Lymphocytic choriomeningitis virus

Viruses are very potent infectious agents and can give a difficult task to the host protective immunity. Viruses are intracellular pathogens that depend on their host cellular machinery to survive and propagate. The spectrum of structural changes caused by a viral invasion is broad.

Cytopathogenic viruses, on one hand, induce cellular or tissue damage and therefore need to be rapidly controlled to ensure survival of the host. Few examples of such viruses include smallpox virus and the neurotropic poliovirus. A widely-used mouse model to study such cytolytic viruses is vesicular stomatitis virus (VSV). Non-cytopathogenic viruses do not damage cells directly and disease is mediated by the host immune response targeting infected cells and leading to pathology (immunopathology). Some examples of non-cytolytic viruses in humans are hepatitis C virus (HCV), and hepatitis B virus (HBV).

In order to investigate host-pathogen interactions, we exploited the lymphocytic choriomeningitis virus (LCMV) infection model. Research on LCMV infections in rodents contributed to several key immunological concepts that fundamentally shaped our current understanding of anti-viral immune responses. Some examples being persistent infections, MHC restriction, T cell tolerance, T cell memory, T cell exaustion and immune pathology107,108,109. Given the defined immunodominant and subdominant epitopes to TCR specificities and the availability of cognate tetramers for specific T cell enumeration, the use of LCMV as an experimental model to extend our understanding of host-pathogen interactions remains an active platform for immunological research worldwide.

4.1 Virus origin

LCMV is a natural pathogen of rodents that is a prototypic member of the arenaviridae family. LCMV is a non-cytolytic virus and clinical symptoms associated with LCMV infection are solely mediated by the resulting immune response (immunopathology). Arenaviruses contain several agents causing human fatal hemorrhagic fever, such as Lassa virus (LASV). The arenaviridae family is divided into two geographically and genetically distinct groups: the “Old World” and the “New World”

arenaviruses. The old-world viruses are typically found in the eastern hemisphere (Europe, Asia or Africa). By contrary, the New World viruses are exclusively found in the western hemisphere (South

and North Americas). LCMV is considered to belong to the old-world arenaviruses, although it is encountered worldwide.

Transmission of LCMV in adult rodents is thought to occur via urine, feces and infected saliva. In addition, infected mothers directly transmit the virus to their offspring via transplacental or congenital routes.

Charles Armstrong first identified LCMV during a meningitis epidemic in 1934, giving his name to this particular LCMV strain. The wildtype LCMV Armstrong strain (LCMVwt) is neurotropic and spreads rapidly in the CNS. Nowadays, numerous different LCMVwt strains with diverse immunopathogenic potentials have been described, such as Clone-13, which can, in a dose-dependent manner, cause a widespread and chronic infection in mice, or Traub and WE¹¹⁰. Importantly, the CD8⁺ T cell epitopes that are generated upon LCMV infection in C57Bl/6 mice are well defined.

In rodents, LCMVwt infection can cause lymphocytic choriomeningitis, a potentially fatal disease represented as encephalitis (transient inflammation in the brain) or aseptic meningitis (inflammation of the meninges, the surrounding membrane protecting the brain). In rare cases, LCMV infection in humans can occur but is generally resolved in a clinically silent manner.

Nevertheless, LCMV infection in immuno-compromised individuals can sometimes provoke headache, fever and, more rarely, lead to seizures and fatal meningitis^111,112.

4.2 Virus structure

The general structure of LCMV consists of nucleic acids encapsulated in a core glycoprotein coat (Figure 4A). The LCMV genome contains two negative-sense single-stranded RNA segments that are designated S (small, 3.4 kb) and L (large, 7.2 kb). Each segment encodes for two genes in an ambisense orientation, separated by a non-coding intergenic region (IGR) that folds into a hairpin structure^113,114 (Figure 4B). The L segment encodes for a matrix protein containing a RING-domain (Z, involved in particle assembly and budding) and for the viral RNA-dependent RNA polymerase (L)¹¹⁵. The S segment encodes for the viral nucleoprotein (NP) and the surface glycoprotein precursor. The glycoprotein precursor is post-translationally cleaved to form mature glycoproteins GP-1 and GP-2¹¹⁶. The viral RNA interacts with NP and L to form ribonucleoprotein complexes that are active in replication and transcription and thus form the minimal infectious unit¹¹⁷.

Each segment is flanked by highly conserved 5’ and 3’ untranslated regions (UTR) that are complementary to each other within each segment and form the LCMV genomic promoter¹¹⁸. Thus, together with the L and NP genes, the conserved UTR sequences are also part of the minimal requirements for efficient RNA replication and transcription¹¹⁹.

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Figure 4. Schematic representation of lymphocytic choriomeningitis virus (LCMV) structure and genome. (A) Scheme of LCMV structure. Two single-strands RNA segments, L (large) and S (small), as well as viral proteins (NP, L and Z) are encapsulated in a glycoprotein (GP) coat. (B) Scheme of LCMVwt (top), rLCMV/INDG (middle) and rLCMV/lsGP (bottom) genomes. All are formed by two single-strands RNA segments, S and L, encoding for two ambisense genes separated by a hairpin-shaped intergenic region (IRG). The L segment in all strains encodes for the RING-domain-containing protein (Z) and viral RNA polymerase (L). In LCMVwt, the S segment encodes for the LCMV glycoprotein (GP) and nucleoprotein (NP) genes. In rLCMV/INDG, the LCMV glycoprotein is replaced by the VSV glycoprotein from the Indiana serotype (VSV-G). In rLCMV/lsGP, the leader sequence of LCMV glycoprotein is followed by the VSV glycoprotein from the Indiana serotype (VSV-G) and there is a point mutation in the NP sequence (N400S) that abrogates the immunodominant NP396-404 epitope binding to the MHC-I. LCMVwt = wildtype LCMV Armstrong strain. Adapted from¹²⁰.

The most common cellular receptor for LCMV as well as other members of the arenavirus family to invade host is α-dystroglycan (α-DG), an extracellular membrane protein expressed on mammalian cells that binds to extracellular matrix^121,122.Viral tetramers of GP-1 and GP-2 interact with cellular glycosylated α-DG, and in particular O-mannosylation structures¹²³, meaning that potentially all cell types can be infected by LCMV.