Genomic analysis and pathogenic characteristics of lymphocytic choriomeningitis virus strains isolated in Japan

Lymphocytic choriomeningitis virus (LCMV) is a zoonotic pathogen of which mice are the natural reservoir. Different strains and clones of LCMV show different pathogenicity in mice. Here we determined the complete genomic sequences of 3 LCMV strains (OQ28 and BRC which were isolated from mice in Japan and WE(ngs) which was derived from strain WE). Strains OQ28 and BRC showed high sequence homology with other LCMV strains. Although phylogenetic analyses placed these 2 Japanese strains in different subclusters, they belonged to same cluster of LCMV isolates. WE(ngs) and WE had many sequence substitutions between them but fell into same subcluster. The pathogenicity of the 3 new LCMV isolates was examined by inoculating ICR mice with 10² and 10⁴ TCID₅₀ of virus. ICR mice infected with OQ28 or WE(ngs) exhibited severe clinical signs, and some of the infected mice died. In contrast, all ICR mice infected with BRC showed no clinical signs and survived infection. Virus was detected in the blood, organs, or both of most of the surviving ICR mice inoculated with either OQ28 or WE(ngs). However, virus was below the level of detection in all ICR mice surviving infection with strain BRC. Therefore, LCMV strains OQ28 and BRC were genetically classified in the same cluster of LCMV strains but exhibited very different pathogenicity.Abbreviations: dpi, days postinfection; GP, viral glycoprotein; h, hydrophobic region; IFA, indirect fluorescent antibody assay; L, viral RNA-dependent RNA polymerase; LCMV, lymphocytic choriomeningitis virus; NP, nucleocapsid protein; UTR, untranslated region; Z, zinc-finger proteinLymphocytic choriomeningitis virus (LCMV) is a member of the genus Arenavirus in the family Arenaviridae. The genus Arenavirus is divided into 2 groups (Old World and New World arenaviruses) according to genetic and antigenic characteristics.⁴ LCMV is a member of the Old World arenavirus group, which also includes Lassa, Mopeia, Mobala, and Ippy viruses.^4,10 The LCMV genome contains 2 negative-sense single-stranded RNA segments, designated S RNA and L RNA, with approximate sizes of 3.4 kb and 7.2 kb, respectively.^30,31 Each RNA segment has an ambisense coding strategy, encoding 2 different proteins in opposite orientations. S RNA encodes the nucleocapsid protein and glycoprotein, and L RNA encodes the viral RNA-dependent RNA polymerase and a small zinc finger protein.^25,30LCMV is a zoonotic agent that is transmitted to humans via urine or saliva of infected mice (Mus musculus), which are a natural reservoir of the virus.⁴ The prevalence of LCMV in mice is 7.0% to 25.9% in Japan and 4% to 9% in Europe.^{5,17,19,20,35} Mice are naturally infected by either vertical or horizontal transmission of the virus, and infected mice usually show no clinical signs. In contrast, experimentally infected mice inoculated intraperitoneally or intracerebrally can exhibit clinical signs such as ruffled fur, half-closed eyes, hunched posture, immobility, and neurologic deficits.^4,12,19 Although human LCMV infections are generally either asymptomatic or mild, immunodeficient persons can develop spontaneous abortion, severe birth defects, aseptic meningitis, or fatal infections.^1,2,13,22,27 Therefore, LCMV is an important agent that should be monitored in facilities housing and breeding mice.LCMV strains Armstrong, Traub, and WE were isolated during the 1930s from laboratory mice and humans working in a mouse facility.⁴ Many other LCMV strains and clones used in research originated from these 3 isolates. Strains Aggressive and Docile are clones (variants) of strain UBC, which was derived from the parental strain WE, and strains E350, CA1371, 53b, and clone 13 were all derived from strain Armstrong.⁴ The lethality of strains Aggressive and Docile varies between mouse strains.³⁸ Mice inoculated with 53b develop acute infections, whereas those inoculated with clone 13 mount chronic infections, even though both of the strains were derived from strain Armstrong.²⁹ Furthermore, strain Armstrong produces more severe disease in C3H mice than do strains WE and Traub.⁴ Therefore, previous studies indicate that mice infected with different strains of LCMV exhibit differences in clinical signs and lethality.^4,7 LCMV is a noncytolytic virus and causes immune-mediated viral disease.¹² The clinical signs and lethal disease arise because virus-specific T cells attack infected cells on critical organs in infected mice.¹²Here we report the characterization of 2 LCMV strains recently isolated in Japan (strains OQ28 and BRC) and a passaged isolate of strain WE. The complete genomic sequences of these 3 strains were determined, and their phylogenetic relationship to other LCMV strains was assessed. We also evaluated the pathogenicity in ICR mice of these isolates.