, species specificity, natural host D efining the natural host species of a virus is often hampered by the fact that an evolutionarily well-adapted virus tends to lose virulence toward its natural host (1, 2). In the process of coevolution, many DNA viruses integrated host genes into their own genome. This feature can provide hints to trace the original host species of a given virus (3). On the other hand, RNA viruses have a limited coding capacity and therefore do not generally integrate host-derived genes. In this case, host prediction can be aided by phylogenic proximity to viruses of known species origin. In addition, species-specific activities of certain viral proteins can help to identify the original host. For instance, Theiler's murine encephalomyelitis virus (TMEV) (or Theiler's virus) encodes the accessory L* protein that inhibits host RNase L, a well-characterized eff...

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A novel mechanism of RNase L inhibition: Theiler's virus L* protein prevents 2-5A from binding to RNase L

“…Interestingly, RNase L inhibition by L* is a highly species-specific process. Indeed, L* of the mouse TMEV strain DA inhibits mouse RNase L but not its orthologues from other tested species including rat [ 12 ], whereas L* of the rat TMEV strain RTV-1 inhibits rat but not human or even mouse RNase L [ 13 ].…”